Rick D. Saylor

Is ozone pollution affecting crop yields in China

Newly available data from non-urban locations in China along with regional model simulations suggest that ground-level ozone may be sufficiently high to affect Chinas winter wheat production. As non-urban ozone increases with industrialization, its effects on crops could hinder efforts to meet increasing food demands in the coming decades, in China.

The current state and future direction of Eulerian models in simulating the tropospheric chemistry and transport of trace species: a review

Abstract Limitations on comprehensive tropospheric chemistry/transport models are discussed within the context of a set of issues currently facing the environmental scientific and policy-making communities. A number of central improvements are discussed in a prioritized manner, with consideration of the key progress necessary to include feedback processes between meteorology and chemistry, aerosol formation, in cloud development with subsequent effects on wet removal, dry deposition and surface exchange processes, and impacts of chemical perturbations on radiation, climate, and weather. These improvements would result in a “third-generation model”. The computational framework for this code is outlined, and estimates of required computer resources presented.

The Southeastern Aerosol Research and Characterization Study, Part 3: Continuous Measurements of Fine Particulate Matter Mass and Composition

Abstract Deployment of continuous analyzers in the Southeastern Aerosol Research and Characterization Study (SEARCH) network began in 1998 and continues today as new technologies are developed. Measurement of fine particulate matter (PM2.5) mass is performed using a dried, 30 °C tapered element oscillating microbalance (TEOM). TEOM measurements are complemented by observations of light scattering by nephelometry. Measurements of major constituents include: (1) SO4 2− via reduction to SO2; (2) NH4 + and NO3 − via respective catalytic oxidation and reduction to NO, (3) black carbon (BC) by optical absorption, (4) total carbon by combustion to CO2, and (5) organic carbon by difference between the latter two measurements. Several illustrative examples of continuous data from the SEARCH network are presented. A distinctive composite annual average diurnal pattern is observed for PM2.5 mass, nitrate, and BC, likely indicating the influence of traffic-related emissions, growth, and break up of the boundary layer and formation of ammonium nitrate. Examination of PM2.5 components indicates the need to better understand the continuous composition of the unmeasured “other” category, because it contributes a significant fraction to total mass during periods of high PM2.5 loading. Selected episodes are presented to illustrate applications of SEARCH data. An SO2 conversion rate of 0.2%/hr is derived from an observation of a plume from a coal-fired power plant during early spring, and the importance of local, rural sources of NH3 to the formation of ammonium nitrate in particulate matter (PM) is demonstrated.

The reformulation and numerical solution of certain nonclassical initial-boundary value problems

Several physical phenomena are modeled by nonclassical parabolic or hyperbolic initial-boundary value problems in one space variable which involve an integral over the spatial domain of a function of the desired solution or of its first spatial derivative. This integral may appear in the boundary conditions and/or the governing partial differential equation itself. In this paper, several examples of problems of this type, which arise in such diverse areas as chemical diffusion and heat conduction processes, thermoelasticity, population dynamics, vibration problems, nuclear reactor dynamics, and certain biological processes, are presented. In general, these problems are either not in the form required by widely available general-purpose software packages or the application of such software would be awkward and/or inefficient. It is shown how the example problems can be converted to a form to which existing software or standard numerical procedures may be applied in a relatively straightforward manner. Special attention is devoted to one particular problem, the diffusion equation subject to the specification of mass in a portion of the domain, which has been studied quite extensively, both analytically and numerically, in recent years. As an example of an effective method for the solution of the reformulated version of this problem, a finite-difference method based on Kellers box scheme is considered. Results of numerical experiments are presented which compare this scheme with a Crank-Nicolson finite-element Galerkin method proposed recently.

A model investigation of summertime diurnal ozone behavior in rural mountainous locations

The STEM-II pollutant transport, transformation, and deposition model has been used to simulate diurnal patterns of surface ozone concentration at idealized high-elevation mountaintop locations and low-elevation plains and valley sites. Two-dimensional hydrostatic mesoscale simulations, initialized with atmospheric conditions representative of sunrise over the southern Appalachians on 19 June, were used to generate meteorological data to drive STEM-II simulations. Sensitivity of surface ozone concentrations has been investigated against input initial and background vertical profiles, synoptic wind speed, photochemistry, and surface removal. The model reproduced typical surface ozone concentration diurnal patterns observed at several high- and low-elevation sites in the United States and central Europe. Results indicate that the nighttime high ozone concentrations at high-elevation mountainous locations are primarily due to the occurrence of local topographically induced wind systems which transport ozone-rich air from aloft down mountain slopes. The simulations indicate that in some situations higher ozone concentrations may also be observed at mountaintop locations due to transport of residual ozone-rich air masses above the nocturnal boundary layer to high-elevation locations.

Role of anthropogenic NOxand VOC as ozone precursors: A case study from the SOS Nashville/Middle Tennessee Ozone Study

Using data collected on July 11, 1995, from the Tennessee Valley Authority (TVA) instrumented Bell 205 helicopter during the 1995 SOS Nashville/Middle Tennessee Ozone Study, we examine the relative roles of NOx and volatile organic compound (VOC) emissions on ozone photochemical production in the Nashville area. On this day, instruments onboard the helicopter separately sampled air from within a plume emanating from a nearby coal-fired power plant and air from the Nashville urban plume. Using SO2 concentrations and latitude to discriminate between the plumes, we compare the effects on O3 production of only adding NOx to the background air (i.e., the power plant plume) to that of adding both NOx and VOC (i.e., the urban plume). Our analysis indicates that NOx was the prime anthropogenic precursor causing the generation of O3 during the experiment, with anthropogenic VOC perhaps playing a secondary but significant role. The addition of VOC emissions from the urban area had no appreciable effect on peak O3 concentrations, clearly demonstrating that anthropogenic NOx emissions by themselves can produce elevated O3 concentrations in the Nashville area. Anthropogenic VOC emissions, however, may have enhanced the ozone production efficiencies and the rate at which O3 reached its peak values. While not conclusive, the data also suggested that the highest O3 concentrations in the Nashville area on July 11, 1995, appeared within embedded plumes that combined the effects of both urban and power plant precursor emissions.

An estimate of the potential significance of heterogeneous loss to aerosols as an additional sink for hydroperoxy radicals in the troposphere

Abstract A simple analysis has been performed to estimate the potential significance for tropospheric chemistry of heterogeneous loss of hydroperoxy (H0 2) radicals to aerosols. By comparing the magnitude of the heterogeneous loss as a function of mass accommodation coefficient and particle number density to the gas-phase chemical loss of H0 2 radicals for urban and remote marine scenarios, it is shown that this process may be an important loss mechanism for H0 2 radicals in the troposphere. Under typical urban conditions, the magnitude of heterogeneous loss of H0 2 may be a significant fraction of the gas-phase chemical loss. This loss process may also be important for remote tropospheric chemistry in that for typical remote marine conditions where NO x and 0 3 concentrations are low, heterogeneous loss may be nearly the same order of magnitude as the gas-phase chemical loss. Two major uncertainties are inherent in the current analysis. One is the value of the mass accommodation coefficient for H0 2 uptake to aerosols. Measurements (Hanson et al. , 1992; Mozurkewich et al. , 1987) indicate that this parameter is most likely > 0.05 and may, in fact, approach unity; however, more laboratory investigations are necessary, especially ones specifically designed to investigate this loss process under typical tropospheric conditions. The second uncertainty is the fate of H0 2 radicals upon being accommodated into an aerosol particle. The current analysis is based on the simplifying assumption that H0 2 is destroyed irreversibly immediately upon entering the aerosol phase. Potential mechanisms for this aerosol-phase destruction have been identified but not definitively tested. A more detailed analysis, including quantitative estimates of these aerosol-phase HO 2 reactions, is necessary to further evaluate the feasibility of this process for tropospheric conditions. Additionally, analysis of coincident measurements of gas-phase species concentrations and aerosol physical and chemical properties are necessary to further examine the significance of this loss process in tropospheric photochemistry.

On the comparison of numerical methods for the integration of kinetic equations in atmospheric chemistry and transport models

Abstract The integration of systems of ordinary differential equations (ODEs) that arise in atmospheric photochemistry is of significant concern to tropospheric and stratospheric chemistry modelers. As a consequence of the stiff nature of these ODE systems, their solution requires a large fraction of the total computational effort in three-dimensional chemical model simulations. Several integration techniques have been proposed and utilized over the years in an attempt to provide computationally efficient, yet accurate, solutions to chemical kinetics ODES. In this work, we present a comparison of some of these techniques and argue that valid comparisons of ODE solvers must take into account the trade-off between solution accuracy and computational efficiency. Misleading comparison results can be obtained by neglecting the fact that any ODE solution method can be made faster or slower by manipulation of the appropriate error tolerances or time steps. Comparisons among ODE solution techniques should therefore attempt to identify which technique can provide the most accurate solution with the least computational effort over the entire range of behavior of each technique. We present here a procedure by which ODE solver comparisons can achieve this goal. Using this methodology, we compare a variety of integration techniques, including methods proposed by Hesstvedt et al. (1978, Int. J. Chem. Kinet. 10, 971–994), Gong and Cho (1993, Atmospheric Environment 27A, 2147–2160), Young and Boris (1977, J. phys. Chem. 81, 2424–2427) and Hindmarsh (1983, In Scientific Computing (edited by Stepleman R. S. et al.), pp. 55–64. North-Holland, Amsterdam). We find that Gear-type solvers such as the Livermore Solver for ordinary differential equations (LSODE) and the sparse-matrix version of LSODE (LSODES) provide the most accurate solution of our test problems with the least computational effort.

Implications of the new ozone National Ambient Air Quality Standards for compliance in rural areas

The U.S. Environmental Protection Agency (EPA) has recently promulgated new National Ambient Air Quality Standards (NAAQS) for ozone (O3). The new standard is based on an 8-hour average O3 concentration instead of a 1-hour average, as was the previous standard. Analysis of O3 concentrations measured at rural sites in the eastern United States in the Aerometric Information Retrieval System (AIRS) network, the Southern Oxidant Studys Spatial Ozone Network (SON), and EPAs Clean Air Status and Trends Network (CASTNet) during 1993–1995 indicates that 30–50% of these sites would have been noncompliant under the new standard, as compared with only 2–12% under the old standard. These results suggest that the new standard will thus require a major shift in emphasis in the nations pollution control strategy from an urban-centered focus to a more regional emphasis. This in turn will require the development of a more comprehensive, truly rural air quality monitoring network in coming years.

Relationships between regional ozone pollution and emissions of nitrogen oxides in the eastern United States

This study examines the relationships between regional ozone (O 3 ) pollution and emissions of nitrogen oxides (NO x ) in the eastern United States during summer. Using measurements from rural sites during the summer of 1995, three 4-day time periods are identified during which significant enhancements of surface O 3 occurred on spatial scales ranging from 0.5 to 1.8 million km 2 . Each of these episodes was characterized by relatively stagnant meteorological conditions conducive to the photochemical formation and accumulation of O 3 in the boundary layer. The surface ozone accumulation efficiency (SOAE), a parameter which relates the O 3 accumulation to the NO x emission density in a given region, is estimated to range from 1 to 2 ppbv O 3 kg -1 N km -2 in the eastern United States during summer. This result is discussed in the context of regional NO x -based O 3 control strategies. In addition, the net ozone production efficiency (OPE) is estimated to range from 2 to 3 ppbv O 3 ppbv -1 NO x in this region.