Summertime Photochemistry of the Troposphere at High Northem Latitudes

The budgets of 03, NOx CNO+NO2), reactive nitrogen CNOy), and acetic acid in the 0-6 km column over western Alaska in summer are examined by photochemical modeling of aircraft and ground-based measurements from the Arctic Boundary Layer Expedition (ABLE 3A). It is found that concentrations of O3 in the region are regulated mainly by input from the stratosphere, and losses of comparable magnitude from photochemistry and deposition. The concentrations of NOx (10-50 ppt) are sufficiently high to slow down 03 photo- chemical loss appreciably relative to a NOx-free atmosphere; if no NOx were present, the lifetime of 03 in the 0-6 km column would decrease from 46 to 26 days because of faster photochemical loss. The small amounts of NOx present in the Arctic troposphere have thus a major impact on the regional 03 budget. Decomposition of peroxyacetyl nitrate (PAN) can account for most of the NOx below 4-km altitude, but for only 20% at 6-km alti- tude. Decomposition of other organic nitrates might supply the missing source of NOx. The lifetime of NOy in the ABLE 3A flight region is estimated at 29 days, implying that organic nitrate precursors of NOx could be supplied from distant sources including fossil fuel combustion at northem mid4atitudes. Biomass fire plumes sampled during ABLE 3A were only marginally enriched in 03; this observation is attributed in part to low NOx emissions in the fires, and in part to rapid conversion of NO(cid:127) to PAN promoted by low atmospheric temperatures. It appears that fires make little contribution to the regional 03 budget. Only 30% of the acetic acid concentrations measured during ABLE 3A can be accounted for by reactions of CH3CO3 with HO2 and CH302. There remains a major unidentified source of acetic acid in the atmosphere. (PAN), HNO3, total reactive nitrogen (NO(cid:127)), CO, non-methane between input the stratosphere, and of comparable hydrocarbons (NMHCs), and organic acids. We examine in this A major point of the present paper is to show that anthropogen- mented by the ABLE 3A data, with focus on the budgets of 03, NO(cid:127) of NO,(cid:127). The low concentrations of NOx measured the lifetime. We show below that decomposition of PAN can account for most of the NO,(cid:127) measured below 4-km altitude, but nitrates might provide the missing source of NO,(cid:127) at high altitude. Negative NOx to un- tions of RNO(cid:127) reflect uncertainties in the measurements. Results are from certainties on the identities, concentrations, and reactivities of the the 475 points in the data base for flights 11-25, ranked in order of increas-RNO,(cid:127) species. We test the hypothesis that ing RNO,, and 50-point clusters. The

appears to be small. The aged fire plumes sampled during ABLE 3A were only slightly enriched in O3 [Wofsy et al., this issue]. The AO3/ACO ratios in the plumes, where A represents the concentration enrichment relative to background, were in the range 0.04-0.18. In comparison, AO3/ACO ratios in the range 0.3-0.5 were observed in urban plumes sampled off the east coast of the United States during ABLE 3A transit flights. Andreae et al. [1988] previously documented AO3/ACO ratios in the range 0.01-0.09 for biomass fire plumes over Amazonia, as compared to 0.34 in the Manaus urban plume. Andreae et al. [1992] reported an average ratio of 0.14 in biomass fire plumes over the Congo. It appears that O3 production from biomass fires, when normalized 600N B•rin• • to CO emissions, is low compared to production from fossil fuel ,a• combustion. We will explain this result as due to low NO,./CO t ."'• and NOx/NIVIHC emission ratios in biomass fires; O3 production is 180ø 16õøw lõOøw NO,.-limited, and NO,. is rapidly oxidized to organic nitrates. The Fig. 1. ABLE 3A sampling region over western Alaska on flights 11-25 relatively low NO,. emissions in biomass fires may result from low (July 19 to August 7, 1988). The dashed area around the town of Bethel bum temperatures, particularly under smoldering conditions, and was heavily sampled; additional flight tracks outside that area are shown as also at high latitudes from the low nitrogen content of vegetation thick lines. [Chapin and Shaver, 1985].

Talbot et al. [this issue] measured acetic acid concentrations in chemical steady state (including OH, peroxy species, and NO2
). the range 100-400 ppt during ABLE 3A. Acetic acid is produced Steady state is assumed also for oxygenated hydrocarbons with by CH3CO3 + peroxy reactions [Moortgaat et al., 1989a, b]. If lifetimes of a few days or less (carbonyls, peroxides) and for other these reactions were the dominant sources of acetic acid in the at-short-lived compounds (e.g., HNO2, HNO4). A fixed acetone conmosphere, as has been suggested by Madronich et al. [1990], then centration of 120 ppt is adopted [Arnold et al., 1986]. The UV raacetic acid would be an interesting tracer of photochemical activi-diation field is computed on the basis of the local altitude, solar ty. However, we report below that only -30% of the acetic acid zenith angle, and albedo, assuming clear-sky conditions (see apmeasured in ABLE 3A can be accounted for in tlmt manner. pendix). Proper accounting of cloud effects is not possible from There remains a major unidentified source of acetic acid in the at-the data available; averaging over a large number of points should mosphere.
at least reduce the associated uncertainty. The paper is organized in two sections. In section 2 we con-The 475 points in the data base were selected on the basis of stmct budgets of 03, NO,., NO•, and acetic acid in the ABLE 3A NIVIHC data availability. Each NMHC grab sample [Blake et al., flight region, using photochemical model statistics based on the this issue] was matched with 10-s average data for 03, CO, and aircraft observations. In section 3 we use a Lagrangian model to meteorological variables [Gregory et al., (Table 1). Initial conditions for the calculations (Table 1)  cal of biomass fires in general, as indicated by data for selva and of NO,, on the first day produces roughly equal proportions of cerrado fires in Brazil [Greenberg et al., 1984], and for a chaparral HNO3 and PAN, plus small mounts of other organic nitrates fire in California [Cofer et aL, 1989]. The speciation of NMHCs (Figure 9b). The high yield of PAN reflects the low NO,`/NMHC among alkanes, alkenes, and aromatic species is taken from emission ratio and the low temperatures. As the plumes age on Greenberg et al. [1984]. Initial concentrations of all secondary the second day, slow decomposition of PAN takes place, because species (including O3 and PAN) are assumed equal to background. of the paucity of NO,,, shifting the composition of the NO• pool Fixed temperature (268 K) and dew point (263 K) are adopted towards HNO3 and RNO,,. The APAN/ANO• ratios after 2 days from aircraft measurements. The chemical evolution of the plume are 0.28 in the diluted plume and 0.38 in the diluting plume; both is simulated for 48 hours using the mechanism described in the appendix. The simulations are initialized at noon; initialization at midnight produced no significant differences in results. ing plume as the plume ages is due to entrainment of background air. Simulated enrichments AO3 after 2 days of travel are 4 ppb in both plumes, consistent with observations. Photochemical production of O3 in the model plumes is strongly NO,`-limited. Increasing NO,` emissions by a factor of 10 causes AO3 to increase by a factor of 5, while increasing NMHC emissions has little effect on O3 (Table 2). This resuk reflects the low NO,`/NMHC emission ratio in the fire (0.034), which can be compared to typical NO,`/NMHC emission ratios of 0.1-1 in U.S. , 1989]. The NO,`/CO emission ratio in the fire (0.0034) is also low compared to typical urban values (0.05-0.1). Our finding that O3 production in the ABLE 3A fire plumes was NO,`-limited may be applicable to biomass fire plumes in general. The review of biomass burning emissions by Crutzen and Andreae [1990] gives NO,`/CO emission ratios in the range 0.002-0.05 for various types of fires; these values are low compared to urban pollution. A likely explanation is that temperatures in biomass fires are relatively low. The particularly low NO,`/CO emission ratios in the ABLE 3A fires may reflect in addition the low nitrogen content of vegetation at high latitudes [Chapin and Shaver, 1985].

cities [Environmental Protection Agency
The above discussion implies that the relatively low AO3/ACO ratios previously reported for biomass fire plumes in the tropics [Andreae et al., 1988[Andreae et al., , 1992 can be explained simply by low NO,`/CO emission ratios. The AO3/ANO • ratio is an alternate measure of O3 production in the plumes. Assuming that O3 and NO• are conserved in the plume, and that NO• is emitted as NO,`, then the AO3/ANO• ratio measures the number of O3 molecules produced per molecule of NO,` emitted, i.e., the "03 production efficiency" [Liu et al., 1987;Linet al., 1988]. The AO3/ANO• ratios measured in the ABLE 3A fire plumes ranged from 12 to 21 [Wofsy et al., 1991], and our model gives a value of 13 (Table 2). Table 1. Part of the difference appears to reflect the low temperatures in the ABLE 3A plumes, promoting conversion of NO,` to PAN.

These values are low compared to the O3 production efficiencies of 30-40 reported by Lin et al. [ 1988] from photochemical simulations of pollution plumes with same initial inputs of NO,` and NMHCs as in
We can estimate roughly the contribution of biomass fires plumes to the regional O3 budget at high northern latitudes by assuming an O3 production efficiency of 13 in the plumes, and a re-  ABLE 3A, suggesting that the overall influence of fires on the regional 03 budget remains minor.

CONCLUSIONS
Modeling of observations from the ABLE 3A expedition indicates that the Os concentrations in the summertime Arctic troposphere reflect mainly a balance between input from the stratosphere, and losses of comparable magnitude from photochemistry and deposition. The observed concentrations of NO,, (10-50 ppt) are sufficiently high to reduce the Os photochemical loss rate by a factor of 2.5 relative to a NO,,-free atmosphere. We estimate an atmospheric lifetime of 46 days for Os in the 0-6 km column sampled during ABLE 3A; this lifetime would drop to 26 days if no NO,, were present. The small mounts of NO,, observed in ABLE 3A have thus a major effect on the regional Os budget.
We find that decomposition of PAN can account for most of the NO,, observed in ABLE 3A below 4-km altitude, but for only 20% at 6 km altitude. The missing source of NO,, at high altitudes may be due to decomposition of unidentified organic nitrates. The atmospheric lifetime of NO• is estimated at 29 days, implying that