Photochemistry in the arctic free troposphere: Ozone budget and its dependence on nitrogen oxides and the production rate of free radicals

Local ozone production and loss rates for the arctic free troposphere (58-85° N, 1-6 km, February-May) during the Tropospheric Ozone Production about the Spring Equinox (TOPSE) campaign were calculated using a constrained photochemical box model. Estimates were made to assess the importance of local photochemical ozone production relative to transport in accounting for the springtime maximum in arctic free tropospheric ozone. Ozone production and loss rates from our diel steady-state box model constrained by median observations were first compared to two point box models, one run to instantaneous steady-state and the other run to diel steady-state. A consistent picture of local ozone photochemistry was derived by all three box models suggesting that differences between the approaches were not critical. Our model-derived ozone production rates increased by a factor of 28 in the 1-3 km layer and a factor of 7 in the 3-6 km layer between February and May. The arctic ozone budget required net import of ozone into the arctic free troposphere throughout the campaign; however, the transport term exceeded the photochemical production only in the lower free troposphere (1-3 km) between February and March. Gross ozone production rates were calculated to increase linearly with NO_x mixing ratios up to ∼300 pptv in February and for NO_x mixing ratios up to ∼500 pptv in May. These NO_x limits are an order of magnitude higher than median NO_x levels observed, illustrating the strong dependence of gross ozone production rates on NO_x mixing ratios for the majority of the observations. The threshold NO_x mixing ratio needed for net positive ozone production was also calculated to increase from NO_x ∼ 10 pptv in February to ∼25 pptv in May, suggesting that the NO_x levels needed to sustain net ozone production are lower in winter than spring. This lower NO_x threshold explains how wintertime photochemical ozone production can impact the build-up of ozone over winter and early spring. There is also an altitude dependence as the threshold NO_x needed to produce net ozone shifts to higher values at lower altitudes. This partly explains the calculation of net ozone destruction for the 1-3 km layer and net ozone production for the 3-6 km layer throughout the campaign.