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

We present the first study of the weekly cycles (WCs) of chemically speciated and size-resolved particulate matter (PM) in Metro Manila, Philippines, a coastal megacity located within a highly complex meteorological environment that is subject to both anthropogenic and natural sources. To measure PM, Micro-Orifice Uniform Deposit Impactors (MOUDIs) were deployed in Metro Manila from August 2018 to October 2019 and samples were analyzed for ionic and elemental species, including black carbon (BC). The WC in Metro Manila varied remarkably across seasons, linked to shifts in meteorology, transport, and aerosol source. Identified aerosol sources were traffic, local and regional burning, dust, sea salt, and secondary aerosol formation. Direct emissions induced a late workweek peak, while secondary aerosol formation led to a weekend peak in response to precursor buildup mainly from traffic. Seasonal analysis revealed that local burning from solid waste management and agricultural fires induced a strong WC peak while regional burning emissions from the Maritime Continent (MC) and possibly the Asian continent elevated seasonal baseline concentrations of the WC. BC showed a seasonally persistent WC, consistent in magnitude, weekly peak timing, and particle size. The dominant submicrometer WC and the contribution of BC across seasons have important ramifications on public health and policymaking, which are also discussed. As many of the observed WC patterns are undetectable when using only bulk PM, this study demonstrates that a seasonal, size-resolved, and chemically speciated characterization is required to more fully understand the driving mechanisms governing WCs.