Michael Lewandowski

Organosulfate Formation in Biogenic Secondary Organic Aerosol

Organosulfates of isoprene, alpha-pinene, and beta-pinene have recently been identified in both laboratory-generated and ambient secondary organic aerosol (SOA). In this study, the mechanism and ubiquity of organosulfate formation in biogenic SOA is investigated by a comprehensive series of laboratory photooxidation (i.e., OH-initiated oxidation) and nighttime oxidation (i.e., NO3-initiated oxidation under dark conditions) experiments using nine monoterpenes (alpha-pinene, beta-pinene, d-limonene, l-limonene, alpha-terpinene, gamma-terpinene, terpinolene, Delta(3)-carene, and beta-phellandrene) and three monoterpenes (alpha-pinene, d-limonene, and l-limonene), respectively. Organosulfates were characterized using liquid chromatographic techniques coupled to electrospray ionization combined with both linear ion trap and high-resolution time-of-flight mass spectrometry. Organosulfates are formed only when monoterpenes are oxidized in the presence of acidified sulfate seed aerosol, a result consistent with prior work. Archived laboratory-generated isoprene SOA and ambient filter samples collected from the southeastern U.S. were reexamined for organosulfates. By comparing the tandem mass spectrometric and accurate mass measurements collected for both the laboratory-generated and ambient aerosol, previously uncharacterized ambient organic aerosol components are found to be organosulfates of isoprene, alpha-pinene, beta-pinene, and limonene-like monoterpenes (e.g., myrcene), demonstrating the ubiquity of organosulfate formation in ambient SOA. Several of the organosulfates of isoprene and of the monoterpenes characterized in this study are ambient tracer compounds for the occurrence of biogenic SOA formation under acidic conditions. Furthermore, the nighttime oxidation experiments conducted under highly acidic conditions reveal a viable mechanism for the formation of previously identified nitrooxy organosulfates found in ambient nighttime aerosol samples. We estimate that the organosulfate contribution to the total organic mass fraction of ambient aerosol collected from K-puszta, Hungary, a field site with a similar organosulfate composition as that found in the present study for the southeastern U.S., can be as high as 30%.

Epoxide Pathways Improve Model Predictions of Isoprene Markers and Reveal Key Role of Acidity in Aerosol Formation

Isoprene significantly contributes to organic aerosol in the southeastern United States where biogenic hydrocarbons mix with anthropogenic emissions. In this work, the Community Multiscale Air Quality model is updated to predict isoprene aerosol from epoxides produced under both high- and low-NOx conditions. The new aqueous aerosol pathways allow for explicit predictions of two key isoprene-derived species, 2-methyltetrols and 2-methylglyceric acid, that are more consistent with observations than estimates based on semivolatile partitioning. The new mechanism represents a significant source of organic carbon in the lower 2 km of the atmosphere and captures the abundance of 2-methyltetrols relative to organosulfates during the simulation period. For the parametrization considered here, a 25% reduction in SOx emissions effectively reduces isoprene aerosol, while a similar reduction in NOx leads to small increases in isoprene aerosol.

Organosulfates as Tracers for Secondary Organic Aerosol (SOA) Formation from 2-Methyl-3-Buten-2-ol (MBO) in the Atmosphere

2-Methyl-3-buten-2-ol (MBO) is an important biogenic volatile organic compound (BVOC) emitted by pine trees and a potential precursor of atmospheric secondary organic aerosol (SOA) in forested regions. In the present study, hydroxyl radical (OH)-initiated oxidation of MBO was examined in smog chambers under varied initial nitric oxide (NO) and aerosol acidity levels. Results indicate measurable SOA from MBO under low-NO conditions. Moreover, increasing aerosol acidity was found to enhance MBO SOA. Chemical characterization of laboratory-generated MBO SOA reveals that an organosulfate species (C5H12O6S, MW 200) formed and was substantially enhanced with elevated aerosol acidity. Ambient fine aerosol (PM2.5) samples collected from the BEARPEX campaign during 2007 and 2009, as well as from the BEACHON-RoMBAS campaign during 2011, were also analyzed. The MBO-derived organosulfate characterized from laboratory-generated aerosol was observed in PM2.5 collected from these campaigns, demonstrating that it is a molecular tracer for MBO-initiated SOA in the atmosphere. Furthermore, mass concentrations of the MBO-derived organosulfate are well correlated with MBO mixing ratio, temperature, and acidity in the field campaigns. Importantly, this compound accounted for an average of 0.25% and as high as 1% of the total organic aerosol mass during BEARPEX 2009. An epoxide intermediate generated under low-NO conditions is tentatively proposed to produce MBO SOA.

Contribution of Primary and Secondary Sources to Organic Aerosol and PM2.5 at SEARCH Network Sites

Abstract Chemical tracer methods for determining contributions to primary organic aerosol (POA) are fairly well established, whereas similar techniques for secondary organic aerosol (SOA), inherently complicated by time-dependent atmospheric processes, are only beginning to be studied. Laboratory chamber experiments provide insights into the precursors of SOA, but field data must be used to test the approaches. This study investigates primary and secondary sources of organic carbon (OC) and determines their mass contribution to particulate matter 2.5 µm or less in aerodynamic diameter (PM2.5) in Southeastern Aerosol Research and Characterization (SEARCH) network samples. Filter samples were taken during 20 24-hr periods between May and August 2005 at SEARCH sites in Atlanta, GA (JST); Birmingham, AL (BHM); Centerville, AL (CTR); and Pensacola, FL (PNS) and analyzed for organic tracers by gas chromatography-mass spectrometry. Contribution to primary OC was made using a chemical mass balance method and to secondary OC using a mass fraction method. Aerosol masses were reconstructed from the contributions of POA, SOA, elemental carbon, inorganic ions (sulfate [SO4 2−], nitrate [NO3 −], ammonium [NH4 +]), metals, and metal oxides and compared with the measured PM2.5. From the analysis, OC contributions from seven primary sources and four secondary sources were determined. The major primary sources of carbon were from wood combustion, diesel and gasoline exhaust, and meat cooking; major secondary sources were from isoprene and monoterpenes with minor contributions from toluene and β-caryophyllene SOA. Mass concentrations at the four sites were determined using source-specific organic mass (OM)-to-OC ratios and gave values in the range of 12–42 µg m−3. Reconstructed masses at three of the sites (JST, CTR, PNS) ranged from 87 to 91% of the measured PM2.5 mass. The reconstructed mass at the BHM site exceeded the measured mass by approximately 25%. The difference between the reconstructed and measured PM2.5 mass for nonindustrial areas is consistent with not including aerosol liquid water or other sources of organic aerosol.

Influence of Aerosol Acidity on the Formation of Secondary Organic Aerosol from Biogenic Precursor Hydrocarbons

Secondary organic carbon (SOC) concentrations in steady-state aerosol were measured in a series of alpha-pinene/NOx and one series of beta-caryophyllene/NOx irradiation experiments. The acidity of the inorganic seed aerosol was varied while the hydrocarbon and NOx concentrations were held constant in each series of experiments. Measurements were made for acidity levels and SOC concentrations much closer to ambient levels than had been previously achieved for alpha-pinene, while there are no previous measurements for SOC increases due to acidity for beta-caryophyllene. The observed enhancement in SOC concentration linearly increases with the measured hydrogen ion concentration in air for each system. For the conditions of these studies, SOC increased by 0.04% per nmol H+ m(-3) for alpha-pinene under two conditions where the organic carbon concentration differed by a factor of 5. For alpha-pinene, this level of response to acidic aerosol was a factor of 8 lower than was reported by Surratt et al. for similar series of experiments for SOC from the photooxidation of isoprene/NOx mixtures. By contrast, SOC from beta-caryophyllene showed an increase of 0.22% per nmol H+ m(-3), roughly two-thirds of the response in the isoprene system. Mass fractions for SOC particle-phase tracers for alpha-pinene decreased slightly with increasing aerosol acidity, although remaining within previously stated uncertainties. Below 200 nmol H+ m(-3), the mass fraction of beta-caryophyllenic acid, the only identified tracer for beta-caryophyllene SOC, was constant although beta-caryophyllenic acid showed a substantial decrease for acidities greater than 400 nmol H+ m(-3).

Secondary organic aerosol characterisation at field sites across the United States during the spring–summer period

Sources of secondary organic carbon at 15 field sites across the United States (U.S.) during the years 2003–2010 have been examined. Filter samples have been taken for 24-h at a site in Research Triangle Park, NC; at SEARCH sites in southeastern U.S. during May and August 2005; at LADCO sites from Mar 2004–Feb 2005; Riverside, CA during SOAR in 2005; Cleveland, OH during CMAPS; and Pasadena and Bakersfield, CA during CalNex (see text for acronyms.) Samples were extracted, derivatised, and analysed for organic tracers by GC-MS. The mass fraction method described by Kleindienst et al. was used to determine the contributions of the tracers to secondary organic carbon mass. Secondary organic aerosol masses were determined using laboratory-derived values for the organic mass–organic carbon (OM/OC) ratio. Results from the analysis show secondary organic carbon in the eastern and midwestern U.S. to be consistently dominated by SOA from biogenic emissions during the spring–summer period. SOA from biogenic emissions are far less important in the western U.S. during the same period with isoprene emissions being particularly weak. These sites in the western U.S. are in more densely populated, polluted regions of California and are probably not representative of sites in the rural western U.S. The ratio of tracers from monoterpenes can also provide information regarding presumed sources. Similarly, the ratio of isoprene tracers can provide information on reaction pathways (NOX vs. non-NOX) leading to the formation of SOA in the atmosphere. Updated tables for the identity and fragmentation of SOA molecular tracers and for mass fractions of four biogenic class types (isoprene, monoterpenes, sesquiterpenes, 2-methyl-3-buten-2-ol) and two anthropogenic class types (aromatic hydrocarbons and 2-ring PAHs) are given.

Collection Efficiency of the Aerosol Mass Spectrometer for Chamber-Generated Secondary Organic Aerosols

The collection efficiency (CE) of the aerosol mass spectrometer (AMS) for chamber-generated secondary organic aerosol (SOA) at elevated mass concentrations (range: 19–207 μg m−3; average: 64 μg m−3) and under dry conditions was investigated by comparing AMS measurements to scanning mobility particle sizer (SMPS), Sunset semi-continuous carbon monitor (Sunset), and gravimetric filter measurements. While SMPS and Sunset measurements are consistent with gravimetric filter measurements throughout a series of reactions with varying parent hydrocarbon/oxidant combinations, AMS CE values were highly variable ranging from unity to <15%. The majority of mass discrepancy reflected by low CE values does not appear to be due to particle losses either in the aerodynamic lens system or in the vacuum chamber as the contributions of these mechanisms to CE are low and negligible, respectively. As a result, the largest contribution to CE in the case of chamber-generated SOA appears to be due to particle bounce at the vaporizer surface before volatilization, which is consistent with earlier studies that have investigated the CE of ambient and select laboratory-generated particles. CE values obtained throughout the series of reactions conducted here are also well correlated with the f 44/f 57 ratio, thereby indicating both that the composition of the organic fraction has an important impact on the CE of chamber-generated SOA and that this effect may be linked to the extent to which the organic fraction is oxidized. Copyright 2013 American Association for Aerosol Research

Secondary organic aerosol formation from the oxidation of a series of sesquiterpenes: α-cedrene, β-caryophyllene, α-humulene and α-farnesene with O3, OH and NO3 radicals

Environmental context Sesquiterpenes, chemicals emitted by terrestrial vegetation, are oxidised in the ambient atmosphere leading to the formation of secondary organic aerosol. Although secondary organic aerosol can have significant effects on air quality from local to global scales, considerable gaps remain in our understanding of their various sources and formation mechanisms. We report studies on the oxidation of sesquiterpenes aimed at improving aerosol parameterisation for these reactions for incorporation into future air quality models. Abstract A series of sesquiterpenes (SQT) were individually oxidised under a range of conditions, including irradiation in the presence of NOx, reactions with O3 or reactions with NO3 radicals. Experiments were conducted in either static mode to observe temporal evolution of reactants and products or in dynamic mode to ensure adequate collection of aerosol at reasonably low reactant concentrations. Although some measurements of gas-phase products have been made, the focus of this work has been particle phase analysis. To identify individual products, filter samples were extracted, derivatised and analysed using gas chromatography mass spectrometry techniques. The results indicate that secondary organic aerosol (SOA) is readily formed from SQT oxidation. The high reactivity of these systems and generally high conversion into SOA products gives rise to high SOA levels. SOA yields (ratio of SOA formed to hydrocarbon reacted) averaged 0.53 for ozonolysis, 0.55 for photooxidation and 1.19 for NO3 reactions. In select experiments, SOA was also analysed for the organic matter/organic carbon (OM/OC) ratio, and the effective enthalpy of vaporisation (ΔHvapeff). The OM/OC ranged from 1.8 for ozonolysis and photooxidation reactions to 1.6 for NO3 reactions, similar to that from SOA generated in monoterpene systems. ΔHvapeff was measured for β-caryophyllene–NOx, β-caryophyllene–O3, β-caryophyllene–NO3, α-humulene–NOx and α-farnesene–NOx systems and found to be 43.9, 41.1, 44.9, 48.2 and 27.7 kJ mol–1. Aerosol yields and products identified in this study are generally in good agreement with results from several studies. A detailed examination of the chamber aerosol for the presence of chemical tracer compounds was undertaken. Only β-caryophyllinic acid, observed mainly under β-caryophyllene photooxidation and ozonolysis experiments, was detected in ambient aerosol. Chemical analysis yielded compounds having oxygen and nitrogen moieties present, which indicates continued evolution of the particles over time and presents high dependence on the SQT–oxidant system studied. This study suggests that SOA from laboratory ozonolysis experiments may adequately represent ambient aerosol in areas with SQT emissions.

A review of selected engineered nanoparticles in the atmosphere: sources, transformations, and techniques for sampling and analysis.

Abstract A state-of-the-science review was undertaken to identify and assess sampling and analysis methods to detect and quantify selected nanomaterials (NMs) in the ambient atmosphere. The review is restricted to five types of NMs of interest to the Office of Research and Development Nanomaterial Research Strategy (US Environmental Protection Agency): cerium oxide, titanium dioxide, carbon nanostructures (carbon nanotubes and fullerenes), zero-valent iron, and silver nanoparticles. One purpose was determining the extent to which present-day ultrafine sampling and analysis methods may be sufficient for identifying and possibly quantifying engineered NMs (ENMs) in ambient air. Conventional sampling methods for ultrafines appear to require modifications. For cerium and titanium, background levels from natural sources make measurement of ENMs difficult to quantify. In cases where field studies have been performed, identification from bulk analysis samples have been made. Further development of methods is needed to identify these NMs, especially in specific size fractions of ambient aerosols.

Characterization of polar organosulfates in secondary organic aerosol from the green leaf volatile 3-Z-hexenal.

Evidence is provided that the green leaf volatile 3-Z-hexenal serves as a precursor for biogenic secondary organic aerosol through the formation of polar organosulfates (OSs) with molecular weight (MW) 226. The MW 226 C6-OSs were chemically elucidated, along with structurally similar MW 212 C5-OSs, whose biogenic precursor is likely related to 3-Z-hexenal but still remains unknown. The MW 226 and 212 OSs have a substantial abundance in ambient fine aerosol from K-puszta, Hungary, which is comparable to that of the isoprene-related MW 216 OSs, known to be formed through sulfation of C5-epoxydiols, second-generation gas-phase photooxidation products of isoprene. Using detailed interpretation of negative-ion electrospray ionization mass spectral data, the MW 226 compounds are assigned to isomeric sulfate esters of 3,4-dihydroxyhex-5-enoic acid with the sulfate group located at the C-3 or C-4 position. Two MW 212 compounds present in ambient fine aerosol are attributed to isomeric sulfate esters of 2,3-dihydroxypent-4-enoic acid, of which two are sulfated at C-3 and one is sulfated at C-2. The formation of the MW 226 OSs is tentatively explained through photooxidation of 3-Z-hexenal in the gas phase, resulting in an alkoxy radical, followed by a rearrangement and subsequent sulfation of the epoxy group in the particle phase.