Constraining Hydrocarbon Emissions over the Indian Subcontinent Based on HCHO Observations from OMI and GOME-2

The Indian subcontinent is a global hotspot for air pollution-related mortality, and among the most under-studied regions of the world in terms of atmospheric composition. A primary obstacle has been a lack of atmospheric measurements to diagnose the sources and processes driving air quality over India. Formaldehyde, as a high-yield product of atmospheric VOC oxidation, can be used as a proxy to understand the emissions of its reactive precursors from biogenic, pyrogenic and anthropogenic sources. In this study, we combine a new high-resolution GEOS-Chem simulation nested over the Indian sub-continent with space-based observations of HCHO from OMI to derive new constraints on VOC emission sources in India. We apply two inversion strategies to better quantify Indian VOC emissions, separately exploiting the seasonal and spatial constraints afforded by the satellite data. Domain-wide, our a posteriori biogenic VOC emission estimates are within ~30% of the bottom-up estimates during all seasons outside of the monsoon. On the other hand, we find that total anthropogenic and biomass burning VOC emissions over India are underestimated (1.5-3x) throughout the non-monsoon seasons. Significant regional differences also emerge, with model overestimates over Eastern and Central India versus underestimates over the Indo-Gangetic plain as well as Western and Southern India. Evaluation of the a posteriori estimates using independent data from GOME-2 shows significant model improvement except during the monsoon season when uncertainties due to cloudiness are higher. We explore the implications of these findings for our understanding of VOC source processes in this region.