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. Issue 13 (15th July 2017) - Zhu, Lei
Abstract: 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 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 U.S. 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 U.S. 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. Plain Language Summary: We use satellite observations to diagnose long‐term trends in HCHO columns across North America from 2005 to 2014. HCHO generally increased from 2005–2009 to 2010–2014 but decreased in the southeastern U.S. We find significant regional trends in excess of 20% related to decreases in urban anthropogenic VOC emissions (Houston metropolitan area) and increases in oil/gas production (oil sands in western Canada). Significant regional trends in the northwestern U.S. and in the Midwest may be driven by afforestation and agricultural activity. The impact of declining NO x emission over the U.S. on HCHO columns is likely small over this time frame. Key Points: We detect significant HCHO trends over North America from space We see evidence of changing emissions of VOCs from space, in particular from oil/gas operations The impact of declining NO x emissions on HCHO columns is greatest in the southeastern U.S.