REAL-TIME CHEMICAL CHARACTERIZATION OF ATMOSPHERIC ORGANIC AEROSOL IN THE SOUTHEASTERN UNITED STATES BY AEROSOL MASS SPECTROMETRY

The chemical characteristics and sources of atmospheric organic aerosol (OA) in the southeastern United States (U.S.) are least known compared to the other areas due to the complex mixture of anthropogenic and biogenic emissions in this area. The chemical characterization of atmospheric OA requires real-time continuous measurements over different seasons to capture the variability of precursor emissions as well as changes in meteorological conditions. Previous studies in this region were limited by low time resolution and low-mass identification. Continuous approaches, such as the newly developed Aerodyne Aerosol Chemical Speciation Monitor (ACSM) applied here, have the benefit of high time resolution and immediate chemical analysis of ambient non-refractory fine aerosol using electron ionization followed by quadrupole aerosol mass spectrometry. The ACSM is designed for long-term monitoring, making it suitable for identifying sources of atmospheric OA based on chemical composition and temporal variations. This dissertation demonstrates that the ACSM is capable of stable and reproducible operation over extended measurement periods. ACSM measurements compared well with established air-monitoring data in urban Atlanta, GA, and rural Look Rock, TN. Source apportionment of summer OA measured by the ACSM using positive matrix factorization in Atlanta yielded four sources (factors), namely hydrocarbon-like OA (HOA), semi-volatile oxygenated OA (SV-OOA), low-volatility oxygenated OA (LV-OOA), and isoprene epoxydiol (IEPOX)-OA. The IEPOX-OA was firstly attributed to the heterogeneous chemistry of IEPOX in this study and was found to be significant source in the southeast U.S. that contributes to the total OA mass on average of 33%. Strong association between the IEPOX-OA and sulfate is consistent between urban and rural sites. Despite no clear association of IEPOX-OA with locally estimated aerosol acidity and liquid water content, box model calculations of IEPOX uptake accounting for the role of acidity and aerosol water result in predictions of IEPOX-derived SOA tracers at the Look Rock site moderately correlated with observations. The remarkable consistency of IEPOX-OA contribution across this region reveals the importance of heterogeneous chemistry of IEPOX in forming OA. Findings from this dissertation could help in developing policy and regulation for mitigating air pollution in this region.Doctor of Philosoph

Seasonal characterization of submicron aerosol chemical composition and organic aerosol sources in the southeastern United States: Atlanta, Georgia,and Look Rock, Tennessee

A year-long near-real-time characterization of non-refractory submicron aerosol (NR-PM1) was conducted at an urban (Atlanta, Georgia, in 2012) and rural (Look Rock, Tennessee, in 2013) site in the southeastern US using the Aerodyne Aerosol Chemical Speciation Monitor (ACSM) collocated with established air-monitoring network measurements. Seasonal variations in organic aerosol (OA) and inorganic aerosol species are attributed to meteorological conditions as well as anthropogenic and biogenic emissions in this region. The highest concentrations of NR-PM1 were observed during winter and fall seasons at the urban site and during spring and summer at the rural site. Across all seasons and at both sites, NR-PM1 was composed largely of OA (up to 76 %) and sulfate (up to 31 %). Six distinct OA sources were resolved by positive matrix factorization applied to the ACSM organic mass spectral data collected from the two sites over the 1 year of near-continuous measurements at each site: hydrocarbon-like OA (HOA), biomass burning OA (BBOA), semi-volatile oxygenated OA (SV-OOA), low-volatility oxygenated OA (LV-OOA), isoprene-derived epoxydiols (IEPOX) OA (IEPOX-OA) and 91Fac (a factor dominated by a distinct ion at m∕z 91 fragment ion previously observed in biogenic influenced areas). LV-OOA was observed throughout the year at both sites and contributed up to 66 % of total OA mass. HOA was observed during the entire year only at the urban site (on average 21 % of OA mass). BBOA (15–33 % of OA mass) was observed during winter and fall, likely dominated by local residential wood burning emission. Although SV-OOA contributes quite significantly ( ∼  27 %), it was observed only at the urban site during colder seasons. IEPOX-OA was a major component (27–41 %) of OA at both sites, particularly in spring and summer. An ion fragment at m∕z 75 is well correlated with the m∕z 82 ion associated with the aerosol mass spectrum of IEPOX-derived secondary organic aerosol (SOA). The contribution of 91Fac to the total OA mass was significant (on average 22 % of OA mass) at the rural site only during warmer months. Comparison of 91Fac OA time series with SOA tracers measured from filter samples collected at Look Rock suggests that isoprene oxidation through a pathway other than IEPOX SOA chemistry may contribute to its formation. Other biogenic sources could also contribute to 91Fac, but there remains a need to resolve the exact source of this factor based on its significant contribution to rural OA mass.</html

Overcoming the lack of authentic standards for the quantification of biogenic secondary organic aerosol markers

Liquid chromatography coupled to electrospray ionisation high resolution mass spectrometry is an extremely powerful technique for both targeted and non-targeted analysis of organic aerosol. However, quantification of biogenic secondary organic aerosol species (BSOA) is hindered by a lack of commercially available authentic standards. To overcome the lack of authentic standards, this study proposes a quantification method based on the prediction of relative ionisation efficiency (RIE) factors to correct concentrations obtained via calibration using a proxy standard. RIE measurements of 89 commercially available standards were made relative to cis-pinonic acid and coupled to structural descriptors. A regularised random forest predictive model was developed using the authentic standards (R2 = 0.66, RMSE = 0.59). The model was then used to predict the RIE’s of 87 biogenic organic acid markers from α-pinene, limonene and β-caryophyllene without available authentic standards. The predicted RIE’s ranged from 0.27 to 13.5, with a mean ± standard deviation of 4.2 ± 3.9. 25 markers were structurally identified in chamber samples and ambient aerosol filter samples collected in summertime Beijing. The markers were quantified using a cis-pinonic acid calibration and then corrected using the predicted RIE factors. This resulted in the average BSOA concentration decreasing from 146 ng m−3 to 51 ng m−3, respectively. This change in concentration is highlighted to have an impact on the types of average aerosol metrics commonly used to describe bulk composition. This study is the first of its kind to use predicted ionisation efficiency factors to overcome known differences in BSOA concentrations due to the inherent lack of authentic standards in aerosol chemistry

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

Isoprene Epoxydiols as Precursors to Secondary Organic Aerosol Formation: Acid-Catalyzed Reactive Uptake Studies with Authentic Compounds

Isoprene epoxydiols (IEPOX), formed from the photooxidation of isoprene under low-NOx conditions, have recently been proposed as precursors of secondary organic aerosol (SOA) on the basis of mass spectrometric evidence. In the present study, IEPOX isomers were synthesized in high purity (> 99%) to investigate their potential to form SOA via reactive uptake in a series of controlled dark chamber studies followed by reaction product analyses. IEPOX-derived SOA was substantially observed only in the presence of acidic aerosols, with conservative lower-bound yields of 4.7–6.4% for β-IEPOX and 3.4–5.5% for δ-IEPOX, providing direct evidence for IEPOX isomers as precursors to isoprene SOA. These chamber studies demonstrate that IEPOX uptake explains the formation of known isoprene SOA tracers found in ambient aerosols, including 2-methyltetrols, C5-alkene triols, dimers, and IEPOX-derived organosulfates. Additionally, we show reactive uptake on the acidified sulfate aerosols supports a previously unreported acid-catalyzed intramolecular rearrangement of IEPOX to cis- and trans-3-methyltetrahydrofuran-3,4-diols (3-MeTHF-3,4-diols) in the particle phase. Analysis of these novel tracer compounds by aerosol mass spectrometry (AMS) suggests that they contribute to a unique factor resolved from positive matrix factorization (PMF) of AMS organic aerosol spectra collected from low-NOx, isoprene-dominated regions influenced by the presence of acidic aerosols

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

New formation and fate of Isoprene SOA markers revealed by field data-constrained modeling

Particulate 2-methyltetrols (2-MT) and 2-methylglyceric acid (2-MG) are typically used to indicate the abundance of isoprene-derived secondary organic aerosols (SOA). However, their formation and fate are not fully understood. In this study, we showed that particulate 2-MT and 2-MG collected at multiple monitoring sites under a wide range of atmospheric and emission conditions, with concentrations spanning six orders of magnitudes, are well reproduced with an expanded isoprene-SOA scheme implemented into the Community Multiscale Air Quality (CMAQ) model. The scheme considers their three-phase (gas-aqueous-organic phase) partitioning, formation from acid-driven multiphase reactions, and degradation by OH radicals in the gas and aqueous phases. The model results reveal that a non-aqueous formation pathway or direct biogenic emission is needed to supplement the commonly assumed acid-driven multiphase reaction process to explain the observed 2-MT concentrations. This missing pathway contributes to 20–40% of 2-MT in areas with aerosol pH<2 and more than 70% under less acidic conditions (pH~2–5), such as those encountered in the western US and China. The typical summertime gas-phase photochemical lifetimes of 2-MT and 2-MG are estimated to be 4–6 and 20–30 h, respectively, and their aqueous lifetimes are approximately 20–40 h. Our simulations show that predicted 2-MT is mainly influenced by its aqueous phase loss to OH, but 2-MG is more sensitive to gas phase OH loss due to the preferential partitioning of the two tracers in the aqueous and gas phases, respectively