Department of Earth System Science

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.