Chemical composition of rainwater under two events of aerosol transport: A Saharan dust outbreak and wildfires.

Abstract A one-year campaign of joint sampling of aerosols and precipitation, carried out in Leon, Spain, allowed the study the impact of two special events that affected the air quality in northern Spain, on rainfall in the city: a period with wildfires and a Saharan dust intrusion. The wildfires that occurred in northern Portugal and northwestern Spain in August 2016 affected the chemistry of rainfall on 15 August 2016, causing an increase in concentrations of NH4+, Na+, Cl−, K+, Mg2+, Ca2+, SO42− and NO3− and in the concentrations of organic acids, reflected in the concentration of soluble and insoluble organic carbon and, consequently, leading to acidification of rainwater (pH = 4.8). The second precipitation event was registered between 11 and 14 February 2017, during which the rainwater was collected in four daily fractions (P1, P2, P3 and P4). The rain sample of 12 February (P2) coincided with a Saharan dust intrusion that reached northern Iberia that day. The chemical composition of P2 showed an increase in the Ca2+ (>800%), Mg2+ (71%), Cl− (62%), and SO42− (33%) concentrations, with respect to P1. The input of crustal elements to the atmosphere helped to neutralize the P2 rainwater, causing pH values higher than 6.5. Once the dust intrusion left the north of the Peninsula, the composition of rainwater P3 and P4 revealed a mixture of marine contribution with local anthropogenic emissions, as well as a decrease in ion concentrations and conductivity, and an increase in pH values.