2014 Progress Report: Sensitivity of Organic Aerosol Concentrations and Forcing to Anthropogenic Emissions

EPA Grant Number: R835405
Title: Sensitivity of Organic Aerosol Concentrations and Forcing to Anthropogenic Emissions
Investigators: Pandis, Spyros N. , Donahue, Neil
Institution: Carnegie Mellon University
EPA Project Officer: Peterson, Todd
Project Period: April 1, 2013 through March 31, 2016
Project Period Covered by this Report: April 1, 2014 through March 31,2015
Project Amount: $399,998
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

Objective:

Despite its importance for human health and climate change, organic aerosol (OA) remains one of the least understood aspects of atmospheric chemistry. We propose to continue the development of an innovative new framework for the description of OA in chemical transport and climate models that will be able to overcome the challenges posed by the chemical complexity of OA while capturing its essential features.

The objectives of the project are to: (i) experimentally investigate the mixing of biogenic and anthropogenic components; (ii) quantify the effects of NOx and SO2 on the formation of biogenic SOA; (iii) measure the OA formed per unit of biogenic and anthropogenic SOA precursor added to typical air masses; (iv) develop parameterizations of the above processes in the OA volatility-oxygen content coordinate system; (v) evaluate this new module in the regional chemical transport model PMCAMx against some of the best available (or soon to be available) datasets (SOAS and SENEX-2013 in the United States and EUCAARI and PEGASOS in Europe); and (vi) use PMCAMx to quantify the response of the biogenic OA and its climatic effects (direct effect and also cloud condensation nuclei concentrations) in the Eastern United States for different scenarios of anthropogenic emission changes.

Progress Summary:

Organic Aerosol Mixing: The partitioning of organic constituents plays a critical role in determining the chemical composition and physical characteristics of particles. Most regional scale chemical transport models assume an internally mixed aerosol. This assumption means that all particles of the same size have the same chemical composition. In this work, we have developed experimental techniques for performing aerosol mixing experiments using single particle mass spectrometry for determining the mixing of progressively more complex organic aerosol systems. The various stages of the mixing experiments are shown in Figure 1.

Single-particle mass spectra from the high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) are collected and their degree of similarity with time was examined. Depending on the similarity, the mixing behavior and timescales can be determined (Figure 2). Docosane and docosane-d46 (22 carbon linear solid alkane) did not show any signs of mixing, but squalane and squalane-d62 (30 carbon branched liquid alkane) mixed on the time scale expected from a condensational-mixing model. Docosane and docosane-d46 were driven to mix when the chamber temperature was elevated above the melting point for docosane. Docosane vapors were shown to mix into squalane-d62, but not the other way around. These results are consistent with low diffusivity in the solid phase of docosane particles. We performed mixing experiments on secondary organic aerosol (SOA) surrogate systems finding that SOA derived from toluene-d8 (a surrogate for anthropogenic SOA [aSOA]) does not mix into squalane (a surrogate for hydrophobic primary organic aerosol [POA]) but does mix into SOA derived from α-pinene (biogenic SOA [bSOA] surrogate). For the aSOA/POA, the volatility of either aerosol does not limit gas-phase diffusion, indicating that the two particle populations do not mix simply because they are immiscible. In the aSOA/bSOA system, the presence of toluene-d8-derived SOA molecules in the α-pinene-derived SOA provides evidence that the diffusion coefficient in α-pinene-derived SOA is high enough for mixing on the time scale of 1 minute.

An analysis of the formation and evaporation of mixed particles containing squalane (a surrogate for hydrophobic primary organic aerosol, POA) and secondary organic aerosol (SOA) is presented. In these experiments, one material (d62-squalane or SOA from α-pinene + O3) was prepared first to serve as surface area for condensation of the other, forming the mixed particles. The mixed particles then were subjected to a heating ramp from 22 to 44 °C. We were able to determine that (1) almost all of the SOA mass is comprised of material less volatile than d62-squalane; (2) AMS collection efficiency in these mixed-particle systems can be parameterized as a function of the relative mass fraction of the components; and (3) the vast majority of d62-squalane is able to evaporate from the mixed particles, and does so on the same time scale regardless of the order of preparation. We also performed two population mixing experiments to directly test whether d62-squalane and SOA from α-pinene + O3 form a single solution or two separate phases. We find that these two OA types are immiscible, which informs our inference of the morphology of the mixed particles. If the morphology is core-shell and dictated by the order of preparation, these data indicate that squalane is able to diffuse relatively quickly through the SOA shell, implying that there are no major diffusion limitations.

Chemical Aging of Organic Aerosol: Monoterpenes are a significant class of volatile organic compounds emitted by vegetation and β-caryophyllene is one of the most important compounds in this class. The SOA production during the oxidation of β-caryophyllene by ozone (O3) and hydroxyl radicals (OH) and the subsequent chemical aging of the products during reactions with OH were investigated. Experiments were conducted with ozone, hydroxyl radicals at low NOx and at high NOx (100s of ppb). The SOA mass yield at 10 μg m-3 of organic aerosol was 27% for the ozonolysis, 20% for the reaction with OH at low NOx and 38% at high NOx under dry conditions, 20 °C, and ozone excess. Parameterizations of the fresh SOA yields have been developed. The average fresh SOA atomic O:C ratio varied from 0.24 to 0.34 depending on the oxidant and the NOx level, while the H:C ratio was close to 1.5 for all systems examined. An average density of 1.06 ± 0.1 μg m-3 of the β-caryophyllene SOA was estimated. The exposure to UV-light had no effect on the β-caryophyllene SOA concentration and Aerosol Mass Spectrometer (AMS) mass spectrum. The chemical aging of the produced β-caryophyllene SOA was studied by exposing the fresh SOA to high concentrations (107 molecules cm-3) of OH for several hours. These additional reactions increased the SOA concentration by 15-40% and the O:C by approximately 25%. A limited number of experiments suggested that there was a significant impact of the relative humidity on the chemical aging of the SOA. The evaporation rates of β-caryophyllene SOA were quantified by using a thermodenuder allowing us to estimate the corresponding volatility distributions and effective vaporization enthalpies. Details about this work can be found in Tassoglou, et al. (2014).

SOA formation from volatile organic compounds in the atmosphere can be thought of as a succession of oxidation steps. The production of later generation SOA via continued oxidation of the first generation products is defined as chemical aging. The chemical aging of SOA produced during the α-pinene ozonolysis was investigated in a series of smog chamber experiments. After the SOA production, the resulting particles and vapors were exposed to OH radicals at both low and high NOx levels. In the former experiments H2O2 photolysis was the OH source, while in the latter HONO was photolyzed producing OH. Up to approximately 30% increase in organic aerosol (OA) mass was observed after an equivalent of 10 h of ambient daytime exposure to OH. A more oxygenated product distribution also was observed after aging based on the increase in aerosol atomic oxygen to carbon ratio. Experiments exposing the first generation α-pinene ozonolysis products to UV lights only showed a small reduction in OA mass, indicating minimum photolysis effect for this system. Experiments performed at elevated relative humidity (RH) of 55% showed no significant difference in additional SOA formation after the aging step compared to those performed at low RH of ≤ 20%. Experiments with OH introduction at different times after the first generation SOA formation yielded similar additional SOA mass, indicating direct vapor loss to the chamber walls is minimal under these circumstances. Experiments at high NOx conditions resulted in general in higher SOA production during the chemical aging steps. Results of a typical experiment are shown in Figure 3. An increase in organic aerosol concentration was observed after the two HONO injections when the lights were turned on. A parameterization for the description of the aging effects on aerosol yield was developed. The SOA concentration increases were modest, however. This work is described in two papers that currently in preparation.

The chemical aging of anthropogenic SOA also was investigated focusing mainly on toluene as a precursor. Photochemical aging clearly influences anthropogenic SOA, and the general trend toward increased SOA mass and reduced volatility is consistent with progressive oxidation driving organic aerosol toward the highly oxidized, low-volatility endpoint observed around the world. There is a strong relationship between exposure to OH and physicochemical properties for SOA formed from the oxidation of toluene and other small aromatic VOCs. Organic nitrogen compounds were a major constituent in the SOA formed. An experiment with higher OH exposure showed higher SOA mass yields, more oxidized SOA, and reduced SOA volatility but only modest differences in hygroscopicity. Volatility varied by a factor of 30 for different OH exposure, and a 10-fold decrease in volatility was associated with a 0.3 increase in carbon oxidation state. The SOA was relatively hygroscopic for organic material, with 0.1 < κ < 0.2 and if anything a slightly negative relationship between kappa and oxidation state, suggesting a possible role for surfactants or oligomeric compounds. Although individual experiments with different OH exposure showed clear aging effects as different oxidation states and OA volatility, these effects were not evident within every single experiment. This suggests that a complex interplay exists between gas-phase processes, including oxidation reactions that both functionalize and fragment condensable organic species as well as photolysis of some species. The composition, hygroscopicity and volatility of organic aerosol do not follow a prescribed relationship, and additional studies are needed to evaluate all of these properties in future laboratory experiments and ambient measurements. This work has been published by Hildebrandt-Ruiz, et al. (2015).

Field Perturbation Experiments: We have developed a mobile dual smog chamber system for field perturbation experiments. The system is shown in Figure 4.

The system can be set-up in less than 2 hours outdoors and experiments can be performed using ambient sunlight, during the night using artificial light, or in the dark during day or night. In these experiments, the starting point is ambient air. Both chambers are filled with ambient air and after its chemical characterization a perturbation is imposed in the first chamber. The second is used as the baseline. Instruments housed in a mobile laboratory sample both chambers.

In the test experiment, the experimental system was deployed in a park. Characterization experiments included blank experiments, measurements of losses of particles, testing of the similarity of the results in the two chambers using only ambient air (no perturbation), etc. The air was characterized with a combination of aerosol (SMPS, APS, AMS) and gas-phase instruments (PTR-MS, O3, NOx, SO2, etc., monitors). In a pilot study α-pinene was added to the first chamber to investigate its SOA formation in a mixture with ambient urban air. The measured organic aerosol concentration is shown in Figure 5. At 16:00 ambient air existed in both chambers. A few ppb of α-pinene was added in Chamber 1 at that point and it was allowed to react with the existing ozone in the dark. The reaction resulted in the formation of a little more than 1 μg m-3 of SOA in Chamber 1 (after the wall loss corrections) while, as expected, there were negligible changes in Chamber 2. The calculated SOAs for this experiment were consistent with laboratory measurements using only α-pinene in clean air.

The system development and evaluation are presented in Kaltsonoudis, et al. (2015).

OA Module Development and Evaluation: The organic aerosol modules in PMCAMx and PMCAMx-Trj (the 1-D version of the CTM using the 2D-VBS) have been updated using the yields and chemical aging parameterizations developed in the first year of the project. Additional updates are expected based on the third year of the experiments. We have continued the evaluation and uncertainty analysis of these CTMs using the REACH dataset collected during the PEGASOS-2012 campaign in Central and Southern Europe. One special feature of the dataset is the use of a Zeppelin for the collection of measurements of pollutant concentrations aloft.

The model evaluation suggests that using the oxygen to carbon (O:C) ratio together with the organic aerosol concentration provides additional constraints to the various organic aerosol schemes that currently are used in CTMs. The original Volatility Basis Set scheme, with its simplistic treatment of SOA production and aging (it simulates only the average net effect of functionalization and fragmentation), performed well in both reproducing the OA levels but also the O:C ratio (Figure 6) both at the ground and aloft. A number of alternative formulations of the 2D-VBS were tested and a number of them could reproduce relatively well the same observations while others failed especially to reproduce the O:C observations. The schemes that reproduced the observations gave similar results about the importance of anthropogenic SOA from VOCs (22 ± 7 % of the total OA), fresh POA (around 10%), SOA from the oxidation of anthropogenic semivolatile compounds (around 10%), but differed on the relative importance of biogenic SOA (from a low of 20 to a high of 50% of the total OA depending on the parameters) and SOA from anthropogenic IVOCs (from 20 to 35%).

Organic Aerosol Formation and Removal: Five case studies (Pittsburgh and Los Angeles in the United States, Athens and Paris in Europe, and Mexico City in Central America) were used to gain insights about the changing levels, sources, and role of atmospheric chemical processes on air quality in large urban areas as they develop technologically. Fine particulate matter was the focus of our analysis. In all cases, reductions of emissions by industrial and transportation sources have resulted in significant improvements in air quality during the last decades. However, these changes have resulted in increasing importance of secondary particulate matter (PM) that dominates over primary in most cases. At the same time, long-range transport of secondary PM from sources located hundreds of kilometres from the city center is becoming a bigger contributor to the urban PM levels in all seasons. “Non-traditional” sources including cooking and residential and agricultural biomass burning contribute an increasing fraction of the now reduced fine PM levels. Atmospheric chemistry is found to change the chemical signatures of a number of these sources relatively fast both during day and night, complicating the corresponding source apportionment (Pandis, et al., 2015).

Key findings of the project so far:

  • The additional SOA formed by the chemical aging of monoterpene and sesquiterpene ozonolysis SOA through reactions with OH results in a modest increase of the original SOA yields (less than 30% in most cases examined so far). This increase is higher when the SOA has been formed under high NOx conditions. 
  • Very oxidized SOA can be formed during the chemical aging reactions of aromatics (toluene, xylenes) with OH.
  • The continued oxidation of the aromatic SOA is accompanied by a net reduction in volatility. A simple relationship has been derived linking the oxidation state of carbon and the change in effective volatility.
  • We have developed a new experimental approach that allows us to investigate chemical and physical processes perturbing ambient air in the field. The mobile experimental system used two smog chambers: one as a baseline and the second as the perturbation chamber.
  • The simplest parameterization of the 2D-VBS framework using the above results reproduces well the OA observations during the PEGASOS-2012 campaign in Southern Europe.
  • We proposed standardizing a naming convention for organic aerosol classification that is relevant to laboratory studies, ambient observations, atmospheric models, and, quite importantly, the public and their leadership. This framework classifies organic material as primary or secondary pollutants and distinguishes among fundamental features important for science and policy questions including emission source, chemical phase and volatility.
  • Organic aerosol exists throughout the troposphere because heterogeneous oxidation by OH radicals is more than an order of magnitude slower than comparable gas-phase oxidation.
  • Secondary aerosol components dominate urban fine PM in most urban areas in the United States and Europe. The importance of “non-traditional” sources of urban pollution (long-range transport, cooking, residential and agricultural biomass burning) has been increasing while that of transportation sources is decreasing.

Future Activities:

During the third year of the project we will:

  1. Perform a series of ambient perturbation experiments focusing on SOA incremental yield determination.
  2. Continue the development of parameterizations of OA for CTMs.
  3. Use PMCAMx and PMCAMx-Trj to simulate the SOAS/SENEX campaign.
  4. Use PMCAMx to quantify the response of the biogenic OA and its climatic effects in the Eastern United States for different scenarios of anthropogenic emission changes. 


Journal Articles on this Report : 7 Displayed | Download in RIS Format

Other project views: All 9 publications 9 publications in selected types All 9 journal articles
Type Citation Project Document Sources
Journal Article Day MC, Zhang M, Pandis SN. Evaluation of the ability of the EC tracer method to estimate secondary organic aerosol carbon. Atmospheric Environment 2015;112:317-325. R835405 (2014)
R835405 (Final)
R835035 (Final)
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  • Journal Article Donahue NM, Chuang W, Epstein SA, Kroll JH, Worsnop DR, Robinson AL, Adams PJ, Pandis SN. Why do organic aerosols exist? Understanding aerosol lifetimes using the two-dimensional volatility basis set. Environmental Chemistry 2013;10(3):151-157. R835405 (2013)
    R835405 (2014)
    R835405 (Final)
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  • Journal Article Hildebrandt Ruiz L, Paciga AL, Cerully KM, Nenes A, Donahue NM, Pandis SN. Formation and aging of secondary organic aerosol from toluene:changes in chemical composition, volatility, and hygroscopicity. Atmospheric Chemistry and Physics 2015;15(14):8301-8313. R835405 (2013)
    R835405 (2014)
    R835405 (Final)
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  • Journal Article Murphy BN, Donahue NM, Robinson AL, Pandis SN. A naming convention for atmospheric organic aerosol. Atmospheric Chemistry and Physics 2014;14(11):5825-5839. R835405 (2013)
    R835405 (2014)
    R835405 (Final)
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  • Journal Article Pandis SN, Skyllakou K, Florou K, Kostenidou E, Kaltsonoudis C, Hasa E, Presto AA. Urban particulate matter pollution: a tale of five cities. Faraday Discussions 2016;189:277-290. R835405 (2014)
    R835405 (Final)
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  • Journal Article Posner LN, Pandis SN. Sources of ultrafine particles in the Eastern United States. Atmospheric Environment 2015;111:103-112. R835405 (2014)
    R835405 (Final)
    R833374 (Final)
    R835035 (2013)
    R835035 (Final)
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  • Journal Article Tasoglou A, Pandis SN. Formation and chemical aging of secondary organic aerosol during the β-caryophyllene oxidation. Atmospheric Chemistry and Physics 2015;15(11):6035-6046. R835405 (2013)
    R835405 (2014)
    R835405 (Final)
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  • Supplemental Keywords:

    Air quality modeling, smog, PM, anthropogenic organic aerosol, biogenic organic aerosol, VOCs, particulate matter, fine particulate matter, SOA, air pollution, NOx, SOx

    Progress and Final Reports:

    Original Abstract
    2013 Progress Report
    Final Report