United States Environmental Protection Agency Atmospheric Research and Exposure Assessment Laboratory Research Triangle Park NC 27711 Research and Development EPA/600/S3-91/015 May 1991 EPA Project Summary A Study Using a Three Dimensional Photochemical Smog Formation Model Under Conditions of Complex Flow: Application of the Urban Airshed Model to the Tokyo Metropolitan Area Shinji Wakamatsu and Kenneth L. Schere The purpose of this study is to evaluate the Urban Airshed Model (UAM), a three-dimensional photo- chemical urban air quality simulation model, using field observations from the Tokyo Metropolitan Area. Empha- sis was placed on the photochemical smog formation mechanism under stagnant meteorological conditions. The UAM produced reasonable calcu- lated results for the diurnal, area! and vertical distributions of O, concentra- tions covering the Tokyo Metropolitan Area. The role and significance of the previous day's secondary pollutants on O3 formation mechanisms were also in- vestigated. During the night time, high values of secondary pollutant concen- trations were predicted above the ra- diation inversion layer. These aged pollutants were then entrained into the mixing layer during the day. in accor- dance with the elevation of the lid. These characteristic features were also observed in the field study. This Project Summary was devel- oped by EPA's Atmospheric Research and Exposure Assessment Laboratory, Research Triangle Park, NC, to an- nounce key findings of the research project that Is fully documented In a separate report of the same title (see Project Report ordering Information at back). Introduction High ozone (O3) concentrations are fre- quently observed in the Tokyo Metropoli- tan ^Area (TMA) in the summer season. The" industrial complexes located in the Tokyo Bay area are the largest in Japan and are the major stationary sources of pollutants (NO, NO2, SO2, hydrocarbons, , aerosols, etc.), while the Tokyo Metropolis is the largest contributor of pollutants from mobile sources. In addition, the topogra- phy of the area is very complicated, which leads to complex wind patterns. Reflecting these circumstances, the relationships between the precursor emissions and fields observed O3 patterns are not clearly un- derstood. Investigating these relationships requires the use of a three-dimensional photochemical air pollution simulation model. Results from such a model allow the analysis of the quantitative effects of the "previous day's secondary pollutants, temporal variation of mixing height, and three-dimensional wind field on O3 forma- tion mechanisms in the atmosphere. The purpose of this study is an evalua- tion of the SAI Urban Airshed Model (UAM), a three-dimensional photochemi- cal urban air quality simulation model, us- ing observations from a 1981 field experi- ment in the Tokyo Metropolitan Area, Ja- pan. From July 15 to July 17 high O3 concentrations were observed in the To- nSy Printed on Recycled Paper ------- kyo Metropolitan Area under conditions of a complex wind field. During this period intensive field measurements of vertical and horizontal winds and pollutants were made. These data provide sufficient infor- mation to evaluate the photochemical smog simulation model for this case-study. Analysis In the first step, input data were se- lected from the data base and set up in a suitable format for the SAI-UAM and a test run was then performed. For the sec- ond step the simulated results were evaluated using the field observed data. Emphasis was placed on the investigation of diurnal variations and area! and vertical distributions of pollutants. Vertical cross- sodions of simulated pollutant patterns are analyzed for the TMA domain, includ- ing the diurnal variation in the patterns. The effects of vertical meteorological con- ditions and the previous day's secondary pollutants are emphasized. Limited O3 field data from aircraft are compared with the simulated results. In the Tokyo Metropolitan Area higher concentrations of O3 are usually observed near the shore in the morning. As the sea breeze layer penetrates inland, this high concentration region travels with it, in- creasing in concentration with time. When the geostrophtc wind is weak, the sea- land breeze circulation and the mountain- valley breeze circulation are the most im- portant meteorological factors in photo- chemical O3 formation inland. A nighttime temperature inversion, which persists into the early morning, prevents the dispersion of primary pollutants emit- ted from the large coastal industrial zones and the Tokyo Metropolitan Area. Two large industrial complexes, Keihin and Keiyo, are located along Tokyo Bay, while the center of Tokyo is the largest con- tributor of pollutants from mobile sources. These pollutants are transported seaward by the land breeze. Around noon time the sea breeze brings these pollutants back inland where they become well mixed un- der thermal convective conditions. These primary pollutants are converted into sec- ondary pollutants and transported inland with the sea breeze penetration. Usually, high O, concentrations are observed aloft (500-1000 m) in the early morning. This is an aged O3 layer, and its associated sec- ondary pollutants are entrained into the growing mixing layer in accordance with the elevation of the lid. This effect is be- lieved to accelerate the formation of sec- ondary pollutants during the next day. During the period of study, July 15 to July 20,1981, the weather conditions were typical of this season in Japan. A pre- dominant northern Pacific high pressure cell covered the Japanese archipelago and the pressure gradient was weak. Under these conditions, especially high oxidant concentrations were observed in the southern part of the Tokyo Metropolitan Area due to the orientation of the sea- land breeze circulation system to the met- ropolitan area. UAM simulations were conducted over 40 hours beginning 0400 JST, July 16, 1981. Data from 62 selected ground monitoring stations, 4 upper-air sounding sites, and 23 pilot balloon sites were used to determine the boundary concentrations and the meteorological parameter input values for the model. The aircraft mea- surements showed a very complex verti- cal and horizontal pollutant distribution mainly caused by the complex wind structure. To simulate this wind field, a three-dimensional mass and momentum- conserving wind model was used to pre- pare the hourly horizontal winds required by the UAM. The Carbon Bond II chemical kinetic scheme was used for the chemis- try calculations in the UAM. Hourly emis- sion rates were calculated from area sources and point sources of NO2 and non- methane hydrocarbons (NMHC) at each grid cell. NMHC emissions must be dis- tributed into particular reactivity classes. To accomplish this the ratio of 18 NMHC species from the five major hydrocarbon source categories (vehicle exhaust, oil re- finery facilities, other petrochemical op- erations, gasoline vapor, and painting sol- vents) were determined. The area modeled around Tokyo was 183 km wide and 172 km long, with each individual cell 6.8 km wide and 5.5 km long. This individual cell size is based on the relative degree length of latitude, lon- gitude of this area. The full horizontal do- main is 27 by 30 cells with the outer ring of cells serving as boundary condition cells. There are five layers in the vertical; the depth of each layer is a function of the time of day. The bottom three layers simulated the mixing layer. Conclusions Usually, in the Tokyo area, high O, concentration areas, which are transported from inland regions by the nighttime land breeze, are observed aloft in the early morning, and these are entrained into the mixing layer in accordance with the el- evation of the lid. This effect is believed to accelerate the formation of secondary pollutants during the next day. These ob- servational results were qualitatively re- produced by the Urban Airshed Model. In this study major efforts were devoted to clarify the effects of vertical meteorologi- cal conditions and the previous day's sec- ondary pollutants on the second day's O, concentrations. Simulated results showed reasonable performance at reproducing a three-dimensionaLp.rpfilei.ofLQ3 concentra- tions. It was found that: 1. The SAI Urban Airshed Model pro- duced reasonable calculated results for the diurnal and area! distributions of O3 concentrations covering the Tokyo Metro- politan Area. 2. Observed vertical O3 profiles were also compared with the simulation results. The simulated vertical profiles were in qualitative agreement with the observa- tions although the coarse vertical resolution and hour averaging in the model may have prevented the prediction of the maximum concentrations accurately. 3. Paraffin (PAR) and aromatic (ARO) concentrations were underestimated. This might be caused from an estimation error in the hydrocarbon emissions. The role and significance of the previ- ous day's secondary pollutants on O3 for- mation mechanisms were investigated using the UAM simulations. The largest differences in the vertical pollutant profiles between the two days studied were in the species O3 and carbonyls (CARB). During the second night of simulation, especially high values of these species were pre- dicted above the radiation inversion layer. These aged pollutants were then entrained into the mixing layer during the day in accordance with the elevation of the lid. In part, because of the higher CARB con- centration levels on the second day, O3 levels increased earlier in time and peaked at higher concentrations than they had on the first day of simulation. The importance of the effects of the previous day's aged pollutants are confirmed using the UAM. . GOVERNMENT PRINTING OFFICE: 1991/548-028/20217 ------- ------- The EPA author, Kenneth L. Schere (also the EPA Project Officer, see below), is on assignment to the Atmospheric Research and Exposure Assessment Laboratory, Research Triangle Park, NO 27711, from the National Oceanic and Atmospheric Administration; Shinji Wakamatsu is with the National Institute of Environmental Studies, Tsukuba, Japan. The complete report, entitled "A Study Using a Three Dimensional Photochemical Smog Formation Model Under Conditions of Complex Flow: Application of the Urban Airshed Model to the Tokyo Metropolitan Area," (Order No. PB91-168 401/AS; Cost: $17.00, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Atmospheric Research and Exposure Assessment Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati, OH 45268 BULK RATE POSTAGE & FEES PAID EPA PERMIT No. G-35 Official Business Penalty for Private Use $300 EPA/600/S3-91/015 ------- |