Regional Similarities and NOx‐Related Increases in Biogenic Secondary Organic Aerosol in Summertime Southeastern United States

During the 2013 Southern Oxidant and Aerosol Study, Fourier transform infrared spectroscopy (FTIR) and aerosol mass spectrometer (AMS) measurements of submicron mass were collected at Look Rock (LRK), Tennessee, and Centreville (CTR), Alabama. Carbon monoxide and submicron sulfate and organic mass concentrations were 15–60% higher at CTR than at LRK, but their time series had moderate correlations (r ~ 0.5). However, NOx had no correlation (r = 0.08) between the two sites with nighttime‐to‐early‐morning peaks 3–10 times higher at CTR than at LRK. Organic mass (OM) sources identified by FTIR Positive Matrix Factorization (PMF) had three very similar factors at both sites: fossil fuel combustion‐related organic aerosols, mixed organic aerosols, and biogenic organic aerosols (BOA). The BOA spectrum from FTIR is similar (cosine similarity > 0.6) to that of lab‐generated particle mass from the photochemical oxidation of both isoprene and monoterpenes under high NOx conditions from chamber experiments. The BOA mass fraction was highest during the night at CTR but in the afternoon at LRK. AMS PMF resulted in two similar pairs of factors at both sites and a third nighttime NOx‐related factor (33% of OM) at CTR but a daytime nitrate‐related factor (28% of OM) at LRK. NOx was correlated with BOA and LO‐OOA for NOx concentrations higher than 1 ppb at both sites, producing 0.5 ± 0.1 μg/m3 for CTR‐LO‐OOA and 1.0 ± 0.3 μg/m3 for CTR‐BOA additional biogenic OM for each 1 ppb increase of NOx.Key PointsAerosol concentration and composition are largely similar at two different forested sites during summertime in the southeastern United StatesFTIR of ambient biogenic SOA factors are similar to isoprene and monoterpene chamber experiment, supporting NOx‐related oxidation pathwaysNOx increases biogenic SOA by 0.5 ± 0.1 μg/m3 for CTR‐LO‐OOA and 1.0 ± 0.3 μg/m3 for CTR‐BOA for each ppb NOx above 1 ppb at Centreville but not at Look Rock (where NOx was usually below 1 ppb)Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146465/1/jgrd54860-sup-0001-SI.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146465/2/jgrd54860.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146465/3/jgrd54860_am.pd

An in situ gas chromatograph with automatic detector switching between PTR- and EI-TOF-MS: isomer-resolved measurements of indoor air

We have developed a field-deployable gas chromatograph (GC) with thermal desorption preconcentration (TDPC), which is demonstrated here with automatic detector switching between two high-resolution time-of-flight mass spectrometers (TOF-MSs) for in situ measurements of volatile organic compounds (VOCs). This system provides many analytical advances, including acquisition of fast time&ndash;response data in tandem with molecular speciation and two types of mass spectral information for each resolved GC peak: molecular ion identification from Vocus proton transfer reaction (PTR) TOF-MS and fragmentation pattern from electron ionization (EI) TOF-MS detection. This system was deployed during the 2018 ATHLETIC campaign at the University of Colorado Dal Ward Athletic Center in Boulder, Colorado, where it was used to characterize VOC emissions in the indoor environment. The addition of the TDPC-GC increased the Vocus sensitivity by a factor of 50 due to preconcentration over a 6&thinsp;min GC sample time versus direct air sampling with the Vocus, which was operated with a time resolution of 1&thinsp;Hz. The GC-TOF methods demonstrated average limits of detection of 1.6&thinsp;ppt across a range of monoterpenes and aromatics. Here, we describe the method to use the two-detector system to conclusively identify a range of VOCs including hydrocarbons, oxygenates, and halocarbons, along with detailed results including the quantification of anthropogenic monoterpenes, where limonene accounted for 47&thinsp;%&ndash;80&thinsp;% of the indoor monoterpene composition. We also report the detection of dimethylsilanediol (DMSD), an organosiloxane degradation product, which was observed with dynamic temporal behavior distinct from volatile organosiloxanes (e.g., decamethylcyclopentasiloxane, D5 siloxane). Our results suggest DMSD is produced from humidity-dependent heterogeneous reactions occurring on surfaces in the indoor environment, rather than formed through gas-phase oxidation of volatile siloxanes. &nbsp;</p

Regional Similarities and NO_x-Related Increases in Biogenic Secondary Organic Aerosol in Summertime Southeastern United States

During the 2013 Southern Oxidant and Aerosol Study, Fourier transform infrared spectroscopy (FTIR) and aerosol mass spectrometer (AMS) measurements of submicron mass were collected at Look Rock (LRK), Tennessee, and Centreville (CTR), Alabama. Carbon monoxide and submicron sulfate and organic mass concentrations were 15–60% higher at CTR than at LRK, but their time series had moderate correlations (r ~ 0.5). However, NO_x had no correlation (r = 0.08) between the two sites with nighttime‐to‐early‐morning peaks 3–10 times higher at CTR than at LRK. Organic mass (OM) sources identified by FTIR Positive Matrix Factorization (PMF) had three very similar factors at both sites: fossil fuel combustion‐related organic aerosols, mixed organic aerosols, and biogenic organic aerosols (BOA). The BOA spectrum from FTIR is similar (cosine similarity > 0.6) to that of lab‐generated particle mass from the photochemical oxidation of both isoprene and monoterpenes under high NO_x conditions from chamber experiments. The BOA mass fraction was highest during the night at CTR but in the afternoon at LRK. AMS PMF resulted in two similar pairs of factors at both sites and a third nighttime NOx‐related factor (33% of OM) at CTR but a daytime nitrate‐related factor (28% of OM) at LRK. NO_x was correlated with BOA and LO‐OOA for NO_x concentrations higher than 1 ppb at both sites, producing 0.5 ± 0.1 μg/m^3 for CTR‐LO‐OOA and 1.0 ± 0.3 μg/m^3 for CTR‐BOA additional biogenic OM for each 1 ppb increase of NO_x

Regional Similarities and NO_x-Related Increases in Biogenic Secondary Organic Aerosol in Summertime Southeastern United States

During the 2013 Southern Oxidant and Aerosol Study, Fourier transform infrared spectroscopy (FTIR) and aerosol mass spectrometer (AMS) measurements of submicron mass were collected at Look Rock (LRK), Tennessee, and Centreville (CTR), Alabama. Carbon monoxide and submicron sulfate and organic mass concentrations were 15–60% higher at CTR than at LRK, but their time series had moderate correlations (r ~ 0.5). However, NO_x had no correlation (r = 0.08) between the two sites with nighttime‐to‐early‐morning peaks 3–10 times higher at CTR than at LRK. Organic mass (OM) sources identified by FTIR Positive Matrix Factorization (PMF) had three very similar factors at both sites: fossil fuel combustion‐related organic aerosols, mixed organic aerosols, and biogenic organic aerosols (BOA). The BOA spectrum from FTIR is similar (cosine similarity > 0.6) to that of lab‐generated particle mass from the photochemical oxidation of both isoprene and monoterpenes under high NO_x conditions from chamber experiments. The BOA mass fraction was highest during the night at CTR but in the afternoon at LRK. AMS PMF resulted in two similar pairs of factors at both sites and a third nighttime NOx‐related factor (33% of OM) at CTR but a daytime nitrate‐related factor (28% of OM) at LRK. NO_x was correlated with BOA and LO‐OOA for NO_x concentrations higher than 1 ppb at both sites, producing 0.5 ± 0.1 μg/m^3 for CTR‐LO‐OOA and 1.0 ± 0.3 μg/m^3 for CTR‐BOA additional biogenic OM for each 1 ppb increase of NO_x

Products and Mechanism of the Reaction of 1‑Pentadecene with NO₃ Radicals and the Effect of a −ONO₂ Group on Alkoxy Radical Decomposition

The linear C₁₅ alkene, 1-pentadecene, was reacted with NO₃ radicals in a Teflon environmental chamber and yields of secondary organic aerosol (SOA) and particulate β-hydroxynitrates, β-carbonylnitrates, and organic peroxides (β-nitrooxyhydroperoxides + dinitrooxyperoxides) were quantified using a variety of methods. Reaction occurs almost solely by addition of NO₃ to the CC double bond and measured yields of β-hydroxynitrate isomers indicate that 92% of addition occurs at the terminal carbon. Molar yields of reaction products determined from measurements, a proposed reaction mechanism, and mass-balance considerations were 0.065 for β-hydroxynitrates (0.060 and 0.005 for 1-nitrooxy-2-hydroxy­pentadecane and 1-hydroxy-2-nitrooxy­pentadecane isomers), 0.102 for β-carbonylnitrates, 0.017 for organic peroxides, 0.232 for β-nitrooxyalkoxy radical isomerization products, and 0.584 for tetradecanal and formaldehyde, the volatile C₁₄ and C₁ products of β-nitrooxyalkoxy radical decomposition. Branching ratios for decomposition and isomerization of β-nitrooxyalkoxy radicals were 0.716 and 0.284 and should be similar for other linear 1-alkenes ≥ C₆ whose alkyl chains are long enough to allow for isomerization to occur. These branching ratios have not been measured previously, and they differ significantly from those estimated using structure–activity relationships, which predict >99% isomerization. It appears that the presence of a −ONO₂ group adjacent to an alkoxy radical site greatly enhances the rate of decomposition relative to isomerization, which is otherwise negligible, and that the effect is similar to that of a −OH group. The results provide insight into the effects of molecular structure on mechanisms of oxidation of volatile organic compounds and should be useful for improving structure–activity relationships that are widely used to predict the fate of these compounds in the atmosphere and for modeling SOA formation and aging

Hygroscopicity of Organic Compounds as a Function of Carbon Chain Length and Carboxyl, Hydroperoxy, and Carbonyl Functional Groups

The albedo and microphysical properties of clouds are controlled in part by the hygroscopicity of particles serving as cloud condensation nuclei (CCN). Hygroscopicity of complex organic mixtures in the atmosphere varies widely and remains challenging to predict. Here we present new measurements characterizing the CCN activity of pure compounds in which carbon chain length and the numbers of hydroperoxy, carboxyl, and carbonyl functional groups were systematically varied to establish the contributions of these groups to organic aerosol apparent hygroscopicity. Apparent hygroscopicity decreased with carbon chain length and increased with polar functional groups in the order carboxyl > hydroperoxy > carbonyl. Activation diameters at different supersaturations deviated from the −3/2 slope in log–log space predicted by Köhler theory, suggesting that water solubility limits CCN activity of particles composed of weakly functionalized organic compounds. Results are compared to a functional group contribution model that predicts CCN activity of organic compounds. The model performed well for most compounds but underpredicted the CCN activity of hydroperoxy groups. New best-fit hydroperoxy group/water interaction parameters were derived from the available CCN data. These results may help improve estimates of the CCN activity of ambient organic aerosols from composition data