The seasonal variation of boundary layer ozone over East Asia is investigated using a regional scale chemical transport model, with the initial and boundary conditions of chemical species obtained from a global chemistry model. Comparisons with observations indicate that the model reproduces the main daily and seasonal features of ozone over eastern Asia. The seasonal variation of ozone has a summer maximum and winter minimum at Waliguan station (on the northeastern edge of the Tibetan Plateau), rather than the spring maximum and summer minimum seen at many other observational sites in the East Asian Pacific rim region. Model results suggest that there is band of a high ozone between 35°N∼45°N from the western boundary (70°E) to 130°E in the summertime. It is concluded that the seasonal transitions associated with the Asian monsoon system and transport from eastern/central China, Central/South Asia and even Europe are significantly responsible for the distinct ozone seasonal cycle at Waliguan.
 The seasonal variation of tropospheric ozone has been documented in many observational studies in the North Hemisphere. These studies indicate that the seasonal cycle of ozone has a spring maximum and summer minimum at many remote surface sites, including East Asia [Monks, 2000; Pochanart et al., 2002, and references therein]. Monks  suggested that the spring ozone maximum occurs widely across mid-latitudes in the North Hemisphere. In the East Asian Pacific Rim region, most monitoring sites are located downwind of the large anthropogenic emission regions of China, Korea and Japan. The seasonal cycle is attributed to long range transport coupled with continental-oceanic air mass exchange forced by the East Asian monsoon [Pochanart et al., 2002]. The recent work of Pochanart et al.  reported an ozone spring maximum-summer minimum at Mondy, a remote site in East Siberia. Modeling studies also reveal such seasonal characteristics of surface ozone in East Asia [Mauzerall et al., 2000]. Wild and Akimoto  indicate that intercontinental transport contributes to the regional-scale photochemical production and transport that results in the spring maximum in northeastern Asia.
 Waliguan station (36°17′N, 100°54′E) is the highest GAW (Global Atmospheric Watch) station, and is located in a remote region on the northeastern boundary of the Qinghai-Tibetan plateau at a height of 3810 meters. However, measurements at Waliguan station show that ozone has a maximum in summer and a minimum in winter [Tang et al., 1995]. This is distinctly different from other sites in East Asia. The purpose of this study is to explain the surface ozone seasonal characteristics at Waliguan station by using a regional air pollution modeling system (RAPMS).
2. Model Description
 The regional air pollution modeling system (RAPMS) is a 3-dimensional regional tropospheric chemical transport model. It consists of two parts, a regional chemical transport model and the Fifth-Generation PSU/NCAR Mesoscale Modeling System, MM5 [Grell et al., 1995], which is used to generate meteorological fields. The initial and boundary conditions for MM5 are obtained from NCAR/NCEP 2.5° × 2.5° reanalysis data sets at 6-hour intervals. The regional chemical transport model was developed by Wang et al. , and has been used to simulate atmospheric trace gases and particles, such as SOx, dust, O3 and acid rain in East Asia [Wang et al., 2002]. In this version, we implemented the calculation of photolysis rates to use the method of the Models-3 Community Multi-scale Air Quality (CMAQ) modeling system [Roselle et al., 1999]. In this study, the model domain (shown in Figure 1) contains 82 × 64 grid boxes in the east-west and south–north directions with 81 km resolution. This region includes a mixture of urban areas, industrial centers, and rural agricultural areas with both remote continental and oceanic regions, and dramatic variations in topography. In the terrain following height coordinate, the model has 20 σz layers unequally spaced from the ground to 15 km, with about 7 layers concentrated in the lowest 1 km of the planetary boundary layer. The chemical transport model includes mass-conserving advection and diffusion processes, parameterization of dry/wet deposition, and the Carbon Bond mechanism IV (CBM-IV) with isoprene chemistry. Anthropogenic emissions of NOx, CO and nonmethane hydrocarbon compounds (NMHC) were obtained from the Emission Database for Global Atmospheric Research (EDGAR) 1° × 1° annual global inventory [Olivier et al., 1996]. NOx emissions from soils and natural hydrocarbon emissions were obtained from the Global Emissions Inventory Activity (GEIA) 1° × 1° monthly global inventory [Benkovitz et al., 1996]. The initial and boundary conditions (lateral and top) of long-lived chemical species (O3, CO, ethane and propane) are taken from the output of a coupled global chemistry-climate model CHASER with roughly 2.8° horizontal resolution. The northern boundary condition of O3 was modified based on observations, as discussed in the next paragraph. The CHASER model has been described and validated by Sudo et al. [2002a, 2002b].
3. Results and Discussion
 We evaluate RAPMS by comparing our model results with observations and other modeling studies. Monthly average O3 mixing ratios and wind fields in the boundary layer simulated by RAPMS for six months in 1996 are shown in Figure 1. The O3 concentrations and distribution pattern in January correspond well with those of CMAQ simulations for the winter of 1997 [Zhang et al., 2002]. Figure 2 compares hourly-averaged model output with observations at two remote locations in January, April, July and October: one site is at Okinawa (26°50′N, 128°15′E), in the East China Sea and the other at Waliguan. Figure 3 shows the observed and calculated (RAPMS and CHASER) monthly-averaged O3 variations at Waliguan, Okinawa and Mondy (51°39′N, 100°55′E) in East Asia. In general, our regional model reproduces well the daily and seasonal variations of O3 caused by large scale atmospheric transport and photochemical reactions. Although the global model (CHASER) approximately reproduces the O3 seasonal features, it cannot capture the daily and seasonal variation of O3 as well because of its coarse resolution. For example, at Mondy CHASER overestimates O3 in the summer and underestimates O3 during the rest of the year, and it overestimates O3 in May at Waliguan. For this reason, for the regional model runs, we decreased the northern boundary O3 value by 8ppbv from CHASER in July and August and increased it by 6ppbv from January to April and October to December.
 From Figures 1 and 3, we can see that there are distinct seasonal distribution features of O3 over East Asia. The monthly mean wind flow patterns shown in Figure 1 highlight the typical pollutant transport pathways in different months. In most rural and remote East Asian Pacific rim regions (such as Okinawa), O3 has a spring maximum and summer minimum. High O3 in spring comes from continental outflow and active photochemistry, and low O3 in summer from the Pacific ocean where background concentrations are low. At Mondy, located at high latitude in central Asia, the spring maximum-summer minimum largely reflects the seasonal impacts of transport from distant upwind some regions [Pochanart et al., 2003; Wild and Akimoto, 2001].
 In contrast with other observational sites, Figures 1 and 3a show a different seasonal cycle at Waliguan station. The O3 has a summer maximum and winter minimum.
 In Figure 1, we see clearly a northward shift in peak O3 over East Asia from winter to summer. From winter to spring, O3 levels increase throughout East Asia because of enhanced photochemical production and outflow from the continent. From April, the southern edge of the band of high O3 shifts northward. In April, the southern edge defined by 60ppbv is at about 22°N, in May at 25°N, in June at 32°N, and it reaches its northmost position at 36°N in July 1996. It is interesting to see that from spring to summer, the area of high O3 (>60 ppbv) shrinks into a narrow band between 35°N∼45°N over 70°E∼125°E (see Figure 1). South Asia and East Asia are the prominent monsoon regions in the world. In East Asia, the summer flow pattern is controlled by the Indian tropical monsoon and East Asian sub-tropic monsoon systems. In the lower troposphere, the southern monsoon flow can sometimes reach as far as 40°N, and brings marine air masses with low background O3 to the north. Most of the monitoring sites in the East Asian Pacific rim region are influenced by marine air masses in summer; this explains the O3 minimum in the summer in the continental rim region in Asia. From September, with the weakening of the southerly wind and the appearance of high pressure over Mongolia and Siberia, the band of high O3 is pushed southwards and the O3 concentration decreases over almost all area north of 35°N (Figure 1, Oct.). A similar seasonal cycle of O3 is seen in global modeling studies [Sudo et al., 2002b; Lelieveld and Dentener, 2000].
Figures 4a and 4b show the monthly mean net transport flux and net O3 photochemical production (ppbv/day) in the boundary layer in July 1996, respectively. Figure 4a shows a band with a high import flux over west of 110°E, corresponding well with the band of high O3. Figure 4b illustrates the strong net photochemical O3 production in summer over the polluted regions in East Asia, including Eastern China, Korea and Japan. Although photochemical O3 production is not weak over polluted regions south of 30°N in the summertime, O3 cannot accumulate because of export by the strong southerly flow. The budget of O3 in the boundary below 1 km in an area 164,025 km2 around Waliguan, Mondy and Okinawa are shown in Table 1. Negative values indicate mass fluxes out of the sample column. The table shows that the O3 sources are mainly controlled by horizontal transport at Waliguan. In summer, vertical transport only contributes a small part to the net transport. The CHASER model indicates that O3 exchange from the stratosphere to the troposphere is minimum in summer. The net O3 photochemical production is small and negative at Waliguan, in agreement with the estimate by Ma et al. . Although Mondy is at almost the same longitude as Waliguan, it is 15° further north and hence experiences lower horizontal transport fluxes than Waliguan. This is the main reason for the different O3 seasonal cycles at these two sites. In Table 1, we see that the transport fluxes are higher at Mondy and Okinawa in spring than in other seasons.
Table 1. Sources and Sinks of O3 in the Boundary Layer in 1996a
Units are 108 mole/month. HT represents the contributions from horizontal transport and diffusion, VT the contributions from vertical transport and diffusion; and WLG, MD, ON denote Waliguan, Mondy and Okinawa, respectively.
 We conclude that to the west of 110°E, there is a mid-latitude band of high O3 in summer over East Asia which is maintained by transport. In polluted regions east of 110°E, however, the O3 concentration is controlled by photochemical production. Normally high levels of O3 are transported from the western boundary north of 35°N because of blocking by the Himalayas and the Tibetan plateau south of 35°N. Many opportunities, high levels of photochemically produced O3 are transported from central and eastern China. Wild and Akimoto  suggested that intercontinental transport from Europe can raise East Asian O3 levels although not by a very large amount in summer. CHASER model simulations [Sudo et al., 2002b] show high O3 levels moving from South Asia (in April) to Central Asia (in July) because of the effect of the Indian summer monsoon. As a result, the transport of high O3 air masses from eastern/central China, Central/South Asia and even Europe contributes significantly to the band of high O3 in Western China in the summertime.
 We have investigated the seasonal variation of boundary layer ozone over East Asia using the regional scale air pollution modeling system (RAPMS). RAPMS is able to reproduce the observed features of surface O3 over East Asia and its daily and seasonal variations. The seasonal distribution of O3 is mainly controlled by the evolution of the East Asian monsoon system in combination with photochemical production. In our simulations, we find that there is a band of high O3 between 35°N∼45°N from 70°E (the western boundary of the model) to 130°E in the summertime. In polluted regions to the east of 110°E, the high O3 levels are mainly controlled by photochemical production and result in regional O3 export. But to the west of 110°E, the band of high O3 is caused by transport, and shows weak photochemical destruction. This O3 maybe transported from eastern/central China, Central/South Asia and even Europe. The results give a reasonable explanation for the distinct seasonal variations of surface O3 (summer maximum and winter minimum) seen at Waliguan station (on the northeastern edge of the Tibetan plateau).
 We thank Dr. P. Pochanart of the Frontier Research Center for Global Change (FRCGC) for providing ozone measurement data. We acknowledge two anonymous reviewers for useful comments on the manuscript. Dr. Oliver Wild and Dr. Donald D. Lucas of the FRCGC is gratefully acknowledged for suggestions on improving the manuscript.