Assessment of the relationship between tropospheric NO2 and photosynthetic activity across seasonal croplands in India

Tropospheric nitrogen dioxide (NO₂) is an indicator of anthropogenic activity and a key precursor of the phytotoxic surface ozone (O₃), with potential implications for agricultural ecosystems. Recent satellite-based studies have reported negative association between NO₂ and vegetation greenness over agricultural regions, but the persistence of these relationships over longer timescales and their relevance for crop photosynthetic functioning remain unclear. Here, we assess co-variability between NO₂ and photosynthetic activity over Indian croplands (rice and wheat dominated regions) over a decadal scale (2007–2022) using High Spatial-Temporal Coverage Merged tropospheric NO₂ (HSTCM-NO₂) dataset and Solar Induced Fluorescence (SIF) from the Global Solar-induced Chlorophyll Fluorescence (GOSIF) product. Analyses conducted separately for the kharif (July–September) and rabi (January–March) seasons to account for contrasting agro-climatic and photochemical conditions. We find widespread increases in SIF for rabi season croplands (0.0032 W/m²/μm/sr/yr) including wheat-dominated regions, despite spatially heterogeneous NO₂ trends (0.0096×10¹⁵ molec./cm²/yr) that increase across much of the Indo-Gangetic Plain (IGP). However, analysis of detrended interannual variability reveals a significant negative association between NO₂ and SIF during the rabi season (Pearson correlation, r=-0.5, p=0.048), indicating reduced photosynthetic activity in years with elevated pollution. A similar but weaker relationship is observed for kharif season croplands in rice-dominated regions (r=-0.1, p=0.704). The results indicate that variations in NO₂ pollution may modulate interannual crop performance by influencing photosynthetic activity, even in systems where long-term productivity trends are primarily driven by management and technology.

Keywords: Tropospheric NO₂; Pollution; Croplands; Photosynthetic activity; SIF; India