Betty K. Pun

Secondary organic aerosol 2. Thermodynamic model for gas/particle partitioning of molecular constituents

A model that predicts secondary organic aerosol (SOA) formation based on the thermodynamic equilibrium partitioning of secondary organic oxidation products has been developed for implementation into atmospheric models. Hydrophobic secondary products are assumed to partition to an absorbing organic aerosol consisting of primary organic aerosol (POA) and other secondary hydrophobic organics according to an equilibrium partitioning coefficient calculated iteratively for each secondary compound present. The hydrophobic module is evaluated by studying the partitioning of octadecanoic acid to surrogate POA species. As expected, the amount of octadecanoic acid predicted to be present in the aerosol phase increases as the total amount of absorbing material increases or as the total amount of acid present increases. Hydrophilic secondary compounds partition to an aqueous phase via Henrys law; the fraction of each compounds mass that partitions is determined by its Henrys law constant and its acid dissociation constant(s). The available liquid water content (LWC) of the aerosol is determined iteratively between an inorganic aerosol module and the hydrophilic module, which is evaluated by studying the partitioning of glyoxalic and malic acids. While glyoxalic acid tends to remain in the gas phase, malic acid partitions strongly to the aqueous phase, with ions being the dominant form in the aqueous phase. As expected, an increase in relative humidity increases the amount of water associated with the organics (ΔLWC), and a lower aerosol pH favors molecular solutes over ionized forms. Increasing pH results in higher effective Henrys law constants for the acids, yielding higher organic aerosol concentrations. Results also indicate that increasing ΔLWC induces additional partitioning of inorganics to the aqueous phase.

Application and Evaluation of Two Air Quality Models for Particulate Matter for a Southeastern U.S. Episode

Abstract The Models-3 Community Multiscale Air Quality (CMAQ) Modeling System and the Particulate Matter Comprehensive Air Quality Model with extensions (PMCAMx) were applied to simulate the period June 29–July 10, 1999, of the Southern Oxidants Study episode with two nested horizontal grid sizes: a coarse resolution of 32 km and a fine resolution of 8 km. The predicted spatial variations of ozone (O3), particulate matter with an aerodynamic diameter less than or equal to 2.5 μm (PM2.5), and particulate matter with an aerodynamic diameter less than or equal to 10 μm (PM10) by both models are similar in rural areas but differ from one another significantly over some urban/suburban areas in the eastern and southern United States, where PMCAMx tends to predict higher values of O3 and PM than CMAQ. Both models tend to predict O3 values that are higher than those observed. For observed O3 values above 60 ppb, O3 performance meets the U.S. Environmental Protection Agencys criteria for CMAQ with both grids and for PMCAMx with the fine grid only. It becomes unsatisfactory for PMCAMx and marginally satisfactory for CMAQ for observed O3 values above 40 ppb. Both models predict similar amounts of sulfate (SO4 2−) and organic matter, and both predict SO4 2− to be the largest contributor to PM2.5. PMCAMx generally predicts higher amounts of ammonium (NH4 +), nitrate (NO3 −), and black carbon (BC) than does CMAQ. PM performance for CMAQ is generally consistent with that of other PM models, whereas PMCAMx predicts higher concentrations of NO3 −,NH4 +, and BC than observed, which degrades its performance. For PM10 and PM2.5 predictions over the southeastern U.S. domain, the ranges of mean normalized gross errors (MNGEs) and mean normalized bias are 37–43% and –33–4% for CMAQ and 50–59% and 7–30% for PMCAMx. Both models predict the largest MNGEs for NO3 − (98–104% for CMAQ, 138–338% for PMCAMx). The inaccurate NO3 − predictions by both models may be caused by the inaccuracies in the ammonia emission inventory and the uncertainties in the gas/particle partitioning under some conditions. In addition to these uncertainties, the significant PM overpredictions by PMCAMx may be attributed to the lack of wet removal for PM and a likely underprediction in the vertical mixing during the daytime.

Day-of-week behavior of atmospheric ozone in three U.S. cities

Abstract The weekly cycles of atmospheric ozone (O3) are of interest because they provide information about the response of O3 to changes in anthropogenic emissions from week-days to weekends. The weekly behavior of O3 in Chicago, IL; Philadelphia, PA; and Atlanta, GA, is contrasted. In Chicago and Philadelphia, maximum 1-hr average O3 increases on weekends. In Atlanta, O3 builds up from Mondays to Fridays and declines during weekends. In all three areas, volatile organic compound (VOC)/nitrogen oxides (NOx) ratios are higher during weekends, resulting from greater than proportionate decreases in NOx relative to VOC emissions. The VOC/NOx ratios correlate with maximum 1-hr O3 concentrations in Chicago, a response consistent with a VOC-sensitive airshed. A weak correlation between O3 concentrations and VOC/NOx ratios in Philadelphia suggests the impact of transported O3, which is formed in upwind VOC-sensitive locations that may be hundreds of kilometers away. Ozone concentrations in Atlanta do not correlate with VOC/NOx ratios but with concentrations of NOx and total reactive nitrogen (NOy) carried over from the previous day. When data from 1986–1990 and 1995–1999 are compared, only small differences in the weekly behavior of O3 are observed in Chicago and Philadelphia. The day-of-week differences in O3 are amplified in the more recent period in Atlanta, a possible result of urban growth.

Determination of Water Activity in Ammonium Sulfate and Sulfuric Acid Mixtures Using Levitated Single Particles

Water activities of ammonium sulfate-sulfuric acid mixtures with ammonium to sulfate molar ratios between 0 and 2 were measured by a spherical void electrodynamic levitator at relative humidities (RH) of 0.18–0.90. Since the composition of the solid particles is subject to uncertainty, solution properties were determined relative to the known properties at RH about 0.75–0.90. The data were compared with other measurements and the estimates from the Zdanovskii-Stokes-Robinson (ZSR) method and SCAPE, a newly developed gas-particle equilibrium model, and generally were found to be in good agreement.

Understanding particulate matter formation in the California San Joaquin Valley: conceptual model and data needs

Abstract Quantitative information from the 1995 Integrated Monitoring Study (IMS95) is used to develop a conceptual model, which describes the chemical characteristics and the physical processes responsible for the accumulation of PM in the San Joaquin Valley of California. One significant finding of the conceptual model is the sensitivity of ammonium nitrate (46% of winter PM2.5) and nitric acid to oxidants, which may be VOC-sensitive rather than NOx-sensitive. Key gaps in current knowledge are identified using the conceptual model, e.g., the relative sensitivity of winter oxidants to VOC and NOx, mechanistic details of secondary organic aerosol formation, mechanisms of dispersion under calm conditions, and the importance of dry deposition. Some recommendations are also provided for the formulation of air quality models suitable to address the accumulation of PM in the San Joaquin Valley.

Modeling regional haze in the BRAVO study using CMAQ-MADRID: 1. Model evaluation

The Big Bend Regional Aerosol and Visibility Observational (BRAVO) study was a multiyear monitoring and assessment study of the causes of regional haze in Big Bend National Park (BBNP), Texas. The Community Multiscale Air Quality model augmented with the Model of Aerosol Dynamics, Reaction, Ionization and Dissolution (CMAQ-MADRID) was used to simulate tracers and regional haze in Big Bend National Park for the full BRAVO period of July-October 1999. The BRAVO monitoring network provided an opportunity to conduct a comprehensive evaluation of CMAQ-MADRID over a 4-month period. Tracer simulations revealed uncertainties in tlie model representation of advection and diffusion processes and the effects of uncertainties in meteorological fields on transport simulations. Results improved with the implementation of a more diffusive horizontal diffusion scheme. The 12-km resolution provided better results than the 4- and 36-km resolution simulations for 15-25 August 1999 and the 36-km resolution provided better results for 5-15 October. Model performance for tracers suggests that as currently formulated, grid-based Eulerian models are not well suited to simulate the impacts of long-range transport of individual point source emissions at specific receptors. Nonetheless, they are suitable for resolving the contributions of source regions that may contain multiple area and point sources. Using a 36-km resolution for a 4-month simulation, the model performance was good in comparison with contemporary models for sulfate (the major PM 2.5 component) in the region of interest (i.e., BBNP), with a low bias and coefficient of determination better than 0.5. However, the model overestimated sulfate and total sulfur significantly in other parts of the modeling domain. For organic particulate matter (OM, the second most prevalent PM 2.5 component at BBNP), the model correctly reproduced the dominance of secondary organic aerosols and explained most of the variance in the OM concentrations; however, it underestimated OM concentrations consistently. Model performance was poor for the less prevalent components of PM 2.5 (i.e., nitrate and black carbon) at BBNP. Diagnostic analyses suggest that the discrepancies between model simulation results and observations are due not only to limitations in the model formulation but also to uncertainties in the model inputs, including emissions, meteorology, and boundary conditions.

Ozone Formation in California's San Joaquin Valley: A Critical Assessment of Modeling and Data Needs

ABSTRACT Data from the 1990 San Joaquin Valley Air Quality Study/ Atmospheric Utility Signatures, Predictions, and Experiments (SJVAQS/AUSPEX) field program in Californias San Joaquin Valley (SJV) suggest that both urban and rural areas would have difficulty meeting an 8-hr average O3 standard of 80 ppb. A conceptual model of O3 formation and accumulation in the SJV is formulated based on the chemical, meteorological, and tracer data from SJVAQS/ AUSPEX. Two major phenomena appear to lead to high O3 concentrations in the SJV: (1) transport of O3 and precursors from upwind areas (primarily the San Francisco Bay Area, but also the Sacramento Valley) into the SJV, affecting the northern part of the valley, and (2) emissions of precursors, mixing, transport (including long-range transport), and atmospheric reactions within the SJV responsible for regional and urban-scale (e.g., downwind of Fresno and Bakersfield) distributions of O3. Using this conceptual model, we then conduct a critical evaluation of the meteorological model and air quality model. Areas of model improvements and data needed to understand and properly simulate O3 formation in the SJV are highlighted.