Long-term (2005–2014) trends in formaldehyde (HCHO) columns across North America as seen by the OMI satellite instrument: Evidence of changing emissions of volatile organic compounds

Satellite observations of formaldehyde (HCHO) columns provide top-down information on emissions of highly reactive volatile organic compounds (VOCs). We examine the long-term trends in HCHO columns observed by the Ozone Monitoring Instrument (OMI) from 2005 to 2014 across North America. Biogenic isoprene is the dominant source of HCHO and its emission has a large temperature dependence. After correcting for this dependence, we find a general pattern of increases in much of North America but decreases in the southeastern US. Over the Houston-Galveston-Brazoria industrial area, HCHO columns decreased by 2.2% a^-1 from 2005 to 2014, consistent with trends in emissions of anthropogenic VOCs. Over the Cold Lake Oil Sands in the southern Alberta in Canada, HCHO columns increased by 3.8% a^-1, consistent with the increase in crude oil production there. HCHO variability in the northwestern US and Midwest could be related to afforestation and corn silage production. Although NO_x levels can affect the HCHO yield from isoprene oxidation, we find that decreases in anthropogenic NO_x emissions made only a small contribution to the observed HCHO trends.