Researchers from Harvard University, Rice University, NASA Goddard Space Flight Center and the Harvard-Smithsonian Center for Astrophysics, all in the US, used satellite observations of formaldehyde (HCHO) columns as an indicator for AHRVOCs over Houston, Texas.

AHRVOCs are difficult to monitor using satellites because they are short-lived, with a lifetime of less than a day. They are an important precursor of ozone and organic aerosols; usually their sources are quantified using bottom-up emission inventories. Formaldehyde is formed from atmospheric oxidation of HRVOCs and is easier to monitor using satellites.

However, measuring AHRVOCs remotely in this way is challenging because emissions come from urban regions, industrial areas and oil/gas fields, and satellite resolution is often not fine enough to detect them. Also, urban signals for formaldehyde can be masked by large regional emissions of isoprene, the dominant biogenic HRVOC contributing to formaldehyde levels.

The researchers solved this problem by using a technique called oversampling. "Oversampling refers to temporal averaging of the satellite data on a spatial grid finer than the pixel resolution on the instrument," said Lei Zhu of Harvard University. "The technique achieves high signal-to-noise ratio at high spatial resolution by sacrificing temporal resolution. [It] has been successfully used to detect SO2 and NO2 from urban and point sources, and we wanted to see if we could use it to monitor formaldehyde and therefore quantify AHRVOCs over Houston, Texas."

Zhu and his colleagues used summertime data for 2005–2008 from the Ozone Monitoring Instrument (OMI) on the Aura satellite. Using the oversampling technique on the OMI data, they estimated an anthropogenic formaldehyde source of 250±140 kmol per hour. "Our result suggests that total AHRVOC emissions are about five times higher than reported by the US Environmental Protection Agency inventory," said Zhu. "But our figures are consistent with field studies such as flight data and ground measurements."

Previous analyses of formaldehyde data in Houston have reached contradictory conclusions on whether most of the formaldehyde is primary (directly emitted) or secondary (produced from alkene oxidation). The distinction is important because primary formaldehyde would accumulate at night and photolyze in early morning, providing a source of radicals to initiate ozone formation.

Zhu and his colleagues were unable to detect the Houston urban plume in winter from the OMI data. "This would suggest that anthropogenic formaldehyde is mainly produced by photochemical oxidation of alkenes rather than directly emitted," he said.

The scientists, who reported the study in Environmental Research Letters (ERL), now plan to use satellite data and the oversampling technique to study trends in AHRVOC emissions from other US urban regions and oil/gas operations in the past decade.

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