Response to reviewer #1

Abstract

Here we use satellite observations of HCHO vertical column densities (VCD) from the TROPOspheric Monitoring Instrument (TROPOMI), ground-based and aircraft measurements, combined with a nested regional chemical transport model (GEOS-Chem at 0.5° × 0.625° resolution), to understand the variability and sources of summertime HCHO better in Alaska. We first evaluate GEOS-Chem with in-situ airborne measurements during Atmospheric Tomography Mission 1 (ATom-1) aircraft campaign and ground-based measurements from Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS). We show reasonable agreement between observed and modeled HCHO, isoprene and monoterpenes. In particular, HCHO profiles show spatial homogeneity in Alaska, suggesting a minor contribution of biogenic emissions to HCHO VCD. We further examine the TROPOMI HCHO product in Alaska during boreal summer, which is in good agreement with GEOS-Chem model results. We find that HCHO VCDs are dominated by free-tropospheric background in wildfire-free regions. During the summer of 2018, the model suggests that the background HCHO column, resulting from methane oxidation, contributes to 66–80 % of the HCHO VCD, while wildfires contribute to 14 % and biogenic VOC contributes to 5–9 % respectively. For the summer of 2019, which had intense wildfires, the model suggests that wildfires contribute to 40 to 65 %, and the background column accounts for 30 to 50 % of HCHO VCD in June and July. In particular, the model indicates a major contribution of wildfires from direct emissions of HCHO, instead of secondary production of HCHO from oxidation of larger VOCs. We find that the column contributed by biogenic VOC is often small and below the TROPOMI detection limit. The source and variability of HCHO VCD above Alaska during summer is mainly driven by background methane oxidation and wildfires. This work discusses challenges for quantifying HCHO and its precursors in remote pristine regions.