Chemical Signatures and Sensitivity of Ozone and Particulate Nitrate from Multi-Satellite and Ground Observations over Northern India

Ozone and particulate nitrate, a key component of PM2.5, form through non-linear chemical interactions, with ozone formation governed by nitrogen oxides (NOx) and volatile organic compounds (VOCs), and particulate nitrate involving NOx (forming HNO3) and ammonia (NH3). This study utilise a multi-satellite observational approach to understand the formation response of ozone and particulate nitrate to their precursors over Delhi, world’s most polluted capital. We utilize Level-2 satellite data for NO2 and HCHO from TROPOMI (Sentinel-5P) and GEMS and NH3 from IASI (MetOp-B) over the spatial domain of 28°-29°N and 76.5°-77.8°E for 2023. Surface observations of ozone and PM2.5 were obtained from CPCB monitoring stations across Delhi after applying multi-level filtration. High resolution maps (1km × 1km) of NO2, HCHO, and NH3 along with their ratios (HCHO/NO2 and NH3/NO2) were generated, and their time series were extracted around each site locations (<10 km radius) to examine spatio-temporal patterns. Strong spatial and seasonal variability, with NO2 columns peaking at ~0.5 DU in winter and HCHO reaching up to ~0.85 DU in autumn is observed. HCHO/NO2 ratio shows VOC-limited and transitional ozone regimes during winter, which evolve into predominantly NOx-limited regimes during spring, summer-monsoon, and autumn. During spring, ozone concentrations peaks, ranging from 30–75 ppb, reflecting intense photochemical ozone production while time series of NO2 and HCHO columns around 0.2-0.3 DU, and 0.6-0.7 DU respectively. Ozone exceedance days (MDA > 50 ppbv) during spring shows elevated NO2 (~0.1 DU) and HCHO (~0.2 DU) levels, confirming photochemical production. Diurnal variation in NO2 and HCHO from GEMS, highlights seasonal and meteorological influence, with HCHO bias indicating a predominantly VOC limited regime.  The spatial gradient of NO2 (NO2/distance) highlights strong sinks in hotspot regions, particularly in winter and spring. A notable decline of 10-60% in ozone concentrations from spring to winter in these sink areas suggests substantial NOx-driven titration under low sunlight conditions. Site classification into urban, rural, and highway-proximal (within 500 m) categories shows consistently higher NO2 and HCHO levels in urban areas across all seasons, followed by highway sites. For particulate pollution, particulate nitrate, a secondary inorganic aerosol was found to significantly contribute to PM2.5 across seasons. PM2.5 levels peaked in autumn at all sites, followed by winter. Binned HCHO averages were higher in autumn, aligning with PM2.5 peaks and suggesting a biogenic contribution during extreme pollution events. Conversely, elevated NO2 during winter points towards a dominant inorganic and anthropogenic influence on PM2.5 enhancement in form of particulate nitrate.