Alessandro Franchin

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.

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.

Airborne and ground-based measurements of aerosol concentrations, chemical composition, and gas-phase precursors were obtained in three valleys in northern Utah (USA). The measurements were part of the Utah Winter Fine Particulate Study (UWFPS) that took place in January–February 2017. Total aerosol mass concentrations of PM ₁ were measured from a Twin Otter aircraft, with an aerosol mass spectrometer (AMS). PM ₁ concentrations ranged from less than 2 µg m ⁻³ during clean periods to over 100 µg m ⁻³ during the most polluted episodes, consistent with PM _2.5 total mass concentrations measured concurrently at ground sites. Across the entire region, increases in total aerosol mass above ∼2 µg m ⁻³ were associated with increases in the ammonium nitrate mass fraction, clearly indicating that the highest aerosol mass loadings in the region were predominantly attributable to an increase in ammonium nitrate. The chemical composition was regionally homogenous for total aerosol mass concentrations above 17.5 µg m ⁻³ , with 74±5 % (average ± standard deviation) ammonium nitrate, 18±3 % organic material, 6±3 % ammonium sulfate, and 2±2 % ammonium chloride. Vertical profiles of aerosol mass and volume in the region showed variable concentrations with height in the polluted boundary layer. Higher average mass concentrations were observed within the first few hundred meters above ground level in all three valleys during pollution episodes. Gas-phase measurements of nitric acid (HNO ₃ ) and ammonia (NH ₃ ) during the pollution episodes revealed that in the Cache and Utah valleys, partitioning of inorganic semi-volatiles to the aerosol phase was usually limited by the amount of gas-phase nitric acid, with NH ₃ being in excess. The inorganic species were compared with the ISORROPIA thermodynamic model. Total inorganic aerosol mass concentrations were calculated for various decreases in total nitrate and total ammonium. For pollution episodes, our simulations of a 50 % decrease in total nitrate lead to a 46±3 % decrease in total PM ₁ mass. A simulated 50 % decrease in total ammonium leads to a 36±17 % µg m ⁻³ decrease in total PM ₁ mass, over the entire area of the study. Despite some differences among locations, our results showed a higher sensitivity to decreasing nitric acid concentrations and the importance of ammonia at the lowest total nitrate conditions. In the Salt Lake Valley, both HNO ₃ and NH ₃ concentrations controlled aerosol formation.

Measuring aerosols and molecular clusters below the 3 nm size limit is essential to increase our understanding of new particle formation. Instruments for the detection of sub-3 nm aerosols and clusters exist and need to be carefully calibrated and characterized. So far calibrations and laboratory tests have been carried out using mainly electrically charged aerosols, as they are easier to handle experimentally. However, the charging state of the cluster is an important variable to take into account. Furthermore, instrument characterization performed with charged aerosols could be biased, preventing a correct interpretation of data when electrically neutral sub-3 nm aerosols are involved. This article presents the first steps to generate electrically neutral molecular clusters as standards for calibration. We show two methods: One based on the neutralization of well-known molecular clusters (mobility standards) by ions generated in a switchable aerosol neutralizer. The second is based on the controlled neutralization of mobility standards with mobility standards of opposite polarity in a recombination cell. We highlight the challenges of these two techniques and, where possible, point out solutions. In addition, we give an outlook on the next steps toward generating well-defined neutral molecular clusters with a known chemical composition and concentration. Published with license by American Association for Aerosol Research