Source Apportionment of sub-Arctic Pollutants at Denali National Park, Alaska

Abstract:

The “bromine (Br) explosion” in the springtime Arctic region, associated with rapid ozone depletion events, is now a well-documented phenomenon. The source of Br appears to be sea salt but its cycling between the aerosol and gas phase is not well understood. In this study, we observed the occurrence of elevated aerosol-phase Br concentrations in springtime IMPROVE network PM_2.5 measurements in the sub-Arctic Denali National Park (DNP; site elevation, 658 MASL). Episodic elevated aerosol Br levels were observed from February to May in all years in the data record, 1988 to 2013. Anti-correlation (R=-0.54) between O₃ and Br for high-concentration Br samples (Br>2 ng/m³, 130 out of 730 springtime samples) implied its possible link to ozone depletion events in the Arctic region. To further identify the sources influencing aerosol observed at DNP, source apportionment using the EPA Positive Matrix Factorization 5.0 model was applied to the entire PM_2.5 speciated data from the DNP site. Six sources were derived, including secondary sulfate, a factor containing both Br and NO₃^-, dust, sea salt, smelting, and a mixture of wildfire and other combustion sources. Concentration weighted trajectory analysis, which was employed to identify the possible source origins, suggested that the Br/NO₃^- factor originated from northern Alaska in the springtime, and the secondary sulfate was largely associated with Asian sources that included Russian Norilsk Nickel. Sea salt, also originating from northern and northwestern coastal Alaska, was highest in the wintertime when high surface winds and low surface temperatures are expected. Dust, generally enhanced in April, May and June, was traced back to Eurasian sources. The smelting factor had a decreasing trend from 1988 to 2013, consistent with other studies of aerosol metal concentrations in the Arctic. The combustion factor was usually highest in the summertime, originated from near the surface in central Alaska, and was linked to variations in PM_2.5 wildfire emissions. Our study results suggested that Arctic haze (e.g., aerosol derived from secondary sulfate production and smelting activities) extends to sub-Arctic Alaska (~60°N) in the springtime, as do particles enriched in Br that appear to be associated with Arctic ozone depletion events.