Radical Precursors in Wintertime Fairbanks, AK: Observations of O3, HCHO, and HONO Vertical Concentration Profiles During ALPACA

High-latitude cities experience wintertime air pollution under unique environmental conditions. Emissions from traffic and residential heating, in combination with weak vertical mixing, lead to high trace gas concentrations in a shallow surface layer. Small actinic fluxes, especially in the UV, and shorter daytime periods impact gas-phase chemistry. Snow increases actinic fluxes and provides both a sink and a source of trace gases. The Alaskan Layered Pollution and Chemical Analysis (ALPACA) field experiment was performed in January and February 2022 in Fairbanks, Alaska to better understand the processes controlling wintertime air pollution. Fairbanks experiences the northern extremes of wintertime conditions, amplifying many of the above-mentioned processes, and making it an ideal location to study wintertime air pollution.Here we present long-path Differential Optical Absorption Spectroscopy observations of vertical concentration profiles of O3, NO2, SO2, HCHO, and HONO in the lowest 200m of the Fairbanks atmosphere during ALPACA, together with in-situ measurements of O3, NO/NO2, meteorology, and actinic fluxes. The observations show the accumulation of primary pollutants, for example NOx and SO2, in a shallow surface layer under calm conditions. HCHO and HONO are also elevated in this surface layer, while O3 concentrations are low near the surface. Background ozone concentrations are commonly found above 20m altitude, while HCHO and HONO concentrations are considerably lower. The presence of HOx radical precursors in the Fairbanks atmosphere allows for the photolytical formation of HOx and can drive photochemistry under high-latitude wintertime conditions. We use measured photolysis frequencies in combination with O3, HCHO, and HONO concentrations to assess the role of OH chemistry in wintertime Fairbanks as a function of altitude. Information on the sources of HCHO and HONO provided by the vertical concentration profiles and the potential role of snow will also be discussed.