Abstract
Physical and chemical properties of the atmosphere at 0-8 km were measured
during the 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 HOx and O3 photochemistry during the spring transition period. The radical
chemistry is driven primarily by photolysis of O3 and the subsequent reaction of O(1D)
and H2O, the rate of which increases rapidly during spring. Unlike in other tropospheric
experiments, observed H2O2 concentrations are a factor of 2-10 lower than 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 H2O2 implies a large loss of HOx resulting in decreases
in O3 production (10-20%) and OH concentrations (20-30%). Photolysis of CH2O, either
transported into the region or produced by unknown chemical pathways, appears to
provide a significant HOx source at 6-8 km at high latitudes. The rapid increase of in situ
O3 production in spring is fueled by concurrent increases of the primary HOx 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 NOx to HNO3 and PAN throughout
the spring suggesting that these long-term NOx reservoirs do not provide a net source for
NOx in the region. In situ O3 chemical loss is dominated by the reaction of O3 and HO2
not that of O(1D) and H2O. At mid latitudes, there is net in situ chemical production of O3
from February to May. The lower free troposphere (1-4 km) is a region of significant net
O3 production. The net production peaks in April coinciding with the observed peak of
column O3 (0-8 km). The net in situ O3 production at mid latitudes can explain much of
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the observed column O3 increase, although it alone cannot explain the observed April
maximum. In contrast, there is a net in situ O3 loss from February to April at high
latitudes. Only in May is the in situ O3 production larger than loss. The observed
continuous increase of column O3 at high latitudes throughout the spring is due to
transport from other tropospheric regions or the stratosphere not in situ photochemistry.