Measurement report: Closure analysis of aerosol–cloud composition in tropical maritime warm convection

Abstract

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.

An Aerosol Climatology via Remote Sensing over Metro Manila, Philippines

“…3b) from 2009 to 2018 over the Manila Observatory was observed from July (~1.4) to September (~1.3), during the southwest monsoon. This period is associated with the biomass burning southwest of the Philippines (Oanh et al, 2018;Stahl et al, 2021;Crosbie et al, 2022). The median (per month) EAE ranged from ~0.9 in November to ~1.4 in August, a range which is within the values from previous studies collected from mixed sites and urban/industrial areas with both fine and coarse particles (Eck et al, 2005;Giles et al, 2012).…”

“…1b). The highest median level of relative humidity for a month was in August (86.5 %) during the summer southwest monsoon, which is also the time of the year (June to August) when rainfall peaks in the region where the sampling station (Manila Observatory) is located (Coronas, 1920;Cruz et al, 2013). The highest mean hourly precipitation (Fig.…”

“…Studying this area is informative owing to the wide dynamic range in aerosol particle and weather conditions, which are interconnected. Aerosol particle lifecycle in the region is impacted by Philippine weather that is marked by two distinct monsoons, typhoons, and impacts from El Niño-Southern Oscillation and Madden-Julian Oscillation (Cruz et al, 2013;Xian et al, 2013;Reid et al, 2012;Reid et al, 2015;Hilario et al, 2021b).…”

Reducing Aerosol Forcing Uncertainty by Combining Models With Satellite and Within‐The‐Atmosphere Observations: A Three‐Way Street

“…A suborbital ACI measurement program could be considerably more complex. Reviews of past modeling and measurement work (e.g., Rosenfeld et al, 2014;Mülmenstädt & Feingold, 2018;Bellouin et al, 2020) and experience from recent aircraft campaigns aimed at characterizing aerosol particles, clouds, and their interactions in specific regions (Behrenfeld et al, 2019;Crosbie et al, 2022;Sorooshian et al, 2019Sorooshian et al, , 2021 provide some indication of what would be involved. Both aerosol and cloud properties would need to be measured for this application, on spatial scales ranging from 10 −7 to 10 6 m and temporal scales of minutes to hours or more.…”

Influence of below‐cloud evaporation on stable isotopes of precipitation in the Yellow River source region

“…When rain falls through unsaturated air, some or all of the droplets may evaporate before reaching the ground, so the tracer in the rain may be released back into the atmosphere closer to the ground (Crosbie et al, 2022) under-cloud evaporation leads to an increase in the global aerosol burden, and the increase in the burden of different types of aerosols is about 7.8-15%. When considering raindrop diameter and evaporation rate (Gong et al, 2006), the release of aerosols is greatly reduced, and in combination with the results observed in 1992 in the Mainz vertical wind tunnel and in a 4-meter-high shaft (Mitra et al, 1992), the aerosol particles do not break up during raindrop evaporation and are converted into coarse particles that enhance removal by settling, resulting in a lower aerosol load.…”