Noncombustion Emissions of Organic Acids at a Site\nnear Boise, Idaho

Gas-phase organic\nacids are ubiquitous in the atmosphere with mixing\nratios of several species, such as formic acid and acetic acid, often\nas high as several parts per billion by volume (ppbv). Organic acids\nare produced via photochemical reactions and are also directly emitted\nfrom various sources, including combustion, microbial activity, vegetation,\nsoils, and ruminants. We present measurements of gas-phase formic,\nacetic, propionic, pyruvic, and pentanoic acids from a site near Boise,\nIdaho, in August 2019 made by iodide-adduct chemical ionization mass\nspectrometry (CIMS). The site is adjacent to a major interstate highway\nand beyond the urban/suburban core is surrounded by national forests\nto the north and northeast and by farmland to the west and south.\nMaximum mixing ratios of formic, acetic, propionic, and pentanoic\nacid were typically near 10, 3, 0.4, and 0.2 ppbv, respectively. Observed\ndaytime concentrations of these acids were mostly consistent with\nother studies, but concentrations were persistently the highest at\nnight between 20:00 to 8:00 (local standard time). Such elevated nighttime\nconcentrations are unlike most other reported organic acid measurements.\nAlthough there were times when organic acid concentrations were enhanced\nby mobile source emissions, the organic acid concentrations appear\nto be mainly controlled by noncombustion surface primary emissions.\nSource apportionment by positive matrix factorization (PMF) supports\nthe importance of significant noncombustion, nonphotochemical emissions.\nTwo agricultural surface sources were identified and estimated to\ncontribute to greater than half of total observed concentrations of\nformic and acetic acid. In contrast to the other measured organic\nacids, but in agreement with all other reported measurements in the\nliterature, pyruvic acid concentrations peaked during the daytime\nand were largely controlled by photochemistry.