Size-resolved composition and morphology of particulate matter during the southwest monsoon in Metro Manila, Philippines

Abstract

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.

Characterizing Weekly Cycles of Particulate Matter in a Coastal Megacity: The Importance of a Seasonal, Size‐Resolved, and Chemically Speciated Analysis

“…Corroborating with independent measurements from Kecorius et al (2017), our measurements show that BC in Metro Manila resides primarily in the 0.18-to 0.32-μm range. In addition to the findings of M. T. Cruz et al (2019), which reported BC only for July-October 2018, the current study demonstrates the seasonal persistence of 10.1029/2020JD032614…”

“…Each MOUDI substrate was extracted in water prior to chemical composition analysis using a combination of ion chromatography (IC; Thermo Scientific Dionex ICS-2100 system) and triple quadrupole inductively coupled plasma mass spectrometry (ICP-QQQ; Agilent 8,800 Series). Species relevant to this study that were detected by IC include the following: Cruz et al (2019). In contrast to other studies which reported the WC in terms of average percent departure from the weekly mean (Perrone et al, 2019) or the difference between the highest and lowest weekly value (Stjern, 2011), we present the WC in terms of mass concentration in order to highlight the interseasonal variability and size-resolved nature of the WC.…”

“…An intensive characterization experiment reported that black carbon (BC) concentrations in Metro Manila are significantly higher than those in European cities (Kecorius et al, 2017), attributed largely to high levels of traffic and poor street configuration (Alas et al, 2018). Subsequent analysis by M. T. Cruz et al (2019) identified five sources of aerosol particles during the southwest monsoon season: transported aerosol, sea-salt, combustion, vehicular/resuspended dust, and waste processing. Although previous studies have demonstrated the importance of anthropogenic sources, a WC study has yet to be conducted in Metro Manila.…”

Aerosol Particle and Black Carbon Emission Factors of Vehicular Fleet in Manila, Philippines

“…That being said, we estimated the weighted average particle density based on the fraction of BC in Metro Manila. Using published [49] densities of atmospheric aerosols (OC-1.2 g/cm 3 ; BC-2.0 g/cm 3 ) and BC fraction in Metro Manila [50], particle density was found to be in a range between 1.7 and 1.8 g/cm 3 .…”

Long-range aerosol transport and impacts on size-resolved aerosol composition in Metro Manila, Philippines

“…As recent studies have indicated a decline in SWM rainfall in the western Philippines and an increase in no-rain days during the typical SWM season (Cruz et al, 2013), the potential for wet scavenging of aerosol during these time periods could be decreasing. Furthermore, decreases in monsoonal rainfall in other parts of Asia, including India (Dave et al, 2017) and China , have been linked to increases in aerosol, especially those of anthropogenic origin. Reinforcing mechanisms in these interactions, such as decreased rainfall reducing wet scavenging, leading to higher aerosol concentrations that in turn suppress precipitation, and the corresponding climatic changes in monsoonal rain in the western Philippines underscore the need to better understand the processes governing atmospheric aerosol characteristics and sources, especially during the monsoonal season.…”