The Missoula, Montana PM2.5 speciation study—seasonal average concentrations

Abstract

Abstract The city of Missoula, located in a high mountain valley in western Montana, contains one of the largest populations of residents in the entire Rocky Mountain region completely enclosed by mountains. Due to the surrounding topography and meteorological conditions, Missoula often experiences elevated concentrations of PM 2.5 . Recently, dry summers have exacerbated August particulate loads by immersing the valley in smoke from forest fires. Through a serendipitous set of circumstances, concentrations of PM 2.5 (including chemical speciation) were measured throughout a yearlong program that captured both traditional urban particulates plus late summer forest fire particulates. This paper presents the results of the yearlong PM 2.5 sampling program in which filter samples were collected over 24-h periods every 12 days at two locations. For the first time, concentrations of the fine fraction were simultaneously measured on the opposite end of the valley in Frenchtown (nearly 18 miles west of Missoula), thus providing information on the dispersive properties of PM 2.5 in an enclosed airshed, and the spatial and seasonal variations within the Missoula Valley. Organic carbon (OC), elemental carbon (EC), trace elements, and ions were determined from the collected PM 2.5 filter samples. The average mass of ambient PM 2.5 , as well as concentrations of OC, EC, elements, and ions measured over the entire year (22/3/00–17/3/01) were nearly identical on both sides of the valley. The annual average of PM 2.5 was 15.4 μg m −3 at both Missoula's Boyd Park and at Frenchtown, with the highest concentrations of PM 2.5 measured during the summer (26/6/00–18/9/00) and winter (23/12/00–17/3/01) seasons. The summer PM 2.5 measurements were dominated by smoke from regional wildfires in Montana and Idaho, while thermal inversions in the valley contributed to the elevated winter concentrations.