Size-resolved Composition and Morphology of Particulate Matter During the Southwest Monsoon in Metro Manila, Philippines
Authors: Cruz Melliza Templonuevo , Bañaga Paola Angela , Betito Grace , Braun Rachel A. , Stahl Connor , Aghdam Mojtaba Azadi , Cambaliza Maria Obiminda , Dadashazar Hossein , Hilario Miguel Ricardo , Lorenzo Genevieve Rose , Ma Lin , MacDonald Alexander B. , Pabroa Preciosa Corazon , Yee John Robin , Simpas James Bernard , Sorooshian Armin 
Published: April 2019

Abstract. This paper presents novel results from size-resolved particulate matter (PM) mass, composition, and morphology measurements conducted during the 2018 Southwest Monsoon (SWM) season in Metro Manila, Philippines. Micro-Orifice Uniform Deposit Impactors (MOUDIs) were used to collect PM sample sets that were analyzed for mass, morphology, black carbon (BC), and composition of the water-soluble fraction. The bulk of the PM mass was between 0.18–1.0 µm with a dominant mode between 0.32–0.56 µm. Similarly, most of the black carbon (BC) mass was found between 0.10–1.0 µm (the so-called Greenfield gap), peaking between 0.18–0.32 µm, where wet scavenging by rain is inefficient. In the range of 0.10–0.18 µm, BC constituted 78.1 % of the measured mass. Comparable contributions of BC (26.9 %) and the water-soluble fraction (31.3 %) to total PM were observed and most of the unresolved mass, which in total amounted to 41.8 %, was for diameters exceeding 0.32 µm. The water-soluble ions and elements exhibited an average combined concentration of 8.53 µg m−3, with SO42−, NH4+, NO3−, Na+, and Cl− as the major contributors. Positive Matrix Factorization (PMF) was applied to identify the possible aerosol sources and estimate their contribution to the water-soluble fraction of collected PM. The factor with the highest contribution was attributed to Aged/Transported aerosol (48.0 %) while Sea Salt (22.5 %) and “Combustion” emissions (18.7 %) had comparable contributions. Vehicular/Resuspended Dust (5.6 %) as well as Waste Processing emissions (5.1 %) were also identified. Microscopy analysis highlighted the ubiquity of non-spherical particles regardless of size, which is significant when considering calculations of parameters such as single scattering albedo, asymmetry parameter, and extinction efficiency. Results of this work have implications for aerosol impacts on public health, visibility, and regional climate as each of these depend on physicochemical properties of particles as a function of size. The significant influence from Aged/Transported aerosol to Metro Manila during the SWM season indicates that local sources in this megacity do not fully govern this coastal area's aerosol properties and that PM in Southeast Asia can travel long distances regardless of the significant precipitation and potential wet scavenging that could occur. That the majority of the regional aerosol mass burden is accounted for by BC and other insoluble components has important downstream effects on the aerosol hygroscopic properties, which depend on composition. The results are relevant for understanding the impacts of monsoonal features on size-resolved aerosol properties, notably aqueous processing and wet scavenging. Finally, the results of this work provide contextual data for future sampling campaigns in Southeast Asia such as the airborne component of the Cloud, Aerosol, and Monsoon Processes Philippines Experiment (CAMP2Ex) planned for the SWM season in 2019. Aerosol characterization via remote-sensing is notoriously difficult in Southeast Asia, which elevates the importance of datasets such as the one presented here.