Emission sources and ocean uptake of acetonitrile (CH3CN) in the atmosphere



[1] Airborne measurements of acetonitrile (CH3CN) were made off the U.S. west coast, over California, and during two transfer flights over the U.S. in April and May of 2002. Acetonitrile was strongly enhanced in the plumes from two forest fires, confirming the usefulness of the measurement as an indicator for biomass burning emissions. The emission ratios relative to CO of acetonitrile in the two plumes were slightly higher than previously reported values for fires burning in other fuel types. No significant acetonitrile release was observed in the Los Angeles basin or from other point sources (ships and a power plant). Acetonitrile concentrations were significantly reduced in the marine boundary layer indicating the presence of an ocean uptake sink. Increased loss of acetonitrile was observed close to the coast, suggesting that acetonitrile was efficiently lost by dissolving in the upwelling ocean water, or by biological processes in the surface water.

1. Introduction

[2] Acetonitrile (CH3CN, methyl cyanide) is an inert organic trace gas in the atmosphere with a lifetime of many months. Although there is still relatively little known about its atmospheric sources and sinks, the consensus is that acetonitrile is predominantly emitted from biomass burning and is lost in reaction with OH and by ocean uptake [Arijs and Brasseur, 1986; Hamm and Warneck, 1990; Bange and Williams, 2000]. In these respects, acetonitrile is very similar to hydrogen cyanide [Li et al., 2000], and both compounds can be useful indicators for biomass burning emissions in atmospheric chemistry studies. Recent developments in mass spectrometry allow acetonitrile to be measured with a fast response time [Lindinger et al., 1998; Sprung et al., 2001], which has prompted a series of airborne and ground-based measurements of this compound [Andreae et al., 2001; Lelieveld et al., 2001; de Gouw et al., 2001; Warneke and de Gouw, 2001; Karl et al., 2003]. Livesey et al. [2001] generated further interest by showing that stratospheric acetonitrile can be determined from satellite data.

[3] Arijs et al. [1978] and Arnold et al. [1978] inferred the presence of acetonitrile in the stratosphere from positive ion measurements: acetonitrile has a high proton affinity and readily accepts a proton from common atmospheric ions such as H3O+(H2O)n. Later, Knop and Arnold [1987] used those same ion-molecule reactions to ionize and detect acetonitrile in the stratosphere. Hamm et al. [1989] and Hamm and Warneck [1990] performed the first reliable and detailed measurements of acetonitrile in the lower troposphere. In ship-based measurements on the Atlantic Ocean, these workers found acetonitrile to be a ubiquitous compound at background levels of 100–150 pptv (parts-per-trillion by volume).

[4] Only a few studies have quantified the sources of atmospheric acetonitrile. Laboratory work by Lobert et al. [1990] and later by Holzinger et al. [1999] identified acetonitrile as an important constituent of the emissions of burning and smoldering savanna grasses and agricultural waste. Hamm and Warneck [1990] suggested that automobile exhaust is a second, significant source of acetonitrile. However, later work by Holzinger et al. [2001], who studied acetonitrile in the exhaust of individual cars, proved that transportation is most likely only a minor source.

[5] Little is known about the atmospheric sinks of acetonitrile. The reaction with hydroxyl radicals (OH) is slow (k298 = 2.3 × 10−14 cm3 molecule−1 s−1; DeMore et al. [1997]) which corresponds to an atmospheric lifetime of 1.4 years, assuming a globally averaged OH concentration of 9.7 × 105 molecules cm−3 [Prinn et al., 1995]. Hamm et al. [1984] determined the Henry's law solubility coefficient for acetonitrile and speculated about wet deposition and ocean uptake of acetonitrile. These authors estimated the lifetime against rain-out to be long (3.0 ± 1.5 year), but ocean uptake was shown to be a potentially significant sink if a loss mechanism for acetonitrile exists in the ocean water. Bange and Williams [2000] suggested that enzymes capable of converting nitriles into ammonia are common in coastal and deep ocean sediments, which could provide such a loss mechanism. Warneke and de Gouw [2001] observed a marked decrease of acetonitrile in the marine boundary layer (MBL) in a region with cold, upwelling ocean water. The acetonitrile minimum coincided with a maximum in the mixing ratio of dimethyl sulfide (DMS). de Laat et al. [2001] analyzed results from the Indian Ocean Experiment (INDOEX) using a model calculation and included an ocean uptake sink to obtain the best agreement with the measured data. Finally, Karl et al. [2003] measured acetonitrile at the Mauna Loa Observatory, Hawaii, and found acetonitrile mixing ratios to be lower in air from the MBL, which reached the observatory in updrafts. These observations again suggested the presence of an ocean uptake sink, but the gradient was likely also influenced by the transport of polluted air masses from Asia at higher altitudes.

[6] Here, we present airborne measurements that give further insight into the sources and sinks of acetonitrile. The measurements were made off the west coast of the U.S. and over California in April and May of 2002 as part of the Intercontinental Transport and Chemical Transformation (ITCT2k2) experiment. Measurements in air masses that had traveled across the Pacific Ocean revealed the importance of ocean uptake as a sink for acetonitrile. In addition, we probed several potential emission sources including two forest fires, urban pollution, a power plant and several petrochemical facilities.

2. Airborne Measurements

2.1. Flight Tracks

[7] Eleven flights were made between April 25 and May 17 using a NOAA WP-3 aircraft based in Monterey, California. Flight objectives included the characterization of polluted air masses from Asia, a study of the emissions by ships in the Pacific, and measurements of regional air pollution in the San Francisco, Los Angeles and California Central Valley areas. Additional measurements were performed during the transfer flights from Tampa, Florida, to Monterey on 22 April 2002, and from Monterey to Broomfield, Colorado, on 19 May 2002. Research objectives during the transfers included a study of the emissions from petrochemical plants near Houston, Texas, and from a power plant in southern Utah. The extensive instrumental payload of the NOAA WP-3 allowed a detailed characterization of the chemical and aerosol composition of the atmosphere. A full description of the payload is beyond the scope of this paper and will be presented in subsequent papers on the ITCT2k2 experiment.

2.2. VOC Measurements

[8] The airborne measurements of acetonitrile and other VOCs were made with a proton-transfer-reaction mass spectrometer (PTR-MS; Ionicon Analytik) [Lindinger et al., 1998]. Ambient air is pumped to the PTR-MS instrument through a PFA (perfluoro alkoxy) inlet heated to 45°C to prevent condensation. The downstream end of the inlet is pressure regulated and the inlet flow varies from 500 STP cm3 min−1 (STP = 1 atm; 273 K) at the surface to 100 STP cm3 min−1 at 8 km altitude. The PTR-MS, in short, consists of a discharge ion source, which produces H3O+ ions. Ambient air is continuously pumped through a drift-tube reactor and many VOCs are ionized by proton-transfer reactions with H3O+. The reagent and product ions are extracted from the airflow and analyzed with a quadrupole mass spectrometer (Balzers 422). Even when sampling air without VOCs, many mass signals in the PTR-MS are not zero due to impurities in the PTR-MS and the inlet lines, and to the generation of ions other than H3O+ in the discharge ion source. These “background” levels are determined by passing the inlet flow through a catalyst (Pt-coated quartz wool heated to 350°C), which efficiently removes VOCs, and are subtracted from the results for ambient air. Protonated acetonitrile is detected at ion mass 42 amu. The measurements were calibrated using a standard mixture of acetonitrile and six other VOCs (methanol, acetaldehyde, acetone, benzene, toluene and dimethyl sulfide) in nitrogen. The uncertainty in the calibration is estimated to be around 15%. Acetonitrile is monitored for 2s every 10–15s and the statistical noise (1σ) in the individual data points is less than 20 pptv. The detection limit for the 2s measurement is approximately 25 pptv and improves when the measurements are averaged over a longer time. The overall measurement accuracy per data point is approximately 25% or 25 pptv, whichever is the larger number. Additional data are presented here for toluene, detected at mass 93 amu, and dimethyl sulfide at mass 63 amu. For a detailed description of the PTR-MS method and its use for measurements of the atmospheric composition, the reader is referred to Lindinger et al. [1998] and de Gouw et al. [2003].

[9] In PTR-MS, only the mass of ions produced in the drift reactor is determined. The selectivity needs to be considered especially for forest fire plumes and urban atmospheres, which may contain many different VOCs with the same mass. We have partially addressed this problem by analyzing air samples with a gas chromatographic (GC) column in line before the PTR-MS. This allows, for individual air samples, the contribution of different VOCs to the signal at a certain mass to be separated based on their retention times [de Gouw et al., 2003; Warneke et al., 2003]. As an example, Figure 1 shows the result of a GC-PTR-MS analysis for an urban air sample collected in Boulder, Colorado, that was heavily influenced by automobile emissions. For this measurement, the VOCs in ambient air were directly collected in a cold trap prior to the injection onto the GC column. Sampling artifacts associated with the use of sample canisters or adsorbent filters are therefore avoided. Figure 1 clearly shows that acetonitrile is by far the dominant contributor to the signal at 42 amu. We have examined hundreds of samples both from strongly polluted atmospheres and the background free troposphere using GC-PTR-MS and have always found the contributions from compounds other than acetonitrile to be at a level of a few percent at most. The minor peaks in the chromatogram between 3 and 5 min are reproducible and are attributed, based on the retention times, to the small alkanes as shown in Figure 1. The alkanes can be ionized in the PTR-MS by reactions with O2+ ions, which are formed by the back-flow of air from the drift reactor to the ion source. O2+ ions are typically only present at levels of a few percent relative to H3O+ and, moreover, the C3H6+ fragment is not a common product from the O2+ + alkane reactions. This explains the small response of the PTR-MS with respect to the alkanes at mass 42 amu. We estimate the sensitivity of PTR-MS for the alkanes to be at least two orders of magnitude smaller than for acetonitrile. Warneke et al. [2003] analyzed 75 VOCs contained in standard mixtures with GC-PTR-MS and reported no other interferences at 42 amu. The study included propene, which has a mass of 42 amu, but is detected at 41 and 43 amu in PTR-MS.

Figure 1.

Result of a GC-PTR-MS analysis of an urban air sample taken in Boulder, Colorado: Figure 1a shows the signal at 42 amu versus the retention time of the gas chromatograph; Figure 1b shows the same data with the y axis expanded.

[10] It is likely that some VOCs that can be measured with PTR-MS are not detected by GC-PTR-MS because of losses in the GC column and sampling system. The absence of peaks other than from acetonitrile in a chromatogram such as Figure 1 is therefore only partial proof that the PTR-MS signal at mass 42 is only due to acetonitrile. We have addressed this problem by making a quantitative comparison between the results from PTR-MS measurements and GC-PTR-MS analyses of urban air sampled simultaneously [Warneke et al., 2003]. This study shows that acetonitrile and other VOCs can be selectively measured by PTR-MS in urban air.

2.3. Carbon Monoxide Measurements

[11] Carbon monoxide (CO) measurement results are used in this paper in addition to the PTR-MS data. CO was determined using a vacuum ultraviolet fluorescence measurement as described by Holloway et al. [2000]. The precision of the measurements is estimated to be 2.5%. Variability in the determination of zero levels results in an absolute uncertainty of about 1 ppbv in the values reported. The field standard was compared to NIST Standard Reference Material (SRM) 2612 a (10 μmol/mol nominal CO in air). We estimate that the concentration of our calibration standard is known to within 2%. The overall accuracy of the 1s measurements is thus estimated to be 5%.

3. Results and Discussion

3.1. Acetonitrile in Forest Fire Plumes

[12] On the transfer flight from Monterey (California) to Broomfield (Colorado) on 19 May 2002, a fire in a coniferous forest was observed over southern Utah and the NOAA WP-3 aircraft made a single pass through the edge of the plume (38.54°N–112.15°W, 5100 m altitude). The measurement results for CO and acetonitrile are shown in Figure 2. It is clear that both compounds are strongly enhanced in the plume: CO to above 800 ppbv and acetonitrile to above 1500 pptv. The response time of the PTR-MS of a few seconds at most allows acetonitrile to be measured accurately across the two-minute plume and also shows details within the plume that are correlated with CO. From the locations of the fire and the aircraft, it is estimated that the trace gases were emitted approximately two hours prior to the measurement. This is much shorter than the atmospheric lifetimes of both CO and acetonitrile, and their ratio can be compared with emission ratios determined in the laboratory. From the results in Figure 2, we estimate the ΔCH3CN/ΔCO ratio to be 2.0 ± 0.3 pptv ppbv−1. This result is obtained by fitting the CH3CN versus CO data to a straight line as shown in the insert in Figure 2. The uncertainty is determined by the error in the fit and by the calibration uncertainties of the acetonitrile (15%) and CO (2%) measurements.

Figure 2.

CO and acetonitrile measurement results in a forest fire plume over southern Utah (38.54°N–112.15°W, 5100 m altitude). The insert shows the correlation between acetonitrile and CO.

[13] On the transfer flight from Tampa (Florida) to Monterey (22 April 2002), while flying off the coast of the Gulf of Mexico near Houston (29.26°N–94.48°W), a layer at 1200 m altitude was encountered which showed enhanced mixing ratios of acetonitrile and CO (Figure 3). Back-trajectories indicate that the sampled air mass had traveled over the Yucatan peninsula of Mexico two days prior to the measurement. Satellite images showed extensive fires in the tropical forests of Yucatan in that period, making it plausible that the observed layer originated from forest fires in that area. Other, more reactive trace compounds such as the aromatic hydrocarbons were relatively low in the layer, indicating that the photochemical age of the plume was indeed on the order of a few days. The ΔCH3CN/ΔCO ratio observed in the plume was 2.7 ± 0.4 pptv ppbv−1, as determined from the fit shown in the insert of Figure 3.

Figure 3.

Vertical profiles of CO and acetonitrile measured south of Houston over the Gulf of Mexico (29.26°N–94.48°W). The insert shows the correlation between acetonitrile and CO.

[14] As discussed in section 2.2, the C3–C5 alkanes can be detected by PTR-MS at mass 42 amu and this may interfere with the detection of acetonitrile. The emission ratios to CO for alkanes from forest fires [Friedli et al., 2001] are lower than those for acetonitrile. Combined with the fact that acetonitrile is much more efficiently detected than the alkanes, it is not expected that the measurement results shown in Figures 2 and 3 are significantly influenced by interference from the alkanes.

[15] The two emission ratios determined in this work, 2.0 ± 0.3 pptv ppbv−1 for the Utah fire and 2.7 ± 0.4 pptv ppbv−1 for the Yucatan fire are relatively high as compared with emission ratios previously reported in the literature. Holzinger et al. [1999] reported emission ratios in the range of 0.4–2.5 pptv ppbv−1 for savanna grasses, with a mean of 1.3 pptv ppbv−1. This mean agreed well with the ΔCH3CN/ΔCO ratio of 1.2 ± 0.3 pptv ppbv−1 observed by Andreae et al. [2001] in plumes observed in the upper troposphere from fires in the equatorial region of South America. During the Indian Ocean Experiment (INDOEX), air masses were studied that were heavily influenced by bio-fuel use in South and Southeast Asia. de Gouw et al. [2001] reported a ΔCH3CN/ΔCO ratio of 1.1 ± 0.4 pptv ppbv−1 for these air masses, whereas Lelieveld et al. [2001] found 2 pptv ppbv−1 from a different platform. The latter two numbers were determined from entire data sets gathered in the large-scale, continental outflow from India, which may be less accurate than emission ratios determined from clearly defined, individual plumes.

[16] It is clear that the emission ratios can be strongly dependent on the type of fire and fuel. Indeed, unpublished results from Kleiss et al. at the Max Planck Institute for Chemistry in Mainz, Germany, indicate the high variability of acetonitrile emission ratios: their measurement results varied between 0.5 pptv ppbv−1 for grass from Zambia and Indonesia to 5.9 pptv ppbv−1 for fir duff from Montana and Canada (personal communication, 2002). The emission ratios reported here are within this range. Also, Yokelson et al. [1997] have shown that VOC emission factors often depend strongly on the ratio of flaming to smoldering combustion, which can be characterized by the modified combustion efficiency (MCE = ΔCO2/(ΔCO2 + ΔCO), where the Δ indicates an excess mixing ratio). These authors showed that the ratio between VOCs and CO from smoldering fuels usually increases as MCE decreases. In the present work, the MCE is approximately 0.93 and 0.94 for the Utah and Yucatan fires, respectively. This indicates that we sampled smoke from a less efficient combustion than in the study of Holzinger et al. [1999] (MCE ∼0.957), which may contribute to the higher emission ratios determined in the present work. However, the difference in fuels (savanna grasses versus coniferous and tropical forests) probably accounts for most of the difference in emission ratios noted here.

[17] Holzinger et al. [1999] estimated the biomass burning source of acetonitrile using the emission ratios for savanna grasses determined in the laboratory. From the mean emission ratio of 1.3 pptv ppbv−1, these authors estimated the global source to be 0.4–1.0 Tg year−1, where the reported uncertainty was solely due to the uncertainty in the budget of CO. The present results show that there is considerable uncertainty in this estimate: the emission ratios determined in this work are considerably higher, which could have important implications for the global budget of acetonitrile. More research is needed to determine the emission ratios for different types of vegetation and fire conditions.

3.2. Other Sources of Acetonitrile

[18] On 13 May 2002, the NOAA WP-3 conducted a research flight over the Los Angeles basin. Figure 4 shows the measurement results for CO, toluene and acetonitrile, along with the flight altitude. During the flight, heavily polluted air was encountered: CO had a maximum over 900 ppbv, while toluene was as high as 2.5 ppbv. The degree of correlation between CO and toluene is high: r2 = 0.92 for the data shown in Figure 4. From a fit of the toluene versus CO data in Figure 4, we find a Δtoluene/ΔCO ratio of 2.9 ± 0.4 pptv ppbv−1, which is typical for automobile emissions [Harley et al., 2001]. As expected, the main source for the two trace gases appears to be vehicle exhaust. Also shown in Figure 4 is the measurement result for acetonitrile, which was relatively constant during the flight and was not enhanced much over the background values observed elsewhere in California. The degree of correlation between CO and acetonitrile is low (r2 = 0.28), and from a fit of the acetonitrile versus CO data in Figure 4, we find a ΔCH3CN/ΔCO ratio of approximately 0.1 pptv ppbv−1. Owing to the lack of correlation, we consider this number to be an upper limit. The ΔCH3CN/ΔCO ratio found in the LA basin is at least an order of magnitude smaller than the emission ratios for biomass burning reported above. Globally, the amount of CO emitted from biomass burning is equal to or exceeds the amount released by fossil fuel combustion [Khalil and Rasmussen, 1990]. Combined with the difference in emission ratios, this implies that automobile emissions are a minor source of acetonitrile to the atmosphere, which agrees with the conclusion of Holzinger et al. [2001].

Figure 4.

CO, toluene and acetonitrile measurement results in the boundary layer and lower troposphere over Los Angeles.

[19] Butanes and pentanes are important constituents of automobile exhaust and might interfere with the detection of acetonitrile in urban atmospheres using PTR-MS. However, the urban sample discussed in section 2.2 (and Figure 1) contained higher levels of VOCs from cars than the measurement results from Los Angeles shown in Figure 4: the toluene mixing ratio for the sample analyzed by GC-PTR-MS was 3.0 ppbv, whereas the highest value observed over Los Angeles was 2.5 ppbv. It is not expected therefore that the acetonitrile data shown in Figure 4 are more significantly influenced by interference from alkanes than is shown in Figure 1. The selectivity of toluene measurements by PTR-MS has previously been confirmed by comparing the measurements with GC-FID analyses of canister samples [Warneke et al., 2001] and by GC-PTR-MS analyses of urban air samples [de Gouw et al., 2003; Warneke et al., 2003].

[20] No enhancements of acetonitrile were observed in the plumes from a number of ships in the Pacific, nor in the plume from a power plant in Utah. Apart from the forest fire plumes discussed above, the only other point source that was found to contain significant acetonitrile (550 pptv) was a petrochemical plant (Solutia, Chocolate Bayou) in Alvin, Texas, which is ranked 6th in the U.S. in terms of the environmental release of acetonitrile (Toxics Release Inventory, U.S. Environmental Protection Agency). In contrast, no acetonitrile was found in the emissions from the neighboring Sterling Chemicals plant in Texas City, Texas, ranked 4th in the U.S. Acetonitrile is used in the production of acrylic fibers, polystyrene and latex and is a common solvent. The total release of acetonitrile to the environment is estimated to be 0.01 Tg year−1 for the U.S. only, but only a fraction is directly released to the atmosphere. The industrial source of acetonitrile to the atmosphere is therefore expected to be small compared with the sources from biomass burning, currently estimated to be 0.4–1.0 Tg year−1 [Holzinger et al., 1999].

3.3. Ocean Uptake of Acetonitrile in the Atmosphere

[21] During all the ITCT2k2 flights we observed relatively low acetonitrile mixing ratios in the MBL over the Pacific in comparison with the free troposphere. In contrast, other trace gases such as ozone and carbon monoxide did not show a significant gradient across the top of the MBL, which is easily explained by the similarity of the back-trajectories calculated for the air masses sampled in the MBL and the free troposphere. The average vertical profile of acetonitrile and CO measured over the ocean up to 3 km is given in Figure 5. During some flights, transport of Asian pollution was observed above 3 km altitude and this altitude range has been omitted from the figure. The CO data are relatively constant, while the acetonitrile data increase with altitude. These results agree with the observations by Karl et al. [2003], who measured acetonitrile at the summit of Mauna Loa, Hawaii. These authors found free tropospheric values in the range of 150–250 pptv, compared to significantly lower values between 100–150 pptv when air from the MBL reached the observatory in updrafts.

Figure 5.

Average vertical profiles of CO and acetonitrile observed during ITCT2k2 over the Pacific Ocean in the lowest 3 km, west of −125°W. The solid circles show the averages for each 250 m altitude interval, and the error bars represent the standard deviation.

[22] A particularly pronounced example of low acetonitrile mixing ratios in the MBL is shown in Figure 6, which shows acetonitrile and CO data obtained during a descent performed on 11 May 2002 off the coast of Washington (approximately 48.0°N–125.0°W). Acetonitrile is approximately zero in the MBL (−20 ± 30 pptv, 1σ), whereas CO is relatively independent of altitude and shows little change across the top of the MBL. Back-trajectory calculations show that the MBL air came from the Gulf of Alaska and had not been influenced by continental emissions for more than 10 days.

Figure 6.

Measurement results for CO and acetonitrile observed during part of a flight off the coast of Washington (48.0°N–125.0°W).

[23] On 15 May 2002, the NOAA WP-3 flight path included three vertical profiles over the Pacific at different distances from the coast. Figure 7a shows a map of the area, the flight track of the WP-3 and the three locations where the WP-3 flew in the MBL for about 15 min. Figure 7b gives the mixing ratio of CO for each of the MBL flight legs and the sea surface temperature (SST) inferred from radiation measurements performed onboard the WP-3 using a modified Barnes PRT-5 instrument. Figure 7c shows the mixing ratios of acetonitrile and dimethyl sulfide (DMS) observed in the three MBL flight legs. Back-trajectory calculations, indicated by the dashed lines in Figure 7a, show that all three air masses came from the same area in the Gulf of Alaska three days prior to the measurement flight. Long-lived trace gases such as CO showed little gradient toward the coast (Figure 7b) confirming the chemical similarity of the three air masses. The most easterly air mass had traveled along the Oregon coast and the slight increase in CO could indicate some continental influence. The SST decreased from approximately 15°C at −128°W to 9°C at −124°W, which we attribute to the presence of colder, upwelling ocean water along the west coast of the U.S. In contrast to CO, both acetonitrile and DMS show strong gradients toward the coast (Figure 7c). Acetonitrile decreased from approximately 100 pptv at −128°W to 30 pptv at −124°W, whereas DMS shows the opposite trend and is increased from 10 pptv to 120 pptv. Qualitatively these results are remarkably similar to those found by Warneke and de Gouw [2001], who observed enhanced DMS and decreased acetonitrile in an upwelling zone on the border of the Indian Ocean and the Gulf of Aden.

Figure 7.

East-west gradients observed in the marine boundary layer: Figure 7a shows the flight track of the NOAA WP-3 on 15 May 2002 (solid line), the flight legs in the MBL (solid circles) and the corresponding back-trajectories (dotted line; one triangle per day); Figure 7b shows the mixing ratio of CO and the sea-surface temperature measured in the MBL; Figure 7c shows the MBL data for acetonitrile and DMS. The solid and dotted lines in Figures 7b and 7c show the result of a fit of the SST, CO, acetonitrile and DMS, respectively, versus the longitude.

[24] The results presented in Figures 57 indicate that acetonitrile is lost in the MBL. CO and acetonitrile are both (mostly) continental in origin. The uniform distribution of CO shows that the lowest 3 km of the troposphere was well mixed. The OH lifetime of acetonitrile is much longer than that of CO and based on that, one would expect an even more homogeneous distribution of acetonitrile. The observation that acetonitrile was significantly lower in the MBL indicates the presence of an additional sink, which supports the conclusions of de Laat et al. [2001] and Karl et al. [2003] who speculated about the existence of an ocean uptake sink. The unusually low, near-zero concentrations of acetonitrile measured on some of the flights (Figure 6), and the gradient observed for acetonitrile toward the coast (Figure 7) suggest that the acetonitrile sink in the MBL can be highly dependent on the location. Both here and in the work of Warneke and de Gouw [2001], markedly lower acetonitrile mixing ratios were observed in the MBL over upwelling zones, which suggests that there may be a connection. The upwelling water could be depleted in acetonitrile (because it is lost elsewhere from the ocean water) and the acetonitrile in the MBL could simply be dissolved. On the other hand, the nutrients in the upwelling ocean water give rise to plankton and bacterial growth, evidenced by the enhanced DMS in the MBL observed both here and in the work of Warneke and de Gouw [2001]. The enhanced biological activity in the ocean water could provide a local, biological sink of acetonitrile as suggested by Bange and Williams [2000].

[25] From the gradient of acetonitrile toward the coast (Figure 7c), we can estimate the loss due to ocean uptake. Assuming that (1) the three air masses had the same composition two days prior to the measurement flight, and that (2) the loss of acetonitrile was small for the more westerly air mass, it follows that 70 pptv of acetonitrile was removed from the more easterly air mass in a period of about 2 days. This corresponds to a loss of 1.5 pptv hour−1, which agrees remarkably well with the loss rate of 1.7 pptv hour−1 estimated by Warneke and de Gouw [2001] for the acetonitrile uptake observed in an upwelling zone. These workers determined a deposition velocity of 0.34 cm s−1 from their results assuming a 1000m boundary layer height. A 500m boundary layer height may be more realistic in the present case, however, suggesting a deposition velocity of 0.17 cm s−1. This value is in the range of 0.09–0.24 cm s−1 determined by Karl et al. [2003] from their Mauna Loa measurements, although these results may have to be regarded as upper limits due to the transport of acetonitrile from biomass burning in the free troposphere. The value of 0.17 cm s−1 determined here is somewhat lower than the deposition velocity of 0.5 cm s−1, which was estimated by Hamm et al. [1984] assuming that (1) acetonitrile is lost efficiently in the ocean, and (2) the uptake is limited by the gas-phase transport, and can therefore be regarded as an upper limit. The presently reported value is higher than the deposition velocity of 0.01–0.05 cm s−1 invoked by de Laat et al. [2001] to explain the acetonitrile mixing ratios observed southwest of India during INDOEX. This suggests that surface deposition of acetonitrile in upwelling zones is more efficient than in the “background” MBL by approximately one order of magnitude. It remains to be seen in what fraction of the oceans the enhanced uptake plays a role and thus how significant the enhanced uptake is globally.

4. Conclusions

[26] Airborne measurements of acetonitrile were made during the ITCT2k2 experiment. Strong enhancements were observed in two forest fire plumes but not from other sources, demonstrating the usefulness of acetonitrile measurements as an indicator for biomass burning emissions. The emission ratios for acetonitrile relative to CO were somewhat higher than values reported previously for fires burning in other fuel types. To improve our understanding of the budget of this compound, better knowledge of the acetonitrile emission ratios for different types of biomass burning is needed. No significant acetonitrile release was observed in an urban atmosphere, or in the emissions from ships in the Pacific and a power plant in Utah. Over the Pacific Ocean, significantly lower acetonitrile concentrations were observed in the marine boundary layer, indicating that ocean uptake is a significant sink. Enhanced ocean uptake was observed close to the coast, possibly explained by more efficient dissolution of atmospheric acetonitrile in the upwelling ocean water, or by a biological loss mechanism. More detailed studies are warranted to understand these phenomena in detail.


[27] We thank one of the reviewers for thoughtful comments and suggestions, which helped to improve the manuscript. We thank the crew and support team of the NOAA WP-3 aircraft, and our ITCT collaborators whose data were not directly used in this work. This work was financially supported by the NOAA Office of Global Programs.