Abstract
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 I. 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 SO 4 2 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 Ca 2+ 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. DOI: 10.1111/j.1600-0889.1985.tb00058.x
Highlights
In addition to the orographic enhancement of precipitation at mountain sites, cloud droplet capture can lead to greater pollutant deposition relative to the surrounding lowlands
Our objectives were to characterize the chemical composition of stratus cloudwater and to address the potential that droplet capture may play as a vector for pollutant deposition
We report the composition of rainwater, cloudwater, aerosol, bulk deposition, and throughfall samples collected in a Los Angeles pine forest
Summary
In addition to the orographic enhancement of precipitation at mountain sites, cloud droplet capture can lead to greater pollutant deposition relative to the surrounding lowlands. Fog-derived (sometimes called mist or “occult“) precipitation has been determined to be an important hydrological input to some ecosystems Measurements of cloudwater composition have shown it t o have higher aqueous-phase concentrations compared to precipitation at the same locale These two factors combine t o suggest the potential for significant pollutant deposition in mountain forests impacted by frequent cloud interception. Often omitted from mass-balance calculations or regional monitoring, this pathway may represent an important component of the total deposition. This may be especially significant in urban-impacted environments By accelerating removal of local emissions. this may be especially significant in urban-impacted environments
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