Urban ozone (O3) formation can be limited by NOx, VOCs, or both, complicating the design of effective O3 abatement plans. A satellite-retrieved ratio of formaldehyde to NO2 (HCHO/NO2), developed from theory and modeling, has previously been used to indicate O3 formation chemistry. Here, we connect this space-based indicator to spatiotemporal variations in O3 recorded by on-the-ground monitors over major U.S. cities. High-O3 events vary nonlinearly with OMI HCHO and NO2, and the transition from VOC-limited to NOx-limited O3 formation regimes occurs at higher HCHO/NO2 value (3 to 4) than previously determined from models, with slight intercity variations. To extend satellite records back to 1996, we develop an approach to harmonize observations from GOME and SCIAMACHY that accounts for differences in spatial resolution and overpass time. Two-decade (1996-2016) multisatellite HCHO/NO2 captures the timing and location of the transition from VOC-limited to NOx-limited O3 production regimes in major U.S. cities, which aligns with the observed long-term changes in urban-rural gradient of O3 and the reversal of O3 weekend effect. Our findings suggest promise for applying space-based HCHO/NO2 to interpret local O3 chemistry, particularly with the new-generation satellite instruments that offer finer spatial and temporal resolution. This dataset is not publicly accessible because: The data are publicly available on government-supported servers and are terabytes in size. It can be accessed through the following means: Please refer to the linked publication, visit archives described in the text or contact the corresponding authors for more information. Format: Data are processed as described in the linked publication - 10.1021/acs.est.9b07785 . Data included in the analysis are from the European Quality Assurance for Essential Climate Variables project (QA4ECV; http://www.qa4ecv.eu/ecvs), and EPA/AQS and are publicly available at the time of publication. Satell. This dataset is associated with the following publication: Jin, X., A. Fiore, K.F. Boersma, I. De Smedt, and L. Valin. Inferring changes in summertime surface ozone-NOx-VOC chemistry over U.S. urban areas from two decades of satellite and ground-based observations. International Journal of Environmental Science and Technology. Springer, Heidelburg, GERMANY, 54(11): 6518-6529, (2020).

The dataset was created by University of Houston. For details, please contact Jia Jung at helloiamjia@gmail.com. This dataset is associated with the following publication: Jung, J., Y. Choi, S. Mousavinezhad, D. Kang, J. Park, A. Pouyaei, M. Ghahremanloo, M. Momeni, and H. Kim. Changes in the ozone chemical regime over the contiguous United States inferred by the inversion of NOx and VOC emissions using satellite observation. Atmospheric Research. Elsevier Science BV, Amsterdam, NETHERLANDS, 270: 106076, (2022).

The dataset was created by University of Houston. For details, please contact Jia Jung at helloiamjia@gmail.com. This dataset is associated with the following publication: Jung, J., Y. Choi, S. Mousavinezhad, D. Kang, J. Park, A. Pouyaei, M. Ghahremanloo, M. Momeni, and H. Kim. Changes in the ozone chemical regime over the contiguous United States inferred by the inversion of NOx and VOC emissions using satellite observation. Atmospheric Research. Elsevier Science BV, Amsterdam, NETHERLANDS, 270: 106076, (2022).

EPA, in collaboration with NASA, collected upward-viewing UV/visible solar radiance measurements at air quality stations in the New York City region. From these radiances, NO2 vertical column densities (molecules cm-2) are retrieved for comparison to retrieved NO2 column from a downward viewing, satellite-based UV/visible instrument, the NOAA Ozone Mapping and Profiler Suite (OMPS) . This dataset is associated with the following publication: Huang, X., K. Yang, S. Kondragunta, Z. Weir, L. Valin, J. Szykman, and M. Goldberg. NO2 retrievals from NOAA-20 OMPS: Algorithm, evaluation, and observations of drastic changes during COVID-19. ATMOSPHERIC ENVIRONMENT. Elsevier B.V., Amsterdam, NETHERLANDS, 290: 119367, (2022).

This dataset provides in situ concentrations of nitric oxide (NO), nitrogen dioxide (NO2), total reactive nitrogen oxides (NOy), and ozone (O3) measured by the NOAA Nitrogen Oxides and Ozone (NOyO3) 4-channel chemiluminescence (CL) instrument during airborne campaigns conducted by NASA's Atmospheric Tomography (ATom) mission. NOyO3 provides fast-response, specific, high precision, and calibrated measurements of nitrogen oxides and ozone at a spatial resolution of better than 100 m. ATom deploys an extensive gas and aerosol payload on the NASA DC-8 aircraft for systematic, global-scale sampling of the atmosphere, profiling continuously from 0.2 to 12 km altitude. Flights occurred in each of 4 seasons from 2016 to 2018. Flights originate from the Armstrong Flight Research Center in Palmdale, California, fly north to the western Arctic, south to the South Pacific, east to the Atlantic, north to Greenland, and return to California across central North America. ATom establishes a single, contiguous, global-scale dataset. This comprehensive dataset will be used to improve the representation of chemically reactive gases and short-lived climate forcers in global models of atmospheric chemistry and climate.

NO2 and O3 data from the DISCOVER-AQ field study measured by the United States EPA. This dataset is associated with the following publication: Long, R., M. Beaver, R. Duvall, J. Szykman, S. Kaushik, K. Kronmiller, M. Wheeler, S. Garvey, and J. Crawford. Evaluation and Comparison of Methods for Measuring Ozone and NO2 Concentrations in Ambient Air during DISCOVER-AQ. EM Magazine. Air and Waste Management Association, Pittsburgh, PA, USA, 1-11, (2016).

NO2 and O3 data from the DISCOVER-AQ field study measured by the United States EPA. This dataset is associated with the following publication: Long, R., M. Beaver, R. Duvall, J. Szykman, S. Kaushik, K. Kronmiller, M. Wheeler, S. Garvey, and J. Crawford. Evaluation and Comparison of Methods for Measuring Ozone and NO2 Concentrations in Ambient Air during DISCOVER-AQ. EM Magazine. Air and Waste Management Association, Pittsburgh, PA, USA, 1-11, (2016).

Concentrations alone do not provide answer to the question about how emissions are changing over the surface, in part due to simultaneous changes in transport, emissions, dynamics, photochemistry, and chemical feedback. Therefore simulating a chemical transport model benefiting from a multi-specie inversion framework using well-characterized observations should differentiate those influences enabling to closely focus on the emission part. This has another advantage in that we can, to a certain extent, disentangle the chemical and physical processes involved in the formation of ozone. Accordingly, we jointly constrain NOx and VOC emissions using well-characterized TROPOMI HCHO and NO2 columns during individual months of March, April, and May 2020 (lockdown) and 2019 (baseline). We observe a noticeable decline in the magnitude of NOx emissions in March (14-31%) in several major cities such as Paris, Madrid, and Milan expanding further to Rome, Brussels, Frankfurt, London, Warsaw, Belgrade, Kyiv, and Moscow (34-51%) in April. A large variability associated with changes in NOx emissions is indicative of varying dates and the degree of restrictions enacted to prevent the spread of the virus. For instance, NOx emissions remain in somewhat similar values or even higher in northern Germany and Moscow in March 2020 compared to the baseline. Comparisons against surface monitoring stations indicate that the model estimate of the NO2 reduction is underestimated, a picture that correlates with the number of TROPOMI samples impacted by cloudiness. During the month of April, when ample TROPOMI samples are present, the surface NO2 reductions occurring in polluted areas are described fairly well by the model (model: -21±17%, observation: -29±21%). Changes in VOC emissions are dominantly influenced by eastern European biomass burning activities and biogenic isoprene emissions. In March, however, TROPOMI HCHO sets an upper limit for HCHO changes such that the chemical feedback of NOx on HCHO constrained by TROPOMI NO2 reveals a non-negligible decline in anthropogenic VOC emissions in Paris, Milan, London, and Rome. This striking result is only achievable by jointly incorporating HCHO and NO2 observations into the inversion context. Results support an increase in surface ozone during the lockdown. In April, the constrained model features a reasonable agreement with maximum daily 8 h average (MDA8) ozone changes observed at the surface (r=0.43), specifically over central Europe where ozone enhancements prevail (model: +3.73±3.94%, +1.79 ppbv, observation: +7.85±11.27%, +3.76 ppbv). This dataset is not publicly accessible because: The data belongs to first author. It can be accessed through the following means: please contact the first author (ahsouri@cfa.harvard.edu). Format: netCDF, observation data, satellite data. This dataset is associated with the following publication: Souri, A.H., K. Chance, J. Bak, C.R. Nowlan, G. Gonzalez Abad, Y. Jung, C. Wong, J. Mao, and X. Liu. Unraveling pathways of elevated ozone induced by the 2020 lockdown in Europe by an observationally constrained regional model using TROPOMI. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, GERMANY, 21: 18227–18245, (2021).

Concentrations alone do not provide answer to the question about how emissions are changing over the surface, in part due to simultaneous changes in transport, emissions, dynamics, photochemistry, and chemical feedback. Therefore simulating a chemical transport model benefiting from a multi-specie inversion framework using well-characterized observations should differentiate those influences enabling to closely focus on the emission part. This has another advantage in that we can, to a certain extent, disentangle the chemical and physical processes involved in the formation of ozone. Accordingly, we jointly constrain NOx and VOC emissions using well-characterized TROPOMI HCHO and NO2 columns during individual months of March, April, and May 2020 (lockdown) and 2019 (baseline). We observe a noticeable decline in the magnitude of NOx emissions in March (14-31%) in several major cities such as Paris, Madrid, and Milan expanding further to Rome, Brussels, Frankfurt, London, Warsaw, Belgrade, Kyiv, and Moscow (34-51%) in April. A large variability associated with changes in NOx emissions is indicative of varying dates and the degree of restrictions enacted to prevent the spread of the virus. For instance, NOx emissions remain in somewhat similar values or even higher in northern Germany and Moscow in March 2020 compared to the baseline. Comparisons against surface monitoring stations indicate that the model estimate of the NO2 reduction is underestimated, a picture that correlates with the number of TROPOMI samples impacted by cloudiness. During the month of April, when ample TROPOMI samples are present, the surface NO2 reductions occurring in polluted areas are described fairly well by the model (model: -21±17%, observation: -29±21%). Changes in VOC emissions are dominantly influenced by eastern European biomass burning activities and biogenic isoprene emissions. In March, however, TROPOMI HCHO sets an upper limit for HCHO changes such that the chemical feedback of NOx on HCHO constrained by TROPOMI NO2 reveals a non-negligible decline in anthropogenic VOC emissions in Paris, Milan, London, and Rome. This striking result is only achievable by jointly incorporating HCHO and NO2 observations into the inversion context. Results support an increase in surface ozone during the lockdown. In April, the constrained model features a reasonable agreement with maximum daily 8 h average (MDA8) ozone changes observed at the surface (r=0.43), specifically over central Europe where ozone enhancements prevail (model: +3.73±3.94%, +1.79 ppbv, observation: +7.85±11.27%, +3.76 ppbv). This dataset is not publicly accessible because: The data belongs to first author. It can be accessed through the following means: please contact the first author (ahsouri@cfa.harvard.edu). Format: netCDF, observation data, satellite data. This dataset is associated with the following publication: Souri, A.H., K. Chance, J. Bak, C.R. Nowlan, G. Gonzalez Abad, Y. Jung, C. Wong, J. Mao, and X. Liu. Unraveling pathways of elevated ozone induced by the 2020 lockdown in Europe by an observationally constrained regional model using TROPOMI. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, GERMANY, 21: 18227–18245, (2021).

The Solar Backscattered Ultraviolet (SBUV) from NOAA-19 Level-2 daily product (SBUV2N19L2) contains ozone nadir profile and total column data from retrievals generated from the v8.6 SBUV algorithm. The v8.6 SBUV algorithm estimates the ozone nadir profile and total column from SBUV measurements using 1) the Brion-Daumont-Malicet ozone cross sections, 2) an OMI-derived cloud-height climatology, 3) a revised a priori ozone climatology, and 4) inter-instrument calibration based on comparisons with no local time difference.The SBUV2N19L2 product is written as daily files using the HDF5 format, with file sizes ranging from about 1 to 5 Mbytes. Data are available from February 2009 through July 2013. The SBUV2N19L2 data product was used as input in creating the SBUV2N19L3zm monthly zonal mean data product.