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

摘要

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_3CHO) 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_3CHO observations during TOPSE. When CH_3CHO 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_2O_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_2O_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_2O_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.