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.

Highlights

  • The Arctic and boreal region have undergone dramatic temperature and ecological changes over the past century and the rate of this change has accelerated in recent decades (Cohen et al, 2014).Satellite-based observations of leaf area index (LAI) and normalized difference vegetation index (NDVI) suggest that northern high latitudes shows a significant 45 trend of greening in the past three decades as a result of vegetation growth (Bhatt et al, 2017; Keeling et al, 1996; Myers-Smith et al, 2011; Myneni et al, 1997; Xu et al, 2013; Zhou et al, 2001; Zhu et al, 2016), in part because the temperature is the limiting factor for vegetation growth in this region (Nemani et al, 2003)

  • biogenic VOCs (BVOCs) emissions are further complicated by land cover and LAI changes in this region (Tang et al, 2016)

  • The predominance of combustion HCHO in differential VCD calculated in GEOS-Chem (dVCDGC),Fire is consistent with the strong localization of dVCDGC,Fire enhancement, as the HCHO lifetime is on the order of hours in the presence of sunlight

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Summary

Introduction

The Arctic (north of 66.5°N) and boreal region (between 45°N and 65°N) have undergone dramatic temperature and ecological changes over the past century and the rate of this change has accelerated in recent decades (Cohen et al, 2014).Satellite-based observations of leaf area index (LAI) and normalized difference vegetation index (NDVI) suggest that northern high latitudes shows a significant 45 trend of greening in the past three decades as a result of vegetation growth (Bhatt et al, 2017; Keeling et al, 1996; Myers-Smith et al, 2011; Myneni et al, 1997; Xu et al, 2013; Zhou et al, 2001; Zhu et al, 2016), in part because the temperature is the limiting factor for vegetation growth in this region (Nemani et al, 2003). A number of studies use satellite-based observations of the HCHO column density to quantify regional and global isoprene emissions in regions where BVOC emissions are dominated by isoprene (Guenther et al, 2006; Millet et al, 2008; Palmer et al, 2003, 2006; Stavrakou et al, 2009, 2014), and the interannual variation of BVOC emissions (De Smedt et al, 2010, 2015; Stavrakou et al, 2014, 2015; Zhu et al, 90 2017) Both Bauwens et al(2016) and Stavrakou et al(2018) find an increasing trend in the HCHO column over northern high latitudes, using OMI observations during the period of 20052015. We use satellite-based observations of HCHO VCDs from TROPOMI, ground-based and aircraft measurements, combined with a high-resolution chemical transport model (GEOS-Chem at 0.5° × 0.625° resolution), to understand the sources and variability of summertime HCHO in Alaska better

TROPOMI
ATom-1 aircraft campaign
MAX-DOAS
Nested GEOS-Chem simulation
Model evaluation by ATom-1
Model evaluation by MAX-DOAS
Evaluating TROPOMI HCHO product
Predominance of background chemistry in mild wildfire summer In
Evaluating TROPOMI HCHO dVCD
Wildfire emission impacts HCHO column in Alaska
Uncertainty and capability of TROPOMI in capturing biogenic emission HCHO signals
Findings
Conclusions and discussions

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