20 Jan 2023
20 Jan 2023
Status: this preprint is open for discussion.

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

Rose Marie Miller1, Robert M. Rauber1, Larry Di Girolamo1, Matthew Rilloraza1, Dongwei Fu1,2, Greg M. McFarquhar3,4, Stephen Nesbitt1, Luke D. Ziemba5, Sarah Woods6, and K. Lee Thornhill5,7 Rose Marie Miller et al.
  • 1Department of Atmospheric Science, University of Illinois Urbana-Champaign, Urbana, IL, USA
  • 2Space Science and Engineering Center, University of Wisconsin-Madison, Madison, WI, USA
  • 3Cooperative Institute of Severe and High Impact Weather Research and Operations, University of Oklahoma, 1Norman, OK, USA
  • 4School of Meteorology, University of Oklahoma, OK, USA
  • 5NASA Langley Research Center, Hampton, VA, USA
  • 6National Center for Atmospheric Research, Boulder, CO, USA
  • 7Science Systems and Applications, Inc., Hampton, VA, USA

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.

Rose Marie Miller et al.

Status: open (until 03 Mar 2023)

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Rose Marie Miller et al.

Rose Marie Miller et al.


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Short summary
The influence of human-produced aerosols on clouds remains one of the uncertainties in radiative forcing of Earth’s climate. Measurements of aerosol chemistry from sources around the Philippines illustrate the linkage between aerosol chemical composition and cloud droplet characteristics. Differences in aerosol chemical composition in the marine layer from biomass burning, industrial and ship-produced, and marine aerosol are shown to impact cloud microphysical structure just above cloud base.