Primary and Secondary Formaldehyde (HCHO) in a Mid-Sized Valley Area in the Inland Northwest

The Lewiston-Clarkston Valley area, located in North Central Idaho and Southeastern Washington State saw elevated formaldehyde (15ppbv) and acetaldehyde concentrations in a community monitoring study in 2006/2007. The study found higher concentrations in summer than winter, suggesting a secondary production of the carbonyls. Meanwhile, local industry sources (mainly a pulp mill, a lumber mill and a composting facility) were also believed to be primary sources. Available data was not sufficient to identify sources for the high levels of formaldehyde. Health risks due to the high formaldehyde concentration merit further study in the region. In summers of 2016 and 2017, an intensive sampling of air toxics in the valley was thus conducted. The purpose was to identify sources of HCHO by measuring carbonyls and other anthropogenic/biogenic VOC concentrations at high time resolution and at detailed species level. Measurements were taken at 3 sites (Sunset Park, Hatwai, and Asotin Landfill) over a period of 4 weeks to provide spatial distribution. The Sunset Park site was the main site where a Mobile Atmospheric Chemistry Laboratory (MACL) from Washington State University was located which hosted a suite of instruments that enable the high time resolution sampling. At this site, a proton transfer reaction mass spectrometer (PTR-MS) measured HCHO, acetaldehyde, methanol, benzene, monoterpene and other air toxics on a minute basis. Ozone, CO, SO2 and NOxy species were also monitored. A weather station WXT-520 provided surface meteorology for tracer analysis. For all sites, two-a-day, 12-hr average grab sampling was collected onto DNPH cartridges for HCHO and acetaldehyde analysis and into summa canisters for VOC analysis on a GC-MS. Initial findings of the two-year study show a decrease in the HCHO concentration compared with 2006/2007, with an average of 1.93 ppb for 2016 and 5 ppb for 2017. A strong correlation between HCHO and SO2, an important tracer of industrial combustion, was observed. There was no significant spatial difference among the HCHO and acetaldehyde measured at different sites. Correlation analysis among HCHO and other VOCs and trace gases conducted suggests that the pulp mill could be a contributor to the elevated HCHO concentrations in the valley.