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Oceans emit large quantities of dimethyl sulfide (DMS) to the marine atmosphere. The oxidation of DMS leads to the formation and growth of cloud condensation nuclei (CCN) with consequent effects on Earth's radiation balance and climate. The quantitative assessment of the impact of DMS emissions on CCN concentrations necessitates a detailed description of the oxidation of DMS in the presence of existing aerosol particles and clouds. In the unpolluted marine atmosphere, DMS is efficiently oxidized to hydroperoxymethyl thioformate (HPMTF), a stable intermediate in the chemical trajectory toward sulfur dioxide (SO2) and ultimately sulfate aerosol. Using direct airborne flux measurements, we demonstrate that the irreversible loss of HPMTF to clouds in the marine boundary layer determines the HPMTF lifetime (τHPMTF < 2 h) and terminates DMS oxidation to SO2 When accounting for HPMTF cloud loss in a global chemical transport model, we show that SO2 production from DMS is reduced by 35% globally and near-surface (0 to 3 km) SO2 concentrations over the ocean are lowered by 24%. This large, previously unconsidered loss process for volatile sulfur accelerates the timescale for the conversion of DMS to sulfate while limiting new particle formation in the marine atmosphere and changing the dynamics of aerosol growth. This loss process potentially reduces the spatial scale over which DMS emissions contribute to aerosol production and growth and weakens the link between DMS emission and marine CCN production with subsequent implications for cloud formation, radiative forcing, and climate.

Dimethyl sulfide (DMS), emitted from the oceans, is the most abundant biological source of sulfur to the marine atmosphere. Atmospheric DMS is oxidized to condensable products that form secondary aerosols that affect Earth's radiative balance by scattering solar radiation and serving as cloud condensation nuclei. We report the atmospheric discovery of a previously unquantified DMS oxidation product, hydroperoxymethyl thioformate (HPMTF, HOOCH2SCHO), identified through global-scale airborne observations that demonstrate it to be a major reservoir of marine sulfur. Observationally constrained model results show that more than 30% of oceanic DMS emitted to the atmosphere forms HPMTF. Coincident particle measurements suggest a strong link between HPMTF concentration and new particle formation and growth. Analyses of these observations show that HPMTF chemistry must be included in atmospheric models to improve representation of key linkages between the biogeochemistry of the ocean, marine aerosol formation and growth, and their combined effects on climate.

We report airborne measurements of acetaldehyde (CH3CHO) during the first and second deployments of the National Aeronautics and Space Administration (NASA) Atmospheric Tomography Mission (ATom). The budget of CH3CHO is examined using the Community Atmospheric Model with chemistry (CAM-chem), with a newly-developed online air-sea exchange module. The upper limit of the global ocean net emission of CH3CHO is estimated to be 34 Tg a-1 (42 Tg a-1 if considering bubble-mediated transfer), and the ocean impacts on tropospheric CH3CHO are mostly confined to the marine boundary layer. Our analysis suggests that there is an unaccounted CH3CHO source in the remote troposphere and that organic aerosols can only provide a fraction of this missing source. We propose that peroxyacetic acid (PAA) is an ideal indicator of the rapid CH3CHO production in the remote troposphere. The higher-than-expected CH3CHO measurements represent a missing sink of hydroxyl radicals (and halogen radical) in current chemistry-climate models.

Coarse mineral dust particles have been observed much further from the Sahara than expected based on theory. They have impacts different to finer particles on Earth's radiative budget, as well as carbon and hydrological cycles, though they tend to be under-represented in climate models. We use measurements of the full dust size distribution from aircraft campaigns over the Sahara, Canaries, Cabo Verde and Caribbean. We assess the observed and modelled dust size distribution over long-range transport at high vertical resolution using the Met Office Unified Model, which represents dust up to 63.2 µm diameter, greater than most climate models. We show that the model generally replicates the vertical distribution of the total dust mass but transports larger dust particles too low in the atmosphere. Importantly, coarse particles in the model are deposited too quickly, resulting in an underestimation of dust mass that is exacerbated with westwards transport; the 20–63 µm dust mass contribution between 2 and 3.7 km altitude is underestimated by factors of up to 11 in the Sahara, 140 in the Canaries and 240 in Cabo Verde. In the Caribbean, there is negligible modelled contribution of d > 20 µm particles to total mass, compared to 10 % in the observations. This work adds to the growing body of research that demonstrates the need for a process-based evaluation of climate model dust simulations to identify where improvements could be implemented. [ABSTRACT FROM AUTHOR]

Atmospheric aerosol particles have a profound impact on Earth's climate by scattering and absorbing solar and terrestrial radiation and by impacting the properties of clouds. Research aircraft such as the Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR) Falcon are widely used to study aerosol particles in the troposphere and lower stratosphere. However, transporting a representative sample to the instrumentation inside the aircraft remains a challenge due to high airspeeds and changing ambient conditions. In particular, for high-quality coarse-mode aerosol measurements, knowledge about losses or enhancements in the aerosol sampling system is crucial. In this study, the sampling efficiency of the aerosol inlet aboard the Falcon research aircraft is characterized for the first time with state-of-the art in situ measurements including sizing instruments operated behind the Falcon aerosol inlet and mounted at the aircraft wing not affected by the aerosol inlet. Sampling efficiencies were derived for different true airspeed ranges by comparing the in-cabin and "full"-size-range particle number size distributions during 174 flight sequences with a major contribution of mineral dust particles during the "Absorbing aerosol layers in a changing climate: aging, lifetime and dynamics" project (A-LIFE). Additionally, experimentally derived Stokes numbers were used to calculate the cutoff diameter of the aerosol sampling system for different particle densities as a function of true airspeed. As expected, the results show that the velocity of the research aircraft has a major impact on the sampling of coarse-mode aerosol particles with in-cabin instruments. For true airspeeds up to about 190 ms-1 , aerosol particles larger than about 1 µm are depleted in the sampling system of the Falcon during the A-LIFE project. In contrast, for true airspeeds higher than 190 ms-1 , an enhancement of particles up to a diameter of 4 µm is observed. For even larger particles, the enhancement effect at the inlet is still present, but inertial and gravitational particle losses in the transport system get more and more pronounced, which leads to a decreasing overall sampling efficiency. In summary, aerosol particles are either depleted or enhanced in the Falcon aerosol inlet, whereas transport in sampling lines always leads to a loss of particles. Here, we have considered both effects and determined the cutoff diameter for the A-LIFE transport system (i.e., the sampling lines only), the cutoff diameter of the Falcon aerosol inlet (i.e., the effect of the inlet only), and the combined effect of the inlet and sampling lines. [ABSTRACT FROM AUTHOR]

Uncertainties in the mineral dust's direct radiative effect arise from the variability in its optical properties. The optical properties can also be influenced by mixing processes with anthropogenic aerosols, such as black carbon or fine particles (called 'pollution' in this study). We aimed to investigate the effect of mixing pollution with mineral dust aerosols from different source regions on the intensive aerosol optical properties. Thus, the Ångström exponents of scattering and absorption (i.e., their wavelength dependence), the single scattering albedo, and the asymmetry parameter were determined from direct optical measurements performed during the A-LIFE aircraft field experiment over the Eastern Mediterranean. This location provided access to Arabian and Saharan dust layers mixed with pollution. Our findings indicated significant changes in all the intensive aerosol optical properties with increasing pollution content within mineral dust layers. Interestingly, the differences between Arabian and Saharan dust's intensive aerosol optical properties were negligible. We discussed the implications of these results for identifying mineral dust events and for their direct radiative effect. First, the mixing with pollution masked the mineral dust signal, suggesting that caution is needed when using the Ångström exponents for identifying mineral dust events. However, the Ångström exponents can help estimate the amount of pollution once a mineral dust event is confirmed. Second, our measurements of the asymmetry parameter and single scattering albedo changed from pure to polluted mineral dust layers (e.g., at 525 nm, the median values decreased from 0.67 to 0.56 and from 0.96 to 0.89, respectively). These changes have 15 opposing effects on the short-wave direct radiative effect efficiency (i.e., the direct radiative effect per unit of aerosol optical depth) and may partly cancel out each other. Nevertheless, the impact of mixing with pollution on the mineral dust's direct radiative effect efficiency can differ depending on the surface albedo. In conclusion, accurate quantification of the pollution content within mineral dust layers is crucial. The pollution significantly impacts mineral dust event identification, its optical properties, and the local direct radiative effect. [ABSTRACT FROM AUTHOR]

Cloud condensation nuclei (CCN) can affect cloud properties and therefore Earth's radiative balance.sup.1-3. New particle formation (NPF) from condensable vapours in the free troposphere has been suggested to contribute to CCN, especially in remote, pristine atmospheric regions.sup.4, but direct evidence is sparse, and the magnitude of this contribution is uncertain.sup.5-7. Here we use in situ aircraft measurements of vertical profiles of aerosol size distributions to present a global-scale survey of NPF occurrence. We observe intense NPF at high altitudes in tropical convective regions over both Pacific and Atlantic oceans. Together with the results of chemical-transport models, our findings indicate that NPF persists at all longitudes as a global-scale band in the tropical upper troposphere, covering about 40 per cent of Earth's surface. Furthermore, we find that this NPF in the tropical upper troposphere is a globally important source of CCN in the lower troposphere, where CCN can affect cloud properties. Our findings suggest that the production of CCN as new particles descend towards the surface is not adequately captured in global models, which tend to underestimate both the magnitude of tropical upper tropospheric NPF and the subsequent growth of new particles to CCN sizes. Widespread formation of new particles from condensable vapours observed in the tropical upper troposphere is an important source of cloud condensation nuclei in the lower troposphere, affecting cloud properties., Author(s): Christina J. Williamson [sup.1] [sup.2] , Agnieszka Kupc [sup.2] [sup.3] , Duncan Axisa [sup.4] [sup.9] , Kelsey R. Bilsback [sup.5] , ThaoPaul Bui [sup.6] , Pedro Campuzano-Jost [sup.1] [sup.7] [...]

Heterogeneous chemical cycles of pyrogenic nitrogen and halides influence tropospheric ozone and affect the stratosphere during extreme Pyrocumulonimbus (PyroCB) events. We report field‐derived N2O5 uptake coefficients, γ(N2O5), and ClNO2 yields, φ(ClNO2), from two aircraft campaigns observing fresh smoke in the lower and mid troposphere and processed/aged smoke in the upper troposphere and lower stratosphere (UTLS). Derived φ(ClNO2) varied across the full 0–1 range but was typically <0.5 and smallest in a PyroCB (<0.05). Derived γ(N2O5) was low in agricultural smoke (0.2–3.6 × 10−3), extremely low in mid‐tropospheric wildfire smoke (0.1 × 10−3), but larger in PyroCB processed smoke (0.7–5.0 × 10−3). Aged biomass burning aerosol in the UTLS had a higher γ(N2O5) of 17 × 10−3 that increased with sulfate and liquid water, but that was 1–2 orders of magnitude lower than values for aqueous sulfuric aerosol used in stratospheric models. Plain Language Summary: The injection of reactive material into Earth's atmosphere from fires affects atmospheric composition at regional and hemispheric scales. Reported stratospheric ozone depletion during extreme events, such as the 2020 Australian wildfires, illustrates one example of fire impacts and the role of heterogeneous (gas‐particle) processes. We report field quantification of rates and product yields from airborne observations of smoke. Extremely slow heterogeneous reaction rates on young smoke increase with transport and aging, but upper atmospheric values are still a factor of 10 slower than parameterizations used in stratospheric models. Heterogeneous production of ClNO2, a major lower atmospheric chlorine activation pathway, may be active on biomass burning aerosol in the upper atmosphere. Key Points: ClNO2 formation is active on biomass burning (BB) particles but decreases with transport to the upper troposphere and lower stratosphere (UTLS)N2O5 uptake coefficients are low on young BB smoke and increase with transport through a PyroCB and UTLS agingN2O5 uptake coefficients on aged BB particles in the UTLS are significantly lower than those used in model parameterizations [ABSTRACT FROM AUTHOR]

Aerosols are key players in the Earth's climate system with mineral dust being one major component of the atmospheric aerosol load. While former campaigns focused on investigating the properties and effects of rather pure mineral dust layers, the A-LIFE (A bsorbing aerosol layers in a changing climate: aging, life time and dynamics) campaign in April 2017 aimed to characterize dust in complex aerosol mixtures. In this study we present ground-based lidar measurements that were performed at Limassol, Cyprus, in April 2017. During our measurement period, the measurement site was affected by complex mixtures of dust from different sources and pollution aerosols from local sources as well as long-range transported. We found mean values of the particle linear depolarization ratio and extinction-to-backscatter ratio (lidar ratio) of 0.27 ± 0.02 and 41 sr ± 5 sr at 355 nm and of 0.30 ± 0.02 and 39 sr ± 5 sr at 532 nm for Arabian dust, and of 0.27 ± 0.02 and 55 sr ± 8 sr at 355 nm and of 0.28 ± 0.02 and 53 sr ± 7 sr at 532 nm for Saharan dust. The values found for pollution aerosols of the particle linear depolarization ratio and the lidar ratio are 0.05 ± 0.02 at 355 nm and 0.04 ± 0.02 at 532 nm, and 65 sr ± 12 sr at 355 nm and 60 sr ± 16 sr at 532 nm, respectively. We use our measurements for aerosol typing and compare that to aerosol typing from sun photometer data, in-situ measurements and trajectory analysis. The different methods agree well for the derived aerosol type, but looking at the derived dust mass concentration from different methods, the trajectory analysis frequently underestimate high dust concentration that were found in major mineral dust events. [ABSTRACT FROM AUTHOR]

Vertical profiles of number concentrations of dust particles relevant for ice nucleation in clouds are derived from lidar measurements. The results are compared to coincidental airborne in-situ measurements of particle number and surface area concentrations in the dust layer. The observations were performed in long- range transported Saharan dust at Barbados and Asian dust at Cyprus. The Asian dust data analysis is ongoing. A comparison of Asian and Saharan dust will be given at the conference. Concentrations of ice nucleating particles in the order of 10 to 1000 per cm-3 in the dust layer are derived for a temperature of -25°C at Barbados. The method can be used to continuously monitor the concentration of ice nucleating dust particles vertically resolved from lidar measurements.

Atmospheric aerosol particles have a profound impact on Earth's climate by scattering and absorbing solar and terrestrial radiation and by impacting the properties of clouds. Research aircraft such as the Deutsches Zentrum für Luft-und Raumfahrt e.V. (DLR) Falcon are widely used to study aerosol particles in the troposphere and lower stratosphere. However, transporting representative sample to the instrumentation inside the aircraft remains a challenge due to high airspeeds and changing ambient conditions. In particular, for high-quality coarse mode aerosol measurements, knowledge about losses or enhancements in the aerosol sampling system is crucial. In this study, we present a detailed characterization of the Falcon aerosol sampling system. Aerosol number size distributions were measured during the A-LIFE field campaign simultaneously with in-cabin and out-cabin/wing-mounted instrumentation. Sampling efficiencies were derived for different true airspeed ranges by comparing the in-cabin and the out-cabin particle number size distributions during flight sequences with a major contribution of mineral dust particles in the coarse mode size range. Additionally, experimentally derived Stokes numbers were used to calculate the cut-off diameter of the A-LIFE aerosol sampling system for different particle densities as a function of true airspeed. The results show that the velocity of the research aircraft has a major impact on the sampling of coarse mode aerosol particles with in-cabin instruments. For true airspeeds up to about 190 m s-1, aerosol particles larger than about 1 µm are depleted in the sampling system of the Falcon during the A-LIFE project. In contrast, for true airspeeds higher than 190 m 1, an enhancement of particles up to a diameter of 4 µm is observed. For even larger particles, the enhancement effect at the inlet is still present, but inertial and gravitational particle losses in the transport system get more and more pronounced which leads to a decreasing overall sampling efficiency. In summary, aerosol particles can either be depleted or enhanced at an aerosol inlet, whereas transport in sampling lines always leads to a loss of particles. Therefore, it is important to consider both, inlet and transport efficiency, when quantifying the sampling efficiency of an aerosol sampling system. [ABSTRACT FROM AUTHOR]

Fires emit sufficient sulfur to affect local and regional air quality and climate. This study analyzes SO2 emission factors and variability in smoke plumes from US wild and agricultural fires, and their relationship to sulfate and hydroxymethanesulfonate (HMS) formation. Observed SO2 emission factors for various fuel types show good agreement with the latest reviews of biomass burning emission factors, producing an emission factor range of 0.47–1.2 g SO2 kg-1 C in the emissions. These emission factors vary with geographic location in a way that suggests that deposition of coal burning emissions and application of sulfur-containing fertilizers likely play a role in the larger observed values, which are primarily associated with agricultural burning. A 0-D box model generally reproduces the observed trends of SO2 and total sulfate (inorganic + organic) in aging wildfire plumes. In many cases, modeled HMS is consistent with the observed organosulfur concentrations. However, a comparison of observed organosulfur and modeled HMS suggests that multiple organosulfur compounds are likely responsible for the observations, but that the chemistry of these compounds yield similar production and loss rates to that of HMS, resulting in good agreement with the modeled results. We provide suggestions for constraining the organosulfur compounds observed during these flights and we show that the chemistry of HMS can allow for organosulfur to act as a S(IV) reservoir under conditions of increased pH (>6) and liquid water content (>10-7 g m-3). This can facilitate long-range transport of sulfur emissions resulting in increased SO2 and eventually sulfate in transported smoke. [ABSTRACT FROM AUTHOR]