Assessing the Impact of Emission Reductions on Surface O3 and PM2.5 in India

The rapid industrialization and urbanization across South Asia in recent years have driven significant increases in surface ozone (O₃) and fine particulate matter (PM₂.₅) concentrations, posing serious environmental and public health challenges. This study investigates the potential of precursor emission reductions in mitigating surface O₃ and PM₂.₅ levels over the Indian subcontinent, with a focus on the pre-monsoon season. Using the WRF-Chem model, we applied two chemical mechanisms: MOZCART, representing a simpler approach, and MOZART-MOSAIC, incorporating more complex gas-phase and aerosol interactions.

Through a series of sensitivity experiments, we analysed the effects of a 50% reduction in key precursors—nitrogen oxides (NOₓ), volatile organic compounds (VOCs), carbon monoxide (CO), methane (CH₄), and their combined reductions. The results revealed distinct responses to emission reductions. For surface O₃, MOZCART mechanism showed substantial reductions (~ 8 ppbv) in northern India under curtailed NOₓ emission, highlighting dominant role of NO_x in O₃ production in this region. In contrast, VOC and CO reductions had limited impacts on O₃ levels (~5 ppbv), suggesting a VOC-limited regime in certain areas. The MOZART-MOSAIC mechanism provided deeper insights, revealing substantial PM₂.₅ reductions (~3 ug/m³) under combined precursor reductions, emphasizing the intricate coupling between gas-phase and particulate chemistry.

Further analysis of diurnal and nocturnal variations highlighted differences in chemical dynamics between the two mechanisms. MOZCART indicated an increase in daytime O₃ levels (~ 8 ppbv) under combined precursor reductions, likely due to shifts in photochemical regimes, whereas MOZART-MOSAIC exhibited nighttime O₃ reductions (~ 4 ppbv) driven by changes in nocturnal chemical pathways.

This study underscores the critical importance of multi-pollutant emission reductions to achieve meaningful improvements in surface O₃ and PM₂.₅ levels. It also highlights the complex and region-specific interactions between atmospheric precursors, emphasizing the need for integrated and holistic approaches to air quality management in South Asia.