Origin of tropospheric ozone at remote high northern latitudes in summer

We quantify the tropospheric ozone budget over remote high northern latitudes in summer using chemical and meteorological measurements between 0 and 6‐km made during the summer of 1990 Arctic Boundary Layer Expedition (ABLE 3B). We include all components of the ozone budget, both sinks (in situ photochemical loss and deposition); and sources (in situ photochemical production, advection of pollution ozone into the region, production in biomass wildfire plumes, and downwards transport from the upper troposphere/stratosphere). In situ production and loss of ozone are calculated with a photochemical model. The net influx of pollution ozone from North America and Eurasia is estimated from the average enhancement ratio of ΔO 3 /ΔC 2 Cl 4 observed in pollution plumes and scaled by the net influx of C 2 Cl 4 . The contribution of ozone produced in biomass wildfire plumes is estimated from the average enhancement ratio of ΔO 3 /ΔCO in aged fire plumes. Regional photochemical production and loss in the 0–6 km column are found to be approximately equal; hence, net photochemical production is near zero. However, when ozone production and loss terms are separated, we find that dispersed in situ photochemical production driven by background NO levels (5–10 pptv) is the largest source term in the ozone budget (62%). Influx of stratospheric ozone is of secondary importance (27%), long‐range transport of pollution ozone makes a small contribution (9%), and photochemical production of ozone within biomass wildfire plumes is a relatively negligible term (2%) in the budget. Biomass fires and transport of anthropogenic pollution into the region may however have a major effect on the ozone budget through enhancement of background NO, mixing ratios which increase dispersed photochemical production. Using a 1‐D time‐dependent photochemical model between 0 and 6 km, we obtain good agreement between the observed and model‐generated vertical ozone profiles. We find that in situ photochemistry within the 0–6 km column accounts for nearly 90% of the ozone mixing ratio within the boundary layer, while above 5 km it accounts for only about 40%. Although photochemical production of ozone within the 0–6 km column is larger than the other source terms combined, the 1‐D model results indicate that influx from above is necessary to account for the observed increase in ozone mixing ratios with altitude.