Impacts of regional wind circulations on aerosol pollution and planetary boundary layer structure in Metro Manila, Philippines

Authors: Melliza Templonuevo Cruz, James Bernard Simpas, Armin Sorooshian, Grace Betito, Maria Obiminda L. Cambaliza, Jarl Tynan Collado, Edwin W. Eloranta, Robert Holz, Xzann Garry Vincent Topacio, Jundy Del Socorro, Gerry Bagtasa

Abstract

Fine particulate matter (PM_2.5) concentrations in Metro Manila, Philippines have consistently exceeded the guideline values set by the World Health Organization (WHO). Although there has been much progress in understanding the components and sources of PM_2.5, limited research has been done on the influence of meteorological factors. In particular, the influence of the planetary boundary layer height (PBLH) on PM_2.5 concentration has not been studied due to inadequate observations. From January 2019–June 2020, measurements from a High Spectral Resolution Lidar (HSRL) filled this gap and allowed for PBLH estimation and aerosol typing. This paper investigates the roles of PBLH and regional and local wind circulations on the temporal evolution of aerosol pollution. Results show that daytime and nighttime PBLH variability is associated with solar heating and radiative cooling, respectively. Cloud-free conditions during the dry season yield a higher PBL growth rate than during the wet season when lower daytime and elevated nighttime PBLH are observed. Lower PM_2.5 levels are generally observed during daytime when PBLH is at its maximum. However, the PBLH has a significant inverse correlation with PM_2.5 only in the months of December-January-February. We find that horizontal directional wind shear between synoptic and mesoscale circulations confounds the PM_2.5 – PBLH relationship by creating stagnant conditions conducive to aerosol accumulation. The lower 20% of PM_2.5 concentrations occur during the prevalence of strong monsoon winds. On the other hand, the upper 80% are found during the occurrence of compound mesoscale winds (i.e., sea/land/lake/valley/mountain breezes and channeling monsoon winds). In addition, mountain breeze is found to be associated with lifting of aerosols, resulting in multi-layering within the PBL. The findings in the present study emphasize the role of complex topography and mesoscale scale winds arising from the landscape on aerosol pollution variability.

e University of Wisconsin-Madison (Shipley et al., 1983; Eloranta, 2005; Razenkov and Eloranta, 2018) was installed at the Manila Observatory (MO) and provided the first relatively long-term, continuous, high-temporal and vertical resolution measurements of aerosol optical properties in Metro Manila that may be used to estimate the PBLH and provide information on the aerosol vertical structure as well as the aerosol types present. These measurements were part of the Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP²Ex), which aims to better understand the complex interactions between aerosols, clouds, and radiation in the region. Recent findings from the campaign highlight the importance of meteorological factors in modulating aerosol concentrations in Metro Manila. For instance, size-resolved aerosol measurements during the southwest monsoon (SWM) season revealed that the bulk of the PM and black carbon mass was within the Greenfield Gap (0.10–1.0 μm) where wet scavenging by rain is not efficient (Cruz et al., 2019). Moreover, aerosol concentrations are influenced not only by local sources but also by long-range aerosol transport facilitated by synoptic circulations such as the SWM and the passage of a typhoon (Braun et al., 2020). Precipitation and wind speeds also potentially influence the weekly cycles of aerosols in Metro Manila (Hilario et al., 2020).

In this paper, a combination of ground-based PM_2.5 and meteorological data, Weather Research and Forecasting model with Chemistry (WRF-Chem) simulations, and the extensive HSRL dataset collected from January 2019–June 2020 were used to examine both (1) the influence of PBLH on PM_2.5 concentrations and (2) the effects of synoptic and mesoscale winds on PBLH and the temporal evolution of PM_2.5 concentrations, aerosol types, and aerosol vertical profile in Metro Manila. Aside from providing insights on the extent to which the PBLH and atmospheric circulations modulate PM_2.5 concentrations, outcomes of this research include the first comprehensive characterization of the temporal trends of PBLH in Metro Manila and baseline data on diurnal and seasonal variation of surface PM_2.5 concentrations.