Mechanisms and Pathways for Coordinated Control of Fine Particulate Matter and Ozone.

Purpose of Review: Fine particulate matter (PM2.5) and ground-level ozone (O3) pose a significant risk to human health. The World Health Organization (WHO) has recently revised healthy thresholds for both pollutants. The formation and evolution of PM2.5 and O3 are however governed by complex physical and multiphase chemical processes, and therefore, it is extremely challenging to mitigate both pollutants simultaneously. Here, we review mechanisms and discuss the science-informed pathways for effective and simultaneous mitigation of PM2.5 and O3. Recent Findings: Global warming has led to a general increase in biogenic emissions, which can enhance the formation of O3 and secondary organic aerosols. Reductions in anthropogenic emissions during the COVID-19 lockdown reduced PM2.5; however, O3 was enhanced in several polluted regions. This was attributed to more intense sunlight due to low aerosol loading and non-linear response of O3 to NO x . Such contrasting physical and chemical interactions hinder the formulation of a clear roadmap for clean air over such regions. Summary: Atmospheric chemistry including the role of biogenic emissions, aerosol-radiation interactions, boundary layer, and regional-scale transport are the key aspects that need to be carefully considered in the formulation of mitigation pathways. Therefore, a thorough understanding of the chemical effects of the emission reductions, changes in photolytic rates and boundary layer due to perturbation of solar radiation, and the effect of meteorological/seasonal changes are needed on a regional basis. Statistical emulators and machine learning approaches can aid the cumbersome process of multi-sector multi-species source attribution.

Impact of COVID-19 lockdown on surface ozone build-up at an urban site in western India based on photochemical box modelling

Elevated ozone (O3) near the earth’s surface causes adverse impacts on human health and vegetation, besides impacting air chemistry and climate. Intense lockdown to contain the spread of Coronavirus disease 2019 (COVID-19) offered a rare opportunity to delineate the anthropogenic impact on urban O3 build-up. In this regard, we incorporated observations of chemical species and environmental conditions into a photochemical box model (NCAR Master Mechanism) to study the O3 changes at a semi-arid urban site in western India (Ahmedabad;23°N, 72.6°E). In contrast with primary pollutants, daytime O3 build-up is observed to be enhanced during the lockdown by ~39%. Model, driven by lower nitrogen oxides (NOx) during the lockdown, also simulated enhanced O3 (by ~41%) showing the role of nonlinear dependence of O3 on NOx. Further, a sensitivity simulation unravelled an important role of the meteorological changes in the O3 enhancement (by ~16%) during the lockdown. The results highlight that the lockdown impacts can be modulated profoundly by the complex chemistry plus meteorological changes, offsetting the benefits of lower precursor levels in the context of O3 pollution. [ABSTRACT FROM AUTHOR] Copyright of Current Science (00113891) is the property of Indian Academy of Sciences and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)