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Corrections for first-order particle losses to Teflon chamber walls are important sources of uncertainty in experimental studies of particle formation and aging. Particle size distributions and environmental factors significantly influence wall loss corrections; thus, it is important to characterize size-dependent particle loss profiles under myriad experimental conditions that may alter deposition rates. This work investigated size-dependent loss coefficients of inorganic (ammonium sulfate, AS), organic (sorbitol, C6H14O6), and mixed composition (AS + sorbitol, 1:1 by mole) particles to a Teflon chamber under varying chamber temperature (20–40 °C), relative humidity (RH, <10–80%), illumination (dark vs. 100% chamber lights), particle water (crystalline vs. deliquesced vs. metastable), and chamber usage history conditions (clean chamber vs. following chemical experiments). It was found that temperature and lights had negligible to minor effects on loss rates for all particles, while RH, particle water, and chamber usage history each had major effects under all tested conditions. Particle wall loss rates were higher under humid than dry conditions, and higher for deliquesced particles than for dry particles at similar RH. Chemical conditions that introduced acidic species to chamber walls the day prior to a wall loss experiment were responsible for uncertainties of up to ∼50% in wall loss rate profiles, despite recommended chamber flushing regimens. These data suggest that sensitive OA formation or aging experiments may consider obtaining same-day wall loss profiles from the target experiment. Otherwise, size-dependent corrections for particle wall loss should consider particle composition, particle water, RH, wall usage history, and possibly illumination conditions. 

Atmospheric chemistry models generally assume organic aerosol (OA) to be photochemically inert. Recent mechanisms for the oxidation of biogenic isoprene, a major source of secondary organic aerosol (iSOA), produce excessive OA in the absence of subsequent OA reactivity. At the same time, models underestimate atmospheric concentrations of formic and acetic acids for which OA degradation could provide a source. Here we show that the aqueous photooxidation of an isoprene-derived organosulfate (2-methyltriolsulfate or MTS), an important iSOA component, produces formic and acetic acids in high yields and at timescales competitive with deposition. Experimental data are well fit by a kinetic model in which three sequential oxidation reactions of the isoprene organosulfate produce two molar equivalents of formic acid and one of acetic acid. We incorporate this chemistry and that of 2-methyltetrol, another ubiquitous iSOA component, into the GEOS-Chem global atmospheric chemistry model. Simulations show that photooxidation and subsequent revolatilization of this iSOA may account for up to half of total iSOA loss globally, producing 4 Tg a−1 each of formic and acetic acids. This reduces model biases in gas-phase formic acid and total organic aerosol over the Southeast United States in summer by ∼30% and 60% respectively. While our study shows the importance of adding iSOA photochemical sinks into atmospheric models, uncertainties remain that warrant further study. In particular, improved understanding of reaction dependencies on particle characteristics and concentrations of particle-phase OH and other oxidants are needed to better simulate the effects of this chemistry on the atmospheric budgets of organic acids and iSOA. 

The sulfate anion radical (SO 4 •– ) is known to be formed in the autoxidation chain of sulfur dioxide and from minor reactions when sulfate or bisulfate ions are activated by OH radicals, NO 3 radicals, or iron. Here, we report a source of SO 4 •– , from the irradiation of the liquid water of sulfate-containing organic aerosol particles under natural sunlight and laboratory UV radiation. Irradiation of aqueous sulfate mixed with a variety of atmospherically relevant organic compounds degrades the organics well within the typical lifetime of aerosols in the atmosphere. Products of the SO 4 •– + organic reaction include surface-active organosulfates and small organic acids, alongside other products. Scavenging and deoxygenated experiments indicate that SO 4 •– radicals, instead of OH, drive the reaction. Ion substitution experiments confirm that sulfate ions are necessary for organic reactivity, while the cation identity is of low importance. The reaction proceeds at pH 1–6, implicating both bisulfate and sulfate in the formation of photoinduced SO 4 •– . Certain aromatic species may further accelerate the reaction through synergy. This reaction may impact our understanding of atmospheric sulfur reactions, aerosol properties, and organic aerosol lifetimes when inserted into aqueous chemistry model mechanisms. 

The >5,000‐year radiocarbon age (¹⁴C‐age) of much of the 630 ± 30 Pg C oceanic dissolved organic carbon (DOC) reservoir remains an enigma in the marine carbon cycle. The fact that DOC is significantly older than dissolved inorganic carbon at every depth in the ocean forms the basis of our current framing of the marine DOC cycle, where some component persists over multiple cycles of ocean mixing. As a result,¹⁴C‐depleted, aged DOC is hypothesized to be present as a uniform reservoir with a constant¹⁴C signature and concentration throughout the water column. However, key requirements of this model, including direct observations of DOC with similar¹⁴C signatures in the surface and deep ocean, have never been met. Despite decades of research, the distribution of Δ¹⁴C values in marine DOC remains a mystery. Here, we applied a thermal fractionation method to compare operationally defined refractory DOC (RDOC) from different depths in the North Pacific Ocean. We found that RDOC shares chemical characteristics (as recorded by OC bond strength) throughout the water column but does not share the same¹⁴C signature. Our results support one part of the current paradigm—that RDOC is comprised of structurally related components throughout the ocean that form a “background” reservoir. However, in contrast to the current paradigm, our results are consistent with a vertical concentration gradient and a vertical and inter‐ocean Δ¹⁴C gradient for RDOC. The observed Δ¹⁴C gradient is compatible with the potential addition of pre‐aged DOC to the upper ocean.

 

Abstract. Organic aerosols generated from the smoldering combustion of woodcritically impact air quality and health for billions of people worldwide;yet, the links between the chemical components and the optical or biologicaleffects of woodsmoke aerosol (WSA) are still poorly understood. In thiswork, an untargeted analysis of the molecular composition of smoldering WSA,generated in a controlled environment from nine types of heartwood fuels(African mahogany, birch, cherry, maple, pine, poplar, red oak, redwood, andwalnut), identified several hundred compounds using gas chromatography massspectrometry (GC-MS) and nano-electrospray high-resolution mass spectrometry(HRMS) with tandem multistage mass spectrometry (MSn). The effects ofWSA on cell toxicity as well as gene expression dependent on the aryl hydrocarbon receptor (AhR) and estrogen receptor(ER) were characterized with cellular assays, andthe visible mass absorption coefficients (MACvis) of WSA were measuredwith ultraviolet–visible spectroscopy. The WSAs studied in this work have significantlevels of biological and toxicological activity, with exposure levels inboth an outdoor and indoor environment similar to or greater than those ofother toxicants. A correlation between the HRMS molecular composition andaerosol properties found that phenolic compounds from the oxidativedecomposition of lignin are the main drivers of aerosol effects, while thecellulose decomposition products play a secondary role; e.g., levoglucosanis anticorrelated with multiple effects. Polycyclic aromatic hydrocarbons(PAHs) are not expected to form at the combustion temperature in this work,nor were they observed above the detection limit; thus, biological and opticalproperties of the smoldering WSA are not attributed to PAHs. Syringylcompounds tend to correlate with cell toxicity, while the more conjugatedmolecules (including several compounds assigned to dimers) have higher AhRactivity and MACvis. The negative correlation between cell toxicity andAhR activity suggests that the toxicity of smoldering WSA to cells is notmediated by the AhR. Both mass-normalized biological outcomes have astatistically significant dependence on the degree of combustion of thewood. In addition, our observations support the fact that the visible lightabsorption of WSA is at least partially due to charge transfer effects inaerosols, as previously suggested. Finally, MACvis has no correlationwith toxicity or receptor signaling, suggesting that key chromophores inthis work are not biologically active on the endpoints tested.