Grantee Research Project Results
2004 Progress Report: Secondary and Regional Contributions to Organic PM: A Mechanistic Investigation of Organic PM in the Eastern and Southern United States
EPA Grant Number: R831073Title: Secondary and Regional Contributions to Organic PM: A Mechanistic Investigation of Organic PM in the Eastern and Southern United States
Investigators: Turpin, Barbara , Lim, Ho-Jin , Seitzinger, Sybil
Institution: Rutgers
EPA Project Officer: Chung, Serena
Project Period: September 1, 2003 through August 31, 2006 (Extended to August 31, 2007)
Project Period Covered by this Report: September 1, 2003 through August 31, 2004
Project Amount: $446,061
RFA: Measurement, Modeling, and Analysis Methods for Airborne Carbonaceous Fine Particulate Matter (PM2.5) (2003) RFA Text | Recipients Lists
Research Category: Air , Air Quality and Air Toxics , Particulate Matter
Objective:
The first objective of this research project is to conduct controlled laboratory experiments investigating the secondary formation of organic particulate matter through cloud/fog processing (i.e., kinetics). Results will provide critical information needed to refine predictive models, to identify potential secondary organic carbon (OC) source tracers or process indicators for data analysis and receptor modeling, and to guide the study of regional and local contributions to organic particulate matter (PM) concentrations. The second objective is to analyze samples from the Pittsburgh Supersite for products identified in the first objective, examine eastern Supersite data for evidence of heterogeneous formation, assess the relative importance of this formation process, and identify conditions conducive to secondary formation through cloud processing. The third objective is to examine the suitability of tracers/process indicators suggested above for estimation of primary versus secondary, local versus regional, and/or heterogeneous versus homogeneous contributions to ambient organic PM.
Progress Summary:
Atmospheric (secondary) formation and regional transport are responsible for a large portion of PM2.5 mass in the eastern United States, even in urban areas. In addition, there is growing evidence suggesting that, as for sulfate, organic PM can be formed not only by homogeneous gas phase reactions, but also by heterogeneous (including aqueous-phase) reactions. We hypothesize that atmospheric chemistry and transport models underestimate secondary OC and the regional contribution to OC in the eastern and southern United States because substantial organic PM is formed through heterogeneous processes (i.e., cloud processing) during regional transport. We propose to provide a better understanding of fundamental atmospheric (i.e. aqueous/heterogeneous) processes needed to predict organic PM concentration, organic species composition, and effects from emissions of particles and precursor species (i.e. improve predictive models). Further, we will examine evidence indicating the importance of these secondary processes in the eastern United States using EPA Supersite data and samples. We expect that this initial work will lead to the identification of secondary source tracers or process indicators that can be used in data analysis efforts and receptor modeling to identify the importance of primary versus secondary, local versus transport, and/or homogeneous versus heterogeneous processes. Additionally, this work will improve predictive models and therefore lead to the development of more effective air pollution control strategies.
We began the project by developing a simple cloud chemistry model to guide the laboratory kinetics experiments. While this activity was underway, we constructed the experimental apparatus and optimized the analytical protocols for sample analysis. The cloud chemistry model was constructed using FACIMILE, a differential equation solver, which also will be used to estimate rate constants from the controlled laboratory experiments. At this point, we have submitted a first manuscript on mechanisms and completed a first set of laboratory experiments that clear up a key uncertainty in the mechanistic understanding.
We developed a photochemical box model to investigate secondary organic aerosol (SOA) formation through cloud processing of isoprene. The box model assumes monodispersed cloud droplets, homogeneously mixed species within the interstitial space and within the cloud droplets, no temporal evolution of physical cloud properties, and constant temperature and pressure. The mass balance of a species in the gas and aqueous phase depends on chemical reactions, phase transfers between gas and aqueous phase, emissions, and dry deposition. It neglects aerosol deposition. The application of this model to a remote setting has been submitted for publication in Environmental Science and Technology.
The chemical mechanism in the model is based on previous cloud chemistry mechanisms and a condensed version of isoprene chemistry for the Regional Acid Deposition Model, Version 2. Chemical reactions involving water-soluble carbonyl products of isoprene were added. Briefly, gas phase isoprene oxidation produces glycolaldehyde, glyoxal, and methylglyoxal. These products dissolve into water and react with OH radical to form oxalic acid via glycolic acid, glyoxylic acid, pyruvic acid, and acetic acid. The aqueous phase chemical mechanism is very similar to another recent cloud photochemistry model by Ervens, et al., except for the fate of methylglyoxal. In the Ervens model, the reaction between methylglyoxal and OH yields pyruvic acid, which is further oxidized to acetaldehyde and finally CO2 without forming low volatility organic acids. In our model, methylglyoxal oxidation yields pyruvic acid, acetic acid, glyoxylic acid, and finally oxalic acid. We selected this pathway because it reproduces well the kinetics of methylglyoxal oxidation in previous studies of acetone degradation by H2O2/UV. From the modeling work, we discovered that the importance of isoprene as an in-cloud precursor of SOA depends heavily on the fate of pyruvic acid. For this reason, we decided to focus our laboratory efforts first on pyruvic acid.
Photochemical, batch aqueous-phase reactions of pyruvic acid and hydrogen peroxide were conducted in 1 L borosilicate vessels with quartz immersion wells under conditions encountered by cloud water. The reaction vessels were wrapped in aluminum foil to minimize the influence of ambient UV in the photochemistry. Low-pressure UV lamps with spectral irradiance at 254 nm were used in the experiments to produce hydroxyl radical from hydrogen peroxide for pyruvic acid oxidation. Each photochemical reaction experiment had two control experiments:
- a UV control (i.e., no UV source within the reaction vessel)
- and a H2O2 control (i.e., no H2O2 added to the reaction solution).
A 0.5 percent catalase solution was added to all experimental and control samples right after sampling to destroy remaining hydrogen peroxide and prevent further reaction. The hydrogen peroxide concentration is measured by colorimetry. Organic acids are measured by high performance liquid chromatography (HPLC) and/or electrospray ionization mass spectroscopy. Aldehydes will be measured in future experiments by HPLC. The actual photon flux in the reaction vessel will be measured using ferrioxalate actinometry. Pyruvic acid batch experiments showed degradation with time of pyruvic acid, followed by increased concentrations of acetic (large) and formic (small) acid and subsequent formation of oxalic acid. These results are consistent with the mechanism proposed in our cloud chemistry model and suggest that low volatility acids are important products of isoprene oxidation when cloud processing occurs. Results to date support our hypothesis. The degree to which current models might underpredict secondary organic aerosol formation in the eastern United States, however, is not yet clear.
Future Activities:
We are currently adding aldehyde analyses to the experimental work and analyzing the data to obtain yields and reaction rate constants. This will be followed by experiments at varying pH, with added sulfuric acid, and experiments starting with glyoxal and methylglyoxal. In addition, we have begun to collect data from Atlanta and Pittsburgh Supersite experiments for use in a more sophisticated model that we are developing. We plan to use this model to simulate the cloud processing of organics with multi-day transport under conditions representative of the eastern United States. The modeling effort will help us to understand what to look for to find evidence of heterogeneous formation, assess the relative importance of this formation process, and identify conditions conducive to secondary formation through cloud processing as proposed in the second objective.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 32 publications | 9 publications in selected types | All 9 journal articles |
---|
Type | Citation | ||
---|---|---|---|
|
Lim H-J, Carlton AG, Turpin BJ. Isoprene forms secondary organic aerosol through cloud processing: model simulations. Environmental Science & Technology 2005;39(12):4441-4446. |
R831073 (2004) R831073 (2005) R831073 (2006) R831073 (Final) |
Exit Exit Exit |
Supplemental Keywords:
SOA, secondary organic aerosol, carbonaceous, PM2.5, cloud processing, isoprene, PM sources, air, ecosystem protection/environmental exposure and risk, air pollution effect, air quality, atmospheric sciences, environmental monitoring, air toxics, particulate matter, air modeling, carbon aerosols, emissions, health effects,, RFA, Scientific Discipline, Air, Ecosystem Protection/Environmental Exposure & Risk, Air Quality, particulate matter, air toxics, Environmental Chemistry, Air Pollution Effects, Monitoring/Modeling, Analytical Chemistry, Environmental Monitoring, Engineering, Chemistry, & Physics, Environmental Engineering, organic pollutants, carbon aerosols, air quality modeling, particle size, atmospheric particulate matter, health effects, particulate organic carbon, atmospheric dispersion models, aerosol particles, atmospheric particles, chemical characteristics, PM 2.5, air modeling, air quality models, airborne particulate matter, air sampling, carbon particles, air quality model, emissions, particulate matter mass, ultrafine particulate matter, transport modeling, modeling studies, particle dispersion, aerosol analyzers, measurement methods, chemical speciation sampling, particle size measurementProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.