Oxidation of bromide in aqueous environments initiates the formation of molecular halogen compounds, which is important for the global tropospheric ozone budget. In the aqueous bulk, oxidation of bromide by ozone involves a [Br•OOO−] complex as intermediate. Here we report liquid jet X-ray photoelectron spectroscopy measurements that provide direct experimental evidence for the ozonide and establish its propensity for the solution-vapour interface. Theoretical calculations support these findings, showing that water stabilizes the ozonide and lowers the energy of the transition state at neutral pH. Kinetic experiments confirm the dominance of the heterogeneous oxidation route established by this precursor at low, atmospherically relevant ozone concentrations. Taken together, our results provide a strong case of different reaction kinetics and mechanisms of reactions occurring at the aqueous phase-vapour interface compared with the bulk aqueous phase.

Ageing of particulate organic matter affects the composition and properties of atmospheric aerosol particles. Driven by temperature and humidity, the organic fraction can vary its physical state between liquid and amorphous solid, or rarely even crystalline. These transitions can influence the reaction kinetics due to limitations of mass transport in such (semi-) solid states, which in turn may influence the chemical ageing of particles containing such compounds. We have used coated wall flow tube experiments to investigate the reaction kinetics of the ozonolysis of shikimic acid, which serves as a proxy for oxygenated, water-soluble organic matter and can form a glass at room temperature. Particular attention was paid to how the presence of water influences the reaction, since it acts a plasticiser and thereby induces changes in the physical state. We analysed the results by means of a traditional resistor model, which assumes steady-state conditions. The ozonolysis rate of shikimic acid is strongly increased in the presence of water, a fact we attribute to the increased transport of O-3 and shikimic acid through the condensed phase at lower viscosities. The analysis using the resistor model suggests that the system undergoes both surface and bulk reaction. The second-order rate coefficient of the bulk reaction is 3.7 (+1.5/-3.2) x 10(3) L mol(-1) s(-1). At low humidity and long timescales, the resistor model fails to describe the measurements appropriately. The persistent O-3 uptake at very low humidity suggests contribution of a self-reaction of O-3 on the surface

Abstract. Future sea ice retreat in the Arctic in summer and autumn is expected to affect both natural and anthropogenic aerosol emissions: sea ice acts as a barrier between the ocean and the atmosphere, and reducing it increases dimethyl sulfide and sea salt emissions. Additionally, a decrease in the area and thickness of sea ice could lead to enhanced Arctic ship traffic, for example due to shorter routes of cargo ships. Changes in the emissions of aerosol particles can then influence cloud properties, precipitation, surface albedo, and radiation. Next to changes in aerosol emissions, clouds will also be affected by increases in Arctic temperatures and humidities. In this study, we quantify how future aerosol radiative forcings and cloud radiative effects might change in the Arctic in late summer (July–August) and early autumn (September–October). Simulations were conducted for the years 2004 and 2050 with the global aerosol–climate model ECHAM6-HAM2. For 2050, simulations with and without additional ship emissions in the Arctic were carried out to quantify the impact of these emissions on the Arctic climate. In the future, sea salt as well as dimethyl sulfide emissions and burdens will increase in the Arctic. The increase in cloud condensation nuclei, which is due to changes in aerosol particles and meteorology, will enhance cloud droplet number concentrations over the Arctic Ocean (+10 % in late summer and +29 % in early autumn; in-cloud values averaged between 75 and 90∘ N). Furthermore, both liquid and total water path will increase (+10 % and +8 % in late summer; +34 % and +26 % in early autumn) since the specific humidity will be enhanced due to higher temperatures and the exposure of the ocean's surface. Changes in both aerosol radiative forcings and cloud radiative effects at the top of the atmosphere will not be dominated by the aerosol particles and clouds themselves but by the decrease in surface albedo (and by the increase in surface temperature for the longwave cloud radiative effect in early autumn). Mainly due to the reduction in sea ice, the aerosol radiative forcing will become less positive (decreasing from 0.53 to 0.36 W m−2 in late summer and from 0.15 to 0.11 W m−2 in early autumn). The decrease in sea ice is also mainly responsible for changes in the net cloud radiative effect, which will become more negative in late summer (changing from −36 to −46 W m−2). Therefore, the cooling component of both aerosols and clouds will gain importance in the future. We found that future Arctic ship emissions related to transport and oil and gas extraction (Peters et al., 2011) will not have a large impact on clouds and radiation: changes in aerosols only become significant when we increase these ship emissions by a factor of 10. However, even with 10-fold ship emissions, the net aerosol radiative forcing shows no significant changes. Enhanced black carbon deposition on snow leads to a locally significant but very small increase in radiative forcing over the central Arctic Ocean in early autumn (no significant increase for average between 75 and 90∘ N). Furthermore, the 10-fold higher ship emissions increase the optical thickness and lifetime of clouds in late summer (net cloud radiative effect changing from −48 to −52 W m−2). These aerosol–cloud effects have a considerably larger influence on the radiative forcing than the direct effects of particles (both aerosol particles in the atmosphere and particles deposited on snow). In summary, future ship emissions of aerosols and their precursor gases might have a net cooling effect, which is small compared to other changes in future Arctic climate such as those caused by the decrease in surface albedo.

Abstract. Future sea ice retreat in the Arctic in summer and autumn is expected to affect both natural and anthropogenic aerosol emissions: sea ice acts as a barrier between the ocean and the atmosphere, and reducing it increases dimethyl sulphide and sea salt emissions. A decrease in the area and thickness of sea ice could in addition lead to enhanced Arctic ship traffic, e.g. to shorten the paths of cargo ships. Changes in the emissions of aerosol particles can then influence cloud properties, precipitation, surface albedo, and radiation. Next to changes in aerosol particles, clouds will also be affected by increases in 5 Arctic temperatures and humidities. In this study, we quantified how future aerosol radiative forcing, aerosol-cloud interactions, and cloud radiative effects might change in the Arctic in late summer (July/August) and early autumn (September/October).Simulations were conducted for the years 2004 and 2050 with the global aerosol-climate model ECHAM6-HAM2. In 2050, simulations with and without additional ship emissions in the Arctic were carried out to quantify the impact of these emissions on the Arctic climate. 10We found that aerosol number concentrations in the Arctic will generally increase in the future due to enhanced emissions of sea salt as well as dimethyl sulphide. The increase in cloud condensation nuclei will enhance cloud droplet number concentrations over the Arctic Ocean. Furthermore, both liquid and total water content will increase since the specific humidity will be enhanced due to higher temperatures and the exposure of the ocean's surface.Changes in both aerosol radiative forcings and cloud radiative effects at the top of the atmosphere will not be dominated by 15 the aerosol particles and clouds themselves but by the decrease in surface albedo (and by the increase in surface temperature for the longwave cloud radiative effect). Due to the reduction in sea ice, the aerosol radiative forcing will become less positive and the cloud radiative effect more negative, i.e. the cooling component of both will gain importance in the future.We found that future Arctic ship emissions related to transport and oil/gas extraction (Peters et al., 2011, ACP) will not have a large impact on clouds and radiation: changes in aerosol concentrations only become significant when we increase 20 these ship emissions by a factor of ten. The net aerosol radiative forcing shows only small, non-significant changes. Enhanced black carbon deposition on snow leads to a significant but very small warming over the central Arctic Ocean in early autumn.Furthermore, the tenfold higher ship emissions increase the optical thickness of low clouds and thus induce a small Twomey effect (cooling) in late summer. This Twomey effect has a considerably larger influence on temperature than the direct effect of particles (both aerosol particles in the atmosphere and particles deposited on snow), but it is more uncertain because of the 25 1 Atmos. Chem. Phys. Discuss., https://doi

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<p><strong>Abstract.</strong> Microscopic charcoal particles are fire-specific tracers, which are ubiquitous in natural archives such as lake sediments or ice cores. Thus, charcoal records from lake sediments are nowadays the primary source for reconstructing past fire activity. Microscopic charcoal particles are generated during forest and grassland fires and can be transported over large distances before being deposited into natural archives. In this paper, we implement microscopic charcoal particles into a global aerosol-climate model to better understand the transport of charcoal on the large scale. Atmospheric transport as well as interactions with other aerosol particles, clouds, and radiation are explicitly simulated. To estimate the emissions of the microscopic charcoal particles, we use recent European charcoal observations from lake sediments as a calibration dataset. We found that scaling black carbon fire emissions from the Global Fire Assimilation System (a satellite-based emission inventory) by a factor of ≈ 40 matches the calibration dataset best. The charcoal validation dataset, for which we collected charcoal observations from all over the globe, generally supports this scaling factor. In the validation dataset, we included charcoal particles from lake sediments, peats, and ice cores. The correlation coefficients for both the calibration and the validation dataset are positive and significantly different from zero, showing that the model captures a significant portion of the spatial variability. However, the model fails to reproduce the extreme spatial variability observed in the charcoal data. This can mainly be explained by the coarse spatial resolution of the model and uncertainties concerning fire emissions. Furthermore, charcoal fluxes derived from ice core sites are much lower than the simulated fluxes, which can be explained by the location properties (high altitude and steep topography, which are not well represented in the model) of most of the investigated ice cores. Global modelling of charcoal can improve our understanding of the representativeness of this fire proxy. Furthermore, it might allow to quantitatively validate past fire emissions provided by fire models. This might deepen our understanding of the processes driving global fire activity.</p>

Abstract. Microscopic charcoal particles are fire-specific tracers, which are ubiquitous in natural archives such as lake sediments or ice cores. Thus, charcoal records from lake sediments have become the primary source for reconstructing past fire activity. Microscopic charcoal particles are generated during forest and grassland fires and can be transported over large distances before being deposited into natural archives. In this paper, we implement microscopic charcoal particles into a global aerosol–climate model to better understand the transport of charcoal on a large scale. Atmospheric transport and interactions with other aerosol particles, clouds, and radiation are explicitly simulated. To estimate the emissions of the microscopic charcoal particles, we use recent European charcoal observations from lake sediments as a calibration data set. We found that scaling black carbon fire emissions from the Global Fire Assimilation System (a satellite-based emission inventory) by approximately 2 orders of magnitude matches the calibration data set best. The charcoal validation data set, for which we collected charcoal observations from all over the globe, generally supports this scaling factor. In the validation data set, we included charcoal particles from lake sediments, peats, and ice cores. While only the Spearman rank correlation coefficient is significant for the calibration data set (0.67), both the Pearson and the Spearman rank correlation coefficients are positive and significantly different from zero for the validation data set (0.59 and 0.48, respectively). Overall, the model captures a significant portion of the spatial variability, but it fails to reproduce the extreme spatial variability observed in the charcoal data. This can mainly be explained by the coarse spatial resolution of the model and uncertainties concerning fire emissions. Furthermore, charcoal fluxes derived from ice core sites are much lower than the simulated fluxes, which can be explained by the location properties (high altitude and steep topography, which are not well represented in the model) of most of the investigated ice cores. Global modelling of charcoal can improve our understanding of the representativeness of this fire proxy. Furthermore, it might allow past fire emissions provided by fire models to be quantitatively validated. This might deepen our understanding of the processes driving global fire activity.