The role of OH production in interpreting the variability of CH2O columns in the southeast U.S.

Abstract

AbstractFormaldehyde (CH2O), a key atmospheric oxidation intermediate that is detectable from satellite‐based UV/visible spectrometers, is primarily formed when hydroxyl radical (OH) reacts with volatile organic compounds (VOC) and is removed by photolysis, reaction with OH or deposition. We investigate the influence of OH and VOC variability on the CH2O column using a steady state model and the WRF‐Chem regional chemical transport model over the southeast United States for the summer of 2012 (June–August). The steady state model indicates that the CH2O column primarily depends on OH production rates (POH) at low concentrations of OH (<3 × 106 molecules cm−3), on both POH and VOC reactivity (VOCR: Σiki[VOC]i) at moderate concentrations of OH (3 × 106–7 × 106 molecules cm−3) and on VOCR at high concentrations of OH (>7 × 106 molecules cm−3). When constrained with WRF‐Chem values of boundary layer average POH and VOCR, the steady state model of CH2O explains most of the daily (r2 = 0.93) and average June–August (r2 = 0.97) spatial variance of the fully simulated cloud‐free CH2O column. These findings imply that measurements of the CH2O column offer the potential to better understand the processes affecting oxidation, particularly in remote regions, where OH concentrations are low. The findings also suggest that the inference of VOC emissions based on measurements of CH2O, or any other intermediate oxidation species with a photolytic lifetime that is short relative to removal by reaction with OH (e.g., glyoxal), should carefully account for the influence of OH on the observed pattern, especially where OH concentrations are below 5 × 106 molecules cm−3, as occurs in remote forests, where OH strongly varies, as occurs downwind of large nitrogen oxide (NOx: NO+NO2) emission sources, or where OH sources are potentially biased.