Photochemical production and evolution of selected C2–C5 alkyl nitrates in tropospheric air influenced by Asian outflow

Authors


Abstract

[1] The photochemical production and evolution of six C2–C5 alkyl nitrates (ethyl-, 1-propyl-, 2-propyl-, 2-butyl-, 2-pentyl-, and 3-pentyl nitrate) was investigated using selected data from 5500 whole air samples collected downwind of Asia during the airborne Transport and Chemical Evolution over the Pacific (TRACE-P) field campaign (February–April 2001). Air mass age was important for selecting appropriate field data to compare with laboratory predictions of C5 alkyl nitrate production rates. In young, highly polluted air masses, the ratio between the production rates of 3-pentyl nitrate and 2-pentyl nitrate from n-pentane was 0.60–0.65. These measured ratios show excellent agreement with results from a field study in Germany (0.63 ± 0.06), and they agree better with predicted ratios from older laboratory kinetic studies (0.63–0.66) than with newer laboratory results (0.73 ± 0.08). TRACE-P samples that did not show influence from marine alkyl nitrate sources were used to investigate photochemical alkyl nitrate evolution. Relative to 2-butyl nitrate/n-butane, the measured ratios of ethyl nitrate/ethane and 2-propyl nitrate/propane showed notable deviations from modeled values based on laboratory kinetic data, suggesting additional Asian sources of their alkyl peroxy radical precursors. By contrast, the measured ratios of 1-propyl-, 2-pentyl-, and 3-pentyl nitrate to their respective parent hydrocarbons were fairly close to modeled values. The 1-propyl nitrate findings contrast with field studies in North America, and suggest that air downwind of Asia was not significantly impacted by additional 1-propyl nitrate precursors. The sensitivity of modeled photochemical processing times to hydroxyl radical concentration, altitude, city ventilation times, and dilution is discussed.

1. Introduction

[2] Alkyl nitrates (RONO2) have a relatively low reactivity compared with other components of reactive odd nitrogen (NOy), which allows them to serve as a reservoir for the long-range transport of nitrogen oxides (NOx = NO + NO2). Within the troposphere the concentration of NOx controls ozone (O3) production [Talbot et al., 2000], and alkyl nitrates can be used as tracers of photochemical O3 production from anthropogenic precursors [Flocke et al., 1998]. Whereas studies of continental air masses have shown that alkyl nitrates typically comprise less than 10% of NOy [Buhr et al., 1990; Ridley et al., 1990; Flocke et al., 1991; Shepson et al., 1993], alkyl nitrates have been found to comprise a large component (20–80%) of total NOy in the equatorial marine boundary layer over the Pacific [Talbot et al., 2000; Blake et al., 2003].

[3] Alkyl nitrate sources include marine emissions [Walega et al., 1992; Atlas et al., 1993; Atlas and Ridley, 1996; Hess et al., 1996; Blake et al., 1999, 2003; Chuck et al., 2002] and photochemical production from the oxidation of parent hydrocarbons [Darnall et al., 1976; Atkinson et al., 1982; Roberts, 1990; Atkinson et al., 1995; Arey et al., 2001]. In addition, biomass burning has recently been identified as a source of C1–C4 alkyl nitrates [Simpson et al., 2002]. Although methyl nitrate (MeONO2) was the most abundant alkyl nitrate released from Australian savanna burning, biomass burning likely has only a small impact on the global MeONO2 budget. The importance of the marine alkyl nitrate source decreases with increasing carbon number, and marine emissions are only a minor source of C4 alkyl nitrates [Blake et al., 2003]. The marine source of C5 alkyl nitrates is also likely to be small. In photochemical alkyl nitrate production, whereas the branching ratios leading to alkyl nitrate formation increase with increasing carbon number [Atkinson et al., 1982; Arey et al., 2001], the ambient concentrations of parent n-alkanes decrease with increasing carbon number. Together, the net effect of these factors leads to a maximum abundance of C3–C4 alkyl nitrates from the photochemical source, in particular 2-propyl- and 2-butyl nitrate.

[4] Here we use alkyl nitrate measurements collected during the airborne NASA Transport and Chemical Evolution over the Pacific (TRACE-P) field campaign (February–April 2001) to investigate the photochemical production, transport, and chemical evolution of alkyl nitrates in tropospheric air influenced by Asian outflow. We developed the capability to accurately measure C5 alkyl nitrates shortly before the TRACE-P mission, and the TRACE-P data set provides a timely opportunity to compare field measurements of photochemical C5 alkyl nitrate production rates with newly published values from laboratory kinetic studies. In photochemical alkyl nitrate production the ratio of a daughter alkyl nitrate to its parent hydrocarbon can be used to determine the extent of photochemical processing within an air mass [Bertman et al., 1995; Flocke et al., 1998; Roberts et al., 1998; Stroud et al., 2001]. Ratios of daughter alkyl nitrates to their parent hydrocarbons were used to investigate alkyl nitrate sources, evolution, and air mass age in tropospheric air samples collected between altitudes of 150 m and 12 km.

2. Experiment

[5] The alkyl nitrate measurements presented in this paper result from a collaboration between the University of California, Irvine (UCI) and the National Center for Atmospheric Research (NCAR). During TRACE-P we measured six C2–C5 alkyl nitrates: ethyl nitrate (EtONO2), 1-propyl nitrate (1-PrONO2), 2-propyl nitrate (2-PrONO2), 2-butyl nitrate (2-BuONO2), 2-pentyl nitrate (2-PeONO2), and 3-pentyl nitrate (3-PeONO2). The measurements were made using a combination of airborne whole air sampling, followed by laboratory gas chromatography (GC) analysis with electron capture detection (ECD). Although we also measured MeONO2 during the TRACE-P mission, its photochemical production from methane is extremely slow and we do not consider the photochemical production and evolution of MeONO2 in this paper.

[6] The whole air samples were collected aboard NASA DC-8 and P-3B research aircraft during 38 science flights of the TRACE-P field campaign (24 February–10 April 2001). The air samples were collected at altitudes between 150 m and 12 km over the western and central Pacific Ocean, with main deployment bases in Hong Kong (22°N, 114°E) and Yokota Air Force Base, Japan (36°N, 140°E). A detailed overview of the sampling area, including flight tracks for both aircraft, is given by Jacob et al. [2003]. Individual air samples were collected in 2-L stainless steel canisters each equipped with a stainless steel bellows valve. Prior to the mission, the canisters were conditioned and evacuated, and 10 Torr of degassed, distilled water was added into each canister to quench active surface sites. The air samples were collected roughly every 3–7 min during horizontal flight legs and every 1–3 min during ascents and descents. Horizontal sampling times were approximately 1 min and correspond to a sampling distance of about 12 km. Approximately 160 air samples were collected per flight aboard the DC-8, and 140 were collected aboard the P-3B.

[7] After each flight, the filled canisters were transported back to the UCI laboratory. Within 10 days of being collected, each air sample was analyzed for alkyl nitrates and more than 50 other trace gases comprising nonmethane hydrocarbons (NMHCs), halocarbons, and sulfur compounds. The same canisters were reused during the course of the mission, and two identical analytical systems (sharing the same standards) were operated simultaneously in order to improve the canister turn-around time. Because it takes a few days for the analytical systems to equilibrate, they were operated 24 hours a day throughout the mission in order to generate an internally consistent data set.

[8] The reader is referred to Colman et al. [2001] for a detailed description of our analytical procedure. Briefly, to analyze each TRACE-P whole air sample, the less volatile components contained within 1520 ± 1 cm3 (STP) of canister air (i.e., alkyl nitrates, NMHCs, halocarbons, and DMS) were preconcentrated by passing the air through tubing immersed in a liquid nitrogen bath. The components were revolatilized using a hot water bath and then reproducibly split into five streams, with each stream directed to a different column-detector combination. We have found that the split ratios are highly reproducible as long as the specific humidity of the air is greater than 0.3 kPa at 298 K, which we ensure by the addition of 10 Torr (1.3 kPa) of water into each canister (see above).

[9] The C2–C5 alkyl nitrates were analyzed using two of the five column-detector combinations. The first column-detector combination (abbreviated as “Restek1701/ECD”) was a RESTEK 1701 column (60 m, 0.25 mm I.D., 0.50 μm film thickness) output to an ECD. The second combination (“DB5-Restek1701/ECD”) was a DB-5 column (J&W, 30 m, 0.25 mm I.D., 1 μm film thickness) connected in series to a RESTEK 1701 column (5 m, 0.25 mm I.D., 0.5 μm film thickness) and output to an ECD. The Restek1701/ECD and DB5-Restek1701/ECD combinations received 7.2 and 6.8% of the sample flow, respectively, and were housed in separate GCs (HP-6890). For the C2–C4 alkyl nitrates the mixing ratios determined from both column-detector combinations agreed very well and were averaged to give a single mixing ratio for each sample. The slope calculated for both combinations differed from the 1:1 line by 1% for EtONO2, 2% for 1-PrONO2, 2% for 2-PrONO2, and 4% for 2-BuONO2. In each case the r2 value exceeded 0.99 and the intercept was between 0.01 and 0.04 pptv. For the C5 alkyl nitrates, the two combinations showed very good agreement for 2-PeONO2 (slope = 1.04; r2 = 0.98) and fair agreement for 3-PeONO2 (slope = 1.22; r2 = 0.98). The Restek1701/ECD results were not used for 3-PeONO2 because they showed a poorer correlation with the 2-BuONO2 mixing ratios than did the DB5-Restek1701/ECD results. For similar reasons, the DB5-Restek1701/ECD results were not used for 2-PeONO2.

[10] The alkyl nitrate measurements were calibrated using two whole air working standards that were analyzed every four samples and have been previously calibrated against a synthetic standard prepared at NCAR. The alkyl nitrate measurement precision is conservatively 2% for mixing ratios above 5 parts per trillion by volume (pptv) and 10% for mixing ratios below 5 pptv. The accuracy of the alkyl nitrate measurements is 10–20%, and the alkyl nitrate detection limit is 0.01 pptv.

3. Alkyl Nitrate Chemistry

3.1. Photochemical Production of Alkyl Nitrates

[11] The photochemical production of alkyl nitrates in the troposphere is initiated by the oxidation of parent hydrocarbons (RH) by the hydroxyl radical (OH) leading to the formation of parent alkyl peroxy radicals (RO2):

equation image
equation image

where k1 and k2 are reaction rate constants and the branching ratio α1 quantifies the fractional yield of a specific parent RO2. For example, for C3–C4 parent n-alkanes (propane, i-butane, and n-butane), equation (1) gives rise to two possible RO2 isomers, depending on which hydrogen is abstracted by OH. (The group rate constant for hydrogen atom extraction from alkanes is larger for a tertiary carbon than a secondary carbon and larger for a secondary carbon than a primary carbon (Table 1).) The branching ratio α1 is calculated using the group rate constants and group substituent factors derived by Atkinson [1987] and revised by Kwok and Atkinson [1995] (Table 1).

Table 1. Structure-Activity Parameters for Hydrogen Atom Abstraction From -CH3, -CH2-, and >CH- Groups in Alkane Moleculesa
SourcekprimksecktertF(-CH3)F(-CH2-)F(>CH-)
  • a

    Here, kprim, ksec, and ktert are group rate constants for H-atom abstraction from -CH3, -CH2-, and >CH-, respectively (all k values are for 298 K and have units of 10−12 cm3 molec−1 s−1); F(X) is the group substituent factor for substituent group X (F(-CH3) is 1.00 by definition).

Atkinson [1987]0.1440.8381.831.001.291.29
Kwok and Atkinson [1995]0.1360.9341.941.001.231.23

[12] At NO mixing ratios greater than about 30–40 pptv the subsequent reaction of RO2 with NO dominates over RO2 reaction with nitrogen dioxide (NO2), hydroperoxy radicals (HO2), or other RO2 radicals [Logan, 1985; Finlayson-Pitts and Pitts, 2000]. The reaction of RO2 with NO has two possible outcomes (equations (3a)(3b)), one of which leads to the formation of a daughter alkyl nitrate (equation (3b)):

equation image
equation image

where k3a and k3b are reaction rate constants and the branching ratio α2 quantifies the fractional yield of the daughter alkyl nitrate from the reaction of RO2 with NO:

equation image

Alkyl nitrates are a minor product of RO2 + NO reaction. In laboratory studies, α2 has been found to range from 0.006 for EtONO2 [Ranschaert et al., 2000] to 0.126 for 3-PeONO2 [Arey et al., 2001]. In addition to increasing with increasing carbon number, the magnitude of α2 is also a function of temperature and pressure [Atkinson et al., 1982; Carter and Atkinson, 1989].

3.2. Photochemical Destruction of Alkyl Nitrates

[13] Alkyl nitrates are destroyed by photolysis or by reaction with OH:

equation image
equation image

where JRONO2 and k4 are the reaction rate constants for the loss of alkyl nitrates by photolysis and OH, respectively. Previous studies have found that the photolysis pathway is the dominant loss process for C1–C3 alkyl nitrates [Roberts and Fajer, 1989; Clemitshaw et al., 1997; Talukdar et al., 1997]. For example, Talukdar et al. [1997] calculated that reaction with OH can contribute approximately 20% of the total loss rate for EtONO2 and 2-PrONO2 and less than 10% for MeONO2 (see Talukdar et al. [1997] for assumed conditions).

[14] The rate of RONO2 destruction varies with altitude, season, and latitude. For example, JRONO2 is a function of both solar flux intensity (which increases with altitude) and absorption cross section (which decreases with altitude) [Clemitshaw et al., 1997 and references therein; Talukdar et al., 1997]. Overall, the higher solar flux intensities at higher altitudes cause JRONO2 to increase with height in the troposphere [Talukdar et al., 1997]. For 1 April and 40°N, Clemitshaw et al. [1997] calculated that J-values for selected C2–C5 alkyl nitrates are more than a factor of two higher at 10 km than at the surface. By contrast, the reaction of RONO2 with OH decreases with increasing altitude because of slower rate constants and lower OH concentrations. For conditions comparable to those encountered during TRACE-P (1 April, 40°N), the surface level lifetimes of the C2–C5 alkyl nitrates considered here range from 6 to 17 days (Table 2).

Table 2. Kinetic Data for the Alkyl Nitrates Measured During TRACE-Pa
Alkyl NitrateRONO2 productionRONO2 lossLifetime
k1bkAk4ck4 [OH]JdkB
298 K245 K0 km10 km298 K245 K0 km10 km0 km10 km0 km10 km0 km
  • a

    Units: 10−12 cm3 molec−1 s−1 for k1 and k4; 10−6 s−1 for kA, kB, k4[OH], and J; days for lifetime. We emphasize that the 10 km J-values were calculated without applying temperature-dependent absorption cross-sections and are likely overestimated. Diurnally averaged [OH] of 8.8 × 105 and 3.2 × 105 molec cm−3 were used for calculations at 0 km and 10 km, respectively. “n/a” = not available.

  • b

    Atkinson [1997]. Although an average temperature of 230 ± 5 K was measured between 9.5 and 10.5 km during TRACE-P, we used 245 K to simulate upper tropospheric temperatures because temperature-dependent rate constants are only available over the restricted range 245–330 K.

  • c

    Atkinson et al. [1997]. Entries marked with an asterisk are from Atkinson [1990]. Temperature-dependent rate constants are only available over the restricted temperature range 290–380 K, and k4 could not be calculated for upper tropospheric temperatures.

  • d

    Diurnal J-values were calculated using absorption cross-sections reported by Roberts and Fajer [1989] and conditions appropriate for TRACE-P (1 April; 40°N; ozone column = 350 Dobson Units; see text).

EtONO20.2540.120.220.040.49n/a0.43n/a0.240.550.67n/a17
1-PrONO21.120.720.990.230.73n/a0.64n/a0.330.740.97n/a12
2-PrONO21.120.720.990.230.49n/a0.43n/a0.571.271.00n/a12
2-BuONO22.441.832.150.590.92*n/a0.81n/a0.471.061.28n/a9
2-PeONO24.003.093.520.991.85*n/a1.63n/a0.461.042.09n/a6
3-PeONO24.003.093.520.991.12*n/a0.99n/a0.441.011.43n/a8

3.3. Alkyl Nitrate Evolution

[15] Assuming that OH reaction with the parent hydrocarbon is the rate-limiting step in photochemical alkyl nitrate formation, equations (1)(3) and (5)(6) can be simplified to sequential alkyl nitrate production and destruction [Bertman et al., 1995]:

equation image
equation image

where kA = k1[OH] and kB = JRONO2 + k4[OH] are pseudo-first order rate constants, and the factor β = α1α2 takes into account the branching ratios that lead to the alkyl nitrate formation, assuming no peroxy radical self-reaction. The time evolution of a daughter alkyl nitrate to its parent hydrocarbon in an air mass can then be modeled by [Bertman et al., 1995]:

equation image

where the initial concentration of the alkyl nitrate is assumed to be zero.

[16] If we assume that photochemical losses are negligible compared to production (such as in very fresh air masses) and consider only chemical terms (i.e., dilution acts on each species equally and there is negligible contribution from marine alkyl nitrate sources), then the measured mixing ratios of two daughter alkyl nitrates i and j that originate from the same parent hydrocarbon are related by [Flocke et al., 1998]:

equation image

The left hand side of equation (10) is termed the measured ratio, mmeas, because it can be quantified by atmospheric measurements. The right hand side is the predicted ratio, mpred = βij, which is based on the branching ratios α1 and α2. The C5 alkyl nitrates 2-PeONO2 and 3-PeONO2 both originate from n-pentane, and the slope of 3-PeONO2 vs. 2-PeONO2 is expected to equal β3-PeONO22-PeONO2 for freshly polluted air masses. By contrast, equation (10) does not apply for alkyl nitrates <C4 because they are formed from the decomposition of other, larger alkoxy radicals in addition to their parent hydrocarbons [Flocke et al., 1998; Roberts et al., 1998].

4. Results and Discussion

[17] This paper focuses on three aspects of photochemical alkyl nitrate production and evolution. In all three cases, TRACE-P alkyl nitrate measurements are compared with predictions based on laboratory kinetic studies. First, the photochemical production of 3-PeONO2 and 2-PeONO2 from n-pentane in relatively fresh air masses (where photochemical losses are likely negligible compared to production) are used to assess laboratory predictions of C5 alkyl nitrate production. Second, RONO2/RH ratios are used to quantitatively investigate photochemical processing within air masses sampled during TRACE-P. Third, RONO2/RH ratios are used to detect additional sources of alkyl nitrate precursors in Asian outflow. The two young, polluted plumes used in the first and second investigations are described below, following an overview of the alkyl nitrates sampled during TRACE-P.

4.1. Overview of TRACE-P Alkyl Nitrate Measurements

[18] Of the 5493 whole air samples collected aboard the NASA DC-8 and P-3B research aircraft during TRACE-P, the percentage of measurements in which alkyl nitrates were detectable exceeded 99% for all reported alkyl nitrates ≤C4, 97% for 2-PeONO2, and 84% for 3-PeONO2. The mixing ratios of the C2–C5 alkyl nitrates decreased with increasing altitude (Table 3), consistent with their surface sources and their short atmospheric lifetimes relative to the vertical tropospheric mixing time of ∼1 month. The maximum abundance of 2-PrONO2 and 2-BuONO2 indicates the importance of photochemical alkyl nitrate production during TRACE-P.

Table 3. Alkyl Nitrate Mixing Ratio Statistics (pptv) for All DC-8 and P-3B Flights of the TRACE-P Missiona
CompoundMinimumMaximumMedianMeanStandard Deviation
0–2 km2–7 km7–12 km0–2 km2–7 km7–12 km0–2 km2–7 km7–12 km0–2 km2–7 km7–12 km0–2 km2–7 km7–12 km
  • a

    A total of 5493 samples were collected at altitudes between 150 m and 12 km, mainly over the western and central Pacific. At low altitudes including the marine boundary layer (0–2 km), the number of samples (n) was 1732; in the lower free troposphere (2–7 km) n = 2835; in the upper troposphere and lower stratosphere (7–12 km) n = 926. LOD = limit of detection.

EtONO20.800.120.0519.37.78.04.12.11.44.22.31.61.81.20.9
1-PrONO20.140.01LOD10.63.63.31.60.51.41.70.60.31.00.60.4
2-PrONO20.720.060.0256.422.719.710.73.41.311.14.42.16.43.52.3
2-BuONO20.250.010.0167.129.322.911.92.00.313.03.71.19.04.32.3
2-PeONO20.03LODLOD30.110.07.33.40.50.14.11.00.33.11.30.6
3-PeONO2LODLODLOD20.07.85.72.70.3LOD3.20.70.22.51.10.5

[19] The TRACE-P alkyl nitrate mixing ratios can be compared to studies in urban continental, rural continental, and marine environments (Table 4). For example, in Karachi, Pakistan during winter, Barletta et al. [2002] measured mean ground-level C2–C4 mixing ratios that are 2–3 times larger than the mean values reported between 0 and 2 km during TRACE-P. In Athens, Greece during summer, Glavas and Moschonas [2002] measured mean 2-PeONO2 and 3-PeONO2 mixing ratios that are 5–9 times higher than the mean mixing ratios sampled between 0–2 km during TRACE-P and comparable to the maximum TRACE-P mixing ratios measured between 0 and 2 km. In rural Alabama during summer, Bertman et al. [1995] measured mean alkyl nitrate mixing ratios that are similar to those measured between 0 and 2 km during TRACE-P, with the exception of EtONO2, which was three times higher in the Alabama study. At the remote Mauna Loa Observatory in Hawaii during late spring, Elliot et al. [1992] measured upslope mixing ratios that are generally much smaller than those observed during TRACE-P. Stroud et al. [2001] measured airborne alkyl nitrate mixing ratios over the northeastern plains of Colorado during summer, between altitudes of 0 and 7 km. Their mean alkyl nitrate mixing ratios lie in between the TRACE-P means for 0–2 km and 2–7 km, although the results are not statistically different. Overall, individual alkyl nitrate measurements during the TRACE-P mission covered the range from clean remote air to polluted continental air. The mean TRACE-P measurements were of similar magnitude to alkyl nitrate mixing ratios in rural continental air from the United States.

Table 4. Mean Alkyl Nitrate Mixing Ratios (pptv) Measured at Selected Urban, Rural, and Marine Locationsa
CompoundUrban Karachi Mixing RatiobUrban Athens Mixing RatiocRural Alabama Mixing RatiodMauna Loa Mixing RatioeSTERAO Mixing Ratiof
  • a

    All air samples were collected at ground level with the exception of the Stratospheric Tropospheric Exchange: Radiation, Aerosols, and Ozone (STERAO) measurements, which were airborne samples collected over the northeastern plains of Colorado. Error bars represent 1 standard deviation. n/a = not available.

  • b

    Barletta et al. [2002].

  • c

    Glavas and Moschonas [2001].

  • d

    Bertman et al. [1995].

  • e

    Atlas et al. [1992].

  • f

    Stroud et al. [2001].

EtONO215.5 ± 4.6n/a12n/a2.8 ± 1.7
1-PrONO25.0 ± 1.6n/a20.67 ± 0.251.4 ± 1.0
2-PrONO221.0 ± 7.6n/a131.91 ± 0.848.7 ± 6.6
2-BuONO225.1 ± 12.8n/a141.55 ± 0.815.9 ± 6.4
2-PeONO2n/a3570.37 ± 0.20n/a
3-PeONO2n/a1640.25 ± 0.171.3 ± 1.3

4.2. Shanghai Plume

[20] A fresh, well-defined plume was encountered at an altitude of 330 m on 21 March 2001 from 1335–1355 local time (0435–0455 UTC) during DC-8 Flight 13 (Figure 1). This was a local flight deployed from Yokota Air Force Base and flown over the Yellow and East China Seas. The plume was characterized by high levels of short-lived compounds such as ethene (lifetime ∼ 1–2 days) and propene (lifetime ∼ 8–12 hours) (680–3050 pptv and 5–110 pptv, respectively). However, the very short-lived hydrocarbon trans-2-butene (lifetime ∼ 4 hours) did not exceed its detection limit during the plume encounter. The plume contained the highest pentyl nitrate levels of the mission, with mixing ratios up to 30 pptv (2-PeONO2) and 20 pptv (3-PeONO2) (Figure 2a). These mixing ratios are comparable to mean mixing ratios measured in the city of Athens during summer [Glavas and Moschonas, 2002]. Based on the observed mixing ratios of compounds with major anthropogenic sources, such as carbon monoxide (CO) and the industrial tracer tetrachloroethene (C2Cl4), the Shanghai plume was the most polluted air mass sampled during the TRACE-P mission. During the plume encounter, C2Cl4 mixing ratios exceeded 120 pptv, compared with a median C2Cl4 mixing ratio of 5 pptv during the remainder of the flight.

Figure 1.

Sampling locations of the Shanghai plume (28.8–30.3°N; 125.1–125.2°E) and the Seoul plume (39.2–39.5°N; 135.3–135.4°E), together with their respective 5-day backward trajectories.

Figure 2.

Measured mixing ratios of 3-PeONO2 versus 2-PeONO2 during TRACE-P. (a) Shanghai plume; (b) Seoul plume; (c) all TRACE-P DC-8 and P-3B flights.

[21] Five-day backward trajectories provided by Florida State University (FSU) suggest that the plume had encountered the city of Shanghai on the Asian coast roughly 18 hours prior to sampling. The high levels of short-lived alkenes suggest that the Shanghai plume was relatively fresh and is a good candidate for investigating the photochemical production of C5 alkyl nitrates. The requirement to consider only chemical terms in applying equation (10) is satisfied because the marine source of C5 alkyl nitrates is expected to be small (section 1). In addition, dilution is presumed to act equally on 2-PeONO2 and 3-PeONO2 because of their similar lifetimes (Table 2).

4.3. Seoul Plume

[22] The second-highest pentyl nitrate mixing ratios during TRACE-P were encountered during a spiral into the marine boundary layer aboard P-3B Flight 19, a local flight deployed from Yokota Air Force Base and flown over the Sea of Japan (Figure 1). The spiral occurred on 3 April 2001 between 1310 and 1320 local time (0410–0420 UTC). Mixing ratios as high as 16 pptv (2-PeONO2) and 12 pptv (3-PeONO2) were measured at altitudes below 900 m (Figure 2b). Five-day backward trajectories show that the air mass had traveled over Beijing roughly 1.5 days prior to sampling, then had been advected over Seoul 0.5–1 day prior to sampling. As with the Shanghai plume, mixing ratios of the very short-lived hydrocarbon trans-2-butene did not exceed its detection limit during the plume encounter, but elevated mixing ratios of propene (5–25 pptv) and ethene (140–790 pptv) were again detected.

[23] To investigate possible influence of Beijing air (which would contribute relatively aged components) on the sampled air mass, chemical tracers distinctive of emissions from China (halon 1211 or H-1211, and methyl chloride or CH3Cl [Blake et al., 2001]) and Japan/Korea (methyl bromide or CH3Br) were examined. The plume showed a 150% increase in CH3Br mixing ratios relative to the rest of the flight, with only minor increases in H-1211 (7%) and CH3Cl (8%). By contrast, the Shanghai plume showed large enhancements of both H-1211 (225%) and CH3Cl (115%). These results suggest that the plume encountered during P-3B Flight 19 was strongly influenced by pollution from Seoul, with little influence from Beijing pollution sources further upwind. The backward trajectories transit over Beijing around midnight local time, at altitudes above 700 hPa (2.5 km). We believe that the Seoul plume shows little influence from Beijing because Beijing's pollution was contained within a compressed nighttime boundary layer, and the trajectories passed over Beijing at an altitude above the main pollution layer. The freshness of the Seoul emissions and the large pentyl nitrate mixing ratios suggest that the Seoul plume is also a good candidate for investigating the photochemical production of C5 alkyl nitrates. Detailed evidence that supports using the Shanghai and Seoul plumes in this investigation is given in section 4.5.

4.4. Photochemical Production of C5 Alkyl Nitrates

[24] The measured slopes of 3-PeONO2 vs. 2-PeONO2 during the Shanghai and Seoul plume encounters were 0.60 ± 0.01 pptv/pptv (r2 = 0.998) and 0.65 ± 0.02 pptv/pptv (r2 = 0.996), respectively (Figure 2a2b). That is, for every 100 molecules of 2-PeONO2 that were formed from the oxidation of n-pentane, 60–65 molecules of 3-PeONO2 were produced. The slopes from the Shanghai and Seoul plumes show excellent agreement with a slope mmeas = 0.63 ± 0.06 pptv/pptv from a ground-based field study in Germany [Flocke et al., 1998]. They also lie within the uncertainty range of mmeas = 0.68 ± 0.09 pptv/pptv based on upslope measurements at the Mauna Loa Observatory [Atlas et al., 1992]. However, the Mauna Loa air samples are likely more aged than the Shanghai and Seoul plumes and as a result are not expected to be good candidates for comparison with mpred (see section 4.5.1).

[25] The measured slopes can be compared with the predicted slope mpred = βij, which is based on the branching ratios α1 and α2 (Table 5). Here, mpred was calculated using both old and newly published values of α1 and α2 in order to investigate the magnitude by which mpred is affected by the new branching ratios (Table 6). The branching ratio α1 was calculated from the structure-activity relationships described by Atkinson [1987], using the parameters derived by Atkinson [1987] and updated by Kwok and Atkinson [1995] (Table 1). The reader is referred to Finlayson-Pitts and Pitts [2000] for a detailed sample calculation. For the alkyl nitrates listed in Table 5, the new structure-activity parameters caused the largest change for the primary alkyl nitrates: α1 decreased by 14% for 1-PrONO2, whereas α1 increased by 2–6% for secondary RONO2, and decreased by 2% for 3-PeONO2.

Table 5. Branching Ratios Leading to the Formation of Daughter Alkyl Nitrates From Parent Hydrocarbons (See Equations (1) and (3b))a
Parent RHDaughter RONO2α1bα2cβd
19871995198720011987, 19871995, 1901
  • a

    Here α1 is the formation yield of a parent alkyl radical from the reaction of a parent hydrocarbon with OH; α2 is the formation yield of a daughter alkyl nitrate from the reaction of a parent alkyl peroxy radical with NO; β is α1α2.

  • b

    Here α1 was calculated using rate constants for the abstraction of hydrogen atoms from C3–C5n-alkanes. These rate constants were determined using the group rate constants and group substituent factors derived by Atkinson [1987] and revised by Kwok and Atkinson [1995].

  • c

    Here α2 values are from Atkinson et al. [1987] and Arey et al. [2001].

  • d

    Here β was calculated using both older and newly published values of α1 and α2. For example, “1987, 1987” pairs α1 values from Atkinson [1987] with α2 from Atkinson et al. [1987]. An asterisk denotes that new α2 branching ratios are not available from Arey et al. [2001] for EtONO2 and 1-PrONO2. For EtONO2, 0.006 is the α2 branching ratio reported by Ranschaert et al. [2000] at 298 K. For 1-PrONO2, β has been calculated by pairing newer α1 values from Kwok and Atkinson [1995] with older α2 values from Atkinson et al. [1987].

EthaneEtONO211<0.0140.006*<0.0140.006*
Propane1-PrONO20.3070.2640.020n/a0.0060.005*
Propane2-PrONO20.6930.7360.0420.0390.0290.029
n-Butane2-BuONO20.8530.8720.0900.0840.0770.073
n-Pentane2-PeONO20.5500.5680.1290.1060.0710.060
n-Pentane3-PeONO20.3560.3490.1310.1260.0470.044
Table 6. Measured and Predicted Slopes (mmeas and mpred, Respectively) for 3-PeONO2 Versus 2-PeONO2a
mmeasReferencempredα1 Referenceα2 Reference
  • a

    The measured slopes from TRACE-P were calculated for the young, highly polluted Shanghai and Seoul plumes.

0.63 ± 0.06Flocke et al. [1998]0.66Atkinson [1987]Atkinson et al. [1987]
0.60 ± 0.01Shanghai plume0.63Kwok and Atkinson [1995]Atkinson et al. [1987]
0.65 ± 0.02Seoul plume0.77Atkinson [1987]Arey et al. [2001]
  0.73 ± 0.08Kwok and Atkinson [1995]Arey et al. [2001]

[26] Arey et al. [2001] present new values of alkyl nitrate formation yields from parent RO2 molecules (α2) and from parent RH molecules (α1α2) for selected C5–C8 alkyl nitrates. The C5 values cited by Arey et al. [2001] are based on work by Atkinson et al. [1995]. For hexyl, heptyl, and octyl nitrates, the sum of isomeric alkyl nitrate yields from a given parent n-alkane are about 32, 39, and 29% lower, respectively, than data previously reported from the same laboratory by Atkinson et al. [1982]. Although the new total pentyl nitrates yields are about 10% lower than the earlier data, the results are not statistically different to within the stated uncertainties. Arey et al. [2001] suspect that wall losses (especially during calibration) may have contributed to the higher yields in the earlier study, in contrast to the new studies in which wall losses during calibration were verified to be minor or negligible.

[27] The predicted slope of 3-PeONO2 versus 2-PeONO2 ranges from 0.63 to 0.66 when α1 values from Atkinson [1987] and Kwok and Atkinson [1995] are paired with older α2 values from Atkinson et al. [1987] (Table 6). Flocke et al. [1998] found that within their error limits, their reported value of mmeas = 0.63 ± 0.06 agreed well with mpred = 0.66 based on older α1 and α2 values. When newer α2 values are used, mpred increases by 16% to 0.73–0.77. The increase is caused by the larger decrease of α2 for 2-PeONO2 relative to 3-PeONO2 (Table 5). Pairing the newer values of α1 and α2 gives the lower result of mpred = 0.73. The uncertainty in mpred = 0.73 can be determined from the uncertainties in the formation yields (α1α2) of 3-PeONO2 (0.044 ± 0.006) and 2-PeONO2 (0.060 ± 0.010) from n-pentane [Arey et al., 2001]. The uncertainty in mpred(= 0.044 ÷ 0.060) is given by equation image = 0.08.

[28] The slope of 3-PeONO2 versus 2-PeONO2 during Shanghai plume encounter is significantly smaller than the newest predicted slope from laboratory kinetic studies, whereas the slope measured during the Seoul plume encounter does not significantly differ from mpred = 0.73 ± 0.08 (Table 6). Overall, the ratio between the production rates of 3-PeONO2 and 2-PeONO2 from n-pentane measured during this study (mmeas = 0.60–0.65) shows better agreement with predicted slopes using older values of α2 (mpred = 0.63–0.66) than with predicted pentyl nitrate production rates using the most recent values of α1 and α2 (mpred = 0.73 ± 0.08). These comparisons suggest that the ratio between the production rates of 3-PeONO2 and 2-PeONO2 from n-pentane likely lies near the lower limit of the reported uncertainty range for the most recent value of mpred = 0.73 ± 0.08.

4.5. Evolution of 3-PeONO2/2-PeONO2

[29] In this section we quantitatively show why young air masses such as the Shanghai and Seoul plumes are the best candidates of the TRACE-P data set for comparing the photochemical production of C5 alkyl nitrates with laboratory predictions. The evolution of 3-PeONO2/2-PeONO2 within the Shanghai plume is used as an example. The sensitivity of the modeled 3-PeONO2/2-PeONO2 ratio to the applied initial conditions is also discussed.

4.5.1. Shanghai Plume Evolution

[30] To predict the time evolution of 3-PeONO2/2-PeONO2 within the Shanghai plume, equation (9) was rearranged to solve for the daughter pentyl nitrate:

equation image

We note that dilution is neglected in this equation (see section 4.5.2). Equation (11) was then solved for both 3-PeONO2 and 2-PeONO2 under conditions appropriate for the Shanghai plume, from t = 0 to t = 30 days. The newest values of α1 and α2 from the literature were used to calculate β (Table 5). A diurnally averaged surface-level OH concentration of [8.8 ± 4.7] × 105 molec cm−3 was used in calculating kA and kB. This OH value was determined for TRACE-P conditions (35–41°N, 0–1 km) using a model previously described by Crawford et al. [1999]. Diurnal J-values are also required to determine kB, and they were calculated assuming a clear sky using absorption cross sections reported by Roberts and Fajer [1989] and conditions appropriate for TRACE-P (1 April, 40°N, ozone column = 350 Dobson Units; Table 2), also using the model described by Crawford et al. [1999]. The reader is referred to Clemitshaw et al. [1997] and Talukdar et al. [1997] for a comparison of alkyl nitrate ultraviolet absorption cross-sections obtained from various laboratory studies since the late 1980s.

[31] The time evolution of n-pentane is also required to solve equation (11) and was determined as follows. First, the initial concentration of n-pentane, [n-pentane]0, was calculated using:

equation image

During the Shanghai plume encounter, the average mixing ratio of n-pentane was 365 ± 202 pptv (1 pptv = 2.46 × 107 molec cm−3 at the Earth's surface). The 18-hour transit time of the Shanghai plume was used to approximate the elapsed time t (the age of the Shanghai plume is examined in detail in section 4.5.2). The diurnally averaged OH concentration of 8.8 × 105 molec cm−3 gives kA = 3.5 × 10−6 s−1. Solving equation (12) under these conditions gives [n-pentane]0 ∼ 460 pptv (1.1 × 1010 molec cm−3). Equation (12) was next rearranged to its more familiar form:

equation image

allowing the depletion of n-pentane to be modeled from t = 0 to t = 30 days (Figure 3a). The calculated n-pentane values were then used to solve equation (11) for 2-PeONO2 and 3-PeONO2. We note that once a value of OH has been selected, the only two values that change with time in equation (11) are time itself, t, and [n-pentane], which decreases with time. The way in which [n-pentane] and t interact with each other determines the shape of the 2-PeONO2 and 3-PeONO2 time evolution curves.

Figure 3.

Modeled time evolution of n-pentane, 2-PeONO2 and 3-PeONO2 under selected applied conditions (see text). (a) [OH] = 8.8 × 105 molec cm−3; (b) [OH] = 17.6 × 105 molec cm−3.

[32] 2-Pentyl nitrate peaks both earlier and at a higher value than 3-PeONO2 (Figure 3a). 2-Pentyl nitrate peaks at a higher value than 3-PeONO2 because the combined branching ratio β for the photochemical formation of 2-PeONO2 is larger than that for 3-PeONO2 (Table 5). 2-Pentyl nitrate peaks earlier than 3-PeONO2 because OH reacts more readily with 2-PeONO2 than with 3-PeONO2, whereas the surface-level photolysis rate constants are similar for 2- and 3-PeONO2 (Table 2). Together, the photochemical production and destruction of the pentyl nitrates combine such that the ratio of 3-PeONO2/2-PeONO2 increases with time from its initial predicted value of 0.73 at t = 0 (Figure 4).

Figure 4.

Modeled time evolution of 3-PeONO2/2-PeONO2 using [n-pentane]0 = 460 pptv, [OH] = 8.8 × 105 molec cm−3, and kinetic parameters from Table 1.

[33] This modeled increase in the ratio of 3-PeONO2/2-PeONO2 with photochemical processing is consistent with measured 3-PeONO2/2-PeONO2 ratios during TRACE-P. The slopes of 3-PeONO2/2-PeONO2 during the Shanghai and Seoul plume encounters (0.60 ± 0.01 and 0.65 ± 0.02, respectively) are lower than a measured slope of 0.80 when data points from all the TRACE-P flights (which comprise more aged air masses) are included (Figure 2a2c). In Figure 2c the elevated mixing ratios in the top right quadrant of the graph represent the young Shanghai and Seoul plumes, and they lie below the fitted line.

[34] The key point to take from Figure 4 is that when selecting field data from which to calculate mmeas for comparison to mpred, younger air samples are more appropriate because they are expected to better approximate mpred. Under the conditions used to generate Figure 4, the ratio of 3-PeONO2/2-PeONO2 is predicted to increase 5% after one day of processing and 11% after two days. For the <1 day old Shanghai plume (section 4.5.2), the measured slope of 0.60 is expected to be within 5% of its initial value at t = 0. As a result young, polluted air masses such as the Shanghai and Seoul plumes are considered to be the best candidates of the TRACE-P field data set for comparison with predicted slopes from laboratory kinetic studies. Because the slope of 3-PeONO2/2-PeONO2 is predicted to increase exponentially with time, we suggest that air mass age of less than one or two days is a more useful criterion for applying equation (10) than our earlier, qualitative assumption that photochemical loss is negligible compared to production.

4.5.2. Sensitivity to Initial Conditions

[35] Based on the estimated initial n-pentane mixing ratio of 460 pptv for the Shanghai plume, 2-PeONO2 and 3-PeONO2 are predicted to peak at values of 13 pptv and 11 pptv, respectively (Figure 3a). By comparison, the average mixing ratios of 2-PeONO2 and 3-PeONO2 measured in the Shanghai plume were 17.5 ± 6.9 pptv and 12.4 ± 4.1 pptv, respectively. That is, the modeled peak mixing ratios are similar to values measured within the Shanghai plume, to within the natural variability of the measurements. However, when making these comparisons it is important to recognize that the RONO2 concentrations measured in the Shanghai plume may not be the peak values. Instead, the measured mixing ratios could lie before or after the plume's peak values.

[36] Direct comparison between the measured and modeled results is also impeded because an assumption in the application of equation (11) is that the initial concentration of the alkyl nitrate is zero, whereas in reality alkyl nitrate production would have begun (1) in Shanghai before the ventilation of the plume from the city and (2) possibly further upwind in air that was advected through Shanghai. In the latter case, influence from upwind air appears to be unlikely. Because ethyne is relatively short-lived (lifetime ∼ 2–3 weeks) compared to CO (lifetime ∼ 2 months), the ratio of ethyne/CO can be used to assess the amount of atmospheric processing (photochemical reaction + dynamic mixing) within an air mass [Smyth et al., 1996]. A ratio of 4–5 pptv/ppbv indicates unprocessed air; a ratio of 1–2 represents processed air; and a ratio of less than 1 indicates very processed air (E. Browell, NASA-Langley, personal communication, 2002). The median ethyne/CO ratio during the Shanghai plume encounter was 6.3 pptv/ppbv, with individual values ranging from 5.0 to 9.4 pptv/ppbv (the CO measurements were made by Glen Sachse, NASA-Langley). These results show that the Shanghai plume samples were unprocessed and were not significantly impacted by more aged components from further upwind.

[37] The extent of photochemical processing within Shanghai depends in part on the city's “ventilation time,” the time required for an emitted pollutant to be transported downwind of the city. Streets et al. [1999] report on air quality in Shanghai, and based on their work the most frequent wind speed class in Shanghai during the spring season is 3–8 m s−1, with an average of 5.5 m s−1 or roughly 20 km hr−1 (J.-H. Woo, University of Iowa, personal communication, 2002). Surface winds measured in Shanghai the day before sampling (20 March 2001) were higher than average, at roughly 10 m s−1. Pollutants that are emitted from central Shanghai need to travel roughly 50 km downwind to be ventilated from the city. Assuming a relatively constant wind direction, pollutants emitted in central Shanghai were likely transported downwind of the city in less than 2 hours on 20 March, and the age of the Shanghai plume is expected to be only a few hours older than its 18-hour transit time to the sampling location over the Pacific Ocean. The photochemical production of alkyl nitrates within Shanghai is expected to remain low as a result of Shanghai's multiple daily ventilations. However, a background alkyl nitrate concentration of even a few pptv in Shanghai violates the assumption of zero initial alkyl nitrate concentration in the application of equation (11), given that a few pptv represents a reasonable fraction of the mean 2-PeONO2 and 3-PeONO2 mixing ratios measured within the Shanghai plume (17.5 ± 6.9 pptv and 12.4 ± 4.1 pptv, respectively).

[38] Dilution competes with chemistry on all timescales, and its effect on the Shanghai plume during its 18-hour transit over the ocean must also be considered. In previous studies the ratios of various hydrocarbons have been shown to change as a function of both photochemistry and atmospheric mixing [McKeen and Liu, 1993; McKeen et al., 1996]. In the comparison between 2-PeONO2 and 3-PeONO2, dilution is presumed have a similar effect on both species because of their comparable lifetimes (Table 2), and the effect of dilution is expected to largely cancel in the ratio of 3-PeONO2/2-PeONO2.

[39] Finally, we consider the OH concentration that was applied in equation (11). Based on [OH] = 8.8 × 105 molec cm−3 (section 4.5.1), equation (11) predicts maximum 2-PeONO2 and 3-PeONO2 mixing ratios after 4 and 5 days, respectively. When the concentration of OH used to calculate kA and kB is doubled, the pentyl nitrate lifetimes with respect to OH are halved, and the pentyl nitrate concentrations peak two days earlier (Figure 3b). That is, the time at which the pentyl nitrate peaks are predicted to occur is highly sensitive to the amount of OH that is used to solve equation (11).

[40] The measured OH mixing ratio in the Shanghai plume was 0.12–0.19 pptv (3.0–4.7 × 106 molec cm−3) (H. Harder, Pennsylvania State University, personal communication, 2002). In the Shanghai plume's estimated 18 hour backward trajectory we estimate that roughly 7 hours (from 0630 to 1330 local time) were spent in daylight. Although OH exhibits a strong diurnal variation, its average mixing ratio was fairly constant between 0930 and 1330 during low-altitude flight legs of Flight 13 (0.16 ± 0.02 pptv; http://www-gte.larc.nasa.gov/trace/gte_mrg1.htm#TRACE-P), suggesting a widespread area of high OH concentration over the East China Sea on the day of sampling. If we apply 0.16 pptv from 0930 to 1330, and an estimated average OH concentration of 0.08 pptv from 0630 to 0930 (from 0 pptv at sunrise to 0.16 pptv at 0930), then the average OH concentration during the 18 hour plume travel time was 1.2 × 106 molec cm−3 (1 pptv = 2.47 × 107 molec cm3 at the Earth's surface). This is 35% higher than the applied value of 8.8 × 105 molec cm−3. The plume likely spent a couple hours in Shanghai before being ventilated from the city at roughly 1930 local time on 20 March, and additional reaction with OH is expected to have occurred within Shanghai that late afternoon/early evening. Despite the uncertainties in these calculations (such as variations in daytime OH levels and the exact amount of time spent in Shanghai before ventilation), the average OH level in the Shanghai plume's history was likely larger than the OH value that was applied in the model.

[41] In summary, the modeled pentyl nitrate mixing ratios are highly sensitive to the amount of OH that is assumed in the history of the air mass. In the case of the Shanghai plume, a direct comparison between the measured and modeled pentyl nitrate mixing ratios was not possible because background pentyl nitrate mixing ratios within Shanghai were not quantified, such that the condition of zero initial alkyl nitrate concentration could not be verified. In addition, it was not known whether the Shanghai plume was sampled at/near the peak pentyl nitrate concentrations. These unknowns precluded using the Shanghai plume measurements to validate the time and magnitude of the pentyl nitrate peaks that were predicted using equation (11).

4.6. Evolution of C2–C5 Alkyl Nitrates

[42] In photochemical alkyl nitrate evolution, the ratio of a daughter alkyl nitrate to its parent hydrocarbon increases as an air mass ages because the daughter alkyl nitrate is produced at the expense of its parent hydrocarbon. Here RONO2/RH ratios are calculated for the measured C2–C5 alkyl nitrates and compared to modeled curves based on laboratory kinetic data.

4.6.1. Photochemical Processing Times

[43] Before attempting to quantify the amount of photochemical processing within an air mass using RONO2/RH ratios, dilution needs to be considered, in addition to the sensitivity of the modeled evolution curves to the kinetic data required to solve equation (9) (k1, k4, α1, α2, [OH] and J). In the latter case, the modeled ratios of 2-PeONO2/n-pentane vs. 2-BuONO2/n-butane are used as an example. The modeled slope is found to vary only slightly depending on whether older or newer branching ratios from the literature are used (Figure 5a). Although the position of the line is quite insensitive to changes in the OH concentration, the times associated with a particular position on the line are seen to change with the OH concentration (Figure 5b).

Figure 5.

Modeled time evolution of 2-PeONO2/n-pentane versus 2-BuONO2/n-butane for surface-level conditions (see text). (a) [OH] = 8.8 × 105 molec cm−3; (b) [OH] = 17.6 × 105 molec cm−3. The dashed and solid curves show results that were calculated using older and newer branching ratios from the literature, respectively.

Figure 5.

(continued)

[44] Whereas recent alkyl nitrate field studies have been performed at ground-level [Bertman et al., 1995; Flocke et al., 1998; Roberts et al., 1998], the TRACE-P data set includes samples collected throughout the troposphere from 150 m to 12 km. Stroud et al. [2001] also measured alkyl nitrate mixing ratios in the lower and middle troposphere (0–7 km). At present it is not possible to fully quantify the effect of altitude on alkyl nitrate evolution because k4 values are not available for RONO2 + OH reaction at temperatures colder than 290 K [Atkinson et al., 1997]. By comparison, an average temperature of 230 ± 5 K was measured in the upper troposphere (9.5–10.5 km) during TRACE-P http://www-gte.larc.nasa.gov/trace/gte_mrg1.htm#TRACE-P). However, we can briefly consider the effect of altitude on the remaining kinetic parameters used to solve equation (9). The hydroxyl radical concentration and k1 both decrease with increasing altitude (decreasing temperature), leading to slower alkyl nitrate production (Table 2). Although the lower [OH] values at higher altitude also lead to slower alkyl nitrate destruction by OH, alkyl nitrate J-values increase with altitude (section 3.2). In this study J-values were calculated at 10 km under TRACE-P conditions, although the absorption cross sections were applied assuming no temperature dependence (Table 2). Because including temperature-dependent cross sections causes JRONO2 to increase more slowly with increasing altitude [Talukdar et al., 1997], the J-values calculated at 10 km are expected to be too large and the amount of alkyl nitrate destruction by photolysis at 10 km is likely overestimated. As stated above, temperature-dependent k4 and J-values are required in order to quantify how the pseudo-first order rate constant for alkyl nitrate loss (kB = k4[OH] + JRONO2) changes with altitude.

[45] Because the rate constants k1 and k4 are known for surface temperatures, photochemical ages can be considered for air masses sampled near the Earth's surface in which the initial alkyl nitrate concentration is known to be zero. Although the Shanghai and Seoul plumes were sampled at low altitude, the initial alkyl nitrate concentration within Shanghai and Seoul was likely not zero (section 4.5.2). As a result, the age of these plumes cannot be accurately quantified based on RONO2/RH ratios. Further, in air samples collected over the western Pacific during the Pacific Exploratory Mission (PEM) West A, McKeen et al. [1996] found that dilution was so effective that hydrocarbon ratios should be used as photochemical markers only for hydrocarbons more reactive than n-butane (OH lifetime ∼ 3 days under PEM-West A conditions). Under TRACE-P conditions the lifetimes of 2-PeONO2 and 3-PeONO2 are 6 and 8 days, respectively, and the lifetime of their parent n-pentane is 3–4 days. Therefore for TRACE-P, 2-PeONO2/n-pentane and 3-PeONO2/n-pentane are not good candidates for use as photochemical markers. Instead, the measured ratios of 2-PeONO2/n-pentane vs. 2-BuONO2/n-butane for the Shanghai and Seoul plumes have been superimposed on evolution curves that were generated under surface-level conditions only to illustrate the sensitivity of estimated photochemical ages to the amount of OH used in the model (Figures 5a5b). Under an applied diurnally averaged [OH] of 8.8 × 105 molec cm3, the predicted photochemical ages of the Shanghai and Seoul plumes are roughly 2–3 days and 4–5 days, respectively (Figure 5a). When the applied [OH] is doubled to 1.76 × 106 molec cm3, the estimated ages of the Shanghai and Seoul plumes are roughly 1–2 and 2–3 days, respectively (Figure 5b).

[46] Overall, the amount of photochemical processing within an air mass can be quantified using equation (9) but only under strict conditions. The modeled ages are highly sensitive to the amount of OH that is applied, and the model is limited to surface-level conditions. Although knowing the ventilation time of a city may allow the elapsed time (since the parent hydrocarbon was emitted from a polluting source) to be quantified, equation (9) only applies if the alkyl nitrate concentration prior to t = 0 was zero. In addition, the influence of dilution must be considered. Whereas sensitivity to initial conditions may not allow equation (9) to be applied quantitatively, it can be used qualitatively to study the relative extent of photochemical processing among different air masses, as discussed below.

4.6.2. Literature Comparison

[47] Alkyl nitrate evolution was modeled using equation (9) and the results were compared with observations from all TRACE-P flights. The newer branching ratios from the literature were used in this analysis. To satisfy the condition of zero initial alkyl nitrate concentrations, air samples displaying large oceanic sources of alkyl nitrates were not used. Because oceans are a major source of MeONO2, ratios of MeONO2/RONO2 can be much higher in air masses that have recently been at low altitude over the Pacific Ocean compared to those that have been influenced by urban/industrial emissions [Blake et al., 2003]. Blake et al. used ratios of MeONO2/EtONO2, MeONO2/2-PrONO2, and MeONO2/2-BuONO2 to distinguish between air masses influenced by equatorial oceanic sources and those influenced by northern hemispheric urban/industrial sources. In this study, as in the study by Blake et al., plots of MeONO2/EtONO2, MeONO2/2-PrONO2, and MeONO2/2-BuONO2 showed clear marine and nonmarine “wings” (data not shown), and the marine wings were removed from the data set. As noted in section 4.1, the influence of the marine source on the TRACE-P alkyl nitrates was less prominent compared with their photochemical source.

[48] The ratio of each daughter alkyl nitrate to its parent hydrocarbon was plotted against 2-BuONO2/n-butane using the edited data (Figures 6a6e). The ratio of 2-BuONO2/n-butane is chosen for the abscissa because n-butane is one of the most abundant hydrocarbons emitted and because 2-BuONO2 is among the most abundant alkyl nitrates formed [Roberts et al., 1998]. In addition, 2-BuONO2 is not formed in significant quantities from other decomposition pathways [Roberts et al., 1998]. The dark grey triangles in Figure 6 represent TRACE-P field data collected near the surface (0–1 km), and the light grey squares show the remaining data collected between 1 and 12 km. Lower ratios (in the bottom left of each panel) represent fresh emissions, and higher ratios (in the top right of each graph) indicate photochemically processed air. For the surface data, individual points were grouped into bins each containing 10% of the edited data by sorting the data based on 2-BuONO2/n-butane ratios. An average ratio is plotted for each bin together with error bars that represent ±1σ. The solid line in each panel is the modeled curve based on equation (9). As discussed in section 4.6.1, equation (9) can only be solved for surface-level conditions, and the modeled curves shown in Figure 6 represent surface conditions.

Figure 6.

Measured and modeled time evolution of ratios of daughter alkyl nitrates to their parent hydrocarbons versus 2-BuONO2/n-butane, for TRACE-P samples that do not show a marine alkyl nitrate source. (a) EtONO2/ethane; (b) 1-PrONO2/propane; (c) 2-PrONO2/propane; (d) 2-PeONO2/n-pentane; (e) 3-PeONO2/n-pentane. Dark grey triangles are individual TRACE-P samples collected near the surface (0–1 km); light grey squares show the remaining samples collected between 1 and 12 km; black squares are average ratios each containing 10% of the nonmarine TRACE-P data (±1σ for 2-BuONO2/n-butane in the x-direction, and ±1σ for the plotted RONO2/RH in the y-direction). The solid lines show modeled evolution curves using kinetic data from the literature.

Figure 6.

(continued)

Figure 6.

(continued)

Figure 6.

(continued)

Figure 6.

(continued)

[49] The measured RONO2/RH ratios generally lie above the modeled line for C2–C3 alkyl nitrates (Figures 6a6c) and below the modeled line for C5 alkyl nitrates (Figures 6d6e). Bertman et al. [1995], Roberts et al. [1998], and Stroud et al. [2001] observed similar results during field studies in Pennsylvania and Alabama, in Nova Scotia, and over Colorado, respectively, with the exception of pentyl nitrate results from Pennsylvania which lied above the modeled curve, possibly as a result of chromatographic coelution problems. Whereas the C2–C3 alkyl nitrates have shorter lifetimes than their parent hydrocarbons, the C4–C5 alkyl nitrates have longer lifetimes than their parents [Roberts et al., 1998]. As a result, mixing with older air masses lowers the RONO2/RH ratio for C2–C3 alkyl nitrates, and raises the ratio for C4–C5 alkyl nitrates. However, similar to findings by Roberts et al. [1998], this is opposite to the overall effects that are observed in Figures 6a6e, in which C2–C3 results are larger than predicted whereas C5 results are smaller than predicted.

[50] The average EtONO2/ethane vs. 2-BuONO2/n-butane ratios measured during TRACE-P differ from the modeled curve at short photochemical processing times and show some indication of leveling off at longer residence times, although the scatter in the data are too large to clearly show the approach to a steady state (Figure 6a). By comparison, the EtONO2/ethane vs. 2-BuONO2/n-butane ratios from Alabama were about a factor of 10 above the predicted line and were used as a strong indication of additional sources of EtONO2, such as the decomposition of larger alkoxy radicals to form ethyl radicals [Bertman et al., 1995]. Measured ratios from the Nova Scotia field study lied closer to the predicted line and tended towards it at longer photochemical processing times (higher ratios) [Roberts et al., 1998]. Roberts et al. suggested that the difference between the Alabama and Nova Scotia data sets likely arose because the Nova Scotia site (on the eastern edge of the continent) was more often influenced by aged air than the Alabama site. Older air could contain NOx levels that are below the threshold for RONO2 production, and could mix in air masses with lower EtONO2/ethane ratios. The strong tendency of the TRACE-P data towards the predicted line at longer residence times suggests that these air masses were influenced by mixing with older air masses. The deviation from the line at lower ratios is consistent with an additional source of ethyl radicals, similar to the findings by Bertman et al. [1995] and Roberts et al. [1998]. The presence of additional EtONO2 precursors is consistent with the unexpectedly high levels of acetaldehyde that were observed during TRACE-P (D. Jacob, Harvard University, personal communication, 2002).

[51] For the surface-level data the slope of 1-PrONO2/propane versus 2-BuONO2/n-butane agrees very well with predicted values, with a small (20–30%) offset above the predicted curve (Figure 6b). The largest ratios (oldest air masses) lie closer to the predicted line than smaller ratios, again consistent with mixing with older air masses at longer residence times. However, as with EtONO2/ethane, the scatter of the data is too large to discern a statistically significant trend. By contrast to the TRACE-P results, Roberts et al. [1998] observed a factor of 8–10 offset for 1-PrONO2/propane vs. 2-BuONO2/n-butane, which they attributed to a possible local source of 1-PrONO2 precursors such as 2-methyl pentane. Similarly, Stroud et al. [2001] observed a factor of 2–10 offset above the predicted line. The TRACE-P field measurements do not show a similarly large offset from the predicted line, indicating that impact from additional 1-PrONO2 precursors was not a large factor in air sampled downwind of Asia.

[52] The 2-PrONO2/propane vs. 2-BuONO2/n-butane surface-level data points show a small (25–45%) offset above the modeled curve, whereas data collected between 1 and 12 km lie further above the modeled curve for younger air masses and closer to the line for older air masses (Figure 6c). This suggests that the air masses sampled between 1 and 12 km were influenced by additional sources of 2-PrONO2 precursors, which could also help explain the high acetone levels that were observed during TRACE-P (D. Jacob, Harvard University, personal communication, 2002). We note that although the modeled curve was generated for surface conditions, it can be qualitatively compared to measured ratios at higher altitudes. For example, halving kA and doubling kB causes the modeled RONO2/RH ratios to shift to lower values but still along the same curve. Bertman et al. [1995], Roberts et al. [1998], and Stroud et al. [2001] found that their field data lied above the predicted line by factors of 2–5, 2–3, and 2–4, respectively.

[53] In contrast to the C2–C3 alkyl nitrates, the C5 alkyl nitrate results generally lie below the predicted line, although the offset is small (5–35%; Figures 6d6e). Roberts et al. [1998] and Stroud et al. [2001] also found good agreement between measured and modeled results, with measurements ∼50% lower than the predicted curve. Based on similar findings from measurements in Alabama, Bertman et al. [1995] suggested that the chemistry of C4 and C5 alkyl nitrates is reasonably well understood. Here, the similar results obtained from independent groups sampling over different global source regions suggest that the photochemical sources of 2-butyl-, 2-pentyl-, and 3-pentyl alkyl nitrates are well represented in alkyl nitrate evolution models such as equation (9).

5. Conclusions

[54] The photochemical production and evolution of six C2–C5 alkyl nitrates was investigated using selected data from 5500 whole air samples collected downwind of Asia. The airborne samples covered an altitudinal range of 150 m to 12 km and were collected during the NASA TRACE-P experiment (February–April 2001). 2-Propyl nitrate and 2-BuONO2 were the most abundant C2–C5 alkyl nitrates, indicating the importance of photochemical (rather than marine) alkyl nitrate production during TRACE-P. Individual alkyl nitrate mixing ratios ranged from clean marine to polluted continental, and mean alkyl nitrate mixing ratios were similar to those measured in and over rural areas of the United States.

[55] The TRACE-P field measurements were compared with model predictions of alkyl nitrate production and evolution. The photochemical production and destruction of 2-PeONO2 and 3-PeONO2 combine in such a way that the ratio of 3-PeONO2/2-PeONO2 is predicted to increase exponentially with time. As a result, young unprocessed air masses were found to be the best candidates for comparing photochemical production rates of C5 alkyl nitrates with laboratory predictions. In young, highly polluted air masses such as the Shanghai and Seoul plumes, the ratio of 3-PeONO2/2-PeONO2 was 0.60–0.65. These results agree with predicted ratios from older laboratory kinetic studies (0.63–0.66), and they lie in the lower end of the uncertainty range from the most recent laboratory kinetic studies (0.73 ± 0.08).

[56] Modeled photochemical ages based on RONO2/RH ratios are highly sensitive to OH concentration and are expected to be sensitive to altitude, although the dependence on altitude could not be quantified. During TRACE-P, the influence of dilution and non-zero initial alkyl nitrate concentrations precluded the use of RONO2/RH ratios as quantitative photochemical markers.

[57] The photochemical evolution of alkyl nitrates and their parent hydrocarbons was investigated using TRACE-P data that did not show influence from marine alkyl nitrate sources. Marine influence was determined using the ratios of MeONO2/RONO2, which can be much higher in air masses that have recently been at low altitude over the Pacific as compared to air masses influenced by urban/industrial emissions. Evidence for additional Asian sources of EtONO2 and 2-PrONO2 precursors, beyond what is expected from ethane and propane chemistry, was found based on deviations of measured RONO2/RH ratios from modeled evolution curves. The additional EtONO2 and 2-PrONO2 precursors could help to explain the high levels of acetaldehyde and acetone that were observed during TRACE-P. The measured ratios of 1-PrONO2, 2-PeONO2, and 3-PeONO2 to their respective parent hydrocarbons agreed fairly well with model predictions, suggesting little impact by additional precursors from Asian sources.

Acknowledgments

[58] We gratefully acknowledge excellent work by the UCI team during the TRACE-P mission (Murray McEachern, Barbara Barletta, John Bilicska, Yunsoo Choi, Lambert Doezema, Kevin Gervais, Lissa Giroux, Max Hoshino, Aaron Katzenstein, Aisha Kennedy, Jimena Lopez, Brent Love, Simone Meinardi, Jason Paisley, Aaron Swanson, Clarissa Whitelaw, and Barbara Yu). We wish to thank Hartwig Harder (Pennsylvania State University), Jung-Hun Woo (University of Iowa), David Streets (Argonne National Laboratory), Alan Fried (NCAR), Ivar Isaksen (University of Oslo), and Ed Browell (NASA-Langley) for helpful discussions and information. We also thank Daniel Jacob (Harvard University) for pointing out the possible implications of the missing EtONO2 and 2-PrONO2 sources on the high levels of acetaldehyde and acetone observed during TRACE-P. The carbon monoxide data are courtesy of Glen Sachse (NASA-Langley), and the hydroxyl data are courtesy of William Brune (Pennsylvania State University). Acetone and acetaldehyde were measured by Hanwant Singh (NASA-Ames) and by Eric Apel (NCAR) and Daniel Riemer (University of Miami).

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