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Organic alkoxy (RO) and peroxy (RO 2 ) radicals are key intermediates in multiphase atmospheric oxidation chemistry, though most of the study of their chemistry has focused on the gas phase. To better understand how radical chemistry may vary across different phases, we examine the chemistry of a model system, the 1-pentoxy radical, in three phases: the aqueous phase, the condensed organic phase, and the gas phase. In each phase, we generate the 1-pentoxy radical from the photolysis of n -pentyl nitrite, run the chemistry under conditions in which RO 2 radicals react with NO, and detect the products in real time using an ammonium chemical ionization mass spectrometer (NH 4 + CIMS). The condensed-phase chemistry shows an increase in formation of organic nitrate (RONO 2 ) from the downstream RO 2 +NO reaction, which is attributed to potential collisional and solvent-cage stabilization of the RO 2 -NO complex. We further observe an enhancement in the yield of carbonyl relative to hydroxy carbonyl products in the condensed phase, indicating changes to RO radical kinetics. The different branching ratios in the condensed phase impact the product volatility distribution as well as HO x -NO x chemistry, and may have implications for nitrate formation, aqueous aerosol formation, and radical cycling within atmospheric particles and droplets., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Gas-phase formaldehyde (HCHO) is formed in high yield from the oxidation of many volatile organic compounds (VOCs) and is commonly used as a constraint when testing the performance of VOC oxidation mechanisms in models. However, prior to using HCHO as a model constraint for VOC oxidation in forested regions, it is essential to have a thorough understanding of its foliar exchange. Therefore, a controlled laboratory setup was designed to measure the emission and dry deposition of HCHO at the leaf-level to red oak ( Quercus rubra ) and Leyland cypress ( Cupressus × leylandii ) tree saplings. The results show that HCHO has a compensation point (CP) that rises exponentially with temperature (22-35 °C) with a mean range of 0.3-0.9 ppbv. The HCHO CP results are also found to be independent of the studied tree species and 40-70% relative humidity. Given that HCHO mixing ratios in forests during the daytime are usually greater than 1 ppbv, the magnitude of the CP suggests that trees generally act as a net sink of HCHO. Additionally, the results show that HCHO foliar exchange is stomatally controlled and better matches a reactivity factor ( f 0 ) of 0 as opposed to 1 in conventional dry deposition parametrizations. At 30 °C, daytime HCHO dry deposition fluxes are reduced by upward of 50% when using f 0 = 0 and a nonzero HCHO CP, although deposition remains the dominant canopy sink of HCHO. A reduced deposition sink also implies the increased importance of the gas-phase photolysis of HCHO as a source of HO 2 ., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

One strategy for mitigating the indoor transmission of airborne pathogens, including the SARS-CoV-2 virus, is irradiation by germicidal UV light (GUV). A particularly promising approach is 222 nm light from KrCl excimer lamps (GUV 222 ); this inactivates airborne pathogens and is thought to be relatively safe for human skin and eye exposure. However, the impact of GUV 222 on the composition of indoor air has received little experimental study. Here, we conduct laboratory experiments in a 150 L Teflon chamber to examine the formation of secondary species by GUV 222 . We show that GUV 222 generates ozone (O 3 ) and hydroxyl radicals (OH), both of which can react with volatile organic compounds to form oxidized volatile organic compounds and secondary organic aerosol particles. Results are consistent with a box model based on the known photochemistry. We use this model to simulate GUV 222 irradiation under more realistic indoor air scenarios and demonstrate that under some conditions, GUV 222 irradiation can lead to levels of O 3 , OH, and secondary organic products that are substantially elevated relative to normal indoor conditions. The results suggest that GUV 222 should be used at low intensities and in concert with ventilation, decreasing levels of airborne pathogens while mitigating the formation of air pollutants.

A new configuration of the Community Earth System Model (CESM)/Community Atmosphere Model with full chemistry (CAM-chem) supporting the capability of horizontal mesh refinement through the use of the spectral element (SE) dynamical core is developed and called CESM/CAM-chem-SE. Horizontal mesh refinement in CESM/CAM-chem-SE is unique and novel in that pollutants such as ozone are accurately represented at human exposure relevant scales while also directly including global feedbacks. CESM/CAM-chem-SE with mesh refinement down to ∼14 km over the conterminous US (CONUS) is the beginning of the Multi-Scale Infrastructure for Chemistry and Aerosols (MUSICAv0). Here, MUSICAv0 is evaluated and used to better understand how horizontal resolution and chemical complexity impact ozone and ozone precursors over CONUS as compared to measurements from five aircraft campaigns, which occurred in 2013. This field campaign analysis demonstrates the importance of using finer horizontal resolution to accurately simulate ozone precursors such as nitrogen oxides and carbon monoxide. In general, the impact of using more complex chemistry on ozone and other oxidation products is more pronounced when using finer horizontal resolution where a larger number of chemical regimes are resolved. Large model biases for ozone near the surface remain in the Southeast US as compared to the aircraft observations even with updated chemistry and finer horizontal resolution. This suggests a need for adding the capability of replacing sections of global emission inventories with regional inventories, increasing the vertical resolution in the planetary boundary layer, and reducing model biases in meteorological variables such as temperature and clouds., (© 2022. The Authors. Journal of Advances in Modeling Earth Systems published by Wiley Periodicals LLC on behalf of American Geophysical Union.)

Formaldehyde (HCHO), the simplest and most abundant carbonyl in the atmosphere, contributes to particulate matter (PM) formation via two in-cloud processing pathways. First, in a catalytic pathway, HCHO reacts with hydrogen peroxide (H 2 O 2 ) to form hydroxymethyl hydroperoxide (HMHP), which rapidly oxidizes dissolved sulfur dioxide (SO 2,aq ) to sulfate, regenerating HCHO. Second, HCHO reacts with dissolved SO 2,aq to form hydroxymethanesulfonate (HMS), which upon oxidation with the hydroxyl radical (OH) forms sulfate and also reforms HCHO. Chemical transport model simulations using rate coefficients from laboratory studies of the reaction rate of HMHP with SO 2,aq show that the HMHP pathways reduce the SO 2 lifetime by up to a factor of 2 and contribute up to ∼18% of global sulfate. This contribution rises to >50% in isoprene-dominated regions such as the Amazon. Combined with recent results on HMS, this work demonstrates that the one-carbon molecules HMHP and HCHO contribute significantly to global PM, with HCHO playing a crucial catalytic role., Competing Interests: The authors declare no competing interest., (Copyright © 2022 the Author(s). Published by PNAS.)