Chemical characterization of stratus cloudwater and its role as a vector for pollutant deposition in a Los Angeles pine forest

Highly concentrated, acidic stratus cloudwater was monitored as it intercepted a pine forest (Henninger Flats) 25 km northeast of Los Angeles. Observed pH values ranged from 2.06 to 3.87 for over 100 samples collected in 1982 and 1983 with a median value below pH 3. The ratio of nitrate/sulfate in cloudwater samples was between 1.5 and 2: rainwater at the same site had a ratio of approximately 1. The solute deposition accompanying several light. spring rains (summing to ∼1 % of annual rainfall) was a disproportionate fraction of the annual total: H⁺, NO^-₃ and SO₄² were ∼20% or more. Based on a reasonable estimate of fog precipitation, deposition of sulfate, nitrate and free acidity due to intercepting stratus clouds may be of comparable magnitude as that due to the incident rainfall at Henninger Flats.

Cloudwater that had deposited on local pine needles was collected. It was in general more concentrated than ambient cloudwater but with comparable acidity. Enrichment of K⁺ and Ca²⁺ in those samples and in throughfall is believed to be due to leaching from foliar surfaces. Injury to sensitive plant tissue has been noted in the literature when prolonged exposure to this severe kind of micro-environment has been imposed.