Feasibility of Photosensitized Reactions with Secondary Organic Aerosol Particles in the Presence of Volatile Organic Compounds

Photolytic Mass Loss of Humic Substances Measured with a Quartz Crystal Microbalance

Laboratory studies have shown that photolytic mass loss can be a significant sink for secondary organic aerosol (SOA). Here, we use a quartz crystal microbalance to measure mass loss of Suwannee River Humic Acid (SRHA) and Suwannee River Fulvic Acid (SRFA), surrogates for SOA, exposed to 254, 300, and 405 nm radiation over the course of 24 h. We find that the photolytic mass loss rates of these materials are comparable to those for laboratory-generated limonene and toluene SOA material from the study of Baboomian et al, ACS Earth Space Chem. 2020, 4, 1078. Scaling our results to ambient conditions, we estimate that humic substances in aerosols can lose as much as 8% by mass in the first day of exposure in the atmosphere, equivalent to 0.025% of J NO2 , the photolysis rate of nitrogen dioxide. By using zero air instead of nitrogen, we also find that the presence of oxygen accelerates the photolytic mass loss rate by a factor of 2 to 4 at all wavelengths suggesting a potential role for reactive oxygen species. UV photolysis of an aqueous SRFA solution demonstrated both photobleaching at UV wavelengths and photoenhancement at visible wavelengths. Ultrahigh-resolution mass spectrometric analysis showed that condensed-phase SRFA photolysis led to decreased intensity in the 100-300 m/z range while aqueous SRFA photolysis resulted in an increase in intensity in the same range. This work reaffirms that photolytic mass loss is a potentially significant sink for SOA, but only on the time scale of a day or two and demonstrates that SRHA and SRFA are suitable surrogates for atmospheric SOA with respect to photolytic mass loss.

Sunlight can convert atmospheric aerosols into a glassy solid state and modify their environmental impacts

Secondary organic aerosol is well known to affect Earth's climate, regional weather, visibility, and public health. Once these aerosols are formed, they are transported throughout the atmosphere for days or even weeks. We show that exposure of secondary organic aerosols to UV solar radiation leads to a surprising and remarkable increase in viscosity by as much as five orders of magnitude. We also show that this UV exposure can lead to an increased abundance of aerosols that are in the glassy solid state in the troposphere, with important implications for climate predictions. Overall, our results clearly demonstrate that aging by exposure to solar radiation needs to be considered when predicting the environmental impacts of secondary organic aerosols.

Secondary organic aerosol (SOA) plays a critical, yet uncertain, role in air quality and climate. Once formed, SOA is transported throughout the atmosphere and is exposed to solar UV light. Information on the viscosity of SOA, and how it may change with solar UV exposure, is needed to accurately predict air quality and climate. However, the effect of solar UV radiation on the viscosity of SOA and the associated implications for air quality and climate predictions is largely unknown. Here, we report the viscosity of SOA after exposure to UV radiation, equivalent to a UV exposure of 6 to 14 d at midlatitudes in summer. Surprisingly, UV-aging led to as much as five orders of magnitude increase in viscosity compared to unirradiated SOA. This increase in viscosity can be rationalized in part by an increase in molecular mass and oxidation of organic molecules constituting the SOA material, as determined by high-resolution mass spectrometry. We demonstrate that UV-aging can lead to an increased abundance of aerosols in the atmosphere in a glassy solid state. Therefore, UV-aging could represent an unrecognized source of nuclei for ice clouds in the atmosphere, with important implications for Earth’s energy budget. We also show that UV-aging increases the mixing times within SOA particles by up to five orders of magnitude throughout the troposphere with important implications for predicting the growth, evaporation, and size distribution of SOA, and hence, air pollution and climate.

Fate of Oxygenated Volatile Organic Compounds in the Yangtze River Delta Region: Source Contributions and Impacts on the Atmospheric Oxidation Capacity

The Community Multiscale Air Quality model (CMAQv5.2) was implemented to investigate the sources and sinks of oxygenated volatile organic compounds (OVOCs) during a high O₃ and high PM_2.5 season in the Yangtze River Delta (YRD) region, based on constraints from observations. The model tends to overpredict non-oxygenated VOCs and underpredict OVOCs, which has been improved with adjusted emissions of all VOCs. The OVOCs in the YRD are dominated by ketones, aldehydes, and alcohols. Ketones and aldehydes mainly originate from direct emissions and secondary formation in the northern YRD, and primarily originate from secondary formation in the southern part influenced by biogenic emissions. The concentration of secondary organic aerosols (SOA) produced by OVOCs is 0.5-1.5 μg/m³, with 40-80% originated from organic nitrates, 20-70% originated from dicarbonyls, and 0-20% originated from isoprene epoxydiols. The influences of OVOCs on the atmospheric oxidation capacity are dominated by the OH^• pathway during the day (∼350%) and by the NO₃^• pathway at night (∼150%). Consequently, O₃ is enhanced by 30-70% in the YRD. Aerosols are also enhanced by 50-140%, 20-80%, and ∼20% for SOA, nitrate, and sulfate, respectively.

Environmental photochemistry on plants: recent advances and new opportunities for interdisciplinary research

Plants play a central role in the photochemistry of chemicals in the environment. They represent a major atmospheric source of volatile organic compounds (VOCs) but also an important environmental surface for the deposition and photochemical reactions of pesticides, gaseous and particulate pollutants. In this review, we point out the role of plant leaves in these processes, as a support affecting the reactions physically and chemically and as a partner through the release of natural constituents (water, secondary metabolites). We discuss the influence of the chosen support (leaves, needle surfaces or fruit cuticles, extracted cuticular waxes and model surfaces) and other factors (additives, pesticides mixture, and secondary metabolites) on the photochemical degradation kinetics and mechanisms. We also show how plants can be a source of photochemically reactive species which can act as photosensitizers promoting the photodegradation of pesticides or the formation and aging of secondary organic aerosols (SOA) and secondary organic materials (SOM). Understanding the fate of chemicals on plants is a research area located at the interface between photochemistry, analytical chemistry, atmospheric chemistry, microbiology and vegetal physiology. Pluridisciplinary approaches are needed to deeply understand these complex phenomena in a comprehensive way. To overcome this challenge, we summarize future research directions which have been clearly overlooked until now.

Mechanisms and Kinetic Parameters for the Gas-Phase Reactions of 3-Methyl-3-buten-2-one and 3-Methyl-3-penten-2-one with Ozone

Ozonolysis of unsaturated ketones is a common atmospheric chemical process that plays a significant role in controlling the atmospheric budget of OH and O₃, organic acids, and secondary organic aerosols (SOA). In this work, the detailed reaction mechanism and rate coefficients for the reactions of O₃ with two unsaturated ketones, 3-methyl-3-buten-2-one (MBO332) and 3-methyl-3-penten-2-one (MPO332), were investigated by using density functional theory (DFT) and Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The results indicate that the major products are butanedione and formaldehyde for MBO332, and butanedione and acetaldehyde for MPO332. Possible reaction mechanism and thermodynamic parameters of some complex stable Criegee intermediates (SCIs) RR'COO were also be investigated in this study. Some organic peroxides can be regarded as the main products for the further reactions of SCIs. The rate constants calculated with O₃ are 2.59 × 10^-16 cm³ molecule^-1 s^-1 and 2.28 × 10^-16 cm³ molecule^-1 s^-1 for MBO332 and MPO332 at 298 K and 1 atm. The total rate constant is negatively correlated with temperature (200-400 K) and positively correlated with pressure. The atmospheric half-lives of MBO332 and MPO332 based on O₃ are estimated.

Environmental Processing of Lipids Driven by Aqueous Photochemistry of α-Keto Acids

Sunlight can initiate photochemical reactions of organic molecules though direct photolysis, photosensitization, and indirect processes, often leading to complex radical chemistry that can increase molecular complexity in the environment. α-Keto acids act as photoinitiators for organic species that are not themselves photoactive. Here, we demonstrate this capability through the reaction of two α-keto acids, pyruvic acid and 2-oxooctanoic acid, with a series of fatty acids and fatty alcohols. We show for five different cases that a cross-product between the photoinitiated α-keto acid and non-photoactive species is formed during photolysis in aqueous solution. Fatty acids and alcohols are relatively unreactive species, which suggests that α-keto acids are able to act as radical initiators for many atmospherically relevant molecules found in the sea surface microlayer and on atmospheric aerosol particles.