Deposition of sulphur, nitrogen and acidity in precipitation over Ireland: chemistry, spatial distribution and long-term trends

The chemical composition of pollutant species in precipitation sampled daily or weekly at 10 sites in Ireland for the five-year period, 1994–1998, is presented. Sea salts accounted for 81% of the total ionic concentration. Approximately 50% of the SO42− in precipitation was from sea-salt sources. The proportion of sea salts in precipitation decreased sharply eastwards. In contrast, the concentration of NO3− and the proportion of non-sea-salt SO42− increased eastwards reflecting the closer proximity to major emission sources. The mean (molc) ratio of SO42−:NO3− was 1.6 for all sites, indicating that SO42− was the major acid anion. The spatial correlation between SO42−, NO3− and NH4+ concentrations in precipitation was statistically significant. The regional trend in NO3− concentration was best described by linear regression against easting. SO42− concentration followed a similar pattern. However, the regression was improved by inclusion of elevation. Inclusion of northing in the regression did not significantly improve any of the relationships except for NH4+, indicating a significant increase in concentrations from northwest to southeast. The spatial distribution of deposition fluxes showed similar gradients increasing from west and southwest to east and northeast. However, the pattern of deposition shows the influence of precipitation volume in determining the overall input. Mean depositions of sulphur and nitrogen in precipitation were ≈30 ktonnes S yr−1 and 48 ktonnes N yr−1 over the five-year period, 1994–1998, for Ireland. Least-squares linear regression analysis indicated a slight decreasing trend in precipitation concentrations for SO42− (20%), NO3− (13%) and H+ (24%) and a slight increasing trend for NH4+ (15%), over the period 1991–1998.