Response of natural dust to removal of anthropogenic emissions over South Asia

We use the regional chemistry transport model WRF-Chem (v4.2.1) to analyse the sensitivity of surface dust across Northern India to various emissions' sources. We use idealised model experiments that switch off individual sources in an attempt to apportion dust aerosol concentrations to anthropogenic, biomass-burning, and natural dust sources. However, these experiments show significant non-linear interactions between sources, making simple apportionment difficult. Across the natural dust-dominated western part of the domain, we find that switching off anthropogenic emissions results in an increase in dust. Alternatively, this can be considered in the opposite sense: the presence of anthropogenic pollutants reduces surface dust aerosol concentrations by almost 50 % on average. We diagnose the processes responsible for this somewhat surprising result. Heterogeneous chemical reactions between dust and nitric acid (HNO₃) shorten the dust’s lifetime, increasing the sink for natural dust as HNO₃ is enhanced by anthropogenic NO_x emissions. The modelled dust lifetime is enhanced by nearly 4 hours when anthropogenic precursor emissions are excluded. The model shows a strong anticorrelation between dust and HNO₃, related to the preferential uptake of HNO₃ by dust particles over a broad relative humidity range (10 – 90 %). The strong non-linear response of dust loading to idealised emissions changes shows considerable regional variation. The effect of these dust-pollution interactions on dust lifetime suggest that dust concentrations will increase as anthropogenic NO_x emissions reduce, making control of particulate pollution harder, particularly in regions with high natural dust sources.