Displays

Organic aerosol (OA) is an important component of the atmospheric submicron particulate mass, consisting of a complex mixture of organic compounds from natural and anthropogenic sources. During its lifetime of approximately one week in the atmosphere, OA is exposed to sunlight and thus undergoes atmospheric processing through photochemistry. This photochemical aging mechanism is thought to have a substantial effect on the propensity of OA to participate in cloud-forming processes by increasing its cloud condensation nuclei (CCN) activity. However, this effect is not well-constrained, and the influence of photochemistry on the ice nucleation (IN) activity of OA is uncertain. In this study, we aim to better understand how the photomineralization mechanism changes the cloud-forming properties of OA by measuring the CCN and IN abilities of photochemically aged OA of different sources: (1) Laboratory-generated ammonium sulfate-methylglyoxal (a proxy for secondary OA), and ambient OA bulk solutions collected from (2) wood smoke and (3) urban particulate matter in Padua (Italy). The solutions are exposed to UV-B radiation in a photoreactor for up to 25 hour and subsequently analyzed for their IN ability and, following aerosolization, for their CCN ability. To correlate changes in cloud-forming properties with changes in chemical composition due to photomineralization, we measure total organic carbon, UV-Vis absorbance, and CO, CO₂, acetic acid, formic acid, pyruvic acid and oxalic acid production. Indeed, preliminary data of wood smoke OA highlights photomineralization as an important atmospheric aging process that modifies the CCN ability of OA. By characterizing both the CCN and IN abilities of photochemically aged OA, our study may thus provide important insights into the atmospheric evolution and cloud-forming properties of OA, potentially establishing photomineralization of OA as an important mechanism to consider in regional and global climate model predictions.

How to cite: Müller, S. and Borduas-Dedekind, N.: The effect of the photomineralization mechanism on ambient organic aerosols' cloud condensation nuclei and ice nucleation abilities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-346, https://doi.org/10.5194/egusphere-egu2020-346, 2020.

In a recent study, we showed that photodegradation of dissolved organic sulfur (DOS) from a wide range of natural terrestrial environments releases sulfate (SO₄^2–) and other small and highly oxidized S-containing compounds as degradation products, similar to what had already been reported for dissolved organic carbon, nitrogen and phosphorous. However, the underlying chemical mechanism of photoproduction of sulfate is still unknown.

To fill this knowledge gap, we selected cysteine as a DOS model compound and we investigated its photodegradation to sulfate using model sensitizers as the source of singlet oxygen (¹O₂) and triplet excited states (³Sens*), two photochemically produced reactive species ubiquitous in sunlit surface waters. Using a combination of steady-state photochemistry experiments, kinetic modeling and mechanistic knowledge from the biochemistry literature, we reconstructed the molecular events that likely lead to the release of sulfate. We found that the release of SO₂ via triplet-sensitized fragmentation of cysteine sulfinic acid, a ¹O₂ degradation product of cysteine, is a key step in the reaction mechanism. In the presence of oxygen and a photosensitizer, SO₂ is then rapidly oxidized to SO₄^2–.

Interestingly, nowadays there is great interest in the atmospheric chemistry community on the same transformation (i.e., aqueous phase oxidation of SO₂ to SO₄^2–) in the context of extreme haze events. Triplet-induced SO₂ oxidation has already been proposed as a potential aqueous phase reaction that might explain the mismatch between measured and modelled sulfate concentrations, but the mechanism of this process is still not established. Our work provides an example of how mechanistic knowledge gained on the (photo)chemical behaviour of dissolved organic matter in aquatic systems can offer insights on processes occurring in atmospheric aqueous phases.

How to cite: Ossola, R., Clerc, B., Tolu, J., Winkel, L. H. E., and McNeill, K.: Photochemical production of sulfate from dissolved organic matter and atmospheric aqueous phases: Is there something in common?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-348, https://doi.org/10.5194/egusphere-egu2020-348, 2020.

The aqueous phase of clouds is a complex atmospheric medium containing a multitude of organic and inorganic species with different reactivities. The main oxidant towards organics in the aqueous phase is the OH radical. Many studies have identified biological material as a major fraction of ambient aerosol loading with bacteria being a small fraction. Laboratory experiments in our and other research groups have shown that microbial degradation of small organics (e.g., formic and acetic acids) can efficiently occur in artificial and real cloud water in competition to chemical radical reactions. However, in current models, it is usually assumed that bacteria are not metabolically active in the atmosphere. The aim of our study is to identify conditions, under which biological activity is significant in the multiphase system for specific organic compounds. Using a cloud multiphase process model, we compare the predicted fractions of organics consumed by radicals in the gas and aqueous phases to that by microbial processes of bacteria in the aqueous phase over large ranges of microphysical (e.g., cloud liquid water content, drop number), biological (cell concentration and activity) and chemical parameters (reaction rate constants and Henry’s law constants). We identify the organic properties and cloud parameters under which metabolic processes represent major atmospheric sinks for organics. In our cloud model, we consider the fact that only a small number fraction of droplets contain active bacteria cells. As many other models might not be able to describe such microphysical details, we also suggest simplified model approaches to represent microbial activity in clouds.

How to cite: Khaled, A., Zhang, M., Amato, P., Delort, A.-M., and Ervens, B.: Photochemistry versus biological activity towards organics in cloud water, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1517, https://doi.org/10.5194/egusphere-egu2020-1517, 2020.

Fundamental understanding of the complex chemistry of light absorbing atmospheric aerosols (aka Brown Carbon - BrC), their physico-chemical properties and environmental impacts is a challenging task because no single method of analytical chemistry is capable of providing the full range of analytical chemistry information. Micro-spectroscopy approaches can visualize individual particles and their internal structures; however, they largely exclude molecular-level information, and are limited to elemental and chemical bonding characterization. Contemporary methods of high-resolution mass spectrometry can provide detailed information on the molecular content of BrC, but these methods use bulk particle samples and provide no knowledge of the individual particle composition. Therefore, application of complementary analytical methods of chemical analysis is necessary for comprehensive characterization of aerosol properties ranging from bulk molecular composition of BrC constituents to microscopy level details of individual particles. Combined assessment of the results provided by complementary analytical chemistry techniques offers unique insights to understand the composition and physico-chemical properties of BrC aerosols determining their effects on air quality and climate. This presentation will give an overview of recent field and laboratory studies of BrC with an overall goal to understand fundamental relationship between chemical transformations of airborne particles and their environmental and climate impacts.

How to cite: Laskin, A.: Chemistry of Atmospheric Brown Carbon, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2776, https://doi.org/10.5194/egusphere-egu2020-2776, 2020.

Particulate organic carbon (OC) mass concentrations demonstrate decreasing trends in many regions across the contiguous US (CONUS). We investigate decadal trends in specific total organic carbon (TOC) volatility fractions OC1, OC2, OC3, and OC4 as defined and reported at 121 locations in the Interagency Monitoring of PROtected Visual Environments (IMPROVE) monitoring network from 2005-2016 for 23 chemical climatology regions across the CONUS. Volatility fraction OC2 drives ubiquitous decadal decreases in TOC, and OC3 mass concentrations increase. The largest changes in OC2 and OC3 occur in the eastern US. In four focus regions (Northeast, Appalachia, West Texas, and Northwest), OC fraction mass concentrations are converted to organic mass (OM) using region-specific OM:OC ratios. GEOS-Chem simulations reproduce and correlate strongly (R²>0.7) with OM fraction decadal trends. Decreases in aerosol liquid water (ALW) concentrations are tightly linked to observed change in individual TOC thermal fractions, and aerosol products derived from aqueous-phase isoprene oxidation predicted by GEOS-Chem. These results lend insight to changing chemical regimes with implications for particle phase state, viscosity, and oxidation state.

How to cite: Carlton, A., Christiansen, A., Porter, W., and Flesch, M.: The changing nature of particulate organic carbon and the relation to aerosol liquid water, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3967, https://doi.org/10.5194/egusphere-egu2020-3967, 2020.

Introduction
 Numerous green leaf volatiles (GLVs) are released into the atmosphere due to the stress, cell damage or wounding. Fog forming over vegetation takes up these compounds, promoting their aqueous-phase oxidation to less volatile compounds. The droplets eventually dry out, leaving behind the secondary organic aerosol (SOA). These pathways are still poorly recognized as potentially novel routes for the formation of atmospheric SOA. Kinetic investigations of GLVs in the gas phase have already been reported by Shalamzari et. al. 2014, Davis et. al. 2011 and many others, while there is no kinetic data on the aqueous phase reactions of selected C6 and C5 GLVs. In the present study, we focussed on the kinetic studies of GLVs with the hydroxyl, sulfate and nitrate radicals as a possible source of aqueous SOA.

Experimental method
 The rate constants of reactions of GLVs with atmospherically relevant radicals were studied using a laser flash photolysis-laser long path absorption (LFP-LLPA). Kinetic investigations of GLVs with hydroxyl radicals were performed using competition kinetics, where H₂O₂ (2 x 10^-4 mol L^-1) was used as a radical precursor and KSCN (2 x 10^-5 mol L^-1) as a reference compound. The method is similar to that introduced by Behar, et al. 1972. Kinetic measurements of sulfate and nitrate radicals with GLVs, were done using a direct flash photolysis method, where sodium persulfate (5 x 10^-4 mol L^-1) was the precursor in the generation of SO₄^•ꟷ and sodium nitrate (1 x 10^-1 mol L^-1) and sodium sulfate (3 x 10^-2 mol L^-1) were the precursor for the generation of nitrate radicals.

Conclusions
 In the present study, we explored the kinetics of aqueous-phase reactions of three GLVs- 1-penten-3-ol, cis-2-hexen-1-ol and 2-E-hexenal - with atmospheric radicals SO₄^•ꟷ, ^•OH and NO₃^•. The second-order rate constants were determined for a temperature range of 278 K to 318 K. A weak temperature dependence was observed for the aqueous-phase kinetics of all three GLVs with selected atmospherically relevant radicals. To explain the weak temperature dependence of aqueous-phase reaction of GLVs with atmospheric radicals, rate constants were investigated for the diffusion limitation. The atmospheric significance of the aqueous-phase reaction was evaluated, by calculating aqueous-phase lifetime and their relative rate to the gas phase reactions with respective radicals, which clearly demonstrated their importance above the gas-phase reactions in tropospheric aqueous-phase. The present work is a part of the bigger research project on the aqueous-phase reactions of a series of atmospherically relevant GLVs whereas a next step oxidation products in the aqueous-phase are being investigated at a present stage. 

This project is supported by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 711859 and by financial resources for science in the years 2017-2021 awarded by the Polish Ministry of Science and Higher Education for the implementation of an international co-financed project. The research project was also partially supported by funding under Project CREATE of European Union’s H2020 and ERASMUS PLUS staff mobility programme.

How to cite: Sarang, K., Otto, T., Rudzinski, K., Grgic, I., Nestorowicz, K., Herrmann, H., and Szmigielski, R.: Tropospheric Aqueous-phase Oxidation of Green Leaf Volatiles with Hydroxyl, Sulfate and Nitrate Radicals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4909, https://doi.org/10.5194/egusphere-egu2020-4909, 2020.

The chemical composition of aerosols, in both gas and particle phase, is an important factor regarding their properties influencing weather, climate and human health. Organic compounds are a major fraction of atmospheric aerosols and their composition depends on chemical processing by atmospheric oxidants and photochemical reactions. These processes are complex due to the abundance of possible reactions and reaction partners and rarely studied over a wider range of atmospheric temperatures. To get a better understanding of photochemical processes in the atmosphere we studied different organic test aerosols from simple to more complex systems between 213 K and 293 K in the AIDA simulation chamber at the Karlsruhe Institute of Technology.  Photochemical reactions were studied using a new LED light-source simulating solar radiation in the UV and visible. The organic aerosols were either generated in situ by oxidation of VOC with ozone, OH radicals and NO₃ radicals or by nebulizing aqueous solutions containing the aerosol components.  The aerosols were analysed by a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and a high–resolution time-of-flight chemical ionization mass spectrometer (FIGAERO-HR-ToF-CIMS).  The latter one offers the possibility to study the composition of gas phase and particle phase separately. First results suggest that secondary organic aerosols from single precursors like toluene or α-pinene show no or only very small changes related to photochemistry even when formed in presence of high NOx concentrations. In contrast, aerosol particles containing molecules with larger mesomeric systems or atmospherically relevant photosensitizers show significant changes upon irradiation.

In this presentation, we will discuss the changes that organic aerosols undergo in gas and particle phase, during photochemical aging at temperatures between 213 and 293 K.

How to cite: Vallon, M., Gao, L., Song, J., Jiang, F., and Saathoff, H.: Photochemical aging of organic aerosols at temperatures between 213 K and 293 K, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5433, https://doi.org/10.5194/egusphere-egu2020-5433, 2020.

Formation of organic aerosol by oxidation of gas phase compounds has been intensely studied, and is much better understood than the aerosol ageing transformations during the lifetime of organic aerosol. Aerosol ageing influences how those aerosol particles affect climate and human health and is still not well constrained in current models.

Photochemistry in the condensed phase is an important mechanism responsible for ageing of organic aerosol. In the lower troposphere, where UV light intensity with sufficiently low wavelength to directly photolyze aerosol components is low, indirect photochemistry (catalyzing redox processes of non-absorbing molecules) is especially relevant. Recently we studied transition metal complex photochemistry in single particles levitated in an electrodynamic balance. In particular, we investigated the aqueous iron(III)-citrate/citric acid system and found that irradiation at 473 nm led to rapid and significant degradation of the citric acid. Up to 80% of the initial particle mass was partitioned to the gas phase with the degradation rate depending on kinetic transport limitations of oxygen. These kinetic limitations arise are influenced strongly by the relative humidity dependence of particle viscosity where water acts as a plasticizer.

Here we will report on photochemical degradation experiments adding various salts in different (ammonium sulfate, ammonium bisulfate, etc.) to the reference system iron(III)-citrate/citric acid. Preliminary experiments suggest that pH of the aerosol particle influences the degradation rate in this system significantly.

How to cite: Krieger, U., Bluvshtein, N., and Dou, J.: Effect of ammonium salts on the photochemical degradation of iron containing organic aerosol, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15572, https://doi.org/10.5194/egusphere-egu2020-15572, 2020.