Modeling the Long-Term Frequency Distribution of Regional Ozone Concentrations Using Synthetic Meteorology

A new method is developed to generate the meteorological input fields required for use with photochemical airshed models that seek to predict the effect of pollutant emissions on the long-term frequency distribution of peak O_3 concentrations. Instead of using meteorological fields derived from interpolation of direct weather observations, this method uses synthetically generated meteorological data. These synthetic meteorological fields are created by first constructing a semi-Markov process that generates a time series of large-scale synoptic weather conditions that statistically resemble the occurrence and persistence of synoptic weather patterns during specific months of the year. Then for each day within each synoptic weather category, local weather variables indicative of the meteorological potential for ozone formation are drawn from the approximated joint distribution of the summation of three pressure gradients across the airshed and the 850 mb temperature measured in the early morning. The synthetic initial conditions are combined with boundary values that are extracted from historical days that match the chosen synoptic class, temperature, and pressure gradient values as closely as possible for use in a prognostic mesoscale meteorological model. The prognostic mesoscale meteorological model generates the meteorological input fields necessary for the photochemical airshed model. The airshed model driven by synthetically generated meteorological data is executed for a 31 day period that statistically resembles weather during the month of August in Southern California using pollutant emissions data from the year 1987. The procedure produced a frequency of occurrence of peak 8 h average ozone concentrations that compared well both to that produced by the deterministic model as well as to the O_3 concentrations observed over the August months of the years 1984−1990

Modeling the long-term frequency distribution of regional ozone concentrations

Efficient methods are developed for modeling emissions – air quality relationships that govern ozone and NO_2 concentrations over very long periods of time. A baseline model evaluation study is conducted to assess the accuracy and speed with which the relationship between pollutant emissions and the frequency distribution of O_3 concentrations throughout the year can be computed along with annual average NO_2 values using a deterministic photochemical airshed model driven by automated objective analysis of measured meteorological parameters. Methods developed are illustrated by application to the air quality situation that exists in Southern California. Model performance statistics for O_3 are similar to the results obtained in previous short-term episodic model evaluation studies that were based on hand-crafted meteorological inputs that are supplemented by expensive field measurement campaigns. Model predictions at one of the highest NO_2 concentration sites in the US indicate that measured violation of the US annual average NO_2 air quality standard at that site occurs because other species such as HNO_3 and PAN are measured as if they were NO_2 by the chemiluminescent NO_x monitors in current use

Effect of Emissions Control on the Long-Term Frequency Distribution of Regional Ozone Concentrations

Photochemical airshed models that simulate the pollutant transport and atmospheric chemical reaction processes leading to ozone formation now can be exercised for years at a time, permitting a thorough evaluation of the extent to which urban and regional ozone concentrations can be controlled. The Los Angeles ozone problem serves as a prototype for severe photochemical smog problems elsewhere. In southern California, the occurrence of peak 1-h average ozone concentrations above 0.12 ppm can be reduced to approximately 20 days per year through control of organic vapor and oxides of nitrogen emissions. Calculations show that the number of days per year with 1-h average O_3 concentrations above 0.12 ppm approaches zero more quickly in response to controls than is the case for the number of days with lower but more persistent ozone concentrations; as a result, more than 60 days per year will exceed the new U.S. Federal ozone standard set in 1997 at a level of 0.08 ppm over an 8-h averaging time, even at very stringent levels of emission control. The days with the highest observed ozone concentrations are not necessarily the hardest days to bring below the air quality standards

Effect of Emissions Control on the Long-Term Frequency Distribution of Regional Ozone Concentrations

Photochemical airshed models that simulate the pollutant transport and atmospheric chemical reaction processes leading to ozone formation now can be exercised for years at a time, permitting a thorough evaluation of the extent to which urban and regional ozone concentrations can be controlled. The Los Angeles ozone problem serves as a prototype for severe photochemical smog problems elsewhere. In southern California, the occurrence of peak 1-h average ozone concentrations above 0.12 ppm can be reduced to approximately 20 days per year through control of organic vapor and oxides of nitrogen emissions. Calculations show that the number of days per year with 1-h average O_3 concentrations above 0.12 ppm approaches zero more quickly in response to controls than is the case for the number of days with lower but more persistent ozone concentrations; as a result, more than 60 days per year will exceed the new U.S. Federal ozone standard set in 1997 at a level of 0.08 ppm over an 8-h averaging time, even at very stringent levels of emission control. The days with the highest observed ozone concentrations are not necessarily the hardest days to bring below the air quality standards

Modeling the Long-Term Frequency Distribution of Regional Ozone Concentrations Using Synthetic Meteorology

A new method is developed to generate the meteorological input fields required for use with photochemical airshed models that seek to predict the effect of pollutant emissions on the long-term frequency distribution of peak O_3 concentrations. Instead of using meteorological fields derived from interpolation of direct weather observations, this method uses synthetically generated meteorological data. These synthetic meteorological fields are created by first constructing a semi-Markov process that generates a time series of large-scale synoptic weather conditions that statistically resemble the occurrence and persistence of synoptic weather patterns during specific months of the year. Then for each day within each synoptic weather category, local weather variables indicative of the meteorological potential for ozone formation are drawn from the approximated joint distribution of the summation of three pressure gradients across the airshed and the 850 mb temperature measured in the early morning. The synthetic initial conditions are combined with boundary values that are extracted from historical days that match the chosen synoptic class, temperature, and pressure gradient values as closely as possible for use in a prognostic mesoscale meteorological model. The prognostic mesoscale meteorological model generates the meteorological input fields necessary for the photochemical airshed model. The airshed model driven by synthetically generated meteorological data is executed for a 31 day period that statistically resembles weather during the month of August in Southern California using pollutant emissions data from the year 1987. The procedure produced a frequency of occurrence of peak 8 h average ozone concentrations that compared well both to that produced by the deterministic model as well as to the O_3 concentrations observed over the August months of the years 1984−1990

Effect of alternative boundary conditions on predicted ozone control strategy performance: A case study in the Los Angeles area

The purpose of this paper is to illuminate the importance of assumptions made regarding boundary and initial conditions on the predicted performance of regional ozone control strategies. A computationally efficient approach to depicting the response of an air basin to emission controls is developed. The problem of ozone isopleth generation is addressed using a large Eulerian grid-based photochemical airshed model that is distributed over a grid system of 64 different ROG and NO_x control combination points that were run simultaneously on a parallel computer. This method is used for the Los Angeles area to examine the effect on predicted ozone concentrations of alternative assumptions about how boundary conditions at the edge of the air basin will change as a result of changed recirculation of pollutants from within the airshed plus emission decreases upwind. We show that an accurate forecast of the effect of future emission control programs on pollutant inflows across the boundaries of the Los Angeles modeling region is absolutely critical to selection of a successful ozone control strategy for the Los Angeles area

Effect of alternative boundary conditions on predicted ozone control strategy performance: A case study in the Los Angeles area

The purpose of this paper is to illuminate the importance of assumptions made regarding boundary and initial conditions on the predicted performance of regional ozone control strategies. A computationally efficient approach to depicting the response of an air basin to emission controls is developed. The problem of ozone isopleth generation is addressed using a large Eulerian grid-based photochemical airshed model that is distributed over a grid system of 64 different ROG and NO_x control combination points that were run simultaneously on a parallel computer. This method is used for the Los Angeles area to examine the effect on predicted ozone concentrations of alternative assumptions about how boundary conditions at the edge of the air basin will change as a result of changed recirculation of pollutants from within the airshed plus emission decreases upwind. We show that an accurate forecast of the effect of future emission control programs on pollutant inflows across the boundaries of the Los Angeles modeling region is absolutely critical to selection of a successful ozone control strategy for the Los Angeles area

Mathematical modeling and control of the dry deposition flux of nitrogen-containing air pollutants

An Eulerian grid-based air quality model has been modified to include a resistance-based dry deposition code. The magnitude and spatial distribution of the dry deposition flux of nitrogen-containing pollutants to the surface of the Los Angeles area was calculated as a function of land use. For August 1982 base case conditions, the dry deposition flux was 247 t of N per day (5 from NO, 49 from NO_2, 7 from PAN, 101 from HNO_3, 59 from NH_3, and 26 from NH4_NO_3), which corresponds to more than half of the daily NO_x emissions to the local atmosphere. The effects of emission controls on NO_x and hydrocarbon sources in Southern California as they existed in 1982 were examined. At the highest level of control studied (37% reactive hydrocarbon reduction, 61 % NO_x reduction), the nitrogen dry flux would be 174 t of N per day after control (2 from NO, 20 from NO_2, 7 from PAN, 58 from HNO_3, 75 from NH_3, and 12 from NH_4NO_3