Measurement report: Closure analysis of aerosol–cloud composition in tropical maritime warm convection
Authors: Crosbie Ewan , Ziemba Luke D. , Shook Michael A. , Robinson Claire E. , Winstead Edward L. , Thornhill K. Lee , Braun Rachel A. , MacDonald Alexander B. , Stahl Connor , Sorooshian Armin , van den Heever Susan C. , DiGangi Joshua P. , Diskin Glenn S. , Woods Sarah , Bañaga Paola , Brown Matthew D. , Gallo Francesca , Hilario Miguel Ricardo A. , Jordan Carolyn E. , Leung Gabrielle R. , Moore Richard H. , Sanchez Kevin J. , Shingler Taylor J. , Wiggins Elizabeth B. 
Published: October 2022

Abstract. Cloud droplet chemical composition is a key observable property that can aid
understanding of how aerosols and clouds interact. As part of the Clouds,
Aerosols and Monsoon Processes – Philippines Experiment (CAMP2Ex),
three case studies were analyzed involving collocated airborne sampling of
relevant clear and cloudy air masses associated with maritime warm convection. Two of the cases represented a polluted marine background, with
signatures of transported East Asian regional pollution, aged over water for several days, while the third case comprised a major smoke transport event from Kalimantan fires. Sea salt was a dominant component of cloud droplet composition, in spite of
fine particulate enhancement from regional anthropogenic sources.
Furthermore, the proportion of sea salt was enhanced relative to sulfate in
rainwater and may indicate both a propensity for sea salt to aid warm rain
production and an increased collection efficiency of large sea salt
particles by rain in subsaturated environments. Amongst cases, as
precipitation became more significant, so too did the variability in the sea salt to (non-sea salt) sulfate ratio. Across cases, nitrate and ammonium were fractionally greater in cloud water than fine-mode aerosol particles;
however, a strong covariability in cloud water nitrate and sea salt was suggestive of prior uptake of nitrate on large salt particles. A mass-based closure analysis of non-sea salt sulfate compared the cloud
water air-equivalent mass concentration to the concentration of aerosol
particles serving as cloud condensation nuclei for droplet activation. While
sulfate found in cloud was generally constrained by the sub-cloud aerosol
concentration, there was significant intra-cloud variability that was
attributed to entrainment – causing evaporation of sulfate-containing droplets – and losses due to precipitation. In addition, precipitation
tended to promote mesoscale variability in the sub-cloud aerosol through a
combination of removal, convective downdrafts, and dynamically driven convergence. Physical mechanisms exerted such strong control over the cloud water compositional budget that it was not possible
to isolate any signature of chemical production/loss using in-cloud
observations. The cloud-free environment surrounding the non-precipitating smoke case indicated sulfate enhancement compared to convective mixing
quantified by a stable gas tracer; however, this was not observed in the
cloud water (either through use of ratios or the mass closure), perhaps implying that the warm convective cloud timescale was too short for
chemical production to be a leading-order budgetary term and because
precursors had already been predominantly exhausted. Closure of other
species was truncated by incomplete characterization of coarse aerosol
(e.g., it was found that only 10 %–50 % of sea salt mass found in cloud was
captured during clear-air sampling) and unmeasured gas-phase abundances affecting closure of semi-volatile aerosol species (e.g., ammonium, nitrate and organic) and soluble volatile organic compound contributions to total
organic carbon in cloud water.