Acidic and related constituents in liquid water stratiform clouds

Composition of cloud liquid water and interstitial air is reported for aircraft measurements near Charleston, South Carolina, in February 1982. Cloud water was collected by means of a slotted‐rod impactor. Cloud‐free interstitial air was obtained by a centrifugal separator. Principal ionic species in cloud water were H+, SO4=, NO3−, and NH4+. The relative acidity of cloud water, as measured by the ratio of H+ to NH4+, was much greater than that of the interstitial aerosol or of clear‐air aerosol samples. Gaseous O3, NOx (i.e., NO+NO2), and SO2 were present in interstitial air within clouds, whereas HNO3 and NH3 were below limit of detection (0.4 ppb for both species). Comparison of relative aciditiy of cloud water samples with that of clear‐air samples suggests that clear‐air aerosol and HNO3 are incapable of providing the levels of acidity observed in cloud water. Such acidity would be consistent with in‐cloud conversion of prevailing concentrations of SO2 or SO2+NOx to aqueous strong acid, although other hypotheses cannot be excluded. Comparison of absolute concentrations of cloudwater constituents to concentrations in clear‐air measurements gives no indication of accumulation of sulfur and nitrogen species by clouds.