Abstract. 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.

Overshooting storms are convective systems with updrafts that penetrate through the tropopause into the overlying stratosphere. These storms can rapidly transport a wide variety of chemical species and aerosols from the boundary layer and free troposphere directly to the stratosphere. The central plains of the U.S. and the Sierra Madre Occidental of Mexico are two of the global hotspots for overshooting convection. While the existence of these storms has been known for several decades, the amount of tropospheric air, including water vapor, trace gases, and aerosols, transported across the tropopause is poorly understood, as is their impact on the dynamics, chemistry, and radiative balance of the stratosphere. Climate models suggest that as Earth’s climate continues to warm, overshooting convection over the U.S. may increase, potentially causing changes to stratospheric composition and transport. To address these scientific questions, the NASA ER-2 high-altitude research aircraft flew 31 missions during the summers of 2021 and 2022 to make observations of the outflow from overshooting storms in the stratosphere over North America and the eastern Pacific Ocean as part of the Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) project. The ER-2 carried a payload of 12 instruments to measure meteorological parameters, water and its isotopologues, trace gases, and aerosol properties. Ozone, water vapor, and aerosol sondes were also launched on balloons during the field deployments. This paper describes the science goals of the DCOTSS project, the aircraft measurement strategy, the data produced by the project, and highlights of science results to date.

In situ carbonyl sulfide (OCS) measurements from the Stratospheric Aerosol processes, Budget and Radiative Effects (SABRE) 2023 airborne campaign are used to evaluate the sulfate budget in the Arctic stratosphere during boreal winter. The strong correspondence between these measurements and remote retrievals from the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer provide robust validation of the satellite's capability to monitor stratospheric OCS globally. We demonstrate how trends in the tropical tropopause layer and National Oceanic and Atmospheric Administration OCS surface data reveal a post-2016 similar to 8% global decline in OCS abundance, which is absent from many global climate models. New simulations with a revised planetary boundary layer OCS abundance show improved agreement with remote retrievals and in situ data across multiple stratospheric layers, but remaining model biases highlight the need for additional in situ OCS observations. The revised representation reduces the stratospheric sulfate burden, resulting in an increased shortwave solar flux at the tropical tropopause by as much as 0.3 Wm-2 locally, with implications for stratospheric circulation, radiative forcing, and climate feedbacks.

Perchlorate is a toxic, regulated contaminant in drinking water. According to previous isotopic studies, much of the perchlorate deposited to the Earth's surface is formed in the atmosphere, with 36Cl suggesting a large contribution from the stratosphere. Here, we present measurements of perchlorate in stratospheric aerosol particles and confirm that the stratosphere is an important source of perchlorate, whereas we did not observe production in the troposphere. Mass mixing ratios of aerosol perchlorate in the stratosphere were 1 to 10 parts per trillion by mass (pptm), with the highest concentrations observed in summer and in the Southern Hemisphere. Almost all of the perchlorate is in biomass burning and nitrogen-rich particles, despite those types contributing only a few percent of the aerosol particles. Such particles are less acidic than the majority of sulfuric acid particles. If the formation of perchlorate is sensitive to acidity, then the injection of some materials for solar radiation modification might significantly increase the global production of perchlorate.

The stratospheric aerosol layer plays an essential role in stratospheric chemical and radiative processes. Emissions of SO2 from small-to-midsized volcanic eruptions are typically introduced into the lower stratosphere, where a background aerosol already exists. The radiative and chemical consequences of these eruptive emissions depend in part upon the characteristics of these background particles, whose number originates from the troposphere. In-situ observations of sub-0.1 µm diameter particles in the upper troposphere/lower stratosphere are rare, but are of particular importance in understanding the contribution of upwelling ultrafine particles formed near the tropical tropopause and their subsequent evolution in the stratospheric (Brewer-Dobson) circulation. In the Stratospheric Aerosol processes, Budget and Radiative Effects (SABRE) mission from January to March 2023, we used three particle sizing instruments on the NASA WB-57 high-altitude aircraft to measure the size distribution of aerosol particles in the size range from 0.003 to ~4.0 μm in the lower stratosphere from middle to high latitudes at altitudes up to 19.6 km. The composition of individual aerosol particles >0.1 µm was also measured, along with O3, N2O, SF6 and OCS. Together, these measurements were used to investigate dynamical and chemical processes in the stratosphere that determine the evolution of the stratospheric aerosol as a function of stratospheric age up to several years. We observed a clear bimodal size distribution structure, with a small mode (< 0.1 μm diameter) originating from the troposphere and a larger mode (>0.2 µm diameter) originating from the photolysis of OCS (the classic Junge, or stratospheric, aerosol layer). These two modes evolve as a function of stratospheric age in a manner consistent with coagulation, condensation, and sedimentation. These are the first reported observations of the presence and evolution of this bimodal aerosol structure deep into the stratosphere. The small (tropospheric) particle mode provides a significant condensation sink at young stratospheric ages. SO2 from modest volcanic eruptions emitted into the lower stratosphere may condense on these smaller particles, reducing the amount of light scattering per unit mass. Proposed geoengineering efforts must also account for this tropospheric mode of particles. Models simulating aerosol processes in the stratosphere need to accurately represent tropospheric particles in the lower stratosphere and their evolution with stratospheric age.

We utilize in situ stratospheric measurements of trace gases from two recent high altitude aircraft campaigns, DCOTTS and SABRE, to compare mean ages and long-lived trace gas relationships in the NH stratosphere to those from ER-2 campaigns in the 1990s. The ER-2 campaign data from three decades ago have been a primary reference for in situ-based estimates of mean age in the lower stratosphere from 16-22 km altitude, but very few measurements have been made in this region since then. We use an updated technique to consistently calculate mean ages from simultaneous in situ measurements of SF6, CO2, N2O and CH4, allowing us to compare mean ages and their relationship with N2O between the 1990s and 2020s. The mesospheric loss of SF6 and subsequent old age biases are largely accounted for based on newly developed theory and modeling work. We then use the idealized tropical leaky pipe model to explore stratospheric circulation and mixing changes that are consistent with the observations.

The age of air is an important transport diagnostic that can be derived from trace gas measurements and compared to global chemistry climate model output. We describe a new technique to calculate the age of air, measuring transport times from the Earth's surface to any location in the atmosphere based on simultaneous in situ measurements of multiple key long-lived trace gases. The primary benefits of this new technique include (1) optimized ages of air consistent with simultaneously measured SF6 and CO2; (2) age of air from the upper troposphere through the stratosphere; (3) estimates of the second moment of age spectra that have not been well constrained from measurements; and (4) flexibility to be used with measurements across multiple instruments, platforms, and decades. We demonstrate the technique on aircraft and balloon measurements from the 1990s, the last period of extensive stratospheric in situ sampling, and several recent missions from the 2020s, and compare the results with previously published and modeled values.

Abstract. Recent studies hypothesize that emissions from fires reaching the stratosphere can provide aerosols and aerosol precursors that initiate stratospheric ozone loss and lead to radiative heating of the stratosphere and cooling of the surface. Air from the troposphere enters the stratosphere primarily over the tropical western Pacific (TWP) during boreal winter. We report observations in the TWP of persistent, ubiquitous continental pollution in the tropical tropopause layer (TTL) and lower stratosphere (LS) during the Airborne Tropical TRopopause EXperiment (ATTREX) campaign in February–March 2014. We found concentrations of carbon monoxide (CO) enhanced up to 65 % over background levels in the deep tropics (5° S–15° N, 16 –17 km). Correlations of CO with hydrocarbon and halocarbon species indicated a biomass burning source, with the largest CO enhancements found in warmer, clear air. Satellite observations of CO did not detect the thin pollution layers observed by the aircraft, but did indicate Africa, Indonesia, and the western/central Pacific as geographical hot spots for CO in the TTL. Backward trajectories identified convective encounters in these areas as the dominant sources of polluted air in the TWP. Africa and Indonesia contributed about 60 % of the excess CO, transported to the TWP in two to four weeks. Our study confirms that air in the TTL over the TWP is affected by emissions from distant fires that can rapidly reach the LS in the principal source region for air entering the stratosphere, supporting the view that fires in tropical regions could impact stratospheric ozone and temperatures.

Anthropogenic trace gases often exhibit interhemispheric gradients because of larger emissions in the Northern Hemisphere. Depending on a tracer's emission pattern and sink processes, trace gas observations can thus be used to investigate interhemispheric transport in the atmosphere. Vice versa, understanding interhemispheric transport is important for interpreting spatial tracer distributions and for inferring emissions. We combine several data sets from the upper troposphere (UT) to investigate the interhemispheric gradient of sulfur hexafluoride (SF6) covering latitudes from ∼ 80∘ N to ∼ 60∘ S: canister sampling based measurements from the IAGOS-CARIBIC infrastructure and data from the in-flight gas chromatography instruments GhOST (Gas chromatograph for Observational Studies using Tracers) and UCATS (Unmanned aircraft systems Chromatograph for Atmospheric Trace Species). The interhemispheric gradient of SF6 in the UT is found to be weaker than near the surface. Using the concept of a lag time removes the increasing trend from the time series. At the most southern latitudes, a lag time of over 1 year with respect to the northern mid-latitude surface is derived, and lag times decrease over the period 2006–2020 in the extra-tropics and the southern tropics. Observations are compared to results from the two-dimensional Advanced Global Atmospheric Gases Experiment (AGAGE) 12-box model. Based on Emissions Database for Global Atmospheric Research (EDGAR 7) emissions, fair agreement of lag times is obtained for the Northern Hemisphere, but southern hemispheric air appears too “old”. This is consistent with earlier findings that transport from the northern extra-tropics into the tropics is too slow in many models. The influence of the emission scenario and the model transport scheme are evaluated in sensitivity runs. It is found that EDGAR 7 underestimates emissions of SF6 globally and in the Southern Hemisphere, whereas northern extra-tropical emissions seem overestimated. Faster southward transport from the northern extra-tropics would be needed in the model, but transport from the southern tropics into the southern extra-tropics appears too fast.

Whole Air Samples (WAS) were collected as part of the Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) field campaign in the Upper Troposphere-Lower Stratosphere (UTLS) region during Summer 2021 and 2022. Grid-Rad and satellite imagery were used to identify regions of overshooting convection, and areas with outflow from the overshooting were targeted by the DCOTSS aircraft using trajectory models and in-situ measurements. Because a wide range of trace gases with different atmospheric lifetimes and sources are measured, WAS can provide insight into the processes that influence trace gas composition of the UTLS over North America. We investigate the tropospheric tracer relationships within and around these deep convective regions to determine the extent of penetration of tropospheric gases into the lower stratosphere (LS). Compounds such as ethane and ethyne with short (< 6 months) tropospheric lifetimes do not reach the LS without rapid transport from deep convection, and we observe cases where these gases are elevated above stratospheric background, typically well correlated with other tropospheric tracers (e.g. CO). However, the relationship between enhanced water vapor from overshooting convection and tropospheric tracers is more complex. Further, we show that ratios between different trace gas species can help identify and distinguish air mass types (e.g., biomass burning, oil and gas production, urban influence, etc.). Finally, we determine the Cl- and Br- halogen budgets for 2021 and 2022 stratosphere over N. America and the contribution of very-short-lived organic halogen species.

The radiative balance of the upper atmosphere is dependent on the magnitude and distribution of greenhouse gases and aerosols in that region. Climate models predict that with increasing surface temperature, the primary mechanism for transporting tropospheric air into the stratosphere (known as the Brewer-Dobson circulation) will strengthen, leading to changes in the distribution of atmospheric water vapor, other greenhouse gases, and aerosols. Stratospheric relationships between greenhouse gases and other long-lived trace gases with various photochemical properties (such as N2O, SF6, and chlorofluorocarbons) provide a strong constraint for tracking changes in the stratospheric circulation. Therefore, a cost-effective approach is needed to monitor these trace gases in the stratosphere. In the past decade, the balloon-borne AirCore sampler developed at NOAA's Global Monitoring Laboratory has been routinely used to monitor the mole fractions of CO2, CH4, and CO from the ground to approximately 25 km above mean sea level. Our recent development work adapted a gas chromatograph coupled with an electron capture detector (GC-ECD) to measure a suite of trace gases (N2O, SF6, CFC-11, CFC-12, H-1211, and CFC-113) in the stratospheric portion of AirCores. This instrument, called the StratoCore-GC-ECD, allows us to retrieve vertical profiles of these molecules at high resolution (5-7 hPa per measurement). We launched four AirCore flights and analyzed the stratospheric air samples for these trace gases. The results showed consistent and expected tracer-tracer relationships and good agreement with recent aircraft campaign measurements. Our work demonstrates that the StratoCore-GC-ECD system provides a low-cost and robust approach to measuring key stratospheric trace gases in AirCore samples and for evaluating changes in the stratospheric circulation.

Nitrous oxide (N2O) is the fourth most important greenhouse gas in the atmosphere and is considered the most important current source gas emission for global stratospheric ozone depletion (O3). It has natural and anthropogenic sources, mainly as an unintended by-product of food production activities. This work examines the identification and quantification of trends in the N2O concentration from the middle troposphere to the middle stratosphere (MTMS) by in situ and remote sensing observations. The temporal variability of N2O is addressed using a comprehensive dataset of in situ and remote sensing N2O concentrations based on aircraft and balloon measurements in the MTMS from 1987 to 2018. We determine N2O trends in the MTMS, based on observations. This consistent dataset was also used to study the N2O seasonal cycle to investigate the relationship between abundances and its emission sources through zonal means. The results show a long-term increase in global N2O concentration in the MTMS with an average of 0.89 ± 0.07 ppb/yr in the troposphere and 0.96 ± 0.15 ppb/yr in the stratosphere, consistent with 0.80 ppb/yr derived from ground-based measurements and 0.799 ± 0.024 ppb/yr ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer) satellite measurements.

The NASA Atmospheric Tomography (ATom) mission built a photochemical climatology of air parcels based on in situ measurements with the NASA DC-8 aircraft along objectively planned profiling transects through the middle of the Pacific and Atlantic oceans. In this paper we present and analyze a data set of 10 s (2 km) merged and gap-filled observations of the key reactive species driving the chemical budgets of O3 and CH4 (O3, CH4, CO, H2O, HCHO, H2O2, CH3OOH, C2H6, higher alkanes, alkenes, aromatics, NOx, HNO3, HNO4, peroxyacetyl nitrate, and other organic nitrates), consisting of 146 494 distinct air parcels from ATom deployments 1 through 4. Six models calculated the O3 and CH4 photochemical tendencies from this modeling data stream for ATom 1. We find that 80 %–90 % of the total reactivity lies in the top 50 % of the parcels and 25 %–35 % in the top 10 %, supporting previous model-only studies that tropospheric chemistry is driven by a fraction of all the air. Surprisingly, the probability densities of species and reactivities averaged on a model scale (100 km) differ only slightly from the 2 km ATom 10 s data, indicating that much of the heterogeneity in tropospheric chemistry can be captured with current global chemistry models. Comparing the ATom reactivities over the tropical oceans with climatological statistics from six global chemistry models, we find generally good agreement with the reactivity rates for O3 and CH4. Models distinctly underestimate O3 production below 2 km relative to the mid-troposphere, and this can be traced to lower NOx levels than observed. Attaching photochemical reactivities to measurements of chemical species allows for a richer, yet more constrained-to-what-matters, set of metrics for model evaluation. This paper presents a corrected version of the paper published under the same authors and title (sans “corrected”) as https://doi.org/10.5194/acp-21-13729-2021.

This article provides an overview of the NASA Atmospheric Tomography (ATom) mission and a summary of selected scientific findings to date. ATom was an airborne measurements and modeling campaign aimed at characterizing the composition and chemistry of the troposphere over the most remote regions of the Pacific, Southern, Atlantic, and Arctic Oceans, and examining the impact of anthropogenic and natural emissions on a global scale. These remote regions dominate global chemical reactivity and are exceptionally important for global air quality and climate. ATom data provide the in situ measurements needed to understand the range of chemical species and their reactions, and to test satellite remote sensing observations and global models over large regions of the remote atmosphere. Lack of data in these regions, particularly over the oceans, has limited our understanding of how atmospheric composition is changing in response to shifting anthropogenic emissions and physical climate change. ATom was designed as a global-scale tomographic sampling mission with extensive geographic and seasonal coverage, tropospheric vertical profiling, and detailed speciation of reactive compounds and pollution tracers. ATom flew the NASA DC-8 research aircraft over four seasons to collect a comprehensive suite of measurements of gases, aerosols, and radical species from the remote troposphere and lower stratosphere on four global circuits from 2016 to 2018. Flights maintained near-continuous vertical profiling of 0.15–13-km altitudes on long meridional transects of the Pacific and Atlantic Ocean basins. Analysis and modeling of ATom data have led to the significant early findings highlighted here.

The detection of increasing global CFC-11 emissions after 2012 alerted society to a possible violation of the Montreal Protocol on Substances that Deplete the Ozone Layer (MP). This alert resulted in parties to the MP taking urgent actions. As a result, atmospheric measurements made in 2019 suggest a sharp decline in global CFC-11 emissions. Despite the success in the detection and mitigation of part of this problem, regions fully responsible for the recent global emission changes in CFC-11 have not yet been identified. Roughly two thirds (60 ± 40 %) of the emission increase between 2008–2012 and 2014–2017 and two thirds (60 ± 30 %) of the decline between 2014–2017 and 2019 were explained by regional emission changes in eastern mainland China. Here, we used atmospheric CFC-11 measurements made from two global aircraft surveys – the HIAPER (High-performance Instrumented Airborne Platform for Environmental Research) Pole-to-Pole Observations (HIPPO) in November 2009–September 2011 and the Atmospheric Tomography Mission (ATom) in August 2016–May 2018, in combination with the global CFC-11 measurements made by the US National Oceanic and Atmospheric Administration during these two periods – to derive global and regional emission changes in CFC-11. Our results suggest Asia accounted for the largest fractions of global CFC-11 emissions in both periods: 43 (37–52) % during November 2009–September 2011 and 57 (49–62) % during August 2016–May 2018. Asia was also primarily responsible for the emission increase between these two periods, accounting for 86 (59–115) % of the global CFC-11 emission rise between the two periods. Besides eastern mainland China, temperate western Asia and tropical Asia also contributed significantly to global CFC-11 emissions during both periods and likely to the global CFC-11 emission increase. The atmospheric observations further provide strong constraints on CFC-11 emissions from North America and Europe, suggesting that each of them accounted for 10 %–15 % of global CFC-11 emissions during the HIPPO period and smaller fractions in the ATom period. For South America, Africa, and Australia, the derived regional emissions had larger dependence on the prior assumptions of emissions and emission changes due to a lower sensitivity of the observations considered here to emissions from these regions. However, significant increases in CFC-11 emissions from southern hemispheric lands were not likely due to the observed increase of north-to-south interhemispheric gradients in atmospheric CFC-11 mole fractions from 2012–2017.

The mean age since air was last at the Northern Hemisphere (NH) midlatitude surface is a fundamental property of tropospheric transport. Here we approximate the mean age in terms of an "SF6 age" (Gamma SF6), derived from surface and aircraft measurements of SF6 that are broader in spatial scope and cover a longer time period (1997-2018) than considered previously. At the surface, Gamma SF6 increases from near-zero values north of 30 degrees N to similar to 1.5 years over the Southern Hemisphere (SH) extratropics, with the largest meridional gradients occurring in the tropics. By comparison, vertical gradients in Gamma SF6 are weak throughout, with only slight increases/decreases with height in the NH/SH. The broader spatial coverage of the measurements reveals strong variations in the seasonal cycle of Gamma SF6 within the (sub)tropics that are weaker over the Atlantic and Pacific oceans, compared to over the Indian Ocean. Observations from 2000 to 2018 reveal that the SF6 age at sites in the SH has been decreasing by similar to 0.12 years/dec. However, this decrease is not due to changes in transport but, rather, is likely related to changes in emissions, which have increased globally and reportedly shifted from northern midlatitudes into the subtropics. Simulations, which reproduce the SF6 age trends, show no decreases in an age-of-air tracer, reinforcing the fact that Gamma SF6 represents only an approximation to the mean age. Finally, the modeled SF6 ages are older than observed, by similar to 0.3-0.4 years throughout the southern extratropics. We show that this bias is partly related to an overestimation in simulated SF6 near emissions regions, likely reflecting a combination of uncertainties in emissions and model transport.