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Previous studies have demonstrated volatility-dependent absorption of gas-phase volatile organic compounds (VOCs) to Teflon and other polymers. Polymer–VOC interactions are relevant for atmospheric chemistry sampling, as gas–wall partitioning in polymer tubing can cause delays and biases during measurements. They are also relevant to the study of indoor chemistry, where polymer-based materials are abundant (e.g., carpets and paints). In this work, we quantify the absorptive capacities of multiple tubing materials, including four nonconductive polymers (important for gas sampling and indoor air quality), four electrically conductive polymers and two commercial steel coatings (for gas and particle sampling). We compare their performance to previously characterized materials. To quantify the absorptive capacities, we expose the tubing to a series of ketones in the volatility range 104 – 109 µgm-3 and monitor transmission. For slow-diffusion polymers (e.g., perfluoroalkoxy alkane (PFA) Teflon and nylon), absorption is limited to a thin surface layer, and a single-layer absorption model can fit the data well. For fast-diffusion polymers (e.g., polyethylene and conductive silicone), a larger depth of the polymer is available for diffusion, and a multilayer absorption model is needed. The multilayer model allows fitting solid-phase diffusion coefficients for different materials, which range from 4×10-9 to 4×10-7 cm 2 s -1. These diffusion coefficients are ∼ 8 orders of magnitude larger than literature values for fluorinated ethylene propylene (FEP) Teflon film. This enormous difference explains the differences in VOC absorption measured here. We fit an equivalent absorptive mass (CW , µgm-3) for each absorptive material. We found PFA to be the least absorptive, with CW ∼ 105 µgm-3 , and conductive silicone to be the most absorptive, with CW ∼ 1013 µgm-3. PFA transmits VOCs easily and intermediate-volatility species (IVOCs) with quantifiable delays. In contrast, conductive silicone tubing transmits only the most volatile VOCs, denuding all lower-volatility species. Semi-volatile species (SVOCs) are very difficult to sample quantitatively through any tubing material. We demonstrate a system combining several slow- and fast-diffusion tubing materials that can be used to separate a mixture of VOCs into volatility classes. New conductive silicone tubing contaminated the gas stream with siloxanes, but this effect was reduced 10 000-fold for aged tubing, while maintaining the same absorptive properties. SilcoNert (tested in this work) and Silonite (tested in previous work) steel coatings showed gas transmission that was almost as good as PFA, but since they undergo adsorption, their delay times may be humidity- and concentration-dependent. [ABSTRACT FROM AUTHOR]

Previous studies have demonstrated volatility-dependent absorption of gas-phase volatile organic compounds (VOCs) to Teflon and other polymers. Polymer-VOC interactions are relevant for atmospheric chemistry sampling, as gas-wall partitioning in polymer tubing can cause delays and biases during measurements. They are also relevant to the study of indoor chemistry, where polymer-based materials are abundant (e.g. carpets, paints). In this work, we quantify the absorptive capacities of multiple tubing materials, including 4 non-conductive polymers (important for gas sampling and indoor air quality), as well as 4 electrically-conductive polymers and 2 commercial steel coatings (for gas and particle sampling). We compare their performance to previously-characterized materials. To quantify the absorptive capacities, we expose the tubing to a series of ketones in the volatility range 104–109 μg m−3 and monitor transmission. For slow-diffusion polymers (e.g. PFA Teflon and nylon), absorption is limited to a thin surface layer, and a single layer absorption model can fit the data well. For fast-diffusion polymers (e.g. polyethylene and conductive silicone) a larger depth of the polymer is available for diffusion, and a multilayer absorption model was needed. The multilayer model allows fitting solid phase diffusion coefficients for different materials, which range from 4 × 10−9 to 4 × 10−7 cm2 s−1. These diffusion coefficients are ~8 orders of magnitude larger than literature values for FEP Teflon. This enormous difference explains the differences in VOC absorption measured here. We fit an equivalent absorptive mass (Cw , μg m−3) for each absorptive material. We found PFA to be the least absorptive, with Cw ~105 μg m−3, and conductive silicone to be the most absorptive, with Cw ~1013 μg m−3. PFA transmits VOCs easily, and intermediate-volatility species (IVOCs) with quantifiable delays. In contrast, conductive silicone tubing transmits only the most volatile VOCs, denuding all lower volatility species. Semi-volatile species (SVOCs) are very difficult to sample quantitatively through any tubing material. We demonstrate how to use a combination of slow- and fast-diffusion tubing to separate a mixture of VOCs into volatility classes before analysis. New conductive silicone tubing contaminated the gas stream with siloxanes, but this effect was reduced by 10,000-fold for aged tubing, while maintaining the same absorptive properties. SilcoNert (tested in this work) and Silonite (tested in previous work) steel coatings showed gas transmission that was almost as good as PFA, but since they undergo adsorption, their delays times may be humidity and concentration dependent. [ABSTRACT FROM AUTHOR]

Perchlorate has been observed in many environments on Earth and Mars but its sources remain poorly quantified. In this study, we use a global three‐dimensional chemical transport model to simulate perchlorate's gas‐phase photochemical production, atmospheric transport, and deposition on Earth's surface. Model predictions are compared to newly compiled observations of atmospheric concentrations, deposition flux, and oxygen isotopic composition of perchlorate. We find that the modeled gas‐phase production of perchlorate is consistent with reported stratospheric observations. Nevertheless, we show that this mechanism alone cannot explain the high levels of perchlorate observed at many near‐surface sites (aerosol concentrations >0.1 ng m−3 and deposition fluxes >10 g km−2 yr−1) or the low 17O‐excess observed in perchlorate sampled from pristine environments (<+18.4‰). We discuss four hypotheses to explain the model‐observation discrepancies and recommend laboratory and field observations to address key uncertainties in atmospheric sources of perchlorate. Plain Language Summary: Perchlorate (ClO4−) pollution is an environmental issue because excessive exposure can affect the thyroid and disrupt hormonal balance, especially for infants. Perchlorate on Earth has both human and natural sources. Industrial perchlorate is used for explosives and rocket fuels. Perchlorate also occurs naturally and accumulates in many deserts on Earth and in the soil of Mars. Atmospheric chemistry has long been considered a source of natural perchlorate, but its contribution remains uncertain. In this study, we use a 3‐D atmospheric model to estimate how much of the perchlorate occurrence on Earth can be explained by known and plausible reactions between gases containing chlorine and oxygen. We find that these reactions can explain the abundance of stratospheric perchlorate. However, they cannot explain many tropospheric observations of perchlorate, especially those in Antarctica and urban areas. Our analysis of oxygen isotopic anomalies also suggests that stratospheric chemistry alone cannot account for all the natural perchlorate found in deserts. We discuss four possible explanations for the differences between observations and model predictions. We recommend some future research that can reduce the uncertainties in the sources of atmospheric perchlorate and improve our understanding of the occurrence of natural perchlorate in planetary atmospheres. Key Points: We conduct the first global simulation of atmospheric perchlorate using a three‐dimensional chemical transport modelGas‐phase production of perchlorate in the stratosphere and its subsequent transport cannot explain observations at many surface sitesAnalysis of modeled and observed 17O excess suggests that non‐stratospheric sources are important for the occurrence of natural perchlorate [ABSTRACT FROM AUTHOR]

Satellite-based retrievals of tropospheric NO 2 columns are widely used to infer NO x (≡ NO + NO 2) emissions. These retrievals rely on model information for the vertical distribution of NO 2. The free tropospheric background above 2 km is particularly important because the sensitivity of the retrievals increases with altitude. Free tropospheric NO x also has a strong effect on tropospheric OH and ozone concentrations. Here we use observations from three aircraft campaigns (SEAC 4 RS, DC3, and ATom) and four atmospheric chemistry models (GEOS-Chem, GMI, TM5, and CAMS) to evaluate the model capabilities for simulating NO x in the free troposphere and attribute it to sources. NO 2 measurements during the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC 4 RS) and Deep Convective Clouds and Chemistry (DC3) campaigns over the southeastern U.S. in summer show increasing concentrations in the upper troposphere above 10 km, which are not replicated by the GEOS-Chem, although the model is consistent with the NO measurements. Using concurrent NO, NO 2 , and ozone observations from a DC3 flight in a thunderstorm outflow, we show that the NO 2 measurements in the upper troposphere are biased high, plausibly due to interference from thermally labile NO 2 reservoirs such as peroxynitric acid (HNO 4) and methyl peroxy nitrate (MPN). We find that NO 2 concentrations calculated from the NO measurements and NO–NO 2 photochemical steady state (PSS) are more reliable to evaluate the vertical profiles of NO 2 in models. GEOS-Chem reproduces the shape of the PSS-inferred NO 2 profiles throughout the troposphere for SEAC 4 RS and DC3 but overestimates NO 2 concentrations by about a factor of 2. The model underestimates MPN and alkyl nitrate concentrations, suggesting missing organic NO x chemistry. On the other hand, the standard GEOS-Chem model underestimates NO observations from the Atmospheric Tomography Mission (ATom) campaigns over the Pacific and Atlantic oceans, indicating a missing NO x source over the oceans. We find that we can account for this missing source by including in the model the photolysis of particulate nitrate on sea salt aerosols at rates inferred from laboratory studies and field observations of nitrous acid (HONO) over the Atlantic. The median PSS-inferred tropospheric NO 2 column density for the ATom campaign is 1.7 ± 0.44 × 10 14 molec. cm -2 , and the NO 2 column density simulated by the four models is in the range of 1.4–2.4 × 10 14 molec. cm -2 , implying that the uncertainty from using modeled NO 2 tropospheric columns over clean areas in the retrievals for stratosphere–troposphere separation is about 1 × 10 14 molec. cm -2. We find from GEOS-Chem that lightning is the main primary NO x source in the free troposphere over the tropics and southern midlatitudes, but aircraft emissions dominate at northern midlatitudes in winter and in summer over the oceans. Particulate nitrate photolysis increases ozone concentrations by up to 5 ppbv (parts per billion by volume) in the free troposphere in the northern extratropics in the model, which would largely correct the low model bias relative to ozonesonde observations. Global tropospheric OH concentrations increase by 19 %. The contribution of the free tropospheric background to the tropospheric NO 2 columns observed by satellites over the contiguous U.S. increases from 25 ± 11 % in winter to 65 ± 9 % in summer, according to the GEOS-Chem vertical profiles. This needs to be accounted for when deriving NO x emissions from satellite NO 2 column measurements. [ABSTRACT FROM AUTHOR]

Environmental chamber studies are widely employed to investigate atmospheric chemistry and aerosol formation. However, the large surface area-to-volume ratio of a chamber leads to effects that need to be accounted for in order to apply the results of chamber studies to the ambient atmosphere. These include, but are not limited to, gas-wall partitioning and particle deposition to walls, both of which can impact quantification of reaction products. Here, a low-cost vaporizer was developed to provide a means to rapidly create well-characterized organic vapors and aerosol particles inside the chamber. This in situ approach eliminates the losses to surfaces that can occur when organic gases or particles are created in a device outside the chamber and then transported inside through tubing, thus providing a simple means for achieving quantitative addition. Thermally stable volatile organic compounds can be added to the chamber within ∼1 min, which allows for accurate measurements of gas-wall partitioning timescales and equilibrium using gas chromatography. The vaporizer can also be used to create low-volatility organic aerosol particles with a mean diameter of ∼150 nm and selectable mass concentration as low as 5 µg m−3 via homogeneous nucleation within a few minutes. Such an aerosol can be used to calibrate or evaluate the performance of instruments such as the scanning mobility particle sizer in laboratory and field studies. Copyright © 2020 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Environmental chambers have been playing a vital role in atmospheric chemistry research for seven decades. In last decade, oxidation flow reactors (OFR) have emerged as a promising alternative to chambers to study complex multigenerational chemistry. OFR can generate higher-than-ambient concentrations of oxidants via H2O, O2 and O3 photolysis by low-pressure-Hg-lamp emissions and reach hours to days of equivalent photochemical aging in just minutes of real time. The use of OFR for volatile-organic-compound (VOC) oxidation and secondary-organic-aerosol formation has grown very rapidly recently. However, the lack of detailed understanding of OFR photochemistry left room for speculation that OFR chemistry may be generally irrelevant to the troposphere, since its initial oxidant generation is similar to stratosphere. Recently, a series of studies have been conducted to address important open questions on OFR chemistry and to guide experimental design and interpretation. In this Review, we present a comprehensive picture connecting the chemistries of hydroxyl (OH) and hydroperoxy radicals, oxidized nitrogen species and organic peroxy radicals (RO2) in OFR. Potential lack of tropospheric relevance associated with these chemistries, as well as the physical conditions resulting in it will also be reviewed. When atmospheric oxidation is dominated by OH, OFR conditions can often be similar to ambient conditions, as OH dominates against undesired non-OH effects. One key reason for tropospherically-irrelevant/undesired VOC fate is that under some conditions, OH is drastically reduced while non-tropospheric/undesired VOC reactants are not. The most frequent problems are running experiments with too high precursor concentrations, too high UV and/or too low humidity. On other hand, another cause of deviation from ambient chemistry in OFR is that some tropospherically-relevant non-OH chemistry (e.g. VOC photolysis in UVA and UVB) is not sufficiently represented under some conditions. In addition, the fate of RO2 produced from VOC oxidation can be kept relevant to the troposphere. However, in some cases RO2 lifetime can be too short for atmospherically-relevant RO2 chemistry, including its isomerization. OFR applications using only photolysis of injected O3 to generate OH are less preferable than those using both 185 and 254 nm photons (without O3 injection) for several reasons. When a relatively low equivalent photochemical age (<∼1 d) and high NO are needed, OH and NO generation by organic-nitrite photolysis in the UVA range is preferable. We also discuss how to achieve the atmospheric relevance for different purposes in OFR experimental planning. [ABSTRACT FROM AUTHOR]

Abstract: We describe the University of Washington airborne high‐resolution time‐of‐flight chemical ionization mass spectrometer (HRToF‐CIMS) and evaluate its performance aboard the NCAR‐NSF C‐130 aircraft during the recent Wintertime INvestigation of Transport, Emissions and Reactivity (WINTER) experiment in February–March of 2015. New features include (i) a computer‐controlled dynamic pinhole that maintains constant mass flow‐rate into the instrument independent of altitude changes to minimize variations in instrument response times; (ii) continuous addition of low flow‐rate humidified ultrahigh purity nitrogen to minimize the difference in water vapor pressure, hence instrument sensitivity, between ambient and background determinations; (iii) deployment of a calibration source continuously generating isotopically labeled dinitrogen pentoxide (15N2O5) for in‐flight delivery; and (iv) frequent instrument background determinations to account for memory effects resulting from the interaction between sticky compounds and instrument surface following encounters with concentrated air parcels. The resulting improvements to precision and accuracy, along with the simultaneous acquisition of these species and the full set of their isotopologues, allow for more reliable identification, source attribution, and budget accounting, for example, by speciating the individual constituents of nocturnal reactive nitrogen oxides (NOz = ClNO2 + 2 × N2O5 + HNO3 + etc.). We report on an expanded set of species quantified using iodide‐adduct ionization such as sulfur dioxide (SO2), hydrogen chloride (HCl), and other inorganic reactive halogen species including hypochlorous acid, nitryl chloride, chlorine, nitryl bromide, bromine, and bromine chloride (HOCl, ClNO2, Cl2, BrNO2, Br2, and BrCl, respectively). [ABSTRACT FROM AUTHOR]

Scientific research findings that span large concurrent field campaigns during the summer of 2013 in the southeast United States are synthesized and discussed. [ABSTRACT FROM AUTHOR]

Accurate modeling of the scattering and absorption of ultraviolet and visible radiation by aerosols is essential for accurate simulations of atmospheric chemistry and climate. Closure studies using in situ measurements of aerosol scattering and absorption can be used to evaluate and improve models of aerosol optical properties without interference from model errors in aerosol emissions, transport, chemistry, or deposition rates. Here we evaluate the ability of four externally mixed, fixed size distribution parameterizations used in global models to simulate submicron aerosol scattering and absorption at three wavelengths using in situ data gathered during the 2008 Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign. The four models are the NASA Global Modeling Initiative (GMI) Combo model, GEOS-Chem v9- 02, the baseline configuration of a version of GEOS-Chem with online radiative transfer calculations (called GC-RT), and the Optical Properties of Aerosol and Clouds (OPAC v3.1) package. We also use the ARCTAS data to perform the first evaluation of the ability of the Aerosol Simulation Program (ASP v2.1) to simulate submicron aerosol scattering and absorption when in situ data on the aerosol size distribution are used, and examine the impact of different mixing rules for black carbon (BC) on the results. We find that the GMI model tends to overestimate submicron scattering and absorption at shorter wavelengths by 10-23%, and that GMI has smaller absolute mean biases for submicron absorption than OPAC v3.1, GEOS-Chem v9-02, or GC-RT. However, the changes to the density and refractive index of BC in GCRT improve the simulation of submicron aerosol absorption at all wavelengths relative to GEOS-Chem v9-02. Adding a variable size distribution, as in ASP v2.1, improves model performance for scattering but not for absorption, likely due to the assumption in ASP v2.1 that BC is present at a constant mass fraction throughout the aerosol size distribution. Using a core-shell mixing rule in ASP overestimates aerosol absorption, especially for the fresh biomass burning aerosol measured in ARCTAS-B, suggesting the need for modeling the time-varying mixing states of aerosols in future versions of ASP. [ABSTRACT FROM AUTHOR]

Recent laboratory studies suggest that secondary organic aerosol (SOA) formation rates are higher than assumed in current models. There is also evidence that SOA removal by dry and wet deposition occurs more efficiently than some current models suggest and that photolysis and heterogeneous oxidation may be important (but currently ignored) SOA sinks. Here, we have updated the global GEOSChem model to include this new information on formation (i.e., wall-corrected yields and emissions of semi-volatile and intermediate volatility organic compounds) and on removal processes (photolysis and heterogeneous oxidation). We compare simulated SOA from various model configurations against ground, aircraft and satellite measurements to assess the extent to which these improved representations of SOA formation and removal processes are consistent with observed characteristics of the SOA distribution. The updated model presents a more dynamic picture of the life cycle of atmospheric SOA, with production rates 3.9 times higher and sinks a factor of 3.6 more efficient than in the base model. In particular, the updated model predicts larger SOA concentrations in the boundary layer and lower concentrations in the upper troposphere, leading to better agreement with surface and aircraft measurements of organic aerosol compared to the base model. Our analysis thus suggests that the long-standing discrepancy in model predictions of the vertical SOA distribution can now be resolved, at least in part, by a stronger source and stronger sinks leading to a shorter lifetime. The predicted global SOA burden in the updated model is 0.88 Tg and the corresponding direct radiative effect at top of the atmosphere is -0.33Wm-2, which is comparable to recent model estimates constrained by observations. The updated model predicts a population-weighed global mean surface SOA concentration that is a factor of 2 higher than in the base model, suggesting the need for a reanalysis of the contribution of SOA to PM pollution-related human health effects. The potential importance of our estimates highlights the need for more extensive field and laboratory studies focused on characterizing organic aerosol removal mechanisms and rates. [ABSTRACT FROM AUTHOR]

Field studies in polluted areas over the last decade have observed large formation of secondary organic aerosol (SOA) that is often poorly captured by models. The study of SOA formation using ambient data is often confounded by the effects of advection, vertical mixing, emissions, and variable degrees of photochemical aging. An oxidation flow reactor (OFR) was deployed to study SOA formation in real-time during the California Research at the Nexus of Air Quality and Climate Change (CalNex) campaign in Pasadena, CA, in 2010. A high-resolution aerosol mass spectrometer (AMS) and a scanning mobility particle sizer (SMPS) alternated sampling ambient and reactor-aged air. The reactor produced OH concentrations up to 4 orders of magnitude higher than in ambient air. OH radical concentration was continuously stepped, achieving equivalent atmospheric aging of 0.8 days- 6.4 weeks in 3 min of processing every 2 h. Enhancement of organic aerosol (OA) from aging showed a maximum net SOA production between 0.8-6 days of aging with net OA mass loss beyond 2 weeks. Reactor SOA mass peaked at night, in the absence of ambient photochemistry and correlated with trimethylbenzene concentrations. Reactor SOA formation was inversely correlated with ambient SOA and Ox, which along with the short-lived volatile organic compound correlation, indicates the importance of very reactive (τOH ~0.3 day) SOA precursors (most likely semivolatile and intermediate volatility species, S/IVOCs) in the Greater Los Angeles Area. Evolution of the elemental composition in the reactor was similar to trends observed in the atmosphere (O:C vs. H:C slope ~-0.65). Oxidation state of carbon (OSc) in reactor SOA increased steeply with age and remained elevated (OSC ~2) at the highest photochemical ages probed. The ratio of OA in the reactor output to excess CO (ΔCO, ambient CO above regional background) vs. photochemical age is similar to previous studies at low to moderate ages and also extends to higher ages where OA loss dominates. The mass added at low-to-intermediate ages is due primarily to condensation of oxidized species, not heterogeneous oxidation. The OA decrease at high photochem- ical ages is dominated by heterogeneous oxidation followed by fragmentation/evaporation. A comparison of urban SOA formation in this study with a similar study of vehicle SOA in a tunnel suggests the importance of vehicle emissions for urban SOA. Pre-2007 SOA models underpredict SOA formation by an order of magnitude, while a more recent model performs better but overpredicts at higher ages. These results demonstrate the value of the reactor as a tool for in situ evaluation of the SOA formation potential and OA evolution from ambient air. [ABSTRACT FROM AUTHOR]

Oxidation flow reactors (OFRs) using lowpressure Hg lamp emission at 185 and 254 nm produce OH radicals efficiently and are widely used in atmospheric chemistry and other fields. However, knowledge of detailed OFR chemistry is limited, allowing speculation in the literature about whether somenon-OH reactants are UV radiation, O(1D), O(3P), and O3. In this study, we investigate the relative importance of other reactants to OH for the fate of reactant species in OFR under a wide range of conditions via box modeling. The relative importance of non-OH species is less sensitive to UV light intensity than to water vapor mixing ratio (H2O) and external OH reactivity (OHRext), as both non-OH reactants and OH scale roughly proportionally to UV intensity. We show that for field studies in forested regions and also the urban area of Los Angeles, reactants of atmospheric interest are predominantly consumed by OH. We find that O(1D), O(3P), and O3 have relative contributions to volatile organic compound (VOC) consumption that are similar or lower than in the troposphere. The impact of O atoms can be neglected under most conditions in both OFR and troposphere. We define "riskier OFR conditions" as those with either low H2O (< 0.1 %) or high OHRext (⩾100 s-1 in OFR185 and > 200 s-1 in OFR254). We strongly suggest avoiding such conditions as the importance of non-OH reactants can be substantial for the most sensitive species, although OH may still dominate under some riskier conditions, depending on the species present. Photolysis at nontropospheric wavelengths (185 and 254 nm) may play a significant (> 20 %) role in the degradation of some aromatics, as well as some oxidation intermediates, under riskier reactor conditions, if the quantum yields are high. Under riskier conditions, some biogenics can have substantial destructions by O3, similarly to the troposphere.Working under low O2 (volume mixing ratio of 0.002) with the OFR185 mode allows OH to completely dominate over O3 reactions even for the biogenic species most reactive with O3. Non-tropospheric VOC photolysis may have been a problem in some laboratory and source studies, but can be avoided or lessened in future studies by diluting source emissions and working at lower precursor concentrations in laboratory studies and by humidification. Photolysis of secondary organic aerosol (SOA) samples is estimated to be significant (> 20 %) under the upper limit assumption of unity quantum yield at medium (1×1013 and 1:5×1015 photons cm-2 s-1 at 185 and 254 nm, respectively) or higher UV flux settings. The need for quantum yield measurements of both VOC and SOA photolysis is highlighted in this study. The results of this study allow improved OFR operation and experimental design and also inform the design of future reactors. [ABSTRACT FROM AUTHOR]

Quantifying the concentrations of trace atmospheric species in complex, reactive, and constantly changing gas and particle mixtures is challenging. This article provides a broad overview of recent advances in instrumentation used for analyzing ambient gases and particles continuously and with fast lime resolution during field campaigns. [ABSTRACT FROM AUTHOR]

The atmospheric evolution of organic compounds encompasses many thousands of compounds with varying volatility, polarity, and water solubility. The molecular-level chemical composition of this mixture plays a major, yet uncertain, role in its transformations and impacts. Here we perform a non-targeted molecular-level intercomparison of functionalized organic aerosol from three diverse field sites and a chamber. Despite similar bulk composition, we report large molecular-level variability between multi-hour organic aerosol samples at each site, with 66 ± 13% of functionalized compounds differing between consecutive samples. Single precursor environmental laboratory chamber experiments and fully chemically-explicit modeling confirm this variability is due to changes in emitted precursors, chemical age, and/or oxidation conditions. These molecular-level results demonstrate greater compositional variability than is typically observed in less-speciated measurements, such as bulk elemental composition, which tend to show less daily variability. These observations should inform future field and laboratory studies, including assessments of the effects of variability on aerosol properties and ultimately the development of strategic organic aerosol parameterizations for air quality and climate models. The detailed molecular-level speciation of organic aerosol is a highly challenging task. Here, the authors show an extensive molecular-level intercomparison of functionalized organic aerosol from three diverse field sites revealing large compositional variability between samples at each field site. [ABSTRACT FROM AUTHOR]

O'Dowd et al. reply - McFiggans raises some interesting, but partly speculative, issues about the possibility of additional condensable-iodine-vapour (CIV) precursors being involved in marine aerosol formation from biogenic iodine emissions, and about the relative roles of iodine oxide and sulphuric acid in the marine new-particle formation process. [ABSTRACT FROM AUTHOR]

Glyoxal and methylglyoxal are scavenged by clouds, where a fraction of these compounds are oxidized during the lifetime of the droplet. As a cloud droplet evaporates, the remaining glyoxal and methylglyoxal must either form low-volatility compounds such as oligomers and remain in the aerosol phase, or transfer back to the gas phase. A series of experiments on evaporating aqueous aerosol droplets indicates that over the atmospherically relevant concentration range for clouds and fog (4-1000 μM), 33 ± 11% of glyoxal and 19 ± 13% of methylglyoxal remains in the aerosol phase while the remainder evaporates. Measurements of aerosol density and time-dependent AMS signal changes are consistent with the formation of oligomers by each compound during the drying process. Unlike glyoxal, which forms acetal oligomers, exact mass AMS data indicates that the majority of methylglyoxal oligomers are formed by aldol condensation reactions, likely catalyzed by pyruvic acid, formed from methylglyoxal disproportionation. Our measurements of evaporation fractions can be used to estimate the global aerosol formation potential of glyoxal and methylglyoxal via self-reactions at and 1.6 Tg C yr-1, respectively. This is a factor of 4 less than the SOA formed by these compounds if their uptake is assumed to be irreversible. However, these estimates are likely lower limits for their total aerosol formation potential because oxidants and amines will also react with glyoxal and methylglyoxal to form additional low-volatility products. [ABSTRACT FROM AUTHOR]

Glyoxal, the simplest and most abundant α-dicarbonyl compound in the atmosphere, is scavenged by clouds and aerosol, where it reacts with nucleophiles to form low-volatility products. Here we examine the reactions of glyoxal with five amino acids common in clouds. When glyoxal and glycine, serine, aspartic acid or ornithine are present at concentrations as low as 30 μM in evaporating aqueous droplets or bulk solutions, 1,3-disubstituted imidazoles are formed in irreversible second-order reactions detected by nuclear magnetic resonance (NMR), aerosol mass spectrometry (AMS) and electrospray ionization mass spectrometry (ESI-MS). In contrast glyoxal reacts with arginine preferentially at side chain amino groups, forming nonaromatic five-membered rings. All reactions were accompanied by browning. The uptake of 45 ppb glyoxal by solid- phase glycine aerosol at 50% RH was also studied and found to cause particle growth and the production of imidazole measured by scanning mobility particle sizing and AMS, respectively, with a glyoxal uptake coefficient α = 0.0004. Comparison of reaction kinetics in bulk and in drying droplets shows that conversion of glyoxal dihydrate to monohydrate accelerates the reaction by over 3 orders of magnitude, allowing these reactions to occur at atmospheric conditions. [ABSTRACT FROM AUTHOR]

Iodine Detection in the Upper Troposphere and Lower StratosphereTheodore K. Koenig1,2, Pedro Campuzano Jost1,2, Benjamin A. Nault1,2, Jose L. Jimenez1,2, Alfonso Saiz-Lopez3, Rainer Volkamer1,21 Department of Chemistry, University of Colorado Boulder, Boulder, CO, USA2 Cooperative Institute for Research in Environmental Sciences (CIRES), Boulder, CO, USA3 Atmospheric Chemistry and Climate Group, Institute of Physical Chemistry Rocasolano, CSIC, Madrid, Spain Observations of iodine monoxide (IO) over the Eastern Pacific using scattered sunlight at low solar zenith angle have found significant IO in the tropical tropopause layer (TTL): 0.13±0.04 ppt in the northern hemisphere and 0.15±0.04 ppt in the southern hemisphere (Dix et al., 2013, 2016; Volkamer et al., 2015; Wang et al., 2015). Chemical transport modeling which reproduces these observations infers that between 0.25 to 0.70 ppt of total inorganic gas-phase iodine (Iy) is injected into the lower stratosphere (LS), a factor of approximately 2 to 5 more than the WMO upper limit of <0.15 ppt (Saiz-Lopez et al., 2015). At these levels, Iy is responsible for 30% of ozone destruction in the LS, an effect comparable to or greater than that of very short lived brominated species. WMO estimates currently do not account for iodine because quantitative evidence of iodine in the lower stratosphere is lacking. Previous measurements of iodine in the LS have been conducted at twilight (Bösch et al., 2003; Butz et al., 2009; Pundt et al., 1998; Wennberg et al., 1997). These have found upper limits of <0.1 ppt IO, and have informed the WMO upper limit on Iy. Iodine has been detected but not quantified in aerosol in the LS (Murphy et al., 2006, 2014; Murphy & Thomson, 2000).We present daytime scattered-sunlight limb observations of IO in the TTL and LS the University of Colorado Airborne Multiaxis Differential Optical Absorption Spectroscopy (CU AMAX-DOAS) instrument during the Convection Transport of Active Species in the Tropics (CONTRAST) campaign over the Western Pacific in 2014 (Pan et al., 2017). These include the first quantitative detection of IO in the LS. Utilizing a chemical box-model, we infer Iy from the measured IO. We also present the first quantification of iodine in submicron aerosol in the LS by the High Resolution Aerosol Mass Spectrometer (HR-AMS) from the Atmospheric Tomography Mission (ATom) over the Pacific and Atlantic in 2016 and 2017. Finally, the implications for iodine injection and ozone destruction in the LS from gas-phase and multi-phase catalytic cycles involving inorganic iodine are discussed. [ABSTRACT FROM AUTHOR]