A Monte-Carlo Simulation of the Ozone Attainment Process

A Monte-Carlo simulation of the approach to attainment of the National Ambient Air Quality Standard for ozone has been performed for the California Bay Area Air Quality Management District. Four compliance tests together with different design values are used in the simulation. The results show that the present compliance test requiring a zero-percent chance of violation and the design value represented by the fourth highest value in three years makes both the standard and the control requirement much more stringent than generally assumed. In fact, to attain the standard on a long-term basis would require annual means and annual second-highest values that are close to those of the rural background ozone. The simulation also shows that by taking into account the fluctuation of ozone concentrations in the compliance test, such as a t test, and by using a design value consistent with the test, a standard defined in terms of the three-year mean of the annual second-highest values not only is more consistent with the currently- perceived stringency of the present standard, but may also be attainable with a more reasonable control requirement.     

Weekday-Weekend Pollutant Studies of the Los Angeles Basin

Pollutant data from the Los Angeles Basin were analyzed for weekday-weekend differences for the smog months of June through September 1972 and 1973. The pollutants investigated were oxidant, NO, NO₂, total hydrocarbons (HC), CO, and particulates. In order to maintain the diurnal variation, the concentration percentiles were calculated for each weekday and weekend hour.     

A study of the association between daily mortality and ambient air pollutant concentrations in Pittsburgh, Pennsylvania

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 PM2.5 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.     

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 [PM_{2 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.     

The Need for a More Robust Ozone Air Quality Standard

The present National Ambient Air Quality Standard for ozone has many statistical problems, including use of extreme values which have inherent large fluctuations, a compliance test that can gradually lower the target of the design value below the standard level, and inconsistencies between the number-of-exceedances criterion and the design value. The above problems can be avoided or minimized by using a more robust statistic, such as the 95th percentile, and applying a statistical compliance test, without sacrificing the stringency of the standard. Analysis of EPA’s ozone data shows that the annual 95th percentiles and their three-year means have less variability than the annual second highest values and the fourth highest values in three years, respectively. A t test for the mean of the annual 95th percentiles is proposed for compliance testing not only to preserve the averaging concept of the present standard, but also to take account of ozone concentration fluctuations in order to increase the stability of the compliance status of a site or a Metropolitan Statistical Area. A procedure is provided to adjust the level of the 95th-percentile standard so that the stringency of the present standard is preserved.     

Issues Regarding the Ozone Air Quality Standard and the Design Value

Abstract

Two problems exist in the form and the compliance test of the present National Ambient Air Quality Standard (NAAQS) for ozone. One is the use of the number of exceedances in the form of the standard, which generates confusion and unnecessary complexity when the form is translated to the design value. The other is the requirement of a zero percent chance of violation in the compliance test, which makes the NAAQS considerably more stringent than generally assumed. There are also two sample-size problems in the estimation procedure for the design value. One is the upward creeping of the (n+l)th highest value in n years as n increases from one in the table look-up approach. The other is the infinite-sample-size assumption instead of the number of high-ozone season days per year for the daily maximum ozone concentrations in the distribution fitting approach. Both problems lead to an exaggeration of the design value.

The above problems can be removed in a revised NAAQS by (1) using a statistic that is identical to the design value itself in the form of the standard, (2) defining the design value as an n-year mean of, say, the annual mth highest values rather than the xth highest value in n years, and (3) using a simple compliance test like the t test that compares the design value with the level of the standard, taking into account the year-to-year fluctuation of the annual mth highest values. When the design value of an area is close to the level of the standard, the test provides a natural “too close to call” interval, which adjusts itself with the fluctuation of the annual mth highest values, so that as the fluctuation increases, the ability to assign the compliance status of the area decreases. The inclusion of a “too close to call” interval or category in the standard is critical to reduce the tendency toward ozone attainment flip-flops in areas approaching attainment and to assure that the ozone NAAQS is not more or less stringent than it appears.     

Attainment Flip-Flops and the Achievability of the Ozone Air Quality Standard

Abstract

This paper presents the results of the first reported study on fine particulate matter (PM) chemical composition at Salamanca, a highly industrialized urban area of Central Mexico. Samples were collected at six sites within the urban area during February and March 2003. Several trace elements, organic carbon (OC), elemental carbon (EC), and six ions were analyzed to characterize aerosols. Average concentrations of PM with aerodynamic diameter of less than 10 μm (PM₁₀) and fine PM with aerodynamic diameter of less than 2.5 μm (PM_2.5) ranged from 32.2 to 76.6 μg m^-3 and 11.1 to 23.7 μg m^-3, respectively. OC (34%), SO₄ ⁼ (25.1%), EC (12.9%), and geological material (12.5%) were the major components of PM_2.5. For PM₁₀, geological material (57.9%), OC (17.3%), and SO₄ ⁼ (9.7%) were the major components. Coarse fraction (PM₁₀ –PM_2.5), geological material (81.7%), and OC (8.6%) were the dominant species, which amounted to 90.4%. Correlation analysis showed that sulfate in PM_2.5 was present as ammonium sulfate. Sulfate showed a significant spatial variation with higher concentrations to the north resulting from predominantly southwesterly winds above the surface layer and by major SO₂ sources that include a power plant and refinery. At the urban site of Cruz Roja it was observed that PM_2.5 mass concentrations were similar to the submicron fraction concentrations. Furthermore, the correlation between EC in PM_2.5 and EC measured from an aethalometer was r² = 0.710. Temporal variations of SO₂ and nitrogen oxide were observed during a day when the maximum concentration of PM_2.5 was measured, which was associated with emissions from the nearby refinery and power plant. From cascade impactor measurements, the three measured modes of airborne particles corresponded with diameters of 0.32, 1.8, and 5.6 μm.