Comment on acp-2021-1030

doi:10.5194/acp-2021-1030-rc1

Abstract

Coarse particulate matter (i.e., PM with aerodynamic diameter between 2.5 and 10 micrometers or PMcoarse) has been increasingly recognized of its importance in PM10 regulation because of its growing proportion in PM10 and the accumulative evidence for its adverse health impact. In this work, we present comprehensive PMcoarse speciation results obtained through a one-year long (January 2020–February 2021) joint PM10 and PM2.5 chemical speciation study in Hong Kong, a coastal and highly urbanized city in southern China. The annual average concentration of PMcoarse is 14.9 ± 8.6 μg m–3 (±standard deviation), accounting for 45 % of PM10 (32.9 ± 18.5 μg m–3). The measured chemical components explain ~75 % of the PMcoarse mass. The unexplained part is contributed by unmeasured geological components and residue liquid water content, supported by analyses by positive matrix factorization (PMF) and the thermodynamic equilibrium model ISORROPIA II. The PMcoarse mass is apportioned to four sources resolved by PMF, namely soil dust, copper-rich dust, fresh sea salt, and an aged sea salt factor containing secondary inorganic aerosols (mostly nitrate). Back-trajectory cluster analysis reveals significant variations in source contributions with the air mass origin. Under the influence of marine air mass, PMcoarse is the lowest (average = 8.0 μg m–3) and sea salt is the largest contributor (47 %), followed by the two dust factors (38 % in total). When the site receives air mass from the northern continental region, PMcoarse increased substantially to 21.2 μg m–3, with the two dust factors contributing 90 % of the aerosol mass. The potential dust source areas are mapped using the Concentration-Weighted Trajectory technique, showing either the Greater Bay Area or the greater part of southern China as the origin of fugitive dust emissions leading to elevated ambient PMcoarse loadings in Hong Kong. This study, first of this kind in our region, provides highly relevant guidance to other locations with similar monitoring needs. Additionally, the study findings point to the needs for further research on the sources, transport, aerosol processes, and health effects of PMcoarse.

Highlights

Coarse particulate matter (PMcoarse), defined as PM with aerodynamic diameter of 2.5–10 μm in the World Health Organization’s air quality guidelines, play important roles in air quality, public health, and global climate
The difference is consistent with combustion and secondary aerosol formation processes being the major sources of fine particles, whereas coarse particles are primarily generated by mechanical processes
The results show 435 that ~70 % of the unidentified mass is associated with the two dust factors, while the rest is residue liquid water content as implied from thermodynamic modeling using ISORROPIA II

Summary

Introduction

Coarse particulate matter (PMcoarse), defined as PM with aerodynamic diameter of 2.5–10 μm in the World Health Organization’s air quality guidelines, play important roles in air quality, public health, and global climate. Previous PMcoarse studies in Hong Kong were focused on suburban coastal area (Cohen et al, 2004), roadside environment (Cheng et al, 2015), and public transport micro-environments (Jiang et al, 2017). These studies provide limited representation of the general PMcoarse pollution characteristics given the predisposition to the influence by nearby sources; for example, sea 65 spray in coastal environment or traffic-related emissions in roadside environment. With the robust source apportionment analysis, we found that fugitive dust associated with regional influence is the dominant contributor of high PMcoarse loading in Hong Kong. The methodology and results from this study can serve to provide guidance to other locations with similar monitoring needs

Methods

Results

Conclusion