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Keywords:

  • interferences;
  • oxidation capacity;
  • photochemistry;
  • polar atmosphere

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Setup
  5. 3. Results and Discussion
  6. 4. Conclusion
  7. Acknowledgments
  8. References

[1] HONO was measured by a LOPAP instrument (LOng Path Absorption Photometer) for one month during the OASIS spring 2009 campaign in Barrow, Alaska. HONO concentrations between ≤ 0.4 pptv (DL) and ∼500 pptv were measured. The very high concentrations observed on several days were caused by local direct emissions and were highly correlated with the NOx and CO data. When only “clean days” were considered, average HONO concentrations varied between ≤ 0.4 - 10 pptv. Average HONO/NOx and HONO/NOy ratios of ∼6% and ∼1% were observed, respectively, in good agreement with other remote LOPAP measurement data, but lower than measured in most other polar regions by other methods. The strong correlation between sharp peaks of OH and HONO during daytime, which was not observed for any other measured radical precursor, suggested that HONO photolysis was a major source of OH radicals in Barrow. This was supported by calculated net OH radical production by HONO and O3 photolysis for which the contribution of O3 (2%) could be neglected compared to that of HONO (98%). A net extra HONO/OH source necessary to explain elevated HONO levels during daytime of up to 90 pptv/h was determined, which was highly correlated with the actinic flux. Accordingly, a photochemical HONO source is proposed here, in good agreement with recent studies. From the higher correlation of the net HONO source with image and [NO2] compared to image and [NO3], photosensitized conversion of NO2 on humic acid containing snow surfaces may be a more likely source of HONO in the polar atmosphere of Barrow than nitrate photolysis.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Setup
  5. 3. Results and Discussion
  6. 4. Conclusion
  7. Acknowledgments
  8. References

[2] The hydroxyl radical (OH) is the major oxidant for pollutants in the atmosphere. Accordingly, the identification and quantification of OH radical sources is of high interest. Nitrous acid (HONO) is an OH radical initiation source which is important not only early in the morning [Perner and Platt, 1979] but also during daytime [Kleffmann, 2007, and references therein]:

  • equation image

Based on the results from field and corresponding modeling studies [Neftel et al., 1996; Staffelbach et al., 1997; Zhou et al., 2001; Vogel et al., 2003; Kleffmann et al., 2005; Acker et al., 2006; Elshorbany et al., 2009] strong daytime sources of HONO were proposed. Also in polar regions, unexpectedly high HONO levels have been observed over sunlit snow surfaces [Li, 1994; Zhou et al., 2001; Beine et al., 2001, 2002; Dibb et al., 2002, 2004; Honrath et al., 2002; Jacobi et al., 2004; Amoroso et al., 2006; Liao et al., 2006]. It has been estimated that the photolysis of HONO can be the dominant source of OH radicals in these regions and that it possibly controls the oxidation capacity of the lower polar boundary layer [Li, 1994; Zhou et al., 2001; Yang et al., 2002]. Accordingly, a mechanistic understanding of the HONO source reactions over irradiated snow surfaces and their quantification is of paramount importance for the chemistry of the polar atmosphere.

[3] At present, the mechanisms of HONO formation in polar regions are still not well understood. It was proposed [Zhou et al., 2001; Beine et al., 2001, 2002; Dibb et al., 2002; Honrath et al., 2002] that HONO formation occurs directly through the photolysis of nitrate on snow surfaces [Dubowski et al., 2002]:

  • equation image
  • equation image

which can also explain observed NOx emissions [e.g., Honrath et al., 1999; Cotter et al., 2003; Jacobi and Hilker, 2007]. The proposed photolysis of nitrate is based on the observed correlation between HONO formation and both, the calculated photolysis frequency of nitrate [Honrath et al., 2002] and the concentration of nitrate in snow [Dibb et al., 2002]. However, recent laboratory and accompanying modeling studies imply low direct HONO yields from the nitrate photolysis and suggest formation of HONO by secondary heterogeneous chemistry, for example via the primary formed NO2 [Jacobi and Hilker, 2007]. Such secondary chemistry was also proposed to explain the absence of significant HONO formation from nitrate-containing snow in Antarctic coastal regions [Beine et al., 2006]. One of the proposed secondary reactions on snow surfaces is the reduction of NO2 by photo-sensitized organics, for example, fulvic and humic acids, observed in laboratory studies on pure organic films and particles [George et al., 2005; Stemmler et al., 2006, 2007]:

  • equation image

Reaction (4) was also recently confirmed in laboratory experiments on ice and snow surfaces containing only traces of humic acids [Beine et al., 2008; Bartels-Rausch et al., 2010]. The first order rate coefficient of this reaction is orders of magnitude faster than the one for the heterogeneous disproportionation of NO2 with water on humid surfaces [Finlayson-Pitts et al., 2003, and references therein]:

  • equation image

which was alternatively proposed to explain the high HONO levels over snow surfaces [Zhou et al., 2001; Hellebust et al., 2007]. The much faster kinetics of reaction (4) coupled with the ubiquitous presence of organic compounds in polar regions [Grannas et al., 2004, 2007], makes this source a suitable candidate for HONO formation under polar conditions.

[4] Other proposed HONO sources in polar regions are microbiological oxidation of NH4+ [Amoroso et al., 2010] and formation via NO+ from the hydrolysis of N2O4 [Hellebust et al., 2007]. However, the first mechanism is a dark reaction, whereas elevated HONO levels are typically only observed under sunlight, and the second mechanism implies low formation rates caused by the quadratic reaction kinetics for the formation of the precursor N2O4 and the very low polar NOx levels.

[5] Reliable measurements of gaseous HONO have been made in the atmosphere since the late 1970s [Perner and Platt, 1979]. In addition to the only very recently applied spectroscopic LIF (Laser Induced Fluorescence) technique [Liao et al., 2006], only carbonate denuders [Li, 1994; Beine et al., 2001], mist chambers [Dibb et al., 2002; Honrath et al., 2002] and HPLC (High Performance Liquid Chromatography) techniques [Zhou et al., 2001; Beine et al., 2002, 2006; Amoroso et al., 2010] have been used for polar HONO measurements up to now. The common principle of these “wet chemical instruments” is the sampling of HONO on humid or aqueous surfaces followed by the analysis of nitrite. However, it is well known that many heterogeneous reactions lead to the formation of nitrite on similar surfaces. One of these reactions, typically not considered, is the reduction of NO2 by hydrocarbons during the sampling of HONO [e.g., Gutzwiller et al., 2002]. In addition, for polar measurements, interferences by HO2NO2 have been proposed [Liao et al., 2006; Clemitshaw, 2006]. Chemical interferences were recently observed and quantified for atmospheric HONO measurements using a LOPAP instrument (LOng Path Absorption Photometer) and the relative interferences were demonstrated to correlate inversely with the pollution level [Kleffmann et al., 2006; Kleffmann and Wiesen, 2008]. Thus, while these interferences may be of particular importance at low concentrations under polar conditions, they are typically not corrected for in measurements performed with wet chemical instruments. Besides interferences, artificial heterogeneous HONO formation in sampling lines of up to 30 m length, used for polar measurements [see, e.g., Zhou et al., 2001], may cause an overestimation of atmospheric HONO data.

[6] These potential problems and results from the modeling of experimental HONO, NOx and HOx data, have led to the consensus that HONO concentrations in polar regions were overestimated [Chen et al., 2004; Sjostedt et al., 2005; Bloss et al., 2006, 2010; Grannas et al., 2007; Mauldin et al., 2010]. This notion has been confirmed by the only polar HONO intercomparison study, in which a mist chamber instrument measured HONO values 7 times higher on average compared to a LIF instrument [Liao et al., 2006].

[7] In conclusion, in order to obtain a better understanding of the impact of HONO on the oxidation capacity of the polar atmosphere, there is an urgent need for the accurate quantification of HONO by instruments free of interferences and sampling artifacts. Thus, in the present study, HONO was measured at the polar measurement site Barrow during the winter/spring OASIS 2009 campaign using a validated LOPAP instrument. In addition, the net source strength of OH radicals formed via HONO photolysis in the gas phase was quantified and discussed in view of the proposed HONO source reactions.

2. Experimental Setup

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Setup
  5. 3. Results and Discussion
  6. 4. Conclusion
  7. Acknowledgments
  8. References

2.1. Measurement Site

[8] The measurements were performed from 13 March to 14 April, 2009 at Barrow, Alaska (71°19′N, 156°39′W) during the multidisciplinary Ocean-Atmosphere-Sea-Ice-Snowpack campaign (OASIS), with the objective to gain a better understanding of air-surface chemical interactions in the Arctic. The research site included two trailers and the Barrow Arctic Research Center (BARC), which is located ∼0.8 km from the Chukchi Sea and ∼5 km northeast of the town of Barrow. The trailers were placed ∼0.2 km southeast (117 deg) from BARC facing the expected prevailing wind direction, which was from the northeast, i.e., from the Arctic Ocean. However, during the campaign the field site was often exposed to air masses from the direction of BARC or from the city of Barrow leading to high pollution levels on many days.

2.2. Instruments

[9] HONO was measured with a highly sensitive, long-path absorption photometer (LOPAP®) with a detection limit of 0.4 pptv and a time resolution of 7 min during the campaign, which is explained in detail elsewhere [Heland et al., 2001; Kleffmann et al., 2002; Kleffmann and Wiesen, 2008]. Two sequential channels are used to correct for interferences. In addition, the very acidic sampling conditions (pH = 0) minimize the occurrence of all know interferences, like PAN hydrolysis, or reactions of NO2 with hydrocarbons, which are efficient only under alkaline conditions [see Kleffmann and Wiesen, 2008]. Due to the external sampling unit, no sampling lines were used, therefore also minimizing sampling artifacts. The sampling unit was fixed on the east side of trailer 1 at ∼2 m height and ∼1.2 m away from the container wall. The instrument was successfully validated by the DOAS technique in the atmosphere and in a smog chamber [Kleffmann et al., 2006].

[10] Measurements of OH were made using the technique of selected ion chemical ionization mass spectrometry (SICIMS). The technique has been discussed previously [e.g., Eisele and Tanner, 1991; Tanner et al., 1997; Mauldin et al., 1998] and the reader is directed to those works for details concerning the measurement instrumentation and technique. The nominal limit of detection (LOD) of the OH system is ∼2 × 104 molecule cm−3, for a 5 min integration period. For this integration time, the combined error, reflecting precision plus the bias error, is ± 35% (2σ).

[11] Hydroperoxy radicals (HO2) and organic peroxy radicals (RO2, where R is any organic group), were measured using the Peroxy Radical Chemical Ionization Mass Spectrometer (PeRCIMS). Similar to the OH measurement, peroxy radicals are measured via chemical conversion to H2SO4. By alternating the ratio of [NO]/[O2] in the PeRCIMS inlet with added NO and either O2 or N2, either HO2 or both HO2 and RO2 are chemically converted to OH. The resulting OH reacts with added unlabelled SO2 to form H2SO4, which is quantitatively measured using CIMS. The PeRCIMS instrument is described in detail by Hornbrook et al. [2011].

[12] OH reactivity, kOH, was measured by a discharge flow technique with detection by chemical ionization mass spectrometry (CIMS). The system is composed of a glass flow tube, an injector rod to create and introduce the hydroxyl radicals to the sampled air and the CIMS [McGrath, 2010]. Humidified N2 gas is continuously flowed over a Hg lamp at 184.9 nm inside the moveable injector rod to produce OH and subsequently injected into the flow tube at varying distance from the CIMS detection inlet. OH is detected via conversion to H234SO4 as described above. Assuming pseudo-first order conditions, the logarithm of the OH concentrations against the residence times will produce a decay curve, for which the slope is the inverse of the OH lifetime. The average measurement time of an OH reactivity decay during this study was 150 s, with the injector at nine distances per measurement to determine the decay of OH in ambient air. OH was measured for 8 s at each distance with OH background measurements performed at the start and finish of each measurement. To account for wall losses in the flow tube, OH losses by self reaction and reactions with impurities in the humidified N2 gas, OH reactivity measurements were carried out with zero air in the flow tube instead of ambient air. The decay curve slopes were then averaged yielding a typical background reactivity of ca. 2 s−1 and subtracted from the measured OH reactivity slopes to determine the reactivity of OH in ambient air (typically 0–4 s−1).

[13] Nitrogen oxides (NO, NO2) and total reactive nitrogen (NOy) were measured by NO+O3 chemiluminescence technique. NO2 and NOy were detected as NO using a photolytic converter (NO2) and a heated gold converter (NOy). Details of this instrument are explained elsewhere [Ridley and Grahek, 1990; Ridley et al., 1994].

[14] Ozone (O3) was measured by reverse chemiluminescence technique after reaction with NO in excess [Ridley et al., 1992].

[15] Formaldehyde (HCHO) was detected with a Difference Frequency Generation Absorption Spectrometer (DFGAS), which is described in detail by Weibring et al. [2007], and further details discussing the DFGAS sampling approach during OASIS are provided by Barret et al. [2011]. The limit of detection (LOD at 1σ) was estimated for each 30-s measurement period to be 30 pptv with an estimated systematic uncertainty of 6.9% (1σ) times the ambient mixing ratio.

[16] Carbon monoxide (CO) was measured with a commercial non-dispersed infrared absorption instrument. The instrument was modified to allow for regular measurement of the background signal in the absence of CO [Parrish et al., 1994].

[17] Snow samples were collected approximately 1000 m ESE of the Barrow Alaska Research Centre (BARC). The frozen samples (−20°C) were thawed and analyzed after a few days for their UV-VIS optical absorption and after the end of the campaign for H2O2 and major anions by HPLC and ion chromatography (IC). Details of the sampling and analysis are described by Beine et al. [2011]. The absorption coefficient corrected for the contributions from H2O2 and nitrate (NO3-) was integrated between 300 and 450 nm (Σαλ(residual)300–450 nm) to focus on photo-chemically active chromophores, like humic acids, which are known as active photo-sensitizers for NO2 in ice [Beine et al., 2008; Bartels-Rausch et al., 2010].

[18] Charged-coupled device Actinic Flux Spectroradiometer (CAFS) [Shetter and Müller, 1999] measured spectrally resolved downwelling actinic flux at 0.1 Hz. The upwelling flux was estimated as a function of solar zenith angle using the Tropospheric Ultraviolet and Visible (TUV) radiative transfer model [Madronich and Flocke, 1999] under clear sky conditions. The sum of up- and downwelling actinic flux was used to calculate the total photolysis frequencies, image JHONO, JHCHO(rad) and image used for this study.

[19] Temperature and humidity were measured with a commercial sensor (HMP 45C, Vaisala, Helsinki, Finland) mounted 2.0 m above ground on the micrometeorology tower, 29 m southeast of the trailer. Wind speed and direction were obtained with an ultrasonic anemometer (CSAT3, Campbell Scientific Inc., Logan, UT, USA) at 1.8 m above the ground, and atmospheric pressure was measured with a barometer (Model 61202V, RM Young, Traverse City, MI, USA).

3. Results and Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Setup
  5. 3. Results and Discussion
  6. 4. Conclusion
  7. Acknowledgments
  8. References

3.1. Measurement Data

[20] HONO concentration during the OASIS campaign varied between ≤ 0.4 and ∼500 pptv (see Figure 1) with an average of 27 pptv. While the average value is still on the upper end of the range of concentrations reported for other polar regions [Li, 1994; Zhou et al., 2001; Beine et al., 2001, 2002; Dibb et al., 2002; Honrath et al., 2002] and also for remote mountain sites [Huang et al., 2002; Kleffmann et al., 2002; Beine et al., 2005; Kleffmann and Wiesen, 2008], the high concentrations observed during several days were out of the range for remote conditions. Similarly, also very high NOx (NO+NO2) levels were observed for these days (see Figure 1). The measured NO, NO2 and NOy levels were in the range 0–20 ppbv, 0–15 ppbv and 0.3–32 ppbv, with average concentrations of 0.41 ppbv, 0.51 ppbv and 1.45 ppbv, respectively, which were also much higher, compared to typical remote conditions.

image

Figure 1. HONO, NOx and CO concentrations (10 min averages) during the OASIS campaign in Barrow, Alaska, spring 2009.

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[21] To understand the origin of the high HONO levels observed, the polluted days were investigated in more detail (see Figure 2a). High temporal correlation of HONO with NOx and CO was observed for these “polluted” days, whereas no significant correlation with the actinic flux was evident. Since NOx and HONO formation in polar regions are proposed to originate from photolytic sources, the high correlation of HONO with NOx and CO and the missing correlation with the actinic flux is explained by local direct anthropogenic emissions from combustion processes. There are several emission sources nearby, both BARC and the town of Barrow, 0.2 and 5 km away from the measurement site, respectively. For the two most polluted days shown in Figure 2a, a high correlation of HONO and NOx (R2 = 0.52) with a slope of 1.2% was observed. Such a HONO/NOx ratio is close to the ratio of 0.8% quantified for direct emissions from a vehicle fleet [Kurtenbach et al., 2001] and indicates that HONO was mainly caused by fresh local emissions for the polluted days. Since the aim of this study was the investigation of remote polar atmospheric chemistry and not of local anthropogenic emissions, these polluted days were not further considered.

image

Figure 2. Examples of HONO, NO, NO2, image and CO data (a) from two anthropogenically influenced days (24–25 March) and (b) from a “clean day” (26 March) during the OASIS campaign in Barrow, Alaska, spring 2009.

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[22] Only seven “clean days” from the whole campaign – identified by the absence of CO emission peaks – were used in most of the following data evaluation (26–30 March and 7–8 April, for the 8 April, the period 9:30–13:30 was excluded caused by local emissions). One of these days is shown in Figure 2b as an example. HONO concentrations were typically well-correlated with the actinic flux with the highest HONO values of ∼10 pptv occurring around noon Alaska standard time. This observation is in good agreement with other measurements in polar regions and on remote mountain sites [Zhou et al., 2001; Dibb et al., 2002; Kleffmann et al., 2002; Huang et al., 2002; Kleffmann and Wiesen, 2008] and is explained by photochemical HONO sources. From all seven “clean days” a single diurnal average was calculated from the 10-min mean values of the instruments used. This resulted in one “average clean day,” which is shown in Figure 3 for HONO, HONO/NOx and HONO/NOy.

image

Figure 3. “Average clean day” of the OASIS campaign for HONO, HONO/NOx and HONO/NOy. The HONO error bars reflect the 1σ day-to-day variability.

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[23] For these selected “clean days,” much lower average concentrations of HONO, NO, NO2 and NOy of 4.6 ± 2.5 pptv, 44 ± 28 pptv, 38 ± 21 pptv and 490 ± 90 pptv were determined, respectively. The average HONO/NOx and HONO/NOy ratios of 6.2 ± 4.8% and 1.0 ± 0.6% agreed well with values measured by LOPAP instruments at remote mountain sites (Zugspitze: 2.5% and 1.0%; Jungfraujoch: 4.6% and 1.1% [see Kleffmann and Wiesen, 2008]). However, the HONO/NOx ratio observed was much lower compared to other polar studies or remote mountain sites in which wet chemical instruments were used, having HONO/NOx ratios in the range ∼20–100% [Beine et al., 2001; Huang et al., 2002; Jacobi et al., 2004; Zhou et al., 2007; Jones et al., 2007; Amoroso et al., 2010].

[24] One possible explanation of the different results could be chemical interferences of wet chemical techniques in agreement with conclusions from other studies [Chen et al., 2004; Grannas et al., 2007; Mauldin et al., 2010]. Chemical interferences are measured in the second channel of the LOPAP instrument and are corrected for in the data evaluation (for details to this correction, see Kleffmann and Wiesen [2008]). For example, when only the HONO data < 10 pptv from the present study were considered, the average interference was 97%. Thus, without correction, the low concentration data would have been overestimated on average by a factor of two. In addition, caused by the different sampling conditions applied, i.e., pH = 0 for the LOPAP and pH ≥ 6 for other wet chemical instruments, other instruments are expected to show even higher interferences (for details to possible interfering reactions at high pH, see Kleffmann and Wiesen [2008]). Finally, heterogeneous formation of HONO in sampling lines up to 30 m length [e.g., Zhou et al., 2001] used in other polar HONO studies may have caused artificial overestimation of HONO. In contrast, caused by the external sampling unit of the LOPAP instrument no sampling lines were used in the present study. Alternatively the much lower HONO/NOx ratios observed by LOPAP instruments under remote conditions compared to other wet chemical instruments may also be caused by different snow properties. For example, alkaline snow surfaces will lead to lower HONO emissions as demonstrated for a high mountain site [Beine et al., 2005]. However, it seems unrealistic that only during all the remote LOPAP measurements alkaline snow surfaces were prevailing, whereas in all studies of other wet chemical instruments acidic snow surfaces were studied, especially when also coastal polar regions were investigated [Beine et al., 2001; Jones et al., 2007; Amoroso et al., 2010].

[25] The factor 3–15 lower HONO/NOx ratio observed in the present study compared to other polar studies is in good agreement with the factor of 7 lower HONO concentrations observed during the only polar HONO intercomparison exercise using a LIF compared to a wet chemical mist chamber instrument at South Pole [Liao et al., 2006]. Unfortunately, no simultaneous HONO and NOx concentrations were published for the ANTCI 2003 campaign [Liao et al., 2006; Eisele et al., 2008] to calculate HONO/NOx ratios for comparison. From the results of the intercomparison study (factor 7) and from the present study (factor 3–15), it is concluded that chemical interferences of wet chemical instruments should be studied in detail and further HONO intercomparison studies in polar regions, including previously used HONO instruments, the LOPAP technique and spectroscopic techniques, are recommended.

3.2. Is HONO Still a Net Source of OH Radicals?

[26] For almost all days a high temporal correlation of HONO and OH was observed (e.g., Figure 4a), i.e., each sharp HONO peak observed during daytime was in coincidence with a similar peak in the OH concentration. When all daytime (10–19 h) HONO data < 100 pptv from the OASIS campaign were considered, the correlation coefficient for a plot of OH against HONO was R2 = 0.30. The high variability and coincidence observed for single days could not simply be explained by the variability of the actinic flux (see Figure 4a), although HONO and OH are both proposed to have photochemical sources. This is also confirmed by a smaller correlation (R2 = 0.10) of OH with image for all daytime HONO data < 100 pptv, which is in contrast to studies under rural conditions [see Rohrer and Berresheim, 2006, and references therein]. The correlation was improved when OH and the gross rate of OH radicals formed by HONO photolysis (JHONO × HONO) were considered (Figure 4b), leading to a correlation coefficient of R2 = 0.48 for all daytime HONO data < 100 pptv. Considering this coincidence and correlation, it is proposed here that HONO is a major precursor of OH radicals during the OASIS campaign. Alternatively, the high coincidence observed may also be accidentally caused by a high correlation of HONO with any other precursors of the OH radical, e. g., ozone or HCHO. Interestingly, no coincidences of sharp OH radical peaks (see Figure 4a) with the gas phase concentrations of O3 or HCHO were observed, indicating that these two radical sources are of smaller importance for the OH radical levels compared to the HONO photolysis.

image

Figure 4. (a) HONO, image and OH and (b) HONO × JHONO and OH data from a “clean day” (27 March).

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[27] To investigate which of the two above explanations is more reasonable, the net rate of OH radical initiation by HONO photolysis was calculated and compared to that from ozone photolysis, which is typically proposed as the major OH radical source in the atmosphere where water vapor is not limiting. To calculate the OH radical source by HONO photolysis, besides the OH source reaction (1), the gas phase losses of OH radicals by reaction with NO:

  • equation image

and with HONO:

  • equation image

must also be considered. Assuming that the OH source (1) is equal to the losses (6) and (7), a theoretical photo-stationary-state daytime concentration, HONOPSS, can be calculated, if the photolysis frequency of HONO, JHONO, and the NO and OH radical concentrations are known:

  • equation image

[28] Only when the measured concentration is higher than HONOPSS, should HONO be considered as a net source of OH radicals. Using the NO, OH and JHONO data from the “average clean day” and the temperature dependent rate coefficients for reactions (6) and (7) from the IUPAC database [Atkinson et al., 2004], HONOPSS was calculated for the OASIS campaign. As expected, HONOPSS was much lower compared to the measured HONO concentration (see Figure 5). Thus, an additional HONO source must prevail during daytime. A well known source, still not considered in equation (8), is the heterogeneous conversion of NO2 on humid surfaces in the dark, which was proposed also for snow surfaces [Zhou et al., 2001]. Although the detailed mechanism, either via a slow reaction of NO2 with water (5) [Finlayson-Pitts et al., 2003] or a much faster dark reaction:

  • equation image
image

Figure 5. “Average clean day” of measured HONO and theoretical HONO concentrations considering only known gas chemistry (HONOPSS) and additionally slow heterogeneous nighttime sources image

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[29] (see, e.g., Gutzwiller et al. [2002] and Ammann et al. [2005], and dark experiments from Stemmler et al. [2006]) is still under discussion, both reactions were found to be first order in the NO2 concentration. Accordingly, a first order rate coefficient of khet = 2.7 ± 1.7 × 10−6 s−1 was determined for the heterogeneous NO2 conversion into HONO in the dark using the nighttime increase of the HONO/NO2 ratio from the “average clean day” (for details, see Alicke et al. [2002]). Assuming that this nighttime source is active also during daytime, the theoretical HONO concentration, image was not significantly higher than HONOPSS (Figure 5) and thus, the dark conversion of NO2 could not explain the high HONO daytime concentrations observed.

[30] From the difference between the measured and theoretical HONO concentrations an additional daytime HONO source, d[HONO]/dtextra, was calculated. This source, which is equal to the net OH radical production rate by HONO photolysis, d[OH]/dtHONO net, is shown in Figure 6 together with the rate of OH production by ozone photolysis. It should be pointed out here, that the calculated extra source of HONO can only be applied for the measurement site, since it is based on the source and sinks of OH by reactions (1), (6) and (7), which are calculated by the measured parameters HONO, NO, OH and JHONO. However, in the case of any spatial variations of the involved parameters, like for example vertical gradients, the calculated extra source will be different at other locations. Three important conclusions can be drawn from the figure:

image

Figure 6. “Average clean day” of HONO, net OH production by HONO and O3 photolysis and image during OASIS 2009.

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[31] 1. The net OH initiation rate by HONO photolysis is significantly larger compared to that from the ozone photolysis. An OH source by HONO photolysis of up to 90 pptv/h could be determined, whereas that by the O3 photolysis reached only 2.7 pptv/h during daytime. When considering only these two radical initiation sources, the average relative contributions of HONO and O3 were 98% and 2%, respectively. The small contribution by O3 photolysis is caused by the low average humidity (∼0.06%) and O3 levels (5–14 ppbv) for the “average clean day.” Since other radical initiation sources, like alkene ozonolysis, are not expected to be of significant importance for this polar site, it is concluded that HONO is the major OH radical initiation source in the lower polar boundary layer of Barrow, which is in excellent agreement with the high coincidence and correlation of OH radical concentrations in the gas phase and JHONO × HONO, shown in Figure 4b.

[32] HCHO photolysis was not considered as an OH radical initiation source here, since HO2 radicals are formed by the radical channel of the HCHO photolysis. These HO2 radicals will not be quantitatively converted into OH radicals under the low NOx conditions during the “clean days” caused by peroxyradical cross reactions. In addition, the fraction of HCHO which is formed by the OH-initiated degradation of VOCs in the gas phase, like e.g., methane, will not lead to radical initiation, but is part of the radical propagation cycle, which leads to a net loss of radicals under the prevailing low NOx conditions. And finally, even if all the HCHO were caused by fresh emissions from the snowpack, the total rate of HO2 radical formation by the HCHO photolysis would be on average four times lower compared to the net OH-initiation rate by HONO photolysis. Thus, HCHO is not expected to be a significant source of new OH radicals in Barrow. This conclusion is in excellent agreement with the completely missing correlation of sharp OH radical peaks (see Figure 4) with the concentration of HCHO (not shown).

[33] Another OH radical source which was not considered here is the reaction HO2+NO, which is not a radical initiation source but part of the propagation cycle. Even for urban high NOx conditions, it was demonstrated that this reaction is not a source of new OH radicals and is balanced by the loss of OH radicals by its reaction with VOCs/CO [Elshorbany et al., 2010]. For the low NOx conditions of the “clean days” from the present study, losses of OH radicals by reaction with VOCs/CO will even exceed the OH source by HO2+OH, caused by peroxyradical cross reactions and thus, should not be considered here as an initiation source.

[34] In conclusion, although the HONO/NOx ratio was much lower compared to other studies (see section 3.1), HONO is still a major OH radical initiation source in the polar atmosphere of Barrow.

[35] 2. Since OH and NO are formed by the photolysis of HONO, also a net source of NO of up to 90 pptv/h can be derived during daytime from the results of the present study. Thus, HONO photolysis may be also a significant source of NOx in Barrow.

[36] 3. The extra daytime source of HONO necessary to explain the measured HONO concentrations shows the same diurnal variation as the actinic flux indicating a photochemical origin, which is in agreement with most HONO source reactions proposed recently based on laboratory and field results (see section 1).

3.3. HOx and NOx Consistency Tests

[37] Recently, high importance of HONO as a major source of OH radicals in the polar atmosphere was questioned in model studies based on HOx and NOx balances [Chen et al., 2004; Sjostedt et al., 2005; Bloss et al., 2006, 2010; Grannas et al., 2007; Mauldin et al., 2010]. This was mainly explained by chemical interferences of the instruments used. To verify whether the significant net daytime source of HONO is reasonable for the HONO/NOx data of the present study, HOx and NOx consistency tests were also performed here.

[38] For the HOx balance, the theoretical OH concentration, OHPSS, was compared with the observed OH. For the steady state calculation of OHPSS, it was assumed that the sources of OH radicals are equal to their sinks. For the gas phase OH sources, only the main sources of OH by HONO and O3 photolysis and by the reaction HO2+NO were considered. For the OH radical losses the measured OH reactivity, kOH, was used. OHPSS was calculated by the following equation (10):

  • equation image

[39] Unfortunately only a scarce data set of the OH reactivity was available during OASIS. In addition, caused by the low precision of the reactivity measurements, only reactivity data higher than the precision of 1.3 s−1 were used here. For the remaining data, reasonable agreement between OHPSS and OHmeas. was observed for the “average clean day” (see Figure 7). However, since the OH-radical propagation source by the reaction HO2 + NO was on average 10 times higher than the OH-radical initiation source by the photolysis of HONO, OHPSS was only slightly reduced when HONO photolysis was not considered (Figure 7). The difference was in between the error limit for OHPSS, which depended almost exclusively on the uncertainties in the HO2 and OH reactivity data. In conclusion, the HONO concentrations of the “average clean day” were not in contradiction with the available data on the OH radical balance, in contrast to other studies [Chen et al., 2004; Bloss et al., 2010; Mauldin et al., 2010].

image

Figure 7. Measured OH radical concentration (OHmeas.) and image for the “average clean day” during OASIS 2009. In addition, the theoretical OH concentrations calculated by equation (10) considering all measured OH sources (OHPSS) and excluding OH initiation by HONO photolysis (OHPSS w/o HONO) are shown.

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[40] Another consistency test for the HONO data is the measured HONO/NOx ratio. Assuming a steady state of the sources and sinks of HONO and NOx, and assuming that all NOx is generated by the photolysis of HONO as a lower limit, the HONO/NOx ratio is similar to the ratio of their lifetimes in the atmosphere τHONO/τNOx. The lifetime of HONO is almost completely controlled by its photolysis, which was 27 min for the “average clean day” during OASIS. Since the average HONO/NOx ratio was 6% (see above), this translates into an average lifetime of NOx of 7.5 h, which seems to be reasonable in view of potential NOx sinks, e.g., NO2+OH, NO2+O3, NO2+HO2, NOx deposition, etc.. For example, assuming an OH radical concentration of 1 × 106 cm−3 (see Figure 7), the lifetime of NO2 due only to its reaction with OH is 19 h, which will be further reduced by the other NOx sinks and is thus not expected to be far from the estimated value of 7.5 h based on the HONO/NOx ratio. The estimated lifetime of NOx is in good agreement with NOx lifetime calculations of 6 h under polar conditions [Bauguitte et al., 2009].

[41] In conclusion, since the LOPAP instrument was validated against the DOAS technique both, in the atmosphere and in a simulation chamber even under very complex conditions [Kleffmann et al., 2006], and since the consistency tests using HOx and NOx balances during OASIS do not indicate the opposite, there is no reason to doubt the reliability of the LOPAP HONO data.

3.4. Characterization of the Daytime Source of HONO

[42] For deeper investigation of the light dependency, the extra daytime source of HONO was plotted against image and image for the “average clean day” in Figure 8. Both photolysis frequencies reflect a different wavelength range and thus, the correlations may help to answer which of the two main proposed source reactions on snow surfaces is of greater importance for the polar atmosphere. Whereas the photolysis of nitrate, reaction (2) [e.g., Zhou et al., 2001] with a corresponding nitrate absorption maximum at ∼300 nm, correlates well with image photosensitized conversion of NO2 on humic acids, reaction (4), is active already in the visible and near UV [see Stemmler et al., 2006] and correlates better with image Unfortunately, both plots show the same high correlation, which is in contrast to a recent study at lower latitude [Elshorbany et al., 2009]. Caused by the lower diurnal variability of the solar zenith angle (SZA) during polar spring image and image show a more linear correlation compared to lower latitude measurements. Thus, by simply using the “average clean day” data, the most probable source reaction could not be identified.

image

Figure 8. Correlation of the extra daytime HONO source with (a) image and (b) image for the “average clean day” during OASIS 2009.

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[43] For deeper investigation, similar data evaluation than that shown in Figure 8 was also done for individual days using the daytime (10:00–19:00 h) data. However, besides the seven “clean days,” six additional days were investigated, for which the HONO levels were < 100 pptv (NOx < 1.4 ppbv) for most of the day, to increase the number of data points for the correlations, but still to exclude the very extreme periods with NOx concentrations up to 30 ppbv, for which mainly fresh emissions controlled the observed HONO levels (see above). The slopes of all plots of the extra HONO source against image image and image image showed high day-to-day variability. This is explained, at least in part, by the variability of the precursors involved. Thus, to differentiate between the two proposed source reactions, i.e., photosensitized NO2 conversion on humic acids (4) and nitrate photolysis (2), image was plotted against the corresponding daily averaged NO2 gas phase concentration (see Figure 9a) and image against the corresponding daily averaged snow nitrate concentration (see Figure 9b). For the correlation plots using NO2, it was assumed that the gas phase NO2 concentration is correlated to the NO2 concentration in the snow interstitial air, where reaction (4) is expected to take place. Clearly, the correlation shown for all HONO data < 100 pptv in Figure 9a was much higher compared to Figure 9b, indicating that nitrate photolysis (2) is less likely compared to a NO2 dependent long wavelength range process, like the photosensitized conversion of NO2 on humic acids (4). In contrast, when only the seven “clean days” were considered, a positive correlation between image and the snow nitrate concentration could be inferred (see Figure 9b). However, the very significant negative intercept of this correlation is not in line with the photochemical HONO source observed (see Figure 6 and Figure 8) and with the proposed nitrate photolysis mechanism (2), which could be inferred from this plot. In contrast, for the correlation of image against NO2, the slopes and the negligible intercepts are not depending on the choice of the data in between the experimental errors (see Figure 9a). Thus, a long wavelength range process, like the photosensitized conversion of NO2 on humic acids (4) is proposed to explain the daytime source of HONO in Barrow.

image

Figure 9. Correlations of (a) image and (b) image against the corresponding NO2 and NO3 concentrations, respectively. image and image were determined by plotting the extra HONO source against image and image for each individual day with HONO < 100 pptv (see Figure 8 as an example for the “average clean day”). In addition, all individual “clean days” are also shown. The regression lines correspond to all days < 100 pptv.

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[44] Interestingly, there was no correlation between the concentration of HONO and NO2 for the “average clean day” (R2 = 0.0035). From similar observations of previous rural studies [e.g., Zhou et al., 2007] reaction (4) was excluded. Thus, the present study demonstrates that correlation studies using HONO concentrations instead of net HONO production rates should not be used to prove any HONO formation mechanism in the atmosphere during daytime. The argument against the use of concentrations is that a strong HONO source in the atmosphere will be masked by the short photolytic lifetime of HONO only during daytime.

[45] Unfortunately, only a scarce data of the humic acid concentration in snow were available and thus, a correlation of image against the product NO2 × humic acids was not possible. Light absorbing aromatics, fulvic and humic acids and soot have been identified to photosensitize heterogeneous conversion of NO2 into HONO [George et al., 2005; Stemmler et al., 2006, 2007; Monge et al., 2010]. Since many organic carbon (OC) species will either not absorb in the actinic spectral range or do not form excited triplet states, which were postulated to photosensitize the NO2 conversion [George et al., 2005], the measured OC in snow is not a suitable marker for NO2 photosensitizers. Besides OC, the integrated absorption coefficient of melted snow in the UV/VIS corrected for the H2O2 and nitrate absorption (Σαλ(residual)300–450 nm) was also measured during OASIS, which may be a marker for the photosensitizer species mentioned above. However, the day-to-day variability of the daily averaged Σαλ(residual)300450 nm was very low during OASIS (<factor of two). Thus, there was no further improvement of the correlation of image against the product NO2 × Σαλ(residual)300–450 nm compared to the one shown in Figure 9a.

[46] In conclusion, while the nitrate photolysis (2) could be excluded and the contribution of NO2 to a long wavelength photochemical HONO source is very likely (see Figure 9), the contribution of humic acids in snow cannot be verified based on the existing data set. However, since no other strong photochemical HONO sources besides the excluded nitrate photolysis are yet known, photosensitized conversion of NO2 on humic acids (4) is proposed to explain daytime formation of HONO in the polar atmosphere of Barrow.

[47] A significant uncertainty in the calculations presented here is the fact that no surface HONO fluxes were measured during OASIS. In contrast, an extra HONO source strength was calculated, in which this source was mathematically treated as a gas phase source. However, in the case of an expected surface source, the calculated source strength would inversely depend on the turbulent vertical mixing of the atmosphere, which is induced, at least in part, by wind shearing. Thus, when plotting image against the product of [NO2] × 1/(wind speed) an even higher correlation (R2 = 0.88) compared to the one shown in Figure 9a was observed and thus confirming a ground surface source, in good agreement with previous HONO studies over snow surfaces [Zhou et al., 2001; Honrath et al., 2002, Amoroso et al., 2006; Beine et al., 2008]. However, if only the turbulent mixing were the reason for the correlation with NO2 shown in Figure 9a, the extra daytime source would also correlate with any other ground emitted species. This phenomenon was already discussed in the past to understand nighttime formation of HONO in the urban atmosphere and to explain high correlation of HONO with Radon [Febo et al., 1996], which is definitely not a precursor of HONO. However, when plotting image against any other ground emitted species like NO, NOx, NOy, CO significantly lower correlations compared to NO2 were observed (NO: R2 = 0.42; NOx: R2 = 0.61; NOy: R2 = 0.41; CO: R2 = 0.41). Thus, it is concluded that NO2 is a precursor of the extra daytime source of HONO in Barrow and that the correlations observed are not simply caused by the boundary layer dynamics.

[48] In addition, one may argue, that the high correlation of the extra HONO source with image as shown in Figure 8, is simply caused by the excellent correlation of JHONO with image (R2 = 1.0) [see also Kraus and Hofzumahaus, 1998] and does not reflect any variability of the source process itself. Caused by the low PSS values observed (see Figure 5), JHONO and the HONO concentration are the most important factors in the calculation of the extra daytime source strength. However, it can be expected that the rate coefficient for the production of HONO by any daytime source is much smaller compared to the high photolysis frequency of HONO, JHONO. Thus, in a simplified consecutive process:

  • equation image

[49] the formation of HONO will be the rate limiting step for the net OH radical production. Accordingly, the diurnal profile of the net OH source by HONO photolysis, as shown in Figure 6, is reflecting the diurnal variation of the source of HONO and not necessarily that of JHONO. For example, if HONO were formed by a light-independent process, it would not accumulate as strongly as observed and no maximum of the HONO/NOx ratio (see Figure 3) would appear during daytime. Only the diurnal variation of the source caused these observations. Thus, the high correlation of the HONO source with image see Figure 6 and Figure 8 and with the NO2 concentration, see Figure 9a, strongly supports a daytime HONO source by photosensitized conversion of NO2, for example on humic acids, as proposed from laboratory studies [George et al., 2005; Stemmler et al., 2006, 2007] and recently also confirmed in studies on humic acid doped ice and snow substrates [Beine et al., 2008; Bartels-Rausch et al., 2010].

[50] Another important question which should be raised here is whether the conditions studied, even for the considered “clean days,” are representative for the remote polar atmosphere. If a day before a “clean day” NOx levels reach 30 ppbv (see for example Figures 1 and 2), one cannot expect a pristine snow surface and thus, the HONO source determined in the present study may be much stronger compared to the “real” remote polar atmosphere. Thus, further reliable HONO measurements under more pristine conditions are necessary.

4. Conclusion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Setup
  5. 3. Results and Discussion
  6. 4. Conclusion
  7. Acknowledgments
  8. References

[51] For the “clean days” considered in the present study, low HONO concentrations in the range ≤ 0.4 - 10 pptv were observed during spring 2009 in Barrow, Alaska. Average HONO/NOx and HONO/NOy ratios of 6% and 1% were much lower compared to most other polar studies. The strong correlation between sharp peaks of OH and HONO during daytime, which was not observed for any other measured radical precursor, suggested that HONO photolysis was a major source of OH radicals during OASIS. This was supported by calculated net OH radical production by HONO and O3 photolysis for which the contribution of O3 could be neglected. From the high correlation of the daytime HONO source with the actinic flux a photochemical origin is proposed, in good agreement with recent laboratory and field studies. The higher correlation of the source with image and [NO2] compared to image and [NO3] indicates that photosensitized conversion of NO2, for example on humic acids, is more likely compared to the nitrate photolysis mechanism.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Setup
  5. 3. Results and Discussion
  6. 4. Conclusion
  7. Acknowledgments
  8. References

[52] This work is part of the international multidisciplinary OASIS (Ocean-Atmosphere-Sea Ice-Snowpack) program. G.V. would like to thank the NSF for grant ATM-0807702. The Canadian contribution was supported by the Canadian Federal Program Office for the International Polar Year (project MD-065). The National Center for Atmospheric Research is sponsored by the National Science Foundation. Any opinions, findings and conclusions or recommendations expressed in the publication are those of the authors and do not necessarily reflect the views of the National Science Foundation.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Experimental Setup
  5. 3. Results and Discussion
  6. 4. Conclusion
  7. Acknowledgments
  8. References