Changes in atmospheric oxidants teleconnect biomass burning and ammonium nitrate formation

Abstract

Open biomass burning has major impacts on the Earth system, including on air quality via the emission of primary fine particulate matter (PM_2.5). Its effect on secondary inorganic PM_2.5 formation is comparatively little investigated. Simulations with the EMEP MSC-W WRF atmospheric chemistry transport model reveal that global biomass burning emissions lead to elevated annual mean ammonium nitrate (NH₄NO₃) concentrations in densely populated regions where biomass burning mostly does not occur. These regions include eastern USA, northwestern Europe, the Indo-Gangetic Plain and eastern China, where NH₄NO₃ conditional on biomass burning emissions constitutes between 29% and 51% of the annual mean PM_2.5 conditional on biomass burning emissions. Biomass burning emissions of CO, NO_x (NO and NO₂) and volatile organic compounds perturb the HO_x (OH and HO₂) cycle globally, such that there is increased oxidation of anthropogenic NO_x to HNO₃. This results in additional contributions to local-scale secondary NH₄NO₃ in areas with high emissions of anthropogenic NO_x and NH₃. These teleconnections increase, by up to a factor of two, the contribution of biomass burning emissions to long-term PM_2.5 concentrations, which measurements alone cannot identify as an impact of biomass burning activity. This may become relatively more important as anthropogenic sources of PM_2.5 are reduced and as the wildfire component of biomass burning increases under climate change.