Reem A. Hannun

In recent years, significant perturbations to stratospheric inorganic chlorine have been observed following wildfires or volcanic eruptions, as well as modeled in simulations with elevated water vapor from overshooting convection or organic species in biomass burning aerosol. A detailed evaluation of inorganic chlorine in the stratosphere sampled under various conditions is presented here using high precision in situ aircraft measurements of chlorine monoxide (ClO) and chlorine nitrate (ClONO₂). Data were obtained over North America during the NASA Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) mission. The vertical distributions of ClO and ClONO₂ from all 29 research flights of the 2-year mission are found to be relatively compact and lacking substantial outliers. The peak mixing ratios are approximately 30 ppt and 400 ppt for ClO and ClONO₂, respectively. No chlorine activation was observed in the lower stratosphere in the presence of low temperatures and elevated water vapor from convective injection. Steady-state calculated ClONO₂ is found to be in good agreement with measured values, suggesting that a reduction in the uncertainty of the recommended rate coefficient for ClONO₂ production is possible. HCl is calculated at high time resolution throughout the mission using satellite data, and the resulting evaluation of the inorganic chlorine budget shows excellent agreement, with the ratio of ClO + ClONO₂ + HCl to total inorganic chlorine equal to 0.95 for mixing ratios greater than 500 ppt. No evidence of inorganic chlorine activation was observed during DCOTSS when aerosol organic mass fraction and biomass burning fraction were elevated.

In recent years, significant perturbations to stratospheric inorganic chlorine have been observed following wildfires or volcanic eruptions, as well as modeled in simulations with elevated water vapor from overshooting convection or organic species in biomass burning aerosol. A detailed evaluation of inorganic chlorine in the stratosphere sampled under various conditions is presented here using high precision in situ aircraft measurements of chlorine monoxide (ClO) and chlorine nitrate (ClONO₂). Data were obtained over North America during the NASA Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) mission. The vertical distributions of ClO and ClONO₂ from all 29 research flights of the 2-year mission are found to be relatively compact and lacking substantial outliers. The peak mixing ratios are approximately 30 ppt and 400 ppt for ClO and ClONO₂, respectively. No chlorine activation was observed in the lower stratosphere in the presence of low temperatures and elevated water vapor from convective injection. Steady-state calculated ClONO₂ is found to be in good agreement with measured values, suggesting that a reduction in the uncertainty of the recommended rate coefficient for ClONO₂ production is possible. HCl is calculated at high time resolution throughout the mission using satellite data, and the resulting evaluation of the inorganic chlorine budget shows excellent agreement, with the ratio of ClO + ClONO₂ + HCl to total inorganic chlorine equal to 0.95 for mixing ratios greater than 500 ppt. No evidence of inorganic chlorine activation was observed during DCOTSS when aerosol organic mass fraction and biomass burning fraction were elevated.

Asia accounts for approximately half of global anthropogenic emissions of major pollutants, yet emission inventories remain uncertain and ground-based monitoring is sparse across much of the region. To address these challenges, this study applies a multi-species satellite data assimilation framework to estimate emissions and concentrations of key chemical species during the NASA's ASIA-AQ campaign. The assimilation improves agreement with airborne observations for O₃, NO_x, CO, and CH₂O, with the largest gains for CO (correlation increasing from 0.63–0.64 to 0.77) and CH₂O (biases reduced by 41–70 %). Domain-wide, the optimized emissions show increases of 15 % for NO_x and 9 % for CO, and a 52 % reduction in isoprene. Comparisons with multiple emission inventories reveal large discrepancies, with normalized standard deviation ranging from 11 % for NO_x in mainland China to 68 % for CO in Taiwan. Over Thailand, the assimilation increases fire emissions from 3.6 Tg (GFASv1.2) to 8.3 Tg, while FINNv2.7 produces estimates roughly twice as high, highlighting persistent divergence among fire emission estimates. Source-receptor analysis reveals strong meteorological control on transboundary pollutant: long-range transport contributes up to 62 % of surface O₃ in Manila during strong monsoon conditions, whereas Seoul exhibits local NO_x-saturated chemistry. During a strong transport episode, Indian emissions account for 72–78 % of the free tropospheric O₃ response over Taipei and Chiang Mai, and 24 % over Seoul, highlighting an overlooked transport pathway affecting Asian air quality. These results highlight the value of satellite data assimilation and the need for improved inventories and coordinated action on local and transboundary pollution.

In recent years, significant perturbations to stratospheric inorganic chlorine have been observed following wildfires or volcanic eruptions, as well as modeled in simulations with elevated water vapor from overshooting convection or organic species in biomass burning aerosol. A detailed evaluation of inorganic chlorine in the stratosphere sampled under various conditions is presented here using high precision in situ aircraft measurements of chlorine monoxide (ClO) and chlorine nitrate (ClONO₂). Data were obtained over North America during the NASA Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) mission. The vertical distributions of ClO and ClONO₂ from all 29 research flights of the 2-year mission are found to be relatively compact and lacking substantial outliers. The peak mixing ratios are approximately 30 ppt and 400 ppt for ClO and ClONO₂, respectively. No chlorine activation was observed in the lower stratosphere in the presence of low temperatures and elevated water vapor from convective injection. Steady-state calculated ClONO₂ is found to be in good agreement with measured values, suggesting that a reduction in the uncertainty of the recommended rate coefficient for ClONO₂ production is possible. HCl is calculated at high time resolution throughout the mission using satellite data, and the resulting evaluation of the inorganic chlorine budget shows excellent agreement, with the ratio of ClO + ClONO₂ + HCl to total inorganic chlorine equal to 0.95 for mixing ratios greater than 500 ppt. No evidence of inorganic chlorine activation was observed during DCOTSS when aerosol organic mass fraction and biomass burning fraction were elevated.

In recent years, significant perturbations to stratospheric inorganic chlorine have been observed following wildfires or volcanic eruptions, as well as modeled in simulations with elevated water vapor from overshooting convection or organic species in biomass burning aerosol. A detailed evaluation of inorganic chlorine in the stratosphere sampled under various conditions is presented here using high precision in situ aircraft measurements of chlorine monoxide (ClO) and chlorine nitrate (ClONO2). Data were obtained over North America during the NASA Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) mission. The vertical distributions of ClO and ClONO2 from all 29 research flights of the 2-year mission are found to be relatively compact and lacking substantial outliers. The peak mixing ratios are approximately 30 ppt and 400 ppt for ClO and ClONO2, respectively. No chlorine activation was observed in the lower stratosphere in the presence of low temperatures and elevated water vapor from convective injection. Steady-state calculated ClONO2 is found to be in good agreement with measured values, suggesting that a reduction in the uncertainty of the recommended rate coefficient for ClONO2 production is possible. HCl is calculated at high time resolution throughout the mission using satellite data, and the resulting evaluation of the inorganic chlorine budget shows excellent agreement, with the ratio of ClO + ClONO2 + HCl to total inorganic chlorine equal to 0.95 for mixing ratios greater than 500 ppt. No evidence of inorganic chlorine activation was observed during DCOTSS when aerosol organic mass fraction and biomass burning fraction were elevated.

Wintertime photochemical air pollution in East Asia remains poorly constrained despite its impact on regional air quality. Sources and formation pathways of acyl peroxynitrates (PANs) and ozone (O₃), key photochemical products, are not well understood, hindering effective mitigation strategies. We investigate PANs and O₃ over South Korea using observations from the ASIA-AQ campaign (February–March 2024). PANs reached 5.5 ppbv, strongly correlating with formaldehyde and particulate matter, indicating active winter photochemistry. Median PANs were higher in the mid-southern peninsula (MS; 990 pptv) and Yellow Sea (1200 pptv) than the Seoul Metropolitan Area (840 pptv). Elevated homologue-to-acetyl peroxynitrate ratios over the MS, with enhanced acryloyl peroxynitrate, acrolein, and ethylene oxide, provided tracers for petrochemical emissions and their impacts. Acetaldehyde contributed 53 %–80 % of PAN production. Ethanol was a major precursor of acetaldehyde (∼ 50 %). Strong correlations (r² > 0.8) of ethanol and halocarbons indicate industrial and solvent sources under-represented in inventories. Formaldehyde and C₂₊ aldehydes contributed ∼ 30 % to ozone production. Low ozone production efficiency (< 10) and radical termination dominated by nitric acid and PANs (> 80 %) indicate VOC-limited conditions. The fractional PANs contribution to NO_x loss increased with decreasing OH reactivity ratio of NO₂ to aldehydes, suggesting spatial increases in ozone production following NO_x reductions. These findings demonstrate that a comprehensive understanding of VOC oxidation, particularly oxygenates from industrial sources, is essential for representing winter photochemistry. PANs measurements provide critical constraints on oxidation processes and their implications for emission control.

Since 1991, continuous, consistently calibrated and openly archived ground-based measurements from the Network for the Detection of Atmospheric Composition Change (NDACC) have been collected to investigate processes responsible for decadal-scale changes, anomalies in atmospheric composition, and to validate satellite observations and model simulations. These measurements, from nearly 120 stations, support fundamental research in the area of stratospheric and tropospheric processes impacting ozone chemistry, greenhouse gases, atmospheric radiative forcing, air quality, and interactions with solar radiation and the entire Earth system. NDACC data are supplemented by observations from eleven global Cooperating Networks. The operational principles of Cooperating Networks are well aligned with NDACC objectives and protocols, focusing on data that (a) are high-quality, uniformly processed and traceable to reference standards; and (b) capture short-term (daily to interannual) anomalies and long-term trends. This paper summarizes the NDACC organizational structure. We also review the major accomplishments of NDACC since De Mazière et al. (2018), collaborative research with Cooperating Networks, and interactions with the satellite and modeling communities. Ground-based atmospheric composition monitoring is at a crossroads. Challenges include sustainability of human and financial resources required for complex and intensive data collection, technical issues including aging instrumentation, requirements for FAIR (findable, accessible, interoperable, reusable) data, and lack of data over large parts of Asia, Africa and South America. NDACC is well-positioned to adopt a three-pronged strategy going forward: protecting and modernizing existing stations; promoting the growing use of NDACC data; expanding the number of measured species and network coverage in under-sampled or under-reporting regions.

Asia accounts for approximately half of global anthropogenic emissions of major pollutants, yet emission inventories remain uncertain and ground-based monitoring is sparse across much of the region. To address these challenges, this study applies a multi-species satellite data assimilation framework to estimate emissions and concentrations of key chemical species during the NASA's ASIA-AQ campaign. The assimilation improves agreement with airborne observations for O₃, NO_x, CO, and CH₂O, with the largest gains for CO (correlation increasing from 0.63–0.64 to 0.77) and CH₂O (biases reduced by 41–70 %). Domain-wide, the optimized emissions show increases of 15 % for NO_x and 9 % for CO, and a 52 % reduction in isoprene. Comparisons with multiple emission inventories reveal large discrepancies, with normalized standard deviation ranging from 11 % for NO_x in mainland China to 68 % for CO in Taiwan. Over Thailand, the assimilation increases fire emissions from 3.6 Tg (GFASv1.2) to 8.3 Tg, while FINNv2.7 produces estimates roughly twice as high, highlighting persistent divergence among fire emission estimates. Source-receptor analysis reveals strong meteorological control on transboundary pollutant: long-range transport contributes up to 62 % of surface O₃ in Manila during strong monsoon conditions, whereas Seoul exhibits local NO_x-saturated chemistry. During a strong transport episode, Indian emissions account for 72–78 % of the free tropospheric O₃ response over Taipei and Chiang Mai, and 24 % over Seoul, highlighting an overlooked transport pathway affecting Asian air quality. These results highlight the value of satellite data assimilation and the need for improved inventories and coordinated action on local and transboundary pollution.