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Brown carbon light absorptivity is associated with organic aerosol volatility and elemental carbon concentrations.

 

Abstract. The oxidation of dimethyl sulfide (DMS;CH3SCH3), emitted from the surface ocean, contributes to theformation of Aitken mode particles and their growth to cloud condensationnuclei (CCN) sizes in remote marine environments. It is not clear whetherother less commonly measured marine-derived, sulfur-containing gases sharesimilar dynamics to DMS and contribute to secondary marine aerosolformation. Here, we present measurements of gas-phase volatile organosulfurmolecules taken with a Vocus proton-transfer-reaction high-resolutiontime-of-flight mass spectrometer during a mesocosm phytoplankton bloomexperiment using coastal seawater. We show that DMS, methanethiol (MeSH;CH3SH), and benzothiazole (C7H5NS) account for on averageover 90 % of total gas-phase sulfur emissions, with non-DMS sulfur sourcesrepresenting 36.8 ± 7.7 % of sulfur emissions during the first 9 d of the experiment in the pre-bloom phase prior to major biologicalgrowth, before declining to 14.5 ± 6.0 % in the latter half of theexperiment when DMS dominates during the bloom and decay phases. The molarratio of DMS to MeSH during the pre-bloom phase (DMS : MeSH = 4.60 ± 0.93) was consistent with the range of previously calculated ambient DMS-to-MeSH sea-to-air flux ratios. As the experiment progressed, the DMS to MeSHemission ratio increased significantly, reaching 31.8 ± 18.7 duringthe bloom and decay. Measurements of dimethylsulfoniopropionate (DMSP),heterotrophic bacteria, and enzyme activity in the seawater suggest theDMS : MeSH ratio is a sensitive indicator of the bacterial sulfur demand andthe composition and magnitude of available sulfur sources in seawater. Theevolving DMS : MeSH ratio and the emission of a new aerosol precursor gas,benzothiazole, have important implications for secondary sulfate formationpathways in coastal marine environments. 

Abstract. Atmospheric aerosols are a significant public health hazard and havesubstantial impacts on the climate. Secondary organic aerosols (SOAs) havebeen shown to phase separate into a highly viscous organic outer layersurrounding an aqueous core. This phase separation can decrease thepartitioning of semi-volatile and low-volatile species to the organic phaseand alter the extent of acid-catalyzed reactions in the aqueous core. A newalgorithm that can determine SOA phase separation based on their glasstransition temperature (Tg), oxygen to carbon (O:C) ratio and organic massto sulfate ratio, and meteorological conditions was implemented into theCommunity Multiscale Air Quality Modeling (CMAQ) system version 5.2.1 andwas used to simulate the conditions in the continental United States for thesummer of 2013. SOA formed at the ground/surface level was predicted to bephase separated with core–shell morphology, i.e., aqueous inorganic coresurrounded by organic coating 65.4 % of the time during the 2013 SouthernOxidant and Aerosol Study (SOAS) on average in the isoprene-rich southeasternUnited States. Our estimate is in proximity to the previously reported∼70 % in literature. The phase states of organic coatingsswitched between semi-solid and liquid states, depending on theenvironmental conditions. The semi-solid shell occurring with lower aerosolliquid water content (western United States and at higher altitudes) has aviscosity that was predicted to be 102–1012 Pa s, whichresulted in organic mass being decreased due to diffusion limitation.Organic aerosol was primarily liquid where aerosol liquid water was dominant(eastern United States and at the surface), with a viscosity <102 Pa s.Phase separation while in a liquid phase state, i.e.,liquid–liquid phase separation (LLPS), also reduces reactive uptake ratesrelative to homogeneous internally mixed liquid morphology but was lowerthan aerosols with a thick viscous organic shell. The sensitivity casesperformed with different phase-separation parameterization and dissolutionrate of isoprene epoxydiol (IEPOX) into the particle phase in CMAQ can havevarying impact on fine particulate matter (PM2.5) organic mass, interms of bias and error compared to field data collected during the 2013 SOAS.This highlights the need to better constrain the parameters thatgovern phase state and morphology of SOA, as well as expand mechanisticrepresentation of multiphase chemistry for non-IEPOX SOA formation in modelsaided by novel experimental insights. 

Marine aerosols strongly influence climate through their interactions with solar radiation and clouds. However, significant questions remain regarding the influences of biological activity and seawater chemistry on the flux, chemical composition, and climate-relevant properties of marine aerosols and gases. Wave channels, a traditional tool of physical oceanography, have been adapted for large-scale ocean-atmosphere mesocosm experiments in the laboratory. These experiments enable the study of aerosols under controlled conditions which isolate the marine system from atmospheric anthropogenic and terrestrial influences. Here, we present an overview of the 2019 Sea Spray Chemistry and Particle Evolution (SeaSCAPE) study, which was conducted in an 11 800 L wave channel which was modified to facilitate atmospheric measurements. The SeaSCAPE campaign sought to determine the influence of biological activity in seawater on the production of primary sea spray aerosols, volatile organic compounds (VOCs), and secondary marine aerosols. Notably, the SeaSCAPE experiment also focused on understanding how photooxidative aging processes transform the composition of marine aerosols. In addition to a broad range of aerosol, gas, and seawater measurements, we present key results which highlight the experimental capabilities during the campaign, including the phytoplankton bloom dynamics, VOC production, and the effects of photochemical aging on aerosol production, morphology, and chemical composition. Additionally, we discuss the modifications made to the wave channel to improve aerosol production and reduce background contamination, as well as subsequent characterization experiments. The SeaSCAPE experiment provides unique insight into the connections between marine biology, atmospheric chemistry, and climate-relevant aerosol properties, and demonstrates how an ocean-atmosphere-interaction facility can be used to isolate and study reactions in the marine atmosphere in the laboratory under more controlled conditions. 

Abstract. Western US wildlands experience frequent and large-scale wildfires which arepredicted to increase in the future. As a result, wildfire smoke emissionsare expected to play an increasing role in atmospheric chemistry whilenegatively impacting regional air quality and human health. Understanding theimpacts of smoke on the environment is informed by identifying andquantifying the chemical compounds that are emitted during wildfires and byproviding empirical relationships that describe how the amount andcomposition of the emissions change based upon different fire conditions andfuels. This study examined particulate organic compounds emitted from burningcommon western US wildland fuels at the US Forest Service Fire ScienceLaboratory. Thousands of intermediate and semi-volatile organic compounds(I/SVOCs) were separated and quantified into fire-integrated emission factors(EFs) using a thermal desorption, two-dimensional gas chromatograph withonline derivatization coupled to an electron ionization/vacuum ultraviolethigh-resolution time-of-flight mass spectrometer(TD-GC × GC-EI/VUV-HRToFMS). Mass spectra, EFs as a function ofmodified combustion efficiency (MCE), fuel source, and other definingcharacteristics for the separated compounds are provided in the accompanyingmass spectral library. Results show that EFs for total organic carbon (OC),chemical families of I/SVOCs, and most individual I/SVOCs span 2–5 orders ofmagnitude, with higher EFs at smoldering conditions (low MCE) than flaming.Logarithmic fits applied to the observations showed that log (EFs) forparticulate organic compounds were inversely proportional to MCE. Thesemeasurements and relationships provide useful estimates of EFs for OC,elemental carbon (EC), organic chemical families, and individual I/SVOCs as afunction of fire conditions.