A laboratory and 0D box modelling study of the wintertime formation of HONO from aerosol surfaces in Fairbanks, Alaska

As a high-latitude city, Fairbanks, Alaska, undergoes prolonged, cold winters with limited sunlight, and strong surface temperature inversions. These conditions coupled with its position at the bottom of the Tanana Valley lead to cold, dark, stagnant weather conditions, which when combined with demands for heating and transportation contribute to substantial degradations in air quality. These factors have led to Fairbanks exceeding the Environment Protection Agency PM2.5 standard and being classified as a serious nonattainment area for air quality. 

The ability to mitigate harmful pollution concentrations in Fairbanks is hampered by a lack of knowledge of the physicochemical processes which drive localised extreme pollution episodes during wintertime. For example, low levels of sunlight and ozone concentrations inhibit the well-established formation mechanisms of HO_x via photolysis or radical reactions. However, nitrous acid (HONO) can be a major source of OH radicals even in cold, dark, polluted environments. Despite being a major source of OH radicals, the formation of HONO is poorly represented in models. HONO is directly emitted from vehicles or formed via gas-phase reactions or via heterogeneous reactions such as those occurring from the surface of aerosols.

Using observations made during the Alaska Layered Pollution and Chemical Analysis Campaign (ALPACA), which took place in Fairbanks during January – February 2022, we conducted constrained chemical box model experiments to investigate HONO and oxidant sources during the ALPACA campaign. Our results show that gas-phase only reactions cannot account for observed HONO concentrations nor correctly reproduce diurnal trends. This suggests additional sources of HONO present in Fairbanks, potentially including formation from the surface of aerosols, which is not currently well constrained, especially at temperatures and relative humidities pertinent to wintertime Fairbanks.

Here, we present laboratory results aimed at addressing the lack of studies into HONO formation on the surface of aerosols in cold, dark environments and provide a wider atmospheric context via chemical box modelling constrained to observations from the ALPACA campaign. We purpose-built a chamber designed to reach temperatures similar to wintertime conditions in Fairbanks to study the formation of HONO from aerosol samples collected on filters during the ALPACA campaign, as well as filters collected from idealised single-source emission experiments in the laboratory. The generated HONO was detected in the gas-phase following its photolysis at 355 nm to OH and NO, with the OH detected via OH laser-induced fluorescence spectroscopy. We comprehensively studied HONO formation from aerosol filter samples as a function of aerosol surface area, NO₂ concentration, relative humidity, and temperature under actinic light levels applicable to wintertime conditions in Fairbanks. Inclusion of our experimental results into the chemical box model suggests enhancement of HONO concentrations over gas-phase only reactions alongside improved diurnal trends.