Abstract.Organosulfates are important organosulfur compounds present in atmospheric particles. While the abundance, 10 composition, and formation mechanisms of organosulfates have been extensively investigated, it remains unclear how they transform and evolve throughout their atmospheric lifetime. To acquire a fundamental understanding of how organosulfates chemically transform in the atmosphere, this work investigates the heterogeneous OH radical-initiated oxidation of sodium methyl sulfate (CH 3 SO 4 Na) droplets, the smallest organosulfate detected in atmospheric particles, using an aerosol flow tube reactor at a high relative humidity of 85 %. Aerosol mass spectra measured by a soft atmospheric pressure ionization source 15 (Direct Analysis in Real Time, DART) coupled with a high-resolution mass spectrometer showed that neither functionalization nor fragmentation products are detected. Instead, the ion signal intensity of the bisulfate ion (HSO 4 − ) has been found to increase significantly after OH oxidation. We postulate that sodium methyl sulfate tends to fragment into a formaldehyde (CH 2 O) and a sulfate radical anion (SO 4•− ) upon OH oxidation. The formaldehyde is likely partitioned back to the gas phase due to its high volatility. The sulfate radical anion, similar to OH radical, can abstract a hydrogen atom from 20 neighboring sodium methyl sulfate to form the bisulfate ion, contributing to the secondary chemistry. Kinetic measurements show that the heterogeneous OH reaction rate constant, k, is (3.79 ± 0.19) × 10 -13 cm 3 molecule -1 s -1 with an effective OH uptake coefficient, γ eff , of 0.17 ± 0.03. While about 40 % of sodium methyl sulfate is being oxidized at the maximum OH exposure (1.27 × 10 12 molecule cm −3 s), only a 3 % decrease in particle diameter is observed. This can be attributed to a small fraction of particle mass lost via the formation and volatilization of formaldehyde. Overall, we firstly demonstrate that 25 the heterogeneous OH oxidation of an organosulfate can lead to the formation of sulfate radical anion and produce inorganic sulfate. Fragmentation processes and sulfate radical anion chemistry play a key role in determining the compositional evolution of sodium methyl sulfate during heterogeneous OH oxidation. 35Atmos. Chem. Phys. Discuss., https://doi

Standard climate projections represent future volcanic eruptions by a constant forcing inferred from 1850 to 2014 volcanic forcing. Using the latest ice‐core and satellite records to design stochastic eruption scenarios, we show that there is a 95% probability that explosive eruptions could emit more sulfur dioxide (SO2) into the stratosphere over 2015–2100 than current standard climate projections (i.e., ScenarioMIP). Our simulations using the UK Earth System Model with interactive stratospheric aerosols show that for a median future eruption scenario, the 2015–2100 average global‐mean stratospheric aerosol optical depth (SAOD) is double that used in ScenarioMIP, with small‐magnitude eruptions (<3 Tg of SO2) contributing 50% to SAOD perturbations. We show that volcanic effects on large‐scale climate indicators, including global surface temperature, sea level and sea ice extent, are underestimated in ScenarioMIP because current climate projections do not fully account for the recurrent frequency of volcanic eruptions of different magnitudes.

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&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; Organic compounds present at/near the surface of aqueous droplets can be efficiently oxidized by gas-phase OH radicals, which alter the molecular distribution of the reaction products within the droplet. A change in aerosol composition affects the hygroscopicity and leads to a concomitant response in the equilibrium amount of particle phase water. The variation in the aerosol water content affects the aerosol size and physicochemical properties, which in turn governs the oxidation kinetics and chemistry. To attain better knowledge of the compositional evolution of aqueous organic droplets during oxidation, this work investigates the heterogeneous OH radical initiated oxidation of aqueous methylsuccinic acid (C&lt;sub&gt;5&lt;/sub&gt;H&lt;sub&gt;8&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;) droplets, a model compound for small branched dicarboxylic acids found in atmospheric aerosols, at a high relative humidity of 85&amp;#8201;% through experimental and modelling approaches. Aerosol mass spectra measured by a soft atmospheric pressure ionization source (Direct Analysis in Real Time, DART) coupled with a high-resolution mass spectrometer reveal two major products: a five carbon atom (C&lt;sub&gt;5&lt;/sub&gt;) hydroxyl functionalization product (C&lt;sub&gt;5&lt;/sub&gt;H&lt;sub&gt;8&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt;) and a C&lt;sub&gt;4&lt;/sub&gt; fragmentation product (C&lt;sub&gt;4&lt;/sub&gt;H&lt;sub&gt;6&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;). These two products likely originate from the formation and subsequent reactions (intermolecular hydrogen abstraction and carbon&amp;#8211;carbon bond scission) of tertiary alkoxy radicals resulting from the OH-abstraction occurring at the methyl-substituted carbon site. Based on the identification of the reaction products, a kinetic model of oxidation (a two-product model) coupled with the Aerosol Inorganic Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model is built to simulate the size and compositional changes of aqueous methylsuccinic acid droplets during oxidation. Model results show that at the maximum OH exposure, the droplets become slightly more hygroscopic after oxidation, as the mass fraction of water predicted to increase from 0.362 to 0.424; however, the diameter of the droplets decreases by 6.1&amp;#8201;%. This can be attributed to the formation of volatile fragmentation products that partition to the gas phase, leading to a net loss of organic species and associated particle phase water, and thus a smaller droplet size. Overall, fragmentation and volatilization processes play a larger role than the functionalization process in determining the evolution of aerosol water content and droplet size at high oxidation stages.&lt;/p&gt;

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&lt;p&gt;Large explosive volcanic eruptions can induce global climate impacts on decadal to multi-decadal timescales. In current climate models, future volcanic eruptions are represented in terms of a time-averaged volcanic forcing that ignores the sporadic nature of volcanic eruptions. This conventional representation does not account for how climate change might affect the dynamics of volcanic plumes and the stratospheric sulfate aerosol lifecycle and, ultimately, volcanic radiative forcing. To account for these climate-volcano feedbacks in climate projections, we perform model simulations from 2015 to 2100 with two key innovations: (1) a stochastic resampling approach to generate realistic future eruption scenarios based on historical volcanic eruptions recorded by ice cores and satellites in the past 11,500 years; and (2) a new modelling framework, UKESM-VPLUME, which couples a 1-D eruptive plume model (Plumeria) with an Earth System Model (UKESM) to consider the impacts of changing atmospheric conditions on eruptive plume heights. Our results show that considering sporadic small-magnitude volcanic eruptions in a future warming scenario can lead to a noticeable difference in global surface temperatures as well as on the time at which temperatures exceed 1.5&amp;#176;C above pre-industrial levels. Our study highlights the importance of considering sporadic eruptions and the changes in eruptive plume heights in a future warmer climate. &amp;#160;The UKESM-VPLUME model framework enables us to quantify the impacts of climate change on volcanic radiative forcing in an Earth System model, which in future research allows us to better evaluate the climate impacts of volcanic eruptions under global warming.&lt;/p&gt;

Standard climate projections represent future volcanic eruptions by a constant forcing inferred from 1850-2014 volcanic forcing. Using the latest ice-core and satellite records to design stochastic eruption scenarios, we show that there is a 95% probability that explosive eruptions could emit more sulfur dioxide (SO) into the stratosphere over 2015-2100 than current standard climate projections (i.e., ScenarioMIP). Our simulations using the UK Earth System Model with interactive stratospheric aerosols show that for a median future eruption scenario, the 2015-2100 average global-mean stratospheric aerosol optical depth (SAOD) is double that used in ScenarioMIP, with small-magnitude eruptions (< 3 Tg of SO) contributing 50% to SAOD perturbations. We show that volcanic effects on large-scale climate indicators, including global surface temperature, sea level and sea ice extent, are underestimated in ScenarioMIP because current climate projections do not fully account for the recurrent frequency of volcanic eruptions of different magnitudes.