Changes in atmospheric oxidants teleconnect biomass burning and ammonium nitrate formation

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 (NH 4 NO 3 ) 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 NH 4 NO 3 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 NO 2 ) and volatile organic compounds perturb the HO x (OH and HO 2 ) cycle globally, such that there is increased oxidation of anthropogenic NO x to HNO 3 . This results in additional contributions to local-scale secondary NH 4 NO 3 in areas with high emissions of anthropogenic NO x and NH 3 . 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.