Chemical composition of rainwater at a coastal town on the southwest of Europe: what changes in 20 years?

Rainwater was collected at the Portuguese west coast between September 2008 and September 2009, and analysed for pH, conductivity, and Cl⁻, NO₃⁻, SO₄²⁻, and NH₄⁺ concentrations. Results of rainwater chemical composition were compared with those obtained at the same site between 1986 and 1989. In both collection periods rainwater was predominantly (≈ 80%) associated to oceanic air masses. The rainwater concentration of H⁺ was in the same range as twenty years ago. A clear decrease of non sea salt sulphate (NSS-SO₄²⁻) was observed in 2008-2009 relatively to 1986-1989, not only in samples with origin in central and northern Europe, but also in samples from Atlantic Ocean and Mediterranean. This decrease indicates that SO₂ emissions were reduced, which may be due to a lower content of sulphur in oil by-products. A decrease was also observed in NH₄⁺ concentration in 2008-2009. On the contrary an increase of NO₃⁻ concentration was observed for samples of all origins in 2008-2009 relatively to 1986-1989, which was particularly high (more than 3 fold) for samples with origin in Atlantic Ocean, suggesting the incorporation of this ion by rainout at the sampling site. The contribution of local sources is indeed suggested by the moderate negative correlation of NH₄⁺, NO₃⁻ and NSS-SO₄²⁻ with rainwater volume. The high increase of NO₃⁻ concentration can be attributed to the increase of local vehicular and industrial emissions in the sampling area.