Variations in polycyclic aromatic hydrocarbons concentrations and their contribution to aerosol optical depth during the COVID-19 outbreak: A study in Yokohama using generalized additive model

Abstract

The severe social restrictions aiming at reducing the novel coronavirus disease (COVID-19) transmission during its outbreak significantly affected regional air quality. This study analyzed changes in polycyclic aromatic hydrocarbons (PAHs) concentrations in cyclone-collected powder form particulate matter (PM) samples collected in Yokohama, Japan, from 2018 to 2023. The annual mean ∑PAHs concentration decreased significantly from 20.7 ng/mg in 2019 to 13.2 ng/mg in 2020 after the outbreak. However, the main sources of PAHs remained stable and were dominated by incomplete fossil fuel combustion. A generalized additive model was utilized to analyze the statistical relationships of air pollutants, namely, aerosol particulate matter (PM_2.5), ∑PAHs, ozone (O₃), metal, and meteorological factors such as solar radiation and their impact on aerosol optical depth (AOD). AOD remote sensing data collected by the Terra/MODIS satellite reflects the degree of air turbidity and atmospheric aerosol loading. Despite the small sample size, the model fitting demonstrated a good fit and showed smoothing non-linear interaction effects among the target factors influencing secondary aerosol generation. In conclusion, intense solar radiation, high concentrations of oxidative gases, elevated levels of organic contaminants in aerosols, and the catalytic degradation of PAHs by transition metals contribute to increased atmospheric aerosol loads. This finding enhances the present understanding of the photochemical reaction processes of aerosol particles. However, longer-term observational studies are needed to confirm our findings due to the limited sample size in this study.