Springtime photochemistry at northern mid and high latitudes : Tropospheric Ozone Production about the Spring Equinox (TOPSE)

Physical and chemical properties of the atmosphere at 0-8 km were measured during the Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiments from February to May 2000 at mid (40°-60°N) and high latitudes (60°-80°N). The observations were analyzed using a diel steady state box model to examine HO x and O 3 photochemistry during the spring transition period. The radical chemistry is driven primarily by photolysis of O 3 and the subsequent reaction of O( 1 D) and H 2 O, the rate of which increases rapidly during spring. Unlike in other tropospheric experiments, observed H 2 O 2 concentrations are a factor of 2-10 lower than those simulated by the model. The required scavenging timescale to reconcile the model overestimates shows a rapid seasonal decrease down to 0.5-1 day in May, which cannot be explained by known mechanisms. This loss of H 2 O 2 implies a large loss of HO x resulting in decreases in O 3 production (10-20%) and OH concentrations (20-30%). Photolysis of CH 2 O, either transported into the region or produced by unknown chemical pathways, appears to provide a significant HO x source at 6-8 km at high latitudes. The rapid increase of in situ O 3 production in spring is fueled by concurrent increases of the primary HO x production and NO concentrations. Long-lived reactive nitrogen species continue to accumulate at mid and high latitudes in spring. There is a net loss of NO x to HNO 3 and PAN throughout the spring, suggesting that these long-term NO x reservoirs do not provide a net source for NO x in the region. In situ O 3 chemical loss is dominated by the reaction of O 3 and HO 2 , and not that of O( 1 D) and H 2 O. At midlatitudes, there is net in situ chemical production of O 3 from February to May. The lower free troposphere (1-4 km) is a region of significant net O 3 production. The net production peaks in April coinciding with the observed peak of column O 3 (0-8 km). The net in situ O 3 production at midlatitudes can explain much of the observed column O 3 increase, although it alone cannot explain the observed April maximum. In contrast, there is a net in situ O 3 loss from February to April at high latitudes. Only in May is the in situ O 3 production larger than loss. The observed continuous increase of column O 3 at high latitudes throughout the spring is due to transport from other tropospheric regions or the stratosphere not in situ photochemistry.