Summertime photochemistry of the troposphere at high northern latitudes

The budgets of O 3 , NO x (NO+NO 2 ), reactive nitrogen (NO y ), 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 O 3 in the region are regulated mainly by input from the stratosphere, and losses of comparable magnitude from photochemistry and deposition. The concentrations of NO x (10–50 ppt) are sufficiently high to slow down O 3 photochemical loss appreciably relative to a NO x ‐free atmosphere; if no NO x were present, the lifetime of O 3 in the 0–6 km column would decrease from 46 to 26 days because of faster photochemical loss. The small amounts of NO x present in the Arctic troposphere have thus a major impact on the regional O 3 budget. Decomposition of peroxyacetyl nitrate (PAN) can account for most of the NO x below 4‐km altitude, but for only 20% at 6‐km altitude. Decomposition of other organic nitrates might supply the missing source of NO x . The lifetime of NO y , in the ABLE 3A flight region is estimated at 29 days, implying that organic nitrate precursors of NO x could be supplied from distant sources including fossil fuel combustion at northern mid‐latitudes. Biomass fire plumes sampled during ABLE 3A were only marginally enriched in O 3 ; this observation is attributed in part to low NO x emissions in the fires, and in part to rapid conversion of NO x to PAN promoted by low atmospheric temperatures. It appears that fires make little contribution to the regional O 3 budget. Only 30% of the acetic acid concentrations measured during ABLE 3A can be accounted for by reactions of CH 3 CO 3 with HO 2 and CH 3 O 2 . There remains a major unidentified source of acetic acid in the atmosphere.