Final Report: Organic aerosol formation in the humid, photochemically-active Southeastern US: SOAS experiments and simulations

EPA Grant Number: R835412
Title: Organic aerosol formation in the humid, photochemically-active Southeastern US: SOAS experiments and simulations
Investigators: Turpin, Barbara , Carlton, Annmarie
Institution: Rutgers, The State University of New Jersey
EPA Project Officer: Hunt, Sherri
Project Period: April 1, 2013 through March 31, 2015
Project Amount: $399,928
RFA: Anthropogenic Influences on Organic Aerosol Formation and Regional Climate Implications (2012) RFA Text |  Recipients Lists
Research Category: Air Quality and Air Toxics , Global Climate Change , Climate Change , Air


We conducted controlled experiments with ambient samples and performed modeling as an integral part of the Southern Oxidant and Aerosol Study (SOAS), to improve mechanistic linkages between emissions and organic species (gases and aerosol) in the humid, photochemically active eastern United States (RFA Q#2) – a location influenced by biogenic emissions and with varying impacts from anthropogenic sources (RFA Q#1). In an environment where photochemistry and abundant atmospheric liquid water coexist (e.g., the Southeastern United States), gas followed by aqueous chemistry in clouds and aerosols could be the predominant source of secondary organic aerosol (aqueous SOA). The SOAS campaign was an ideal opportunity to study this. We expect this work to ultimately lead to the development of more effective air quality management through models that better capture critical atmospheric processes.


  1. Compare predicted (funded herein) and measured (funded separately) aerosol liquid water (ALW) concentrations during the SOAS campaign;
  2. Use Community Multiscale Air Quality (CMAQ) model predictions of watersoluble gases in conjunction with the aqueous photooxidation experiments (objective 3) and other SOAS measurements to provide insights into the formation of SOAaq;
  3. Conduct ambient aqueous photooxidation experiments using water-soluble gases scrubbed from the ambient air during the SOAS campaign. Use these results to identify key precursors and products of aqueous chemistry leading to SOA formation during SOAS and evaluate the degree to which this chemistry is captured by our current aqueous chemistry model.
  4. Collaborate, provide intellectual leadership and share data to achieve the SOAS science goals.

Summary/Accomplishments (Outputs/Outcomes):

Objective 1: Aerosol Liquid Water Predictions

CMAQ’s predictive availability for aerosol liquid water (ALW) mass concentrations is not well understood. We used CMAQ to predict ALW concentrations from June 6-June 15, 201, during SOAS and compared model results to measurements made by the Carlton group at Centerville (funded separately by NSF). ALW was modeled with the inorganic thermodynamic partitioning model, ISORROPIA. ISORROPIA was used as a subroutine in EPA’s CMAQ model. Water was a major aerosol component during SOAS. Modeled ALW was correlated with RH and followed measured trends except that from ~ 4:00 am to 1:00 p.m. measured water mass was substantially greater than predicted. An inter-comparison of ALW measured and estimated with independent techniques is ongoing.

Objective 2: Water-Soluble Organic Gas (WSOG) Predictions

Gas-phase concentrations of WSOG species and semi-volatile (SV) species were extracted from the CMAQ run. We found that, over the continental United States and at the SOAS ground site in Brent, Alabama, for both at the surface and over all model layers, predicted WSOG concentrations were an order of magnitude greater than semi-volatile organic gas concentrations. Combined with the consistent presence of aerosol liquid water and knowledge that organic aerosol in this region is largely secondary, this suggests that SOA formation through aqueous chemistry is likely to be important in the southeastern United States.

Gas-phase concentrations of glyoxal, methylglyoxal, and IEPOX (isoprene epoxide) from CMAQ were post-processed and converted into particle-phase potentials (Cip): [Ci]p=HiRTLCi(g) (1) where [Ci]p is the concentration of WSOG species i in the particle-phase, Hi is the Henry’s law constant for species i as used in CMAQ, R is the ideal gas constant, T is temperature, L is ALW concentration in air from CMAQ, and Ci(g) is mass concentration of species i in the gas-phase (converted from mixing ratios using ideal gas law) from CMAQ. This work includes gas-to-particle partitioning but no further aqueous phase reactions. It includes partitioning to aerosols but not partitioning to clouds and fogs. Setschenow constants for the WSOGs in ammonium sulfate (AS) were applied to account for the influence of “salting-in” and “salting-out” in an alternative analysis.

These calculations suggest that glyoxal, methylglyoxal and isoprene epoxide were all present in aerosol water, and IEPOX was the most abundant of these three water soluble gases. Condensed-phase glyoxal potentials approximately double in the eastern United States when the Setschenow coefficient to account for “salting-in” is employed, but they are still second to IEPOX particle-phase potentials. Compared to glyoxal and IEPOX, methylglyoxal was a minor contributor. IEPOX (isoprene epoxide) predominance over glyoxal and methylglyoxal is largely driven by differences in Henry’s Law constants; IEPOX has a factor of 10 and 100 greater solubility than glyoxal and methylglyoxal, respectively. All three species are known to react further in the aqueous phase to form SOA. Thus, IEPOX (isoprene epoxide) is likely to be a more important precursor to aqueous SOA in the Southeast than glyoxal and methylglyoxal. Note that aerosol uptake of these WSOGs will be enhanced by reactions in aerosol water and altered by the resence of other solutes.

IEPOX, an isoprene oxidation product that forms SOA in wet acidic aerosols, had a higher potential to partition to ALW in the eastern US than in other parts of the country. IEPOX accounted for a predicted ~90% of the sum of particle-phase glyoxal, methylglyoxal, and IEPOX in the eastern United States during this 10-day simulation. In contrast, the maximum glyoxal contribution (~ 50%) occured in the midwestern United States. These results provide support for the importance of IEPOX as a dominant source of aqueous SOA in the eastern United States, and significant contributions also from glyoxal. This work contributes to the growing laboratory, field, and modeling evidence of the importance of IEPOX as a precursor to SOA formation via water-mediated pathways, especially in the southeastern United States.

Objective 3: Photooxidation of Ambient WSOG Mixtures

Water-soluble gases were scrubbed from filtered ambient air into water at the Centerville ground site in Brent, Alabama, during SOAS using mist chambers. These ambient aqueous mixtures, with cloud/fog relevant total organic carbon concentrations (approx. 100 μM), were reacted with OH radicals to provide insights into precursors and products of aqueous chemistry. Ion chromatography, electrospray ionization (ESI) mass spectrometry (MS), ultra high resolution FTICR-MS and MS-MS fragmentation were used to provide structural information about reactants and products.

The concentration dynamics in aqueous oxidation experiments conducted with Centerville SOAS samples from across the field campaign were similar, indicating that the water-soluble organics captured from the ambient daytime air in the mist chambers varied little across the study. All precursor ions appeared in the positive ion mode of the ESI-MS, consistent with the presence of carbonyl compounds and polyols. They were odd ions, suggesting they are likely not nitrogen-containing species. Positive ions observed at m/z 125, 129, 143, 173, and 187 in the ESI-MS exhibited reactant-like trends, showing decreasing signal intensity with longer exposure to OH. In control experiments (without OH), the abundance of these ions did not change over time. We expect that in some cases observed ions are hydrated with water or methanol (singly or doubly) and ionized with hydrogen or sodium. With this knowledge, masses and mass fragments, we proposed tentative structures for some ambient aqueous precursors. Most notably, we found evidence consistent with the presence (and aqueous OH oxidation) of gas phase oxidation products of green leaf volatiles. Specifically, we tentatively identified an oxidation product of E-2-hexenal and Z-3-hexenal with the elemental formula C6H12O3 and an oxidation product of (E)-2-methyl-2-butenal with the elemental formula C5H10O3. E-2-hexenal and Z-3-hexenal have frequently been detected during field studies and are emitted to the atmosphere from vegetation due to leaf wounding.

Oxalate and pyruvate formed in OH radical experiments conducted with all samples but not during the control experiments. Acetate + glycolate (which co-elute in the IC) also forms in at least some samples and reacts away in the presence of OH. These observations suggest that oxalate, pyruvate and acetate can form in ambient mixtures of water-soluble gases in the Southeast US in the presence of clouds/fogs and oxidants. Pyruvate and oxalate have been observed primarily in the particle phase in the atmosphere (Saxena and Hildemann, 1996; Limbeck et al., 2001; Yao et al., 2002; Kawamura et al., 2003). Thus, the experiments suggest that aqueous oxidation of ambient (Southeastern US) water-soluble mixtures at cloud/fog relevant concentrations has the potential to form material that remains in the particle-phase species after droplet evaporation (i.e., SOAAQ). The aerosol at the SOAS ground site was acidic (campaign average pH ~ 0.94) (Guo et al., 2015) and as a consequence oxalic acid may remain largely in the gas phase in this environment, but may eventually react on more basic surfaces (e.g., coarse particles), reducing its vapor pressure by orders of magnitude and forming SOA.

Objective 4: SOAS Leadership

Dr. Carlton was onsite throughout the field campaign, providing leadership and solving problems. After the campaign, Dr. Carlton co-hosted the SAS data workshop with the NOAA SENEX lead scientist Joost de Gouw in Boulder, CO March 31- April 2, 2014 ( Exit). The meeting included most of EPA STAR grantees awarded through this call. She has also spearheaded a community-authored SOAS overview paper for submission to the Bulletin of the American Meteorological Society.


Babouka, E.D., Kanakidou, M., and Mihalopoulos, N.: Carboxylic acids in gaad particulte phae aboveth Atlantic Ocean, J. Geophys. Res., 15, 14459 - 14471, 2000.

Gui, H., Xu, L., Bougitioti, A., Cerully, K.M., Cpps, S.L., Hite Jr., J.R., Carloton, A.G., Lee, S.H., Bergn, M.H.., Ng, N.L. and Nenes, A.: Fin-particle water andpH in th southeatn United States. Atmos. Chem. Phys., 15,5211-5228, 2015.

Kawamura, K., Umemoto, >, Mochida, M., Betra, T., Howell, S. and Huebert, B.J.: Water-soluble dicarboxylic acids in th tropospher aerosols cllected over east Asia a western NorthPacific by ACE-Asia C-130 aircraft. J. Geophys. Res., 108(D23), 2003, doi: 10.1029/2002JD003256.

Limbeck, A., Puxbaum, H., Otter, L., Scholes, M.C.: Semivolatile behavior of dicarboxylic acids and other polar organic species at a rural background site (Nylsvley, RSA), Atmos, Environ., 35, 1853-1862, 2001.

Saxena, P. and Hildemann, L.M.: Water-soluble organicsin atmopheric particles: a critical revew of the literature an application of thermodynamics to identify candidat compounds, J. Atmos. Chem., 24, 57-19, 1996.

Ya, X.H., Fang, M., an Chan, C.K.: Size distrbtions and foratiin of dicarboxylics aids in atmospheric paricles. Atmos. Environ., 36, 2099-2107.2002.

Journal Articles on this Report : 3 Displayed | Download in RIS Format

Other project views: All 14 publications 3 publications in selected types All 3 journal articles
Type Citation Project Document Sources
Journal Article Ervens B, Sorooshian A, Lim YB, Turpin BJ. Key parameters controlling OH-initiated formation of secondary organic aerosol in the aqueous phase (aqSOA). Journal of Geophysical Research-Atmospheres 2014;119(7):3997-4016. R835412 (Final)
  • Full-text: Wiley-Full Text PDF
  • Abstract: Wiley-Abstract
  • Journal Article Lim YB, Turpin BJ. Organic peroxide and OH formation in aerosol and cloud water: laboratory evidence for this aqueous chemistry. Atmospheric Chemistry and Physics Discussions 2015;15(12):17367-17396. R835412 (Final)
  • Full-text: Research Gate-Abstract & Full Text PDF
  • Abstract: Harvard-Abstract
  • Journal Article Lim YB, Turpin BJ. Laboratory evidence of organic peroxide and peroxyhemiacetal formation in the aqueous phase and implications for aqueous OH. Atmospheric Chemistry and Physics 2015;15(22):12867-12877. R835412 (Final)
  • Full-text: Atmospheric Chemistry and Physics-Full Text PDF
  • Abstract: Atmospheric Chemistry and Physics-Abstract
  • Supplemental Keywords:

    SOA, secondary organic aerosol, PM2.5, aqueous chemistry, isoprene, ambient air;

    Relevant Websites:

    Earth Observing Laboratory | NCAR | UCAR Exit
    NOAA ESRL CSD Projects: SENEX 2013

    Progress and Final Reports:

    Original Abstract
    2013 Progress Report