Efficient Ozone Production within Indochina Biomass Burning Plumes: From Near-Source to Downwind Regions

This study investigates O 3 production and chemistry within Indochina biomass burning plumes, using ozonesondes, satellite, and reanalysis data (2000–2024). Elevated terrain over the burning region produces a “terrain chimney” effect, lifting plumes into the free troposphere and confining downwind transport largely above the boundary layer. On average, Indochina biomass burning contributes ~30 ppbv of O 3 enhancement in the lower free troposphere (LFT, 2–5 km) over Hanoi and Hong Kong—nearly twice the increase reported for North American wildfires—indicating more efficient O 3 production. Satellite/reanalysis data reveal a NOₓ-limited O 3 formation regime over the Indochina Peninsula and along smoke transport pathways. On peak-fire days, NO 2 surges while CH 2 O increases modestly, distinct from North America. Despite the NOₓ surge, the O 3 formation regime remains NO x -limited across most transport pathways, favoring sustained O 3 production via NOₓ → O 3 . Combined with strong solar radiation and high VOC background, this yields an average O 3 production rate of 17.6 ppbv h⁻¹ over northern Laos, and a dilution-corrected excess LFT O 3 concentration of up to 150 ppbv over the Greater Bay Area on smoke-laden days. Our findings underscore the urgency of biomass burning prevention in Indochina.