David P. Chock

A Study of the Association between Daily Mortality and Ambient Air Pollutant Concentrations in Pittsburgh, Pennsylvania

ABSTRACT We have studied the possible association of daily mortality with ambient pollutant concentrations (PM10, CO, O3, SO2, NO2, and fine [PM2 5] and coarse PM) and weather variables (temperature and dew point) in the Pittsburgh, PA, area for two age groups—less than 75, and 75 and over—for the 3-year period of 1989-1991. Correlation functions among pollutant concentrations show important seasonal dependence, and this fact necessitates the use of seasonal models to better identify the link between ambient pollutant concentrations and daily mortality. An analysis of the seasonal model results for the younger-age group reveals significant multicollinearity problems among the highly correlated concentrations of PM10, CO, and NO2 (and O3 in spring and summer), and calls into question the rather consistent results of the single- and multi-pollutant non-seasonal models that show a significant positive association between PM10 and daily mortality. For the older-age group, dew point consistently shows a significant association with daily mortality in all models. Collinearity problems appear in the multi-pollutant seasonal and non-seasonal models such that a significant, positive PM10 coefficient is accompanied by a significant, negative coefficient of another ambient pollutant, and the identity of this other pollutant changes with season. The PM25 data set is half that of PM10. Identical-model runs for both data sets reveal instability in the pollutant coefficients, especially for the younger age group. The concern for the instability of the pollutant coefficients due to a small signal-to-noise ratio makes it impossible to ascertain credibly the relative associations of the fine- and coarse-particle modes with daily mortality. In this connection, we call for caution in the interpretation of model results for causal inference when the models use fully or partially estimated PM values to fill large data gaps.

Development and application of an efficient moving sectional approach for the solution of the atmospheric aerosol condensation/evaporation equations

Abstract Condensation of gases (H2SO4, NH3, HNO3, low vapor pressure organics, etc.) onto existing aerosols can account for a significant portion of fine particulate matter. Since aerosols affect human health, visibility, and climate change, it is important to be able to model condensation/evaporation accurately in order to predict how particle mass will change over time. (Atmos. Environ. 34 (2000) 2957) proposed adapting the trajectory-grid method, used for solving the transport equation, to solve the condensation/evaporation equation. Their preliminary results showed it to be fast and accurate in simple systems (one component, etc.). The approach of Chock and Winkler has been modified for implementation in both the Hybrid method (Atmos. Environ. 34 (2000) 3617) and the improved multicomponent aerosol dynamics model (MADM) (Aerosol Sci. Technol. 32 (2000) 482) The first improvement in MADM modifies the method for restricting the acidic flux, while the second reduces physically meaningless dry/wet oscillations in the aerosol phase by assuming the aerosol is metastable when these oscillations are present. Measurements in Claremont, CA in August of 1987 are used to evaluate the new method in a one-dimensional model and the October 1995 PM episode in the South Coast Air Basin in CA is used for evaluation in a three-dimensional chemical transport model (PMCAMx). The trajectory-grid method allows the use of a simple scheme for time step selection and provides at least a factor of two or three reduction in computational requirements compared to an ODE solver at the same level of accuracy. The improvements to MADM also have a significant effect on performance, providing over an order of magnitude reduction in computational requirements compared to the original MADM. In the three-dimensional chemical transport model, the Hybrid method of (Atmos. Environ. (2000) 3617) is applied which assumes the smallest particles are in equilibrium while the condensation/evaporation equation is solved for the larger ones. Combined with the improvements to MADM and trajectory-grid method, this Hybrid approach takes just three to four times the computational requirements of assuming bulk equilibrium for all particles, while providing more accurate predictions of the aerosol size distribution.

A particle grid air quality modeling approach: 2. Coupling with chemistry

The particle grid method is applied to a system of 10 reacting chemical species in a two-dimensional rotating flow field with and without diffusion. Two types of chemistry grids are used to describe the chemical reactions: a fixed coarse grid and a moving (the advection case) or stationary (the advection plus diffusion case) fine grid. Two particle number densities are also used: 256 and 576 particles per fixed coarse grid cell. The species mass redistributed back to the particle after each reaction step is assumed to be proportional to the species mass in the particle before the reaction. The simulation results are very accurate, especially in the advection chemistry case. Accuracy improves with the use of a fine grid. A higher particle number density also reduces the concentration fluctuation in the cases involving diffusion. We also show by examples that chemistry can lead to significantly different results from numerical methods for the diffusion equation (let alone the advection equation) which otherwise yield almost identical solutions. The absence of this difficulty in the particle grid method further enhances its attractiveness.

A photochemical extent parameter to aid ozone air quality management

Abstract Interest in semiempirical approaches for relating precursor emissions (VOC and NOx to ozone air quality has been renewed recently. Of particular interest is a semi-empirical approach [called Smog Production Model (SPM) or SP Algorithm] initiated by Johnson and coworkers. SPM defines a photochemical extent parameter, which provides the directional guidance on the relative effectiveness of NOx and VOC controls in reducing ozone levels. In the present paper, a modified version of SPM is introduced, and the parameters involved in SPM are evaluated using urban airshed model (UAM) simulation results. UAM results are also used to examine relationships of extent parameters to ozone reductions resulting from VOC or NOx emission reductions. A modified version of SPM is applied to ambient air quality data. With improved quantified model parameter values and improved measurements of NOy, SPM may provide important information on the relative effectiveness of precursor controls. An attractive feature of SPM is that the required data are simultaneously measured NOx NOy and ozone.

A comparison of advection algorithms coupled with chemistry

Abstract Three-dimensional air quality models face the problem of solving the advection or advection-dominated transport equation. The performance of six advection algorithms coupled with chemistry is assessed here. The six algorithms are the forward-Euler Taylor-Galerkin (FETG) method, the implicit Chapeau-function method, a streamline upwind Petrov-Galerkin (SUPG) method, Smolarkiewiczs method, a semi-Lagrangian (SemiLag6) method, and the accurate space-derivative (ASD) method coupled with periodicity recovery. These methods are coupled with a 10-step chemistry involving 10 species simulating the essence of atmospheric photochemical reactions. The flow field is a two-dimensional, non-divergent rotating velocity field. The domain has 33 × 33 grid cells. Two test cases defined by the initial conditions are considered: (1) non-zero background concentrations for all species except four that have an additional conic profile; (2) same as (1) for a region that covers 11 × 11 grid cells with the conic profile at its center, while the concentrations of all species outside the 11 × 11 concentration platform are set equal to zero. After one full rotation in 24 h, for test case (1), the ASD gives the most accurate results, followed by FETG and the chapeau-function method. The Smolarkiewicz method gives the least-accurate result. For test case (2), no method performs well for NO and OH. For most species, ASD and FETG still yield accurate results. To assure highly accurate advection results for all concentration profiles, however, one may have to resort to a particle-trajectory method such as the particle-grid modelling approach.

Trajectory-grid: An accurate sign-preserving advection-diffusion approach for air quality modeling

Abstract We propose a new method, called the trajectory-grid (T-G) approach, which contains a fully Lagrangian advection scheme coupled with an Eulerian diffusion scheme, to remove the numerical problems associated with the advection equation in air quality modeling. This method is sign preserving, mass conserving, and very accurate. The method assigns the spatial locations of points on a given concentration profile to a set of concentration pulses; then tracks the pulse positions as they move downwind undergoing diffusion and reactions. Multigrid nesting can be conveniently accounted for by increasing the number density of concentration pulses in given areas in the modeling domain. The method can also serve as a tool to test the validity of the observation-based models. While the execution time may be comparable to that of the Accurate Space Derivative scheme, the advection step of the method is highly amenable to multiply-parallel processing.

A trajectory-grid approach for solving the condensation and evaporation equations of aerosols

Abstract The trajectory-grid method (Chock et al., Atmospheric Environment 30 (1996) 857–868) for solving the transport equation in a grid model can be easily adapted to solve the condensation/evaporation equations of internally mixed multicomponent aerosols based on the sectional approach. Compared to the Bott scheme (Monthly Weather Review 117 (1998) 1006–1015, 2633–2636) often used for solving these equations, the method is significantly more accurate, of comparable speed, but gaining in speed as the number of aerosol components increases. Most notably, there is no mismatch in size change of the different components of the same aerosol. The method is quite flexible, can handle an abrupt change in aerosol size due to evaporation of fog particles, and is ideal for multiply-parallel processors.

Sensitivity of urban airshed model results for test fuels to uncertainties in light-duty vehicle and biogenic emissions and alternative chemical mechanisms—Auto/oil air quality improvement research program

Abstract Three sources of uncertainty in the air quality modeling performed for the Auto/Oil Air Quality Improvement: Research Program, Phase I, were investigated to assess their impact on predicted ozone for test fuels in Los Angeles in year 2010. First, quadrupling the estimated total organic gas (TOG) and tripling the CO emissions from light-duty gasoline vehicles in the air quality model increases the predicted peak ozone, as expected. The percent increase in peak ozone for the test fuels, about 25% of total ozone, is essentially the same as the percent increase in TOG emissions, about 25% of the total emissions from all sources. However, there is no important effect on the ranking of the test fuels from lowest to highest in predicted ozone formation. Second, replacing the original biogenic emission inventory with an alternative inventory having substantially lower biogenic emissions reduces the predicted peak ozone. The percent decrease in peak ozone, about 6% of total ozone, is considerably less than the percent decrease in TOG emissions, about 34% of total emissions. Fuel rankings are unchanged except for a reversal of two test gasolines in the ranking based on peak ozone. However, this reversal is not found in fuel rankings based on other measures of ozone formation. Third, replacing the Carbon Bond Mechanism version IV (CBM-IV) in the air quality model with an alternative representation of atmospheric chemistry, the Statewide Air Pollution Research Center (SAPRC) mechanism, increases the peak ozone by about 9%. There are also important changes in fuel rankings. For one research test gasoline, the contribution of light-duty gasoline vehicles to ozone is similar with both chemical mechanisms, but for another test gasoline that gives the lowest ozone with the CBM-IV, the contribution of light-duty gasoline vehicles to ozone is substantially higher with the SAPRC mechanism. With the CBM-IV mechanism, the most promising of the test gasolines studied has lower predicted ozone than any of the cases representing use of methanol fuels in prototype, flexible/variable fuel vehicles. With the SAPRC mechanism, the most promising test gasoline studied has lower predicted ozone than one methanol case and higher ozone than the other methanol case. These changes in fuel rankings are probably due to known differences in the reactivity of toluene and formaldehyde in the two mechanisms.

URBAN OZONE AIR QUALITY IMPACT OF EMISSIONS FROM VEHICLES USING REFORMULATED GASOLINES AND M85

The urban ozone air quality impact of exhaust emissions from vehicles using reformulated gasolines and flexible/variable-fuel vehicles using M85 has been studied using emissions data from the Auto/Oil Air Quality Improvement Research Program and a single-cell trajectory air quality model with two different chemical mechanisms (the updated version of Carbon-Bond-IV (CB4) and the LCC mechanisms). Peak ozone concentrations are predicted for each fuel for all combinations of the following ambient conditions: low and high atmospheric dilution or mixing height, four NMOG/NOx ratios, two each of the initial NMOG concentration, the vehicular contribution to the ambient air, and the NMOG composition of the initial ambient mixture. The ozone impact of a fuel depends strongly on the atmospheric dilution and NMOG/NOx ratio of an area. The differences in ozone impact among fuels are limited under the condition of high atmospheric dilution and a high NMOG/NOx ratio. The ozone-forming potentials (OFPs) for the exhaust emissions based on the maximum incremental reactivities (MIRs) for various fuels are generally well correlated with model-calculated peak ozone levels at a low NMOG/NOx ratio. These OFPs can serve to separate out fuels with rather different reactivities, but not fuels with comparable reactivities. Model-calculated ozone levels for various fuels based on CB4 and LCC mechanisms are relatively well correlated at low NMOG/NOx ratios, but much less so at higher ratios. Fuels with a high aromatic content, including high-toluene fuels, tend to be ranked more favorably by CB4 than by LCC. On the other hand, M85 is ranked more favorably by LCC than by CB4. Fuels with a low 90% boiling point and a low content on aromatics and olefins are generally less reactive. M85 would be an attractive fuel if the formaldehyde emissions could be curtailed significantly. (A)

Effect of grid resolution and subgrid assumptions on the model prediction of a reactive bouyant plume under convective conditions

Abstract We have introduced a new and elaborate approach to understand the impact of grid resolution and subgrid chemistry assumption on the grid-model prediction of species concentrations for a system with highly non-homogeneous chemistry—a reactive buoyant plume immediately downwind of the stack in a convective boundary layer. The Parcel-Grid approach was used to describe both the air parcel turbulent transport and chemistry. This approach allows an identical transport process for all simulations. It also allows a description of subgrid chemistry. The ambient and plume parcel transport follows the description of Luhar and Britter (Atmos. Environ. 23 (1989) 1911, 26A (1992) 1283). The chemistry follows that of the Carbon-Bond mechanism. Three different grid sizes were considered: fine, medium and coarse, together with three different subgrid chemistry assumptions: micro-scale or individual parcel, tagged-parcel (plume and ambient parcels treated separately), and untagged-parcel (plume and ambient parcels treated indiscriminately). Reducing the subgrid information is not necessarily similar to increasing the model grid size. In our example, increasing the grid size leads to a reduction in the suppression of ozone in the presence of a high-NOx stack plume, and a reduction in the effectiveness of the NOx-inhibition effect. On the other hand, reducing the subgrid information (by using the untagged-parcel assumption) leads to an increase in ozone reduction and an enhancement of the NOx-inhibition effect insofar as the ozone extremum is concerned.