Wet-only sequential deposition in a rural area in north-eastern Spain

Abstract

The present work summarises the results of a 2-year study of wet-only sequential deposition in 2 rural areas (Alcanñiz and Morella) located at different distances from a large coal-fired power station. Precipitation chemistry was characterised by relatively high, SO₄^2-, Ca²⁺ and NH⁺₄ average concentrations. Sequential study of single precipitation events showed that concentrations of most of the ions studied decreased exponentially throughout single precipitation events, with a sharper decrease in concentrations at the beginning of the event. Usually, 40 to 80% of the wet-only deposition of major ions occurred in the first 2 mm. pH measurements, ranging from 5.6 to 8.1, showed a decrease in the pH values throughout a precipitation event. Deposition levels of Ca²⁺ and NH+4 accounted for the neutralisation of major acidic species in the precipitation events except in 5 rain episodes sampled at Morella. The sequential study of the evolution of the (SO^2-₄ + NO^-₃)/(Ca²⁺ + NH⁺₄) ratio during a single precipitation allowed us to identify potential acidic rainfall fractions after an initial volume of precipitation, after most of the Ca²⁺ in atmospheric particles had been scavenged. A higher neutralisation capacity was deduced for Alcanñiz owing to the higher atmospheric levels of natural carbonates and NH⁺₄.