Influence of natural and anthropogenic aerosol on cloud base droplet size distributions in clouds over the South China Sea and Western Pacific

Abstract. Cumulus clouds are common over maritime regions. They are important regulators of the global radiative energy budget and global hydrologic cycle, and a key contributor to the uncertainty in anthropogenic climate change projections due to uncertainty in aerosol-cloud interactions. These interactions are regionally specific owing to their strong influences on aerosol sources and meteorology. Here, our analysis focuses on the statistical properties of marine boundary layer (MBL) aerosol chemistry and the relationships of MBL aerosol to cumulus cloud properties just above cloud base as sampled in 2019 during the NASA Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP²Ex). The aerosol and clouds were sampled by instruments on the NASA P-3 aircraft over three distinct maritime regions around the Philippines: the West Pacific, the South China Sea, and the Sulu Sea.

Our analysis show three primary sources influenced the aerosol chemical composition: marine (ocean source), industrial (Southeast Asia, Manila, and cargo and tanker ship emissions), and biomass burning (Borneo and Indonesia). The marine aerosol chemical composition had low values of all sampled chemical signatures, specifically median values of 2.3 µg/m³ of organics (ORG), 6.1 µg/m³ of SO₄, 0.1 µg/m³ of NO₃, 1.4 µg/m³ of NH₄, 0.04 µg/m³ of Cl, and 0.0074 µg/m³ of refractory black carbon (BC). Chemical signatures of the other two aerosol source regions were: industrial, with elevated SO₄ having a median value of 6.1 µg/m³ and biomass burning, with elevated median concentrations of ORG 21.2 µg/m³ and BC 0.1351 µg/m³. The industrial component was primarily from ship emissions based on chemical signatures. The ship emissions were sampled within 60 km of ships and within projected ship plumes. Normalized cloud-droplet size distributions in clouds sampled near the MBL passes of the P-3 showed that clouds impacted by industrial and biomass burning contained higher concentrations of cloud droplets, by as much as 1.5 orders of magnitude for sizes with diameters < 13 µm compared to marine clouds, while at size ranges between 13.0–34.5 µm the median concentrations of cloud droplets in all aerosol categories were nearly an order of magnitude less than the marine category. In the droplet size bins centered at diameters > 34.5 µm concentrations were equal to, or slightly exceeded, the concentrations of the marine clouds. These analyses show that anthropogenic aerosol generated from industrial and biomass burning sources significantly influence cloud base microphysical structure in the Philippine region enhancing the small droplet concentration and reducing the concentration of mid-sized droplets.