Changes in atmospheric oxidising capacity cause teleconnections between biomass burning and NH4NO3 formation

Open biomass burning affects many aspects of the Earth system, including atmospheric chemistry and composition. Due to its impact on human health, we focus on the contribution of biomass burning emissions to fine particulate matter (PM_2.5) concentrations on a global, annual mean basis, particularly the lesser-studied secondary inorganic component. We use the EMEP MSC-W WRF atmospheric chemistry transport model to show that biomass burning leads to increased ammonium nitrate (NH₄NO₃) concentrations in densely populated regions not necessarily associated with large-scale fire activity. This is prominent in the eastern USA, northwestern Europe, the Indo-Gangetic Plane and eastern China, where NH₄NO₃ contributes between 29 and 51% to annual mean biomass burning-derived PM_2.5. Pyrogenic CO and NO_x (NO and NO₂) emissions alter the global-scale oxidising capacity of the atmosphere, affecting how local-scale anthropogenic NO_x and NH₃ emissions lead to formation of NH₄NO₃. These teleconnections can locally increase, by up to a factor of two, the contribution of biomass burning emissions to PM_2.5 concentrations, which measurements alone cannot detect. This will become relatively more important as anthropogenic sources of PM_2.5 are reduced, and with potentially intensified biomass burning occurrences under climate change.