Observing atmospheric formaldehyde from space: validation and implications

Abstract

Formaldehyde (HCHO) column data from satellites are widely used as a proxy for emissions of volatile organic compounds (VOCs), but validation of the data has been extremely limited. Here I use highly accurate HCHO aircraft observations from the NASA SEAC4RS campaign over the Southeast US in 2013 summer to validate and intercompare currently available HCHO retrievals. All retrievals feature a HCHO maximum over Arkansas and Louisiana, consistent with the aircraft observations and GEOS-Chem model results, and reflecting high emissions of biogenic isoprene. The retrievals are also broadly consistent in their spatial (r = 0.4–0.8 on a 0.5° × 0.5° grid) and daily variability (r = 0.5–0.8) over the Southeast US in 2013 summer. Validation results show that HCHO column data provide a reliable proxy for isoprene emission variability but with a low mean bias (20–51%) due both to the spectral fitting and the scattering weights. I apply the corrected OMI data in the following studies. First, I show that temporal oversampling of OMI HCHO column data for 2005–2008 enables detection of urban and industrial plumes in eastern Texas including Houston, Port Arthur, and Dallas/Fort Worth. By spatially integrating the HCHO enhancement in the Houston plume observed by OMI, I estimate an anthropogenic HCHO source of 250 ± 140 kmol h-1, implying that anthropogenic highly reactive VOC emissions in Houston are 4.8 ± 2.7 times higher than reported by the US Environmental Protection Agency inventory (EPA). I then examine the interannual variability of temperature-independent HCHO columns over North America from 2005 to 2014. Significant trends are found to be likely driven by tightening emission controls over the Houston-Galveston-Brazoria area (-2.2% 0.7% a-1), by oil/gas extraction activities over the Floyd shale in Alabama (-1.8% 0.7% a-1) and the Athabasca oil sands in Alberta (4.7% 1.1% a-1), and by reforestation over the northwestern US (4.7% 1.0% a-1). Variation in HCHO columns may also be associated with changing crop cover in the midwest US and with changing wildfire activity in California. Finally, I use 2005–2015 OMI HCHO column data to map surface air HCHO concentrations across the contiguous US. Results are in good agreement with high-quality observations from urban sites and a factor of 2 lower than data from the EPA network. I estimate that up to 6600–13200 people in the US will develop cancer over their lifetimes by exposure to outdoor HCHO. I also find out that NOx emission controls to improve ozone air quality have a co-benefit in reducing HCHO-related cancer risks.