Boundary Layer Ozone Across the Indian Subcontinent: Who Influences Whom?

Boundary layer ozone (BLO3) over the Indian subcontinent (IS) can be region-specific with consequences to health and pollution management strategies. The Goddard Earth Observing System-Chemistry (GEOS-Chem) model was used to estimate the contributions of eight IS regions on each other for BLO3. BLO3 in northern India, Pakistan, and Sri Lanka is mostly (similar to 65-70%) influenced by regions outside the subcontinent. Roughly 30% of BLO3 over eastern India is from within the IS, mostly from the Indo-Gangetic Plain and central India (CI). Southern India is influenced by CI; however, it hardly influences other regions. Northern India and Sri Lanka regions are essentially isolated from other regions in the IS. The meteorology is the primary factor in the redistribution of BLO3 and its precursors. Our results highlight the emerging importance of CI to O-3 pollution in addition to the Indo-Gangetic Plain and shed light on potential strategies for O-3 control in the IS. Plain Language Summary The Indian subcontinent is one of the most densely populated regions in the world with a population of similar to 1.7 billion, with more than 1 billion people in nonurban areas. In the absence of detailed measurements, we use a chemical transport model to estimate the transport of boundary layer ozone and its precursors from eight regions within the Indian subcontinent. The Indo-Gangetic Plain and central India are shown to be significant boundary layer ozone contributors to the neighboring regions as well as eastern (including Bangladesh) and southern India. Not only the precursors but also ozone made in a specific region is transported to other regions within the Indian subcontinent. We show that most of the regions in the Indian subcontinent cannot independently control their boundary layer ozone, since a significant fraction comes from regions beyond their control, with Indo-Gangetic Plain and central India being major contributors.