Springtime photochemistry at northern mid and high latitudes

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.