Tropospheric ozone precursors over the Indian region: Insights into the shift toward a highly NOx-limited regime

Abstract

This study comprehensively analyzes trends in tropospheric ozone precursors over the Indian region from 2018 to 2024, focusing on nitrogen dioxide (NO₂), formaldehyde (HCHO), and glyoxal (CHOCHO), using TROPOMI onboard Sentinel-5 Precursor satellite datasets. Our results reveal contrasting trends: while HCHO and CHOCHO show consistent increases, NO₂ exhibits a slight decrease over the past few years, likely reflecting the impact of emission control measures. The mean growth rate calculated for HCHO Vertical Column Density (VCD) is ∼1.7 % yr⁻¹ and ∼2 % yr⁻¹ for CHOCHO, whereas tropospheric NO₂ VCDs show a slower growth rate of ∼ −0.21 %yr⁻¹, indicating a reduction in NO_x emissions over recent years. The growth rate of HCHO is consistent with findings from previous studies. However, unlike earlier long-term studies, a recent decline in NO₂ has been observed. Long-term OMI dataset measurements are used to corroborate these findings. Analysis of the formaldehyde-to-nitrogen ratio indicates that the Indian mainland region is mostly NO_x-limited in nature, and further shifts toward highly NO_x-limited ozone production are observed in most parts of the study area. Seasonal patterns show pronounced peaks in NO₂ during winter and HCHO and CHOCHO during pre- and post-monsoon seasons, underscoring the significant influence of biomass burning and agricultural activities on precursor levels. The seasonal variation in the Enhanced Vegetation Index (EVI) and fire density was analyzed to assess emissions from biogenic and pyrogenic sources. These findings emphasize the complexity of ozone precursor dynamics in India, highlighting the need for targeted emission controls to mitigate regional ozone pollution.