Chemical analysis of rain samples collected over the Pacific Ocean

Abstract

During seven research cruises in the Pacific Ocean from 1984 to 1989 we collected rain samples for chemical analyses. The geographical and temporal variations of rain chemistry were studied in relation to natural and anthropogenic sources and transport of the constituents. The pH values of rain samples ranged from 3.9 to 7.2, with a mean of 5.6 (n = 104, σ = 0.5). The most acidic rain was found near the big ports, where SO4= and NO3− concentrations in rain were also the highest. The data indicated that wet deposition transports anthropogenic pollution from continents to oceans. Enrichments of Ca++ and K+, which are compared to Na+ in seawater, were observed in all rain samples throughout the ocean. However, their enrichment factors (EF) decrease with distance from land, signifying the extent of transport of crustal elements through cloud process and wet deposition. The EFs of Mg++ were close to unity, indicating its common origin with Na+. The EFs of Cl− in most rain samples were less than l, indicating loss of Cl− in aerosols, cloud droplets, or rain drops. High EFs of SO4= were also observed at the equatorial regions, where biogenic production of dimethyl sulfide is well known. During the 1987 El Niño anomaly the sulfate aerosol distribution and rainfall patterns altered with air trajectories. The effects of sulfate aerosol on cloud condensation and rainfall amount in various regions during normal years and the El Niño anomaly are discussed.