Frank Flocke

Controlling ozone (O3) in rapidly urbanizing megacities in Southeast and East Asia remains a challenge. O3 is a secondary pollutant formed through nonlinear photochemical reactions with its precursors: nitrogen oxides (NOx) and volatile organic compounds (VOCs). Observation-based quantification of precursor sensitivity remains scarce, limiting actionable O3 control. To address this, we leverage airborne observations from the NASA DC-8 during the ASIA-AQ campaign conducted in February and March 2024 across four Asian megacities: Metro Manila, the Seoul Metropolitan Area, the Tainan–Kaohsiung Metropolitan Area, and the Bangkok Metropolitan Region. These extensive measurements of various trace gases were used to constrain a zero-dimensional box model and estimate the net production rates of Ox (POx, Ox = O3 + NO2). Precursor sensitivity regimes were characterized for each megacity by generating isopleths of POx across varying levels of NOx and VOCs. The analysis revealed that Manila and Tainan–Kaohsiung exhibited predominantly NOx-sensitive conditions, favoring NOx reduction as an effective O3 mitigation strategy, while Bangkok showed a more mixed sensitivity, suggesting combined NOx and VOC reductions. In contrast, Seoul exhibited a primarily VOC-sensitive regime associated with its higher NOx conditions relative to the other cities, underscoring the importance of VOC-focused strategies. In addition, to quantitatively assess sensitivity transitions, we computed orthogonal distances from the isopleth transition boundaries for all four study areas. Diurnal analyses of these distances revealed a shift from more VOC-sensitive conditions in the morning toward more NOx-sensitive regimes in the afternoon. These findings provide critical insights for formulating effective, city-specific O3 control policies in urban environments.

Controlling ozone (O₃) in rapidly urbanizing megacities in Southeast and East Asia remains a challenge. O₃ is a secondary pollutant formed through nonlinear photochemical reactions with its precursors: nitrogen oxides (NO_x) and volatile organic compounds (VOCs). Observation-based quantification of precursor sensitivity remains scarce, limiting actionable O₃ control. To address this, we leverage airborne observations from the NASA DC-8 during the ASIA-AQ campaign conducted in February and March 2024 across four Asian megacities: Metro Manila, the Seoul Metropolitan Area, the Tainan-Kaohsiung Metropolitan Area, and the Bangkok Metropolitan Region. These extensive measurements of various trace gases were used to constrain a zero-dimensional box model and estimate the net production rates of O_x (PO_x, O_x = O₃ + NO₂). Precursor sensitivity regimes were characterized for each megacity by generating isopleths of PO_x across varying levels of NO_x and VOCs. The analysis revealed that Manila and Tainan-Kaohsiung exhibited predominantly NO_x-sensitive conditions, favoring NO_x reduction as an effective O₃ mitigation strategy, while Bangkok showed a more mixed sensitivity, suggesting combined NOₓ and VOC reductions. In contrast, Seoul, under colder and low solar irradiance conditions, exhibited a primarily VOC-sensitive regime, underscoring the importance of VOC-focused strategies. In addition, to quantitatively assess sensitivity transitions, we computed orthogonal distances from the isopleth transition boundaries for all four study areas. Diurnal analyses of these distances revealed a shift from more VOC-sensitive conditions in the morning toward more NO_x-sensitive regimes in the afternoon. These findings provide critical insights for formulating effective, city-specific O₃ control policies in urban environments.

The first 5 hours of aging in biomass burning plumes can strongly affect ozone photochemistry. We examine how volatile organic compounds (VOCs), nitrogen oxides, and nitrous acid influence hydroxyl radical, ozone, and peroxyacetyl nitrate (PAN) based on three aircraft campaigns over the United States. Our analyses reveal variable, highly elevated hydroxyl radical concentrations in the first 2 hours, resulting in evident fire-to-fire variability in VOCs oxidation and in ozone and PAN production. About 40 to 70% of the variability is explained by chemical aging. Ozone production in the plumes is usually VOC-limited for the first 2 hours and then nitrogen oxide limited downwind. Box model results for hydroxyl radical, ozone, and most VOCs, using the full, explicit Master Chemical Mechanism (MCM) mechanism, suggest no major gaps in the current best knowledge of gas-phase chemistry. However, the MCM sometimes overestimates PAN due to underestimated nitrogen oxide sinks. GEOS-Chem, a widely used chemical transport model with a reduced mechanism, generally underperforms because of incomplete VOC representation. We identify these critical pathways to guide future model development.

Controlling ozone (O₃) in rapidly urbanizing megacities in Southeast and East Asia remains a challenge. O₃ is a secondary pollutant formed through nonlinear photochemical reactions with its precursors: nitrogen oxides (NO_x) and volatile organic compounds (VOCs). Observation-based quantification of precursor sensitivity remains scarce, limiting actionable O₃ control. To address this, we leverage airborne observations from the NASA DC-8 during the ASIA-AQ campaign conducted in February and March 2024 across four Asian megacities: Metro Manila, the Seoul Metropolitan Area, the Tainan-Kaohsiung Metropolitan Area, and the Bangkok Metropolitan Region. These extensive measurements of various trace gases were used to constrain a zero-dimensional box model and estimate the net production rates of O_x (PO_x, O_x = O₃ + NO₂). Precursor sensitivity regimes were characterized for each megacity by generating isopleths of PO_x across varying levels of NO_x and VOCs. The analysis revealed that Manila and Tainan-Kaohsiung exhibited predominantly NO_x-sensitive conditions, favoring NO_x reduction as an effective O₃ mitigation strategy, while Bangkok showed a more mixed sensitivity, suggesting combined NOₓ and VOC reductions. In contrast, Seoul, under colder and low solar irradiance conditions, exhibited a primarily VOC-sensitive regime, underscoring the importance of VOC-focused strategies. In addition, to quantitatively assess sensitivity transitions, we computed orthogonal distances from the isopleth transition boundaries for all four study areas. Diurnal analyses of these distances revealed a shift from more VOC-sensitive conditions in the morning toward more NO_x-sensitive regimes in the afternoon. These findings provide critical insights for formulating effective, city-specific O₃ control policies in urban environments.

Controlling ozone (O₃) in rapidly urbanizing megacities in Southeast and East Asia remains a challenge. O₃ is a secondary pollutant formed through nonlinear photochemical reactions with its precursors: nitrogen oxides (NO_x) and volatile organic compounds (VOCs). Observation-based quantification of precursor sensitivity remains scarce, limiting actionable O₃ control. To address this, we leverage airborne observations from the NASA DC-8 during the ASIA-AQ campaign conducted in February and March 2024 across four Asian megacities: Metro Manila, the Seoul Metropolitan Area, the Tainan-Kaohsiung Metropolitan Area, and the Bangkok Metropolitan Region. These extensive measurements of various trace gases were used to constrain a zero-dimensional box model and estimate the net production rates of O_x (PO_x, O_x = O₃ + NO₂). Precursor sensitivity regimes were characterized for each megacity by generating isopleths of PO_x across varying levels of NO_x and VOCs. The analysis revealed that Manila and Tainan-Kaohsiung exhibited predominantly NO_x-sensitive conditions, favoring NO_x reduction as an effective O₃ mitigation strategy, while Bangkok showed a more mixed sensitivity, suggesting combined NOₓ and VOC reductions. In contrast, Seoul, under colder and low solar irradiance conditions, exhibited a primarily VOC-sensitive regime, underscoring the importance of VOC-focused strategies. In addition, to quantitatively assess sensitivity transitions, we computed orthogonal distances from the isopleth transition boundaries for all four study areas. Diurnal analyses of these distances revealed a shift from more VOC-sensitive conditions in the morning toward more NO_x-sensitive regimes in the afternoon. These findings provide critical insights for formulating effective, city-specific O₃ control policies in urban environments.

Studying tropospheric ozone over the remote areas of the planet, such as the open oceans and the polar regions, is crucial to understand the role of ozone as a global climate forcer and regulator of atmospheric oxidative capacity. A focus on the pristine oceanic and polar regions complements the available land-based datasets and provides insights into key photochemical and depositional loss processes that control the concentrations and spatiotemporal variability in ozone as well as the physicochemical mechanisms driving these patterns. However, an assessment of the role of ozone over the oceanic and polar regions has been hampered by a lack of comprehensive observational datasets. Here, we present the first comprehensive collection of ozone data over the oceans and the polar regions. The overall dataset consists of 77 ship cruises/buoy-based observations and 48 aircraft-based campaigns. The dataset, consisting of more than 630 000 independent ozone measurement data points covering the period from 1977 to 2022 and an altitude range from the surface to 5000 m (with a focus on the lowest 2000 m), allows systematic analyses of the spatiotemporal distribution and long-term trends over the 11 defined ocean/polar regions. The datasets from ships, buoys, and aircraft are complemented by ozonesonde data from 29 launch sites or field campaigns and by 21 non-polar and 17 polar ground-based station datasets. The datasets contain information on how long the observed air masses were isolated from land, as estimated by backward trajectories from the individual observation points. To extract observations representative of oceanic conditions, we recommend using a subset of the data with an isolation time of 72 h or longer, from the analysis with coincident radon observations. These filtered oceanic and polar data showed typically flat diurnal cycles at high latitudes, whereas daytime decreases in ozone (11 %–16 %) were observed at lower latitudes. The ship/buoy- and aircraft-based datasets presented here will supplement the land-based ones in the TOAR-II (Tropospheric Ozone Assessment Report Phase II) database to provide a fully global assessment of tropospheric ozone. The described dataset is available at https://doi.org/10.17596/0004044 (Kanaya et al., 2025).

Tropospheric ozone (O₃) production from wildfires is highly uncertain; previous studies have identified both production and loss of O₃ in fire-influenced air masses. To capture the total ozone production attributable to a smoke plume, we bridge the gap between near-field fire chemistry and aged smoke in the remote troposphere. Using airborne measurements from several campaigns, we find that fire-ozone production increases with age, with a regime transition from NO_x-saturated to NO_x-limited conditions, showing that O₃ production in aged plumes is controlled by nitrogen oxides (NO_x). Observations in fresh smoke show that suppressed photochemistry reduces O₃ production by ~70% in units of ppb O_x per ppm CO. Anthropogenic NO_x injection into VOC-rich fire plumes drives additional O₃ production, exceeding 50 ppb above background in extreme cases. Using a box model, we explore the sensitivity of O₃ production to fire emissions and chemical parameters, demonstrating the importance of aerosol-induced photochemical suppression over heterogeneous HO₂ uptake, validating HONO's role as an oxidant precursor, and confirming evolving NO_x sensitivity. We evaluate GEOS-Chem's performance against these observations, finding that the model captures fire-induced O₃ enhancements at older ages but overestimates near-field enhancements, fails to capture fire emission magnitude and variability, and misses the chemical regime transition. These discrepancies bias normalized ozone production (∆O₃/∆CO) across plume lifetime. GEOS-Chem attributes 2.4% of the global tropospheric ozone burden and 3.1% of surface ozone concentrations to fire emissions in 2020, with stronger impacts in regions of frequent burning.