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Wildfires inject large amounts of black carbon (BC) particles into the atmosphere, which can reach the lowermost stratosphere (LMS) and cause strong radiative forcing. During a 14-month period of observations on board a passenger aircraft flying between Europe and North America, we found frequent and widespread biomass burning (BB) plumes, influencing 16 of 160 flight hours in the LMS. The average BC mass concentrations in these plumes (∼140 ng·m-3, standard temperature and pressure) were over 20 times higher than the background concentration (∼6 ng·m-3) with more than 100-fold enhanced peak values (up to ∼720 ng·m-3). In the LMS, nearly all BC particles were covered with a thick coating. The average mass equivalent diameter of the BC particle cores was ∼120 nm with a mean coating thickness of ∼150 nm in the BB plume and ∼90 nm with a coating of ∼125 nm in the background. In a BB plume that was encountered twice, we also found a high diameter growth rate of ∼1 nm·h-1 due to the BC particle coatings. The observed high concentrations and thick coatings of BC particles demonstrate that wildfires can induce strong local heating in the LMS and may have a significant influence on the regional radiative forcing of climate.

The air–sea exchange of ozone (O 3) is controlled by chemistry involving halogens, dissolved organic carbon, and sulfur in the sea surface microlayer. Calculations also indicate faster ozone photolysis at aqueous surfaces, but the role of clouds as an ozone sink is currently not well established. Fast-response ozone sensors offer opportunities to measure eddy covariance (EC) ozone fluxes in the marine boundary layer. However, intercomparisons of fast airborne O 3 sensors and EC O 3 fluxes measured on aircraft have not been conducted before. In April 2022, the Technological Innovation Into Iodine and GV Environmental Research (TI 3 GER) field campaign deployed three fast ozone sensors (gas chemiluminescence and a combination of UV absorption with coumarin chemiluminescence detection, CID) together with a fast water vapor sensor and anemometer to study iodine chemistry in the troposphere and stratosphere over Colorado and over the Pacific Ocean near Hawaii and Alaska. Here, we present an instrument comparison between the NCAR Fast O 3 instrument (FO3, gas-phase CID) and two KIT Fast AIRborne Ozone instruments (FAIRO, UV absorption and coumarin CID). The sensors have comparable precision < 0.4 % Hz -0.5 (0.15 ppbv Hz -0.5), and ozone volume mixing ratios (VMRs) generally agreed within 2 % over a wide range of environmental conditions: 10 < O 3 < 1000 ppbv, below detection < NO x < 7 ppbv, and 2 ppmv < H 2 O < 4 % VMR. Both instrument designs are demonstrated to be suitable for EC flux measurements and were able to detect O 3 fluxes with exchange velocities (defined as positive for upward) as slow as - 0.010 ± 0.004 cm s -1 , which is in the lower range of previously reported measurements. Additionally, we present two case studies. In one, the direction of ozone and water vapor fluxes was reversed (vO3 = + 0.134 ± 0.005 cm s -1), suggesting that overhead evaporating clouds could be a strong ozone sink. Further work is needed to better understand the role of clouds as a possibly widespread sink of ozone in the remote marine boundary layer. In the second case study, vO3 values are negative (varying by a factor of 6–10 from - 0.036 ± 0.006 to - 0.003 ± 0.004 cm s -1), while the water vapor fluxes are consistently positive due to evaporation from the ocean surface and spatially homogeneous. This case study demonstrates that the processes governing ozone and water vapor fluxes can become decoupled and illustrates the need to elucidate possible drivers (physical, chemical, or biological) of the variability in ozone exchange velocities on fine spatial scales (∼ 20 km) over remote oceans. [ABSTRACT FROM AUTHOR]

Tropospheric ozone (O3) is an important greenhouse gas that is also hazardous to human health. The formation of O3 is sensitive to the levels of its precursors NOx (≡NO + NO2) and peroxy radicals, for example, generated by the oxidation of volatile organic compounds (VOCs). A better understanding of this sensitivity will show how changes in the levels of these trace gases could affect O3 levels today and in the future, and thus air quality and climate. In this study, we investigate O3 sensitivity in the tropical troposphere based on in situ observations of NO, HO2 and O3 from four research aircraft campaigns between 2015 and 2023. These are OMO (Oxidation Mechanism Observations), ATom (Atmospheric Tomography Mission), CAFE Africa (Chemistry of the Atmosphere Field Experiment in Africa) and CAFE Brazil, in combination with simulations using the EMAC atmospheric chemistry—climate model. We use the metric α(CH3O2) together with NO to investigate the O3 formation sensitivity. We show that O3 formation is generally NOx‐sensitive in the lower and middle tropical troposphere and is in a transition regime in the upper troposphere. By distinguishing observations impacted by lightning or not we show that NO from lightning is the most important driver of O3 sensitivity in the tropics. NOx‐sensitive chemistry predominates in regions without lightning impact, with α(CH3O2) ranging between 0.56 and 0.82 and observed average O3 levels between 35 and 55 ppbv. Areas affected by lightning exhibit strongly VOC‐sensitive O3 chemistry with α(CH3O2) of about 1 and average O3 levels between 55 and 80 ppbv. Plain Language Summary: Ozone (O3) in the troposphere is both an air pollutant and a greenhouse gas. It is formed from nitrogen oxides (NOx) and volatile organic compounds (VOCs). The formation can be sensitive to either of these precursors depending on their abundance. Considering the high relevance of O3 in regard to human health and global warming, it is important to understand this sensitivity of O3 formation, which allows to predict future changes in O3. Here, we investigate O3 formation sensitivity toward NOx and VOCs in the tropical troposphere based on aircraft measurements during four research campaigns between 2015 and 2023, and a global model. We include observations of NO, HO2 (hydroperoxyl radicals) and O3 over South America, the Middle East and the Pacific, Atlantic and Indian Ocean. We find that O3 formation is sensitive to NOx in the lower tropical troposphere. In the upper tropical troposphere, lightning events control O3 chemistry and promote strong VOC‐sensitive O3 formation. Key Points: α(CH3O2) correlated with NO is a powerful metric for indicating O3 sensitivity and is valid throughout the troposphereO3 chemistry in the remote tropical lower troposphere is found to be NOx‐sensitiveNO emissions from lightning drive O3 sensitivity in the tropical upper troposphere and induce highly VOC‐sensitive chemistry [ABSTRACT FROM AUTHOR]

The chemical composition of the upper troposphere/lower stratosphere region (UTLS) is influenced by horizontal transport of air masses, vertical transport within convective systems and warm conveyor belts, rapid turbulent mixing, as well as photochemical production or loss of species. This results in the formation of the extratropical transition layer (ExTL), which is defined by the vertical structure of CO and has been studied until now mostly by means of trace gas correlations. Here, we extend the analysis to include aerosol particles and derive the sulfate–ozone correlation in central Europe from aircraft in situ measurements during the CAFE-EU (Chemistry of the Atmosphere Field Experiment over Europe)/BLUESKY mission. The mission probed the UTLS during the COVID-19 period with significantly reduced anthropogenic emissions. We operated a compact time-of-flight aerosol mass spectrometer (C-ToF-AMS) to measure the chemical composition of non-refractory aerosol particles in the size range from about 40 to 800 nm. In our study, we find a correlation between the sulfate mass concentration and O 3 in the lower stratosphere. The correlation exhibits some variability exceeding the mean sulfate–ozone correlation over the measurement period. Especially during one flight, we observed enhanced mixing ratios of sulfate aerosol in the lowermost stratosphere, where the analysis of trace gases shows tropospheric influence. However, back trajectories indicate that no recent mixing with tropospheric air occurred within the last 10 d. Therefore, we analyzed volcanic eruption databases and satellite SO 2 retrievals from the TROPOspheric Monitoring Instrument (TROPOMI) for possible volcanic plumes and eruptions to explain the high amounts of sulfur compounds in the UTLS. From these analyses and the combination of precursor and particle measurements, we conclude that gas-to-particle conversion of volcanic SO 2 leads to the observed enhanced sulfate aerosol mixing ratios. [ABSTRACT FROM AUTHOR]

ABSTRACT The Polar Stratosphere in a Changing Climate (POLSTRACC) mission employed the German High Altitude and Long Range Research Aircraft (HALO). The payload comprised an innovative combination of remote sensing [...]

The air-sea exchange of ozone is controlled by chemistry involving halogens, dissolved organic carbon and sulfur in the sea surface microlayer. Calculations also indicate faster ozone photolysis at aqueous surfaces, but the role of clouds as ozone sink is currently not well established. Fast response ozone sensors offer opportunities to measure eddy covariance (EC) ozone fluxes in the marine boundary layer. However, intercomparisons of fast airborne O3 sensors, and EC O3 fluxes measured on aircraft have not been conducted before. In April 2022, the TI³GER (Technical Innovation Into Iodine and GV aircraft Environmental Research) field campaign deployed three fast ozone sensors (gas chemiluminescence and a combination of UV absorption with coumarin chemiluminescence detection, CID) together with a fast water vapor sensor and anemometer to study iodine chemistry in the troposphere and stratosphere over Colorado and over the Pacific Ocean near Hawaii and Alaska. Here, we present an instrument comparison between the NCAR Fast O3 instrument (FO3, gas-phase CID) and two KIT Fast AIRborne Ozone instruments (FAIRO, UV absorption and coumarin CID). The sensors have comparable precision <0.4% Hz- 0.5 (0.15 ppbv Hz-0.5), and ozone volume mixing ratios (vmr) generally agreed within 2% over a wide range of environmental conditions: 10 < O3 < 1000 ppbv; below detection < NOx < 7 ppbv; and 2 ppmv < H2O < 4% VMR. Both instrument designs are demonstrated to be suitable for EC flux measurements and were able to detect O3 fluxes with exchange velocities (defined as positive for upward) as slow as -0.010 ± 0.004 cm s-1, which is in the lower range of previously reported measurements. Additionally, we present two case studies: one in which the direction of ozone and water vapor fluxes were reversed (vO3 = +0.134 ± 0.005 cm s-1), suggesting that overhead evaporating clouds could be a strong ozone sink; and another in which ozone fluxes vO3 are negative (varying by a factor of 6-10 from -0.036 ± 0.006 to -0.003 ± 0.004 cm s-1), while the water vapor fluxes are consistently positive due to evaporation from the ocean surface and spatially homogeneous. Future work is needed to better understand the role of clouds as a possibly widespread sink of ozone in the remote marine boundary layer, and to elucidate possible drivers (physical, chemical, or biological) of the variability in ozone exchange velocities on fine spatial scales (~20 km) over remote oceans. [ABSTRACT FROM AUTHOR]

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 datasets 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 and UCATS. 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 tropics and in the southern hemisphere. Observations are compared to results from the two-dimensional AGAGE 12-box model. Based on 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. [ABSTRACT FROM AUTHOR]

Ozone trend estimates have shown large uncertainties in the upper troposphere–lower stratosphere (UTLS) region despite multi-decadal observations available from ground-based, balloon, aircraft, and satellite platforms. These uncertainties arise from large natural variability driven by dynamics (reflected in tropopause and jet variations) as well as the strength in constituent transport and mixing. Additionally, despite all the community efforts there is still a lack of representative high-quality global UTLS measurements to capture this variability. The Stratosphere-troposphere Processes And their Role in Climate (SPARC) Observed Composition Trends and Variability in the UTLS (OCTAV-UTLS) activity aims to reduce uncertainties in UTLS composition trend estimates by accounting for this dynamically induced variability. In this paper, we describe the production of dynamical diagnostics using meteorological information from reanalysis fields that facilitate mapping observations from several platforms into numerous geophysically based coordinates (including tropopause and upper tropospheric jet relative coordinates). Suitable coordinates should increase the homogeneity of the air masses analyzed together, thus reducing the uncertainty caused by spatiotemporal sampling biases in the quantification of UTLS composition trends. This approach thus provides a framework for comparing measurements with diverse sampling patterns and leverages the meteorological context to derive maximum information on UTLS composition and trends and its relationships to dynamical variability. The dynamical diagnostics presented here are the first comprehensive set describing the meteorological context for multi-decadal observations by ozonesondes, lidar, aircraft, and satellite measurements in order to study the impact of dynamical processes on observed UTLS trends by different sensors on different platforms. Examples using these diagnostics to map multi-platform datasets into different geophysically based coordinate systems are provided. The diagnostics presented can also be applied to analysis of greenhouse gases other than ozone that are relevant to surface climate and UTLS chemistry. [ABSTRACT FROM AUTHOR]

Evaluating the global chemistry models in the upper troposphere–lower stratosphere (UTLS) is an important step toward a further understanding of its chemical composition. The latter is regularly sampled through in situ measurements based on passenger aircraft, in the framework of the In-service Aircraft for a Global Observing System (IAGOS) research infrastructure. This study focuses on the comparison of the IAGOS measurements in ozone, carbon monoxide (CO), nitrogen reactive species (NOy) and water vapour, with a 25-year simulation output from the LMDZ-OR-INCA chemistry-climate model. For this purpose, we present and apply an extension of the Interpol-IAGOS software that projects the IAGOS data onto any model grid, in order to derive a gridded IAGOS product and a masked model product that are directly comparable to one another. Climatologies are calculated in the upper troposphere (UT) and in the lower stratosphere (LS) separately, but also in the UTLS as a whole, as a demonstration for the models that do not sort out the physical variables necessary to distinguish between the UT and the LS. In the northern extratropics, the comparison in the UTLS layer suggests that the geographical distribution in the tropopause height is well reproduced by the model. In the separated layers, the model simulates well the water vapour climatologies in the UT, and the ozone climatologies in the LS. The opposite biases in CO in both UT and LS suggest that the cross-tropopause transport is overestimated. The NOy observations highlight the difficulty of the model in parameterizing the lightning emissions. In the tropics, the upper-tropospheric climatologies are remarkably well simulated for water vapour, as the observed CO peaks due to biomass burning in the most convective systems, and the ozone latitudinal variations. Ozone is more sensitive to lightning emissions than to biomass burning emissions, whereas the CO sensitivity to biomass burning emissions strongly depends on the location and on the season. Through this evaluation, the present study demonstrates that the Interpol-IAGOS software is a tool facilitating the assessment of the global model simulations in the UTLS, potentially useful for any modelling experiment involving chemistry-climate and chemistry-transport models. [ABSTRACT FROM AUTHOR]