A Large Contribution of Anthropogenic Organo-Nitrates to Secondary Organic Aerosol in the Alberta Oil Sands

Abstract. The oil sands industry in Alberta, Canada represents a large anthropogenic source of secondary organic aerosol (SOA). Atmospheric emissions from oil sands operations are a complex mixture of gaseous and particulate pollutants. Their interaction can affect the formation and characteristics of SOA during plume dispersion, but their chemical evolution remains poorly understood. Oxidative processing of organic vapours in the presence of NOx can lead to particulate organo-nitrate (pON) formation, with important impacts for the SOA budgets, the nitrogen cycle and human health. We provide the first direct field evidence, from ground and aircraft-based real-time aerosol mass spectrometry, that anthropogenic pON contributed up to half of SOA mass that was freshly produced within the emission plumes of oil sands facilities. Using a top-down emission rate retrieval algorithm constrained by aircraft measurements, we estimate the production rate of pON in the oil sands region to be ~ 15.5 tonnes/day. We demonstrate that pON formation occurs via photooxidation of intermediate-volatility organic compounds (IVOCs) in high NOx environments, providing observational constraints to improve current SOA modelling frameworks. Our ambient observations are supported by laboratory photooxidation experiments of IVOCs from bitumen vapours under high NOx conditions, which demonstrate that pON can account for 30–55 % of the observed SOA mass depending on the degree of photochemical aging. The large contribution of pON to freshly formed anthropogenic SOA illustrates the central role of pON in SOA production from the oil and gas industry, with relevance for other urban and industrial regions with significant IVOC and NO x emissions.