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Abstract To quantify the volatility of organic aerosols (OA), a comprehensive campaign was conducted in the Chinese megacity. Volatility distributions of OA and particle‐phase organic nitrate (pON) were estimated based on five methods: (a) empirical method and (b) kinetic model based on the measurement of a thermodenuder (TD) coupled with an aerosol mass spectrometer; (c) Formula‐based SIMPOL model‐driven method; (d) Element‐based estimations using molecular formula measurements of OA; and (e) gas/particle partitioning. Our results demonstrate that the ambient OA volatility distribution shows good agreement between the two heating methods and the formula‐based method when assuming ambient OA was mainly composed of organic nitrate (pON), organic sulfate and acid groups using the SIMPOL model. However, the element‐based method tends to overestimate the volatility of OA compared to the above three methods, suggesting large uncertainties in the parameterizations or in the representativeness of the molecular measurements that need further refinement. The volatility of ambient OA is generally lower than that of the laboratory‐derived secondary OA, emphasizing the impact of aging. A large fraction at the higher and lower volatility ranges (approximately logC* ≤ −9 and ≥2 μg m⁻³) was found for pON, implying the importance of both extremely low volatile and semi‐volatile species. Overall, this study evaluates different methods for volatility estimation and gives new insight into the volatility of OA and pON in urban areas. 

Climate change will bring about changes in meteorological and ecological factors that are currently used in global-scale models to calculate biogenic emissions. By comparing long-term datasets of biogenic compounds to modeled emissions, this work seeks to improve understanding of these models and their driving factors. We compare speciated biogenic volatile organic compound (BVOC) measurements at the Virginia Forest Research Laboratory located in Fluvanna County, VA, USA, for the year 2020 with emissions estimated by the Model of Emissions of Gases and Aerosols from Nature version 3.2 (MEGANv3.2). The emissions were subjected to oxidation in a 0-D box model (F0AM v4.3) to generate time series of modeled concentrations. We find that default light-dependent fractions (LDFs) in the emissions model do not accurately represent observed temporal variability in regional observations. Some monoterpenes with a default light dependence are better represented using light-independent emissions throughout the year (LDFα-pinene=0, as opposed to 0.6), while others are best represented using a seasonally or temporally dependent light dependence. For example, limonene has the highest correlation between modeled and measured concentrations using an LDF =0 for January through April and roughly 0.74–0.97 in the summer months, in contrast to the default value of 0.4. The monoterpenes β-thujene, sabinene, and γ-terpinene similarly have an LDF that varies throughout the year, with light-dependent behavior in summer, while camphene and α-fenchene follow light-independent behavior throughout the year. Simulations of most compounds are consistently underpredicted in the winter months compared to observed concentrations. In contrast, day-to-day variability in the concentrations during summer months are relatively well captured using the coupled emissions–chemistry model constrained by regional concentrations of NOX and O3. 

Abstract. Isoprene has the largest global non-methane hydrocarbon emission, and the oxidation of isoprene plays a crucial role in the formation of secondary organic aerosol (SOA). Two primary processes are known to contribute to SOA formation from isoprene oxidation: (1) the reactive uptake of isoprene-derived epoxides on acidic or aqueous particle surfaces and (2) the absorptive gas–particle partitioning of low-volatility oxidation products. In this study, we developed a new multiphase condensed isoprene oxidation mechanism that includes these processes with key molecular intermediates and products. The new mechanism was applied to simulate isoprene gas-phase oxidation products and SOA formation from previously published chamber experiments under a variety of conditions and atmospheric observations during the Southern Oxidant and Aerosol Studies (SOAS) field campaign. Our results show that SOA formation from most of the chamber experiments is reasonably reproduced using our mechanism, except when the concentration ratios of initial nitric oxide to isoprene exceed ∼ 2, the formed SOA is significantly underpredicted. The SOAS simulations also reasonably agree with the measurements regarding the diurnal pattern and concentrations of different product categories, while the total isoprene SOA remains underestimated. The molecular compositions of the modeled SOA indicate that multifunctional low-volatility products contribute to isoprene SOA more significantly than previously thought, with a median mass contribution of ∼ 57 % to the total modeled isoprene SOA. However, this contribution is intricately intertwined with IEPOX-derived SOA (IEPOX: isoprene-derived epoxydiols), posing challenges for their differentiation using bulk aerosol composition analysis (e.g., the aerosol mass spectrometer with positive matrix factorization). Furthermore, the SOA from these pathways may vary greatly, mainly dependent on the volatility estimation and treatment of particle-phase processes (i.e., photolysis and hydrolysis). Our findings emphasize that the various pathways to produce these low-volatility species should be considered in models to more accurately predict isoprene SOA formation. The new condensed isoprene chemical mechanism can be further incorporated into regional-scale air quality models, such as the Community Multiscale Air Quality Modelling System (CMAQ), to assess isoprene SOA formation on a larger scale. 

Volatile organic compounds (VOCs) contribute to air pollution both directly, as hazardous gases, and through their reactionswith common atmospheric oxidants to produce ozone, particulate matter, andother hazardous air pollutants. There are enormous ranges of structures andreaction rates among VOCs, and there is consequently a need to accuratelycharacterize the spatial and temporal distribution of individual identifiedcompounds. Current VOC measurements are often made with complex, expensiveinstrumentation that provides high chemical detail but is limited in itsportability and requires high expense (e.g., mobile labs) for spatiallyresolved measurements. Alternatively, periodic collection of samples oncartridges is inexpensive but demands significant operator interaction thatcan limit possibilities for time-resolved measurements or distributedmeasurements across a spatial area. Thus, there is a need for simple,portable devices that can sample with limited operator presence to enabletemporally and/or spatially resolved measurements. In this work, we describenew portable and programmable VOC samplers that enable simultaneouscollection of samples across a spatially distributed network, validate theirreproducibility, and demonstrate their utility. Validation experimentsconfirmed high precision between samplers as well as the ability ofminiature ozone scrubbers to preserve reactive analytes collected oncommercially available adsorbent gas sampling cartridges, supportingsimultaneous field deployment across multiple locations. In indoorenvironments, 24 h integrated samples demonstrate observable day-to-dayvariability, as well as variability across very short spatial scales(meters). The utility of the samplers was further demonstrated by locatingoutdoor point sources of analytes through the development of a new mappingapproach that employs a group of the portable samplers and back-projectiontechniques to assess a sampling area with higher resolution than stationarysampling. As with all gas sampling, the limits of detection depend onsampling times and the properties of sorbents and analytes. The limit of detectionof the analytical system used in this work is on the order of nanograms,corresponding to mixing ratios of 1–10 pptv after 1 h of sampling atthe programmable flow rate of 50–250 sccm enabled by the developed system.The portable VOC samplers described and validated here provide a simple,low-cost sampling solution for spatially and/or temporally variablemeasurements of any organic gases that are collectable on currentlyavailable sampling media. 

Volatile organic compounds (VOCs) range in their reaction rates with atmospheric oxidants by several orders of magnitude. Therefore, studying their atmospheric concentrations across seasons and years requires isomer resolution to fully understand their impact on oxidant budgets and secondary organic aerosol formation. An automated gas chromatograph/flame ionization detector (GC-FID) was developed for hourly sampling and analysis of C 5 –C 15 hydrocarbons at remote locations. Samples are collected on an air-cooled multibed adsorbent trap for preconcentration of hydrocarbons in the target volatility range, specifically designed to minimize dead volume and enable rapid heating and sample flushing. Instrument control uses custom electronics designed to allow flexible autonomous operation at moderate cost, with automated data transfer and processing. The instrument has been deployed for over two years with samples collected mid-canopy from the Virginia Forest Laboratory located in the Pace research forest in central Virginia. We present here the design of the instrument itself, control electronics, and calibration and data analysis approaches to facilitate the development of similar systems by the atmospheric chemistry community. Detection limits of all species are in the range of a few to tens of ppt and the instrument is suitable for detection of a wide range of biogenic, lightly oxygenated, and anthropogenic (predominantly hydrocarbon) compounds. Data from calibrations are examined to provide understanding of instrument stability and quantify uncertainty. In this work, we present challenges and recommendations for future deployments, as well as suggested adaptions to decrease required maintenance and increase instrument up-time. The presented design is particularly suitable for long-term and remote deployment campaigns where access, maintenance, and transport of materials are difficult.