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: August 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
composed of size-resolved measurements at the following aerodynamic cut-point
diameters (Dp): 18, 10, 5.6, 3.2, 1.8, 1.0, 0.56, 0.32, 0.18, 0.10, and
0.056 µm. Each sample set was analyzed for composition of the
water-soluble fraction. Analysis for mass was carried out on two sample sets, whereas
black carbon (BC) and morphology analysis were analyzed on a single sample set.
The bulk of the PM mass was between 0.18 and 1.0 µm with a dominant mode
between 0.32 and 0.56 µm. Similarly, most of the black carbon (BC) mass
was found between 0.10 and 1.0 µm, peaking between 0.18 and 0.32 µm.
These peaks are located in the Greenfield gap, or the size range between
0.10 and 1.0 µm, where wet scavenging by rain is relatively inefficient.
In the range between 0.10 and 0.18 µm, BC constituted 78.1 % of the
measured mass. Comparable contributions of BC (26.9 %) and the
water-soluble fraction (33.4 %) to total PM were observed and most of the
unresolved mass, which amounted to 39.6 % in total, 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 aerosol” (48.0 %), while “sea
salt” (22.5 %) and “combustion” emissions (18.7 %) had comparable
contributions. “Vehicular/resuspended dust” (5.6 %) and “waste
processing” emissions (5.1 %) were also identified. Microscopy analysis
highlighted the ubiquity of nonspherical particles regardless of size,
which is significant when considering calculations of parameters such as
single scattering albedo, the asymmetry parameter, and the extinction efficiency. The significant influence from aged 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. The fact 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.