On the Source and Composition of Cloud Nuclei in a Subsident Air Mass over the North Atlantic

Abstract

Measurements of the concentration of cloud nuclei, which are activated at a supersaturation of 0.75%, were made aboard an aircraft at various altitudes in a subsident air mass over the North Atlantic Ocean and on the cast coast of Barbados, West Indies. The measurements were made on air samples at normal temperatures as well as on air samples heated to various temperatures up to 600C. In this way the volatility of the cloud nuclei was compared to the volatility as measured in a similar manner in the laboratory on nuclei artificially generated and of a known composition. The measurements at Barbados showed that ∼50% of the cloud nuclei were nonvolatile at the temperatures used and thus were similar to artificially generated nuclei composed of sea salt; the remaining nuclei were destroyed by temperatures ≳320C. The aircraft measurements showed the fraction of volatile cloud nuclei to increase with altitude with all nuclei being volatile above the inversion layer. These measurements indicate that in a subsident marine atmosphere only a fraction of the cloud nuclei at the sea surface are composed of sea salt, this fraction decreasing with altitude such that the sea salt nuclei are confined to the lower few kilometers. Based on the work of other investigators it is suggested that the volatile cloud nuclei are sulfates or sulfuric acid particles which result from the oxidation in the atmosphere of S0₂ or H₂S.