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. A 16-month (July 2018–October 2019) dataset of size-resolved aerosolcomposition is used to examine the sources and characteristics of fiveorganic acids (oxalate, succinate, adipate, maleate, phthalate) andmethanesulfonate (MSA) in Metro Manila, Philippines. As one of the mostpolluted megacities globally, Metro Manila offers a view of how diversesources and meteorology impact the relative amounts and size distributionsof these species. A total of 66 sample sets were collected with aMicro-Orifice Uniform Deposit Impactor (MOUDI), of which 54 sets wereanalyzed for composition. Organic acids and MSA surprisingly were lessabundant than in other global regions that are also densely populated. Thecombined species accounted for an average of 0.80 ± 0.66 % of totalgravimetric mass between 0.056 and 18 µm, still leaving 33.74 % ofmass unaccounted for after considering black carbon and water-soluble ionsand elements. The unresolved mass is suggested to consist ofnon-water-soluble metals as well as both water-soluble and non-water-solubleorganics. Oxalate was approximately an order of magnitude more abundant thanthe other five species (149 ± 94 ng m−3 versus others being&lt; 10 ng m−3) across the 0.056–18 µm size range. Bothpositive matrix factorization (PMF) and correlation analysis are conductedwith tracer species to investigate the possible sources of organic acidsand MSA. Enhanced biomass burning influence in the 2018 southwest monsoonresulted in especially high levels of submicrometer succinate, MSA, oxalate,and phthalate. Peculiarly, MSA had negligible contributions from marinesources but instead was linked to biomass burning and combustion. Enhancedprecipitation during the two monsoon seasons (8 June–4 October 2018 and14 June–7 October 2019) coincided with a stronger influence from localemissions rather than long-range transport, leading to notable concentrationenhancements in both the sub- and supermicrometer ranges for some species(e.g., maleate and phthalate). While secondary formation via gas-to-particleconversion is consistent with submicrometer peaks for the organic acids andMSA, several species (i.e., phthalate, adipate, succinate, oxalate)exhibited a prominent peak in the coarse mode, largely owing to theirassociation with crustal emissions (i.e., more alkaline aerosol type) ratherthan sea salt. Oxalate's strong association with sulfate in thesubmicrometer mode supports an aqueous-phase formation pathway for the studyregion. However, high concentrations during periods of low rain and highsolar radiation suggest photo-oxidation is an important formation pathway.

Sources and characteristics of size-resolved particulate organic acids and methanesulfonate in a coastal megacity: Manila, Philippines