INTRODUCTION: Although the carbon budget is often presented in terms of global-scale fluxes, many of the contributing processes occur through localized point sources, which have been challenging to measure from space. Persistent anthropogenic carbon dioxide (CO ₂ ) emissions have altered the natural balance of Earth’s carbon sources and sinks. These emissions are driven by a multitude of individual mobile and stationary point sources that combust fossil fuels, with urban areas accounting for more than 70% of anthropogenic emissions to the atmosphere. Natural point-source emissions are dominated by wildfires and persistent volcanic degassing. RATIONALE: Comprehensive global measurements from space could help to more completely characterize anthropogenic and natural point-source emissions. In global carbon cycle models, anthropogenic point-source information comes from bottom-up emission inventories, whereas natural point-source information comes from a sparse in situ measurement network. Whereas clusters of urban CO ₂ point-source plumes merge together, isolated point sources (e.g., remote power plants, cement production plants, and persistently degassing volcanoes) create localized plumes. Because turbulent mixing and diffusion cause rapid downwind dilution, they are challenging to detect and analyze. Point-source detection from space is complicated by signal dilution: The observed values of Δ X CO 2 (enhancement of the column-averaged dry-air CO ₂ mole fraction) correspond to in situ CO ₂ enhancements of 10-fold or higher. Space-based sensors that detect and quantify CO ₂ in plumes from individual point sources would enable validation of reported inventory fluxes for power plants. These sensors would also advance the detectability of volcanic eruption precursors and improve volcanic CO ₂ emission inventories. RESULTS: Spaceborne measurements of atmospheric CO ₂ using kilometer-scale data from NASA’s Orbiting Carbon Observatory-2 (OCO-2) reveal distinct structures caused by known anthropogenic and natural point sources, including megacities and volcanoes. Continuous along-track sampling across Los Angeles (USA) by OCO-2 at its ~2.25-km spatial resolution exposes intra-urban spatial variability in the atmospheric X CO 2 distribution that corresponds to the structure of the urban dome, which is detectable under favorable wind conditions. Los Angeles X CO 2 peaks over the urban core and decreases through suburban areas to rural background values more than ~100 km away. Enhancements of X CO 2 in the Los Angeles urban CO ₂ dome observed by OCO-2 vary seasonally from 4.4 to 6.1 parts per million (ppm). We also detected isolated CO ₂ plumes from the persistently degassing Yasur, Ambrym, and Aoba volcanoes (Vanuatu), corroborated by near-simultaneous sulfur dioxide plume detections by NASA’s Ozone Mapping and Profiler Suite. An OCO-2 transect passing directly downwind of Yasur volcano yielded a narrow filament of enhanced X CO 2 ( Δ X CO 2 ≈ 3.4 ppm), consistent with plume modeling of a CO ₂ point source emitting 41.6 ± 19.7 kilotons per day (15.2 ± 7.2 megatons per year). These highest continuous volcanic CO ₂ emissions are collectively dwarfed by about 70 fossil fuel–burning power plants on Earth, which each emit more than 15 megatons per year of CO ₂ . CONCLUSION: OCO-2’s sampling strategy was designed to characterize CO ₂ sources and sinks on regional to continental and ocean-basin scales, but the unprecedented kilometer-scale resolution and high sensitivity enables detection of CO ₂ from natural and anthropogenic localized emission sources. OCO-2 captures seasonal, intra-urban, and isolated plume signals. Capitalizing on OCO-2’s sensitivity, a much higher temporal resolution would capture anthropogenic emission signal variations from diurnal, weekly, climatic, and economic effects, and, for volcanoes, precursory emission variability. Future sampling strategies will benefit from a continuous mapping approach with the sensitivity of OCO-2 to systematically and repeatedly capture these smaller, urban to individual plume scales of CO ₂ point sources. OCO-2 detects urban CO ₂ signals with unprecedented detail over Los Angeles.: Individual “footprints” of OCO-2 X CO 2 data from early fall 2014 and summer 2015 over the city of Los Angeles strongly contrast with values over the distant, rural Antelope Valley. X CO 2 is the averaged dry-air molar CO ₂ concentration between the spacecraft and Earth’s surface.

Fast‐response tunable diode laser measurements of water vapor were made over the North Atlantic during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) conducted in the fall of 1997. Resulting water vapor mixing ratios, in conjunction with air temperature and pressure measurements obtained by the DC‐8's Data Acquisition and Distribution System (DADS), were used to calculate the prevalence of ice‐saturated conditions (relative humidity with respect to ice >100% or frost saturation) in the upper troposphere over the SONEX sampling region. Additionally, Appleman theory was applied to the data to determine the subset of ice‐saturated regions which would support formation of contrails. Results suggest that studies to determine the potential climate or radiative impact of contrails and aviation‐induced cirrus clouds should not only consider the climatology of frost‐saturated regions, but should also address whether ambient conditions can support contrail formation within these regions. A separate discussion within the paper describes an in‐flight intercomparison between the SONEX diode laser hygrometer and the Pollution From Aircraft Emissions in the North Atlantic Flight Corridor (POLINAT 2) cryogenic hygrometer and shows that the instruments perform to within their stated accuracies (∼10%) over a range of mixing ratios and altitudes.

Intense and frequent convection occurring during the Asian Summer Monsoon (ASM) rapidly transports surface emissions to the upper troposphere (UT). Depending on their chemical reactivities, pollutants transported to the UT via this mechanism may either undergo chemistry in the UT region or be transported into the lower stratosphere. The Asian Summer Monsoon Chemical and CLimate Impact Project (ACCLIP) used high‐altitude research aircraft to characterize chemistry in the tropopause region within the ASM anticyclone and outflows of monsoon convection during summer 2022. Here, we use measurements of trace gases from ACCLIP and results from a 0‐D model constrained by airborne observations to calculate net ozone production rates in airmasses influenced by recent convection and in the summer monsoon background upper troposphere/lower stratospheric air. We find that ozone production inside the polluted air downstream of recent convection is up to one order of magnitude higher than that in the cleaner background upper troposphere. The ozone production is driven by NO + HO ₂ produced from CO and OVOC oxidation, rather than from organic peroxy radicals produced from volatile organic compounds inside the highly polluted airmasses. Consistent with previous modeling work, we find efficient HO _x (OH + HO ₂ ) cycling is dominated by reactions with CO. Ozone production in this region is primarily NO _x ‐limited and increases with higher NO _x such as during convective events with lightning NO _x production. Further, we find that the dominant impact of enhanced NO _x here is through acceleration of HO _x cycling, and thus an increase in ozone production.