Photochemistry in the arctic free troposphere: NOx budget and the role of odd nitrogen reservoir recycling

Abstract The budget of nitrogen oxides (NO x ) in the arctic free troposphere is calculated with a constrained photochemical box model using aircraft observations from the Tropospheric O 3 Production about the Spring Equinox (TOPSE) campaign between February and May. Peroxyacetic nitric anhydride (PAN) was observed to be the dominant odd nitrogen species (NO y ) in the arctic free troposphere and showed a pronounced seasonal increase in mixing ratio. When constrained to observed acetaldehyde (CH 3 CHO) mixing ratios, the box model calculates unrealistically large net NO x losses due to PAN formation (62 pptv/day for May, 1–3 km). Thus, given our current understanding of atmospheric chemistry, these results cast doubt on the robustness of the CH 3 CHO observations during TOPSE. When CH 3 CHO was calculated to steady state in the box model, the net NO x loss to PAN was of comparable magnitude to the net NO x loss to HNO 3 (NO 2 reaction with OH) for spring conditions. During the winter, net NO x loss due to N 2 O 5 hydrolysis dominates other NO x loss processes and is near saturation with respect to further increases in aerosol surface area concentration. NO x loss due to N 2 O 5 hydrolysis is sensitive to latitude and month due to changes in diurnal photolysis (sharp day–night transitions in winter to continuous sun in spring for the arctic). Near NO x sources, HNO 4 is a net sink for NO x ; however, for more aged air masses HNO 4 is a net source for NO x , largely countering the NO x loss to PAN, N 2 O 5 and HNO 3. Overall, HNO 4 chemistry impacts the timing of NO x decay and O 3 production; however, the cumulative impact on O 3 and NO x mixing ratios after a 20-day trajectory is minimal.