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Measurements of formaldehyde (CH2O) from a tunable diode laser absorption spectrometer (TDLAS) were acquired onboard the NASA DC-8 aircraft during the summer 2004 INTEX-NA campaign to test our understanding of convection and CH2O production mechanisms in the upper troposphere (UT, 6–12 km) over continental North America and the North Atlantic Ocean. The present study utilizes these TDLAS measurements and results from a box model to (1) establish sets of conditions by which to distinguish “background” UT CH2O levels from those perturbed by convection and other causes; (2) quantify the CH2O precursor budgets for both air mass types; (3) quantify the fraction of time that the UT CH2O measurements over North America and North Atlantic are perturbed during the summer of 2004; (4) provide estimates for the fraction of time that such perturbed CH2O levels are caused by direct convection of boundary layer CH2O and/or convection of CH2O precursors; (5) assess the ability of box models to reproduce the CH2O measurements; and (6) examine CH2O and HO2 relationships in the presence of enhanced NO. Multiple tracers were used to arrive at a set of UT CH2O background and perturbed air mass periods, and 46% of the TDLAS measurements fell within the latter category. In general, production of CH2O from CH4 was found to be the dominant source term, even in perturbed air masses. This was followed by production from methyl hydroperoxide, methanol, PAN-type compounds, and ketones, in descending order of their contribution. At least 70% to 73% of the elevated UT observations were caused by enhanced production from CH2O precursors rather than direct transport of CH2O from the boundary layer. In the presence of elevated NO, there was a definite trend in the CH2O measurement–model discrepancy, and this was highly correlated with HO2 measurement–model discrepancies in the UT.

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 diet 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 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 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, and not that of O(1D) and H2O. At midlatitudes, 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 midlatitudes can explain much of 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.