AerosoL heterogeneous Processing as a source of oxidants in Cold winter Atmospheres: application to Alaska and UK (ALPACA-UK)

Air pollution is the leading environmental risk to human health. Wintertime pollution events lead to violations of air quality (AQ) standards for particulate pollution in many regions of the world, including the UK. Despite limited sunlight availability in these events, secondary atmospheric production of aerosol particles appears to make an important contribution to particulate pollutant concentrations, which well-established oxidation mechanisms cannot explain. High levels of sulfate observed during such severe winter pollution episodes cannot be explained by current air quality models. Nitrous acid (HONO) is an important source of the chief atmospheric oxidant, OH, in polluted atmospheres. There is growing evidence that HONO produced via heterogeneous aerosol chemistry occurring on particle surfaces is important in wintertime conditions, where HONO may contribute up to 90% of primary OH production in polluted winter urban regions. There is incomplete understanding of this source and it is not included in air pollution forecasting models. Here we will undertake an ambitious integration of laboratory, field and modelling studies to establish the role of heterogeneous HONO production in a polluted high latitude city (Fairbanks, Alaska) and in London, through exploiting recently-developed laboratory capabilities, data that will become available through a planned international field campaign, and new model schemes for aerosol chemistry. Our approach will involve undertaking laboratory experiments on aerosol samples collected in the field, in which we will directly monitor the production of HONO from aerosol filters as a function of varying light, humidity, temperature and aerosol composition. These results will then be used to produce new model parameterisations for aerosol HONO production that will be tested against field observations from Fairbanks using a chemical box model framework. Finally, regional model simulations using the WRF-Chem model will be used to scale up the impacts of the aerosol HONO source on air quality in wintertime Fairbanks and London, UK.

Our project will deliver new parameterisations and capabilities for including a heterogeneous HONO source in air quality models, including dependencies on aerosol composition, which we will make available to the community via our links with the global and regional WRF-Chem modelling communities. We will link with the new UK Strategic Priority Fund (SPF) Clean Air programme to exploit the newly funded UK Emissions Modelling System (UK-EMS), and to determine the impact of these processes on recent extreme wintertime pollution events in London, and on the efficacy of potential air pollution mitigation measures. Our recent advances in laboratory methods and a planned wintertime air pollution measurement field intensive in Fairbanks, Alaska, provide a timely opportunity to gain new understanding of a heterogeneous HONO source during winter pollution episodes. Outcomes will enable improved understanding and forecasting of wintertime air pollution across many regions of the world, where cold, low sunlight conditions occur in wintertime.