| Summary: | Aerosol hygroscopicity affects Earth’s radiation budget, secondary aerosol formation, cloud formation, and health. It is important for correct estimation of the cooling at Earth’s surface and for remote sensing. Limited aerosol studies in the complex and moisture-rich environment of Southeast Asia, one of the rapidly developing regions in Asia, motivates the need to understand aerosol hygroscopicity there. This is one of the goals of the NASA Cloud, Aerosol, and Monsoon Processes Philippines Experiment (CAMP2Ex) airborne measurement campaign from August to October 2019. As context for CAMP2Ex, surface and column aerosol characteristics were investigated in the largest megacity in the Philippines, Metro Manila. Background surface aerosol hygroscopicity in Metro Manila is relatively low due to the dominance and high levels of black carbon and organics from local sources. Cases of extreme aerosol loading, as in firework emissions during the New Year 2019, increased the inorganic fraction of aerosols, reduced visibility, and doubled hygroscopicity in Metro Manila for several hours. This extremely unhealthy event was short-lived because the hygroscopic inorganic particles derived from the fireworks were washed out by rain and surface particulate levels went back to background levels. Despite the relatively high surface aerosol particle concentrations in Metro Manila, columnar aerosol optical depth (AOD) has been relatively low compared to other megacities in the region and is another motivation to dig deeper in aerosol characteristics. Cluster analysis of ten-years of clear air data (2009-2018) from the NASA AErosol RObotic NETwork (AERONET) suggests that the atmospheric column above Metro Manila was mostly (58% of the time) due to marine and some local (20% of the time) sources, possibly the reason for the low AOD values. AERONET refractive index values were also low suggesting the presence of relatively hygroscopic particles, strengthening the case of marine aerosol particles being the dominant component over Metro ...
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