Annmarie G. Carlton

Model Representation of Secondary Organic Aerosol in CMAQv4.7

Numerous scientific upgrades to the representation of secondary organic aerosol (SOA) are incorporated into the Community Multiscale Air Quality (CMAQ) modeling system. Additions include several recently identified SOA precursors: benzene, isoprene, and sesquiterpenes; and pathways: in-cloud oxidation of glyoxal and methylglyoxal, particle-phase oligomerization, and acid enhancement of isoprene SOA. NO(x)-dependent aromatic SOA yields are also added along with new empirical measurements of the enthalpies of vaporization and organic mass-to-carbon ratios. For the first time, these SOA precursors, pathways and empirical parameters are included simultaneously in an air quality model for an annual simulation spanning the continental U.S. Comparisons of CMAQ-modeled secondary organic carbon (OC(sec)) with semiempirical estimates screened from 165 routine monitoring sites across the U.S. indicate the new SOA module substantially improves model performance. The most notable improvement occurs in the central and southeastern U.S. where the regionally averaged temporal correlations (r) between modeled and semiempirical OC(sec) increase from 0.5 to 0.8 and 0.3 to 0.8, respectively, when the new SOA module is employed. Wintertime OC(sec) results improve in all regions of the continental U.S. and the seasonal and regional patterns of biogenic SOA are better represented.

To what extent can biogenic SOA be controlled

The implicit assumption that biogenic secondary organic aerosol (SOA) is natural and can not be controlled hinders effective air quality management. Anthropogenic pollution facilitates transformation of naturally emitted volatile organic compounds (VOCs) to the particle phase, enhancing the ambient concentrations of biogenic secondary organic aerosol (SOA). It is therefore conceivable that some portion of ambient biogenic SOA can be removed by controlling emissions of anthropogenic pollutants. Direct measurement of the controllable fraction of biogenic SOA is not possible, but can be estimated through 3-dimensional photochemical air quality modeling. To examine this in detail, 22 CMAQ model simulations were conducted over the continental U.S. (August 15 to September 4, 2003). The relative contributions of five emitted pollution classes (i.e., NO(x), NH(3), SO(x), reactive non methane carbon (RNMC) and primary carbonaceous particulate matter (PCM)) on biogenic SOA were estimated by removing anthropogenic emissions of these pollutants, one at a time and all together. Model results demonstrate a strong influence of anthropogenic emissions on predicted biogenic SOA concentrations, suggesting more than 50% of biogenic SOA in the eastern U.S. can be controlled. Because biogenic SOA is substantially enhanced by controllable emissions, classification of SOA as biogenic or anthropogenic based solely on VOC origin is not sufficient to describe the controllable fraction.

Photochemical modeling of the Ozark isoprene volcano: MEGAN, BEIS, and their impacts on air quality predictions.

Biogenic volatile organic compounds (BVOCs) contribute substantially to atmospheric carbon, exerting influence on air quality and climate. Two widely used models, the Model of Emissions of Gases and Aerosols from Nature (MEGAN) and the Biogenic Emission Inventory System (BEIS) are employed to generate emissions for application in the CMAQ air quality model. Predictions of isoprene, monoterpenes, ozone, formaldehyde, and secondary organic carbon (SOC) are compared to surface and aloft measurements made during an intensive study in the Ozarks, a large isoprene emitting region. MEGAN and BEIS predict spatially similar emissions but magnitudes differ. The total VOC reactivity of the emissions, as developed for the CB05 gas-phase chemical mechanism, is a factor of 2 different between the models. Isoprene estimates by CMAQ-MEGAN are higher and more variable than surface and aloft measurements, whereas CMAQ-BEIS predictions are lower. CMAQ ozone predictions are similar and compare well with measurements using either MEGAN or BEIS. However, CMAQ-MEGAN overpredicts formaldehyde. CMAQ-BEIS SOC predictions are lower than observational estimates for every sample. CMAQ-MEGAN underpredicts SOC ∼ 80% of the time, despite overprediction of precursor VOCs. CMAQ-MEGAN isoprene predictions improve when prognostically predicted solar radiation is replaced with the GEWEX satellite product. CMAQ-BEIS does not exhibit similar photosensitivity.

Aerosol liquid water driven by anthropogenic nitrate: Implications for lifetimes of water-soluble organic gases and potential for secondary organic aerosol formation

Aerosol liquid water (ALW) influences aerosol radiative properties and the partitioning of gas-phase water-soluble organic compounds (WSOCg) to the condensed phase. A recent modeling study drew attention to the anthropogenic nature of ALW in the southeastern United States, where predicted ALW is driven by regional sulfate. Herein, we demonstrate that ALW in the Po Valley, Italy, is also anthropogenic but is driven by locally formed nitrate, illustrating regional differences in the aerosol components responsible for ALW. We present field evidence for the influence of controllable ALW on the lifetimes and atmospheric budgets of reactive organic gases and note the role of ALW in the formation of secondary organic aerosol (SOA). Nitrate is expected to increase in importance due to increased emissions of nitrate precursors, as well as policies aimed at reducing sulfur emissions. We argue that the impacts of increased particulate nitrate in future climate and air quality scenarios may be under predicted because they do not account for the increased potential for SOA formation in nitrate-derived ALW, nor do they account for the impacts of this ALW on reactive gas budgets and gas-phase photochemistry.

Aerosols from Fires: An Examination of the Effects on Ozone Photochemistry in the Western United States

This study presents a first attempt to investigate the roles of fire aerosols in ozone (O(3)) photochemistry using an online coupled meteorology-chemistry model, the Weather Research and Foresting model with Chemistry (WRF-Chem). Four 1-month WRF-Chem simulations for August 2007, with and without fire emissions, were carried out to assess the sensitivity of O(3) predictions to the emissions and subsequent radiative feedbacks associated with large-scale fires in the Western United States (U.S.). Results show that decreases in planetary boundary layer height (PBLH) resulting from the radiative effects of fire aerosols and increases in emissions of nitrogen oxides (NO(x)) and volatile organic compounds (VOCs) from the fires tend to increase modeled O(3) concentrations near the source. Reductions in downward shortwave radiation reaching the surface and surface temperature due to fire aerosols cause decreases in biogenic isoprene emissions and J(NO(2)) photolysis rates, resulting in reductions in O(3) concentrations by as much as 15%. Thus, the results presented in this study imply that considering the radiative effects of fire aerosols may reduce O(3) overestimation by traditional photochemical models that do not consider fire-induced changes in meteorology; implementation of coupled meteorology-chemistry models are required to simulate the atmospheric chemistry impacted by large-scale fires.

Microanalysis Methods for Characterization of Personal Aerosol Exposures

ABSTRACT Chemical characterization of particulate matter (PM) to which human populations are exposed is essential to developing an understanding of the risks and associations of PM with health endpoints. In this research we developed microanalysis methods for the characterization of personal exposure to PM and demonstrated the capabilities of sensitive analytical techniques through the analysis of 9 aerosol samples. These techniques will be used in future exposure assessment studies and to conduct source apportionment of personal and community exposures. Aerosol loadings comparable to 24-h personal exposure samples (150–480 μg) were collected on 25 mm Teflon filters with a PM10 personal sampler operating at 4 lpm. Qualitative functional group identification as well as quantitation of five metals (V, Cr, As, Cd, Pb) and 26 polycyclic aromatic hydrocarbons (PAHs) present in the aerosol samples was accomplished using the following techniques: Fourier Transform Infrared (FTIR) Spectroscopy, Inductively-Couple...

Decreasing Aerosol Water Is Consistent with OC Trends in the Southeast U.S.

Water is a ubiquitous and abundant component of atmospheric aerosols. It influences light scattering, the hydrological cycle, atmospheric chemistry, and secondary particulate matter (PM) formation. Despite the critical importance of aerosol liquid water, mass concentrations are not well-known. Using speciated ion and meteorological data from the Southeastern Aerosol Research and Characterization network, we employ the thermodynamic model ISORROPIAv2.1 to estimate water mass concentrations and evaluate trends from 2001 to 2012 in urban and rural locations. The purpose of this study is to better understand the historical trends of aerosol liquid water in the southeast U.S. in the context of improved air quality and recently noted reductions in particulate organic carbon (OC). Aerosol water mass concentrations decrease by ∼79% from 2001 to 2012 in the region. Decreases are more prominent in rural than in urban areas. Fractional contribution of water to PM also decreases during the same time period, and this is consistent with recently noted improvements in visibility. These findings agree with the hypotheses that aerosol liquid water facilitates formation of biogenic secondary organic aerosol (SOA) and that biogenically derived SOA is modulated in the presence of anthropogenic perturbations.

On the implications of aerosol liquid water and phase separation for organic aerosol mass

Organic compounds and liquid water are major aerosol constituents in the southeast United States (SE US). Water associated with inorganic constituents (inorganic water) can contribute to the partitioning medium for organic aerosol when relative humidities or organic matter to organic carbon (OM/OC) ratios are high such that separation relative humidities (SRH) are below the ambient relative humidity (RH). As OM/OC ratios in the SE US are often between 1.8 and 2.2, organic aerosol experiences both mixing with inorganic water and separation from it. Regional chemical transport model simulations including inorganic water (but excluding water uptake by organic compounds) in the partitioning medium for secondary organic aerosol (SOA) when RH > SRH led to increased SOA concentrations,· particularly at night. Water uptake to the organic phase resulted in even greater SOA concentrations as a result of a positive feedback in which water uptake increased SOA, which further increased aerosol water and organic aerosol. Aerosol properties· such as the OM/OC and hygroscopicity parameter (κorg), were captured well by the model compared with measurements during the Southern Oxidant and Aerosol Study (SOAS) 2013. Organic nitrates from monoterpene oxidation were predicted to be the least water-soluble semivolatile species in the model, but most biogenically derived semivolatile species in the Community Multiscale Air Quality (CMAQ) model were highly water soluble and expected to contribute to water-soluble organic carbon (WSOC). Organic aerosol and SOA precursors were abundant at night, but additional improvements in daytime organic aerosol are needed to close the model–measurement gap. When taking into account deviations from ideality, including both inorganic (when RH > SRH) and organic water in the organic partitioning medium reduced the mean bias in SOA for routine monitoring networks and improved model performance compared to observations from SOAS. Property updates from this work will be released in CMAQ v5.2.

Simulating Aqueous-Phase Isoprene-Epoxydiol (IEPOX) Secondary Organic Aerosol Production During the 2013 Southern Oxidant and Aerosol Study (SOAS)

The lack of statistically robust relationships between IEPOX (isoprene epoxydiol)-derived SOA (IEPOX SOA) and aerosol liquid water and pH observed during the 2013 Southern Oxidant and Aerosol Study (SOAS) emphasizes the importance of modeling the whole system to understand the controlling factors governing IEPOX SOA formation. We present a mechanistic modeling investigation predicting IEPOX SOA based on Community Multiscale Air Quality (CMAQ) model algorithms and a recently introduced photochemical box model, simpleGAMMA. We aim to (1) simulate IEPOX SOA tracers from the SOAS Look Rock ground site, (2) compare the two model formulations, (3) determine the limiting factors in IEPOX SOA formation, and (4) test the impact of a hypothetical sulfate reduction scenario on IEPOX SOA. The estimated IEPOX SOA mass variability is in similar agreement (r2 ∼ 0.6) with measurements. Correlations of the estimated and measured IEPOX SOA tracers with observed aerosol surface area (r2 ∼ 0.5-0.7), rate of particle-phase reaction (r2 ∼ 0.4-0.7), and sulfate (r2 ∼ 0.4-0.5) suggest an important role of sulfate in tracer formation via both physical and chemical mechanisms. A hypothetical 25% reduction of sulfate results in ∼70% reduction of IEPOX SOA formation, reaffirming the importance of aqueous phase chemistry in IEPOX SOA production.

The data gap: can a lack of monitors obscure loss of Clean Air Act benefits in fracking areas?

The U.S. is shifting to a greater reliance on natural gas to meet its energy needs, and a large part of this demand is being met by the development of shale gas formations. The increased utilization of natural gas is driven by the supply and thus lower cost, which largely results from new advances in engineering techniques. Primarily, gas production from horizontal drilling and high-volume hydraulic fracturing of shale and other low-porosity rock drives the favorable economics. Discussion of the environmental impacts of these operations has largely focused on water quality issues, but air pollution is also an important potential impact due to emissions associated with drilling, extraction, and associated transportation activities. Recently, air quality impacts have been measured in active oil and gas well areas. The extent to which these increased emissions impact air quality, especially in highly developed shale gas regions where there are no air monitors represents a substantial data gap and hinders effective air quality management. Throughout the United States, ambient concentrations of criteria pollutants have been decreasing in response to the implementation of the Clean Air Act. For example, Russell et al. (2012) find the rate of decrease in ambient NO2 concentrations in the mid 2000s to be, on average, about 4.5% yr−1 for U.S. cities. Using data from the Environmental Protection Agency’s Technology Transfer Network (TTN) (http://www.epa.gov/ ttn/airs/airsaqs/detaildata/downloadaqsdata.htm; data downloaded on September 23, 2013 and processed with R statistical software), we find overall similar decreasing trends in New York, Ohio, Pennsylvania, and West Virginia for SO2, CO, NO2, NOx, and PM2.5. This improvement in air quality through reduction in ambient concentrations of a variety of criteria air pollutants represents a tremendous success of the Clean Air Act. However, there can be local differences. An example is the Marcellus Shale Region of Western Pennsylvania where the amount of gas development activities has recently exploded (e.g., >6000 active wells in Pennsylvania alone). Ambient NOx (NO+NO2) concentrations generally were decreasing since the mid 2000s in most Eastern U.S. locations. However, in some areas a clear decrease is not evident and at a few locations the pollutant concentrations have been increasing in recent years. TTN air quality data indicates that at a monitor in Beaver County in western Pennsylvania, near several active wells in the Marcellus Shale region, monthly mean ambient NOx concentrations have been steadily increasing since 2010. In fact, some recent wintertime monthly average NOx concentrations have reached levels not observed since the implementation of the “Clean Air Interstate Rule” (CAIR) in 2009 (Figure 1). We find a similar trend in Steuben County, New York in a forested area downwind of shale gas activities in PA, where ambient NO2 concentrations have been increasing since 2008. The increase in pollutant concentrations and potential onset of losses in Clean Air Act benefits started in the latter part of the 2000s, coincident with the onset of shale gas activities in the surrounding areas. In addition to these findings, we observe that the individual counties with the highest number of active wells by State in the Marcellus Shale region, that is, Carroll, OH; Marshall, WV, and Bradford, PA have no routine air quality monitors. As a consequence, current trends in criteria air pollutants such as NO2, SO2, CO, Pb, PM2.5 cannot be effectively evaluated with observational data in those locations. This represents a data gap that may be obscuring negative air quality effects from shale gas activities. NOx emissions from electric generating units (EGUs) in Pennsylvania have been decreasing and are unlikely to explain the increases in ambient NOx concentrations measured at these sites. Continuous emissions monitoring (CEM) data from EPA clean air markets division (www.epa.gov/airmarket downloaded November 9, 2013) indicates that statewide NOx emissions from EGUs in Pennsylvania peaked in 2008. The CEM data suggest year-to-year variability in NOx emissions exists, possibly as a combination of meteorology and NOx credit trading price,