Ozone Source Apportionment Technology Research Articles

Insights into soil NO emissions and the contribution to surface ozone formation in China

Abstract. Elevated ground-level ozone concentrations have emerged as a major environmental issue in China. Nitrogen oxide (NOx) is a key precursor to ozone formation. Although control strategies aimed at reducing NOx emissions from conventional combustion sources are widely recognized, soil NOx emissions (mainly as NO) due to microbial processes have received little attention. The impact of soil NO emissions on ground-level ozone concentration is yet to be evaluated. This study estimated soil NO emissions in China using the Berkeley–Dalhousie Soil NOx Parameterization (BDSNP) algorithm. A typical modeling approach was used to quantify the contribution of soil NO emissions to surface ozone concentration. The brute-force method (BFM) and the Ozone Source Apportionment Technology (OSAT) implemented in the Comprehensive Air Quality Model with Extensions (CAMx) were used. The total soil NO emissions in China for 2018 were estimated to be 1157.9 Gg N, with an uncertainty range of 715.7–1902.6 Gg N. Spatially, soil NO emissions are mainly concentrated in Central China, North China, Northeast China, the northern Yangtze River Delta (YRD), and the eastern Sichuan Basin, with distinct diurnal and monthly variations that are mainly affected by the temperature and timing of fertilizer application. Both the BFM and OSAT results indicate a substantial contribution of soil NO emissions to the maximum daily 8 h (MDA8) ozone concentrations by 8.0–12.5 µg m−3 on average for June 2018, with the OSAT results being consistently higher than the BFM results. The results also showed that soil NO emissions led to a relative increase in ozone exceedance days by 10.5 %–43.5 % for selected regions. Reducing the soil NO emissions resulted in a general decrease in monthly MDA8 ozone concentrations, and the magnitude of ozone reduction became more pronounced as reductions increased. However, even with complete reductions in soil NO emissions, approximately 450.3 million people are still exposed to unhealthy ozone levels, necessitating multiple control policies at the same time. This study highlights the importance of soil NO emissions for ground-level ozone concentrations and the potential for reducing NO emissions as a future control strategy for ozone mitigation in China.

Improving photochemical indicators for attributing ozone sensitivities in source apportionment analysis

Comprehensive Air Quality Model with extensions (CAMx)–Decoupled Direct Method (DDM) simulations of first-order ozone (O3) sensitivity to nitrogen oxides (NOx) and volatile organic compounds (VOCs) emissions were performed and combined with modelled PH2O2/PHNO3 ratios to obtain a range of thresholds for determining O3-sensitivity regimes for different areas of China. Utilising the new threshold ranges for photochemical indicators, the method for determining O3 formation in the Ozone Source Apportionment Technology (OSAT) module within CAMx was improved by a dynamically varied threshold of PH2O2/PHNO3 ratio. The O3 concentration contributions in the newly added transition regime were apportioned to NOx and VOCs emissions in proportion to the relationship between the PH2O2/PHNO3 ratio and first-order O3 sensitivity. The source contributions of O3 concentrations from different emission sectors from June to September 2019 were compared using the original and improved CAMx-OSAT. The results showed that the O3 concentration contributions changed significantly in the NOx-limited regime, with a maximum decrease of 21.89%, while the contributions increased by up to 7.57% in the VOC-limited regime, and were within 15 µg/m3 in the transition regime. The modified OSAT module enabled a more sophisticated attribution of O3 to precursor emissions and may have far-reaching implications for informing O3 pollution control policy.

A comprehensive investigation on source apportionment and multi-directional regional transport of volatile organic compounds and ozone in urban Zhengzhou

To understand the characteristics, source apportionment, and regional transport of volatile organic compounds (VOCs) and ozone (O3) in a typical city with severe air pollution in central China, we observed and analyzed 115 VOC species at an urban site in Zhengzhou from 29 July to 26 September 2021. During this period, observation- and emission-based approaches revealed that Zhengzhou was in a VOC-limited regime. The average concentration of total VOCs (TVOCs) was 162.25 ± 71.42 μg/m3, dominated by oxygenated VOCs (OVOCs, 34.49%), alkanes (24.29%), and aromatics (19.49%). Six VOC sources were identified using positive matrix factorization (PMF) model, including paint solvent usage (25.32%), secondary production (24.11%), industrial production (19.22%), vehicle exhaust (16.18%), biogenic emission (8.87%), and combustion (6.30%). To assess the regional contribution and source apportionment of VOCs and O3, Comprehensive Air Quality Model with Extensions (CAMx) with the Ozone Source Apportionment Technology (OSAT) was used for simulation. Results showed that the VOCs were significantly affected by local emissions (about 70%), while O3 was mainly attributed to regional and super-regional transport. Regarding multi-directional regional transport of VOCs and O3, dominant contributions were from the northeast and east-northeast directions, and O3 contributions were also predominantly from the east and east-southeast directions. In terms of source apportionment, the transportation and industrial sectors (including solvent usage) were the major contributors to O3 and VOCs. To alleviate VOCs and O3 pollution, transportation and industrial emission reduction should be strengthened, and regional coordination, especially from the northeast to east-southeast directions, should be emphasized in addition to local management.

Comparison of ozone formation attribution techniques in the northeastern United States

Abstract. The Integrated Source Apportionment Method (ISAM) has been revised in the Community Multiscale Air Quality (CMAQ) model. This work updates ISAM to maximize its flexibility, particularly for ozone (O3) modeling, by providing multiple attribution options, including products inheriting attribution fully from nitrogen oxide reactants, fully from volatile organic compound (VOC) reactants, equally from all reactants, or dynamically from NOx or VOC reactants based on the indicator gross production ratio of hydrogen peroxide (H2O2) to nitric acid (HNO3). The updated ISAM has been incorporated into the most recent publicly accessible versions of CMAQ (v5.3.2 and beyond). This study's primary objective is to document these ISAM updates and demonstrate their impacts on source apportionment results for O3 and its precursors. Additionally, the ISAM results are compared with the Ozone Source Apportionment Technology (OSAT) in the Comprehensive Air-quality Model with Extensions (CAMx) and the brute-force method (BF). All comparisons are performed for a 4 km horizontal grid resolution application over the northeastern US for a selected 2 d summer case study (9 and 10 August 2018). General similarities among ISAM, OSAT, and BF results add credibility to the new ISAM algorithms. However, some discrepancies in magnitude or relative proportions among tracked sources illustrate the distinct features of each approach, while others may be related to differences in model formulation of chemical and physical processes. Despite these differences, OSAT and ISAM still provide useful apportionment data by identifying the geographical and temporal contributions of O3 and its precursors. Both OSAT and ISAM attribute the majority of O3 and NOx contributions to boundary, mobile, and biogenic sources, whereas the top three contributors to VOCs are found to be biogenic, boundary, and area sources.

Nonlinear Response Relationship Between Ozone and Precursor Emissions in the Pearl River Delta Region Under Different Transmission Channels

With the rapid development of urbanization, ozone (O3) pollution is an ongoing occurrence in the Pearl River Delta (PRD) region in China. The effective control of O3 pollution is a great challenge owing to the nonlinear relationship between O3 and precursor emissions and the effect of meteorological conditions. Based on the regional air quality model CAMx-OSAT (ozone source apportionment technology), O3 formation regimes were determined, and inter-city transportation across PRD was quantified under different transmission channels. The results showed that spatial differences were observed for the O3 formation regimes under different transmission channels. The VOCs-sensitive regime was mainly located in the central areas of the PRD region, and the NOx-sensitive regime was distributed in the suburban areas of the PRD regions under calm wind conditions. When the northeast wind was prevailing, the polluted air mass of the urban agglomeration was transmitted southwesterly downward, resulting in the downwind areas being transformed to VOCs-sensitive; the upwind areas were still NOx-sensitive. Under the southeast wind, the VOCs-sensitive regime had a banding distribution along the southeast-northwest direction, and the remaining areas were NOx-sensitive. With the influence of transmission channels, downwind cities were significantly affected by the transmission of upwind urban agglomerations (41%-87%), whereas the local formation was the main contributor under the calm wind conditions (60%-87%). To explore the relationship between O3 and precursor emissions, a series of sensitivity tests were designed. The results showed that maximized areas (20%-36%) with reductions in O3 can be achieved by reducing VOCs and NOx in the corresponding sensitive regimes, and the maximized level with the reduction in O3 can be fulfilled by reducing VOCs in the VOCs-sensitive regime. For the typical city Jiangmen, the area that met the standard increased the most under the calm wind (11%) and southeast wind (8%) conditions when VOCs and NOx were reduced in the corresponding sensitive regimes. Additionally, under northeast wind conditions, reducing VOCs in the VOCs-sensitive regime can more effectively control O3, as the area up to the standard increased by 140%.

Source impact and contribution analysis of ambient ozone using multi-modeling approaches over the Pearl River Delta region, China

Quantification of source impacts and contributions is a key element for the design of effective air pollution control policies. In this study, O3 source impacts and contributions were comprehensively assessed over the Pearl River Delta (PRD) region of China using brute-force method (BFM), response surface modeling with BFM (RSM-BFM) and differential method (RSM-DM) respectively, high-order decoupled direct method (HDDM), and ozone source apportionment technology (OSAT). The multi-modeling comparison results indicated that under typical nonlinear atmospheric conditions during the O3 formation, BFM, RSM-BFM, and HDDM seemed to be appropriate for assessing the impact of single source emissions; however, the results of HDDM could deviate from those of BFM when the emission reduction ratio was higher than 50 %. Under multi-source control scenarios, the results of source contribution analyses obtained from RSM-DM and OSAT were reasonably well, but the performance of OSAT was limited by its capability in representing the nonlinearity of O3 response to emission reductions of its precursors, particularly NOx. The results of this pilot study in the PRD showed that the RSM-DM appeared to replicate the nonlinearity of O3 chemistry reasonably well (e.g., O3 disbenefits due to local NOx emission reductions in Guangzhou city). Based on the source contribution results, on-road mobile (including both NOx and VOC emissions) and industrial process (mainly VOC emissions) sources were identified as two major contribution sectors by both RSM-DM and OSAT, contributing an average of 31.5 % and 11.4 % (estimated by RSM-DM) and 29.2 % and 13.0 % (estimated by OSAT) respectively to O3 formation in 9 cities of the PRD. Therefore, the reinforced emission reductions on NOx and VOC from on-road mobile and industrial process sources in the central cities (i.e., Guangzhou, Foshan, Dongguan, Shenzhen, and Zhongshan) were suggested to effectively mitigate the ambient O3 levels in the PRD.

CAMx simulations of the control of anthropogenic emissions on the reduction of ozone formation in Southeast Texas of USA

The elevated point and on-road emission sources have been identified to be the dominant anthropogenic categories contributing to ozone formation in Southeast Texas of USA. In this study, additional CAMx simulations were carried out to quantify the reductions of ozone concentration in response to the reductions of the two emission sources in the amount of 25, 50 and 75% from the Southeast region based on the summertime Ozone Episode provided by the Texas Commission on Environmental Quality (TCEQ). The simulation results were analyzed and reported in the form of Empirical Kinetics Modeling Approach (EKMA) diagrams and further confirmed through additional Ozone Source Apportionment Technology (OSAT) simulations available in CAMx. While the results are observed to be highly depending on the monitor station, they have indicated that the control of elevated point emissions is slightly more effective in ozone reduction than that of on-road emissions. Averagely, a sole reduction of 48.3% from the elevated point source, or a sole reduction of 61.0% from the on-road source, or a combined reduction of 30.0% each from the two sources are required to meet a potential future 8-h ozone NAAQS of 65 ppb. The current study has demonstrated that CAMx simulations are powerful for the practice and the developed methodology is beneficial to potential future practice when new ozone episodes become available.

Source apportionment simulations of ground-level ozone in Southeast Texas employing OSAT/APCA in CAMx

Ground-level ozone is a criteria air pollutant which can be transported into an area or produced locally through photochemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. For an ozone nonattainment area, it is critically essential to know the sources of the observed ozone before a strategic implementation plan can be established for effective reduction of ozone pollution. In this study, source apportionment simulations of ground-level ozone were performed for the Beaumont-Port Arthur (BPA) Area in Southeast Texas employing CAMx (Comprehensive Air Quality Model with Extensions) through its two built-in tools, i.e., OSAT (Ozone Source Apportionment Technology) and APCA (Anthropogenic Precursor Culpability Assessment), involving the 2012 Ozone Episode developed by TCEQ (Texas Commission on Environmental Quality). The OSAT results have indicated that local emission sources play a major role (82%) in the observed local high ozone events in the BPA area; in the meantime, 14% and 4% of the observed ozone have been identified to have transported into the area from the Southwest Louisiana and Gulf of Mexico, respectively. The APCA results have indicated that the Elevated Point, On-road and Low Point emissions are three major anthropogenic sources contributed to the BPA ozone productions with 60%, 19.5% and 19.5%, respectively. The simulation results provide needed information for the development of an implementation plan for effective reduction of ozone pollution in an ozone nonattainment area.

Characterization and sensitivity analysis on ozone pollution over the Beaumont-Port Arthur Area in Texas of USA through source apportionment technologies

Ground-level ozone pollution will harm the human health and ecological environment. With the increasingly rigorous ozone standard in the U.S., it is important to quantitatively characterize ozone pollution contributions from different spatial domains and emission sources to help air-quality controls in chemical industrial regions, such as Beaumont-Port Arthur (BPA) area in Texas of USA. In this study, both methods of ozone source apportionment technology (OSAT) and anthropogenic precursor culpability assessment (APCA) associated with the comprehensive air quality modeling with extensions (CAMx) have been employed to perform ozone source apportionment modeling to study the effect of the emission sources and domains on ozone formations in the BPA area. OSAT results indicate that local emission sources play the major role (34.8% ozone) in local high ozone events; meanwhile, emission sources and direct ozone transportations from its surrounded regions also provide notable contributions (e.g., West Louisiana and Gulf of Mexico contributes 24.0% and 20.6% ozone, respectively). APCA results indicates that elevated point emissions and on-road emissions are major anthropogenic sources for local ozone formations, which account for 43.2% and 38.2%, respectively. This study could be helpful to provide scientific and technological support for the future development of air-quality management and ozone pollution control strategies in industrial concentrated regions.

Summertime ozone pollution in the Yangtze River Delta of eastern China during 2013–2017: Synoptic impacts and source apportionment

Severe ozone (O3) pollution in China has magnified global concerns over the past years. Exploring the synoptic impacts and quantifying the source contributions are important for mitigating O3 pollution. This study focuses on the summertime O3 pollution over the Yangtze River Delta (YRD) in eastern China. We identify six predominant synoptic weather patterns (SWPs) over the YRD during 2013–2017 using the self-organising map (SOM) approach. We conduct O3 source attribution based on ozone source apportionment technology (OSAT) in a regional transport model. Surface O3 pollution is found to be sensitive to the predominant SWPs, including four O3-polluted types (northeasterly, northerly and southwesterly and anticyclone) and two O3-clean types (cyclone and the Meiyu front). The integrated influences of local chemistry and regional transport are important in O3 pollution under different SWPs. Daily maximum 8-h average (MDA8) O3 holds strong negative and positive correlations with relative humidity and sunshine duration, respectively. Among the six SWPs, the concentration and exceedance of MDA8 O3 reach their maxima under the northerly type on a regional average. Higher O3 levels, particularly under the northerly and northeasterly types, are primarily related to regional transport on various spatial scales. Local production accounts for the largest proportion of O3 sources under the anticyclone type. Under the southwesterly type, favourable weather conditions superimposed on regional transport lead to severe O3 pollution in coastal cities. The impact of super-regional transport is most pronounced along the eastern coast under the cyclone type. Source category analysis shows that transportation and industrial emissions play a prime role in surface O3 formation. This study illustrates the imperative implementation of joint emission control over eastern China to reduce O3 pollution.

Ozone source apportionment over the Yangtze River Delta region, China: Investigation of regional transport, sectoral contributions and seasonal differences

With the stringent emission control measures implemented in recent years, the overall air quality in China has been improved greatly. However, high ozone (O3) concentrations have become a more and more severe air pollution issue in the rapidly developing Yangtze River Delta (YRD) region of eastern China including Shanghai, Jiangsu, Zhejiang and Anhui provinces. The 90th percentile of O3-8 h concentration in the YRD region increased from 144 μg/m3 in 2013 to 168 μg/m3 in 2017. To investigate the sources of O3 within the YRD region, we use a tagged-species method in the Comprehensive Air Quality Model with Extensions (CAMx) called Ozone Source Apportionment Technology (OSAT) to quantify O3 contributions by source regions and sectors in Spring, Summer and Fall. We find that the O3 pollution in Shanghai, southern Jiangsu and northern Zhejiang is caused by the joint effect of local to regional precursor emissions combined with super-regional O3 transport. However, these contributions vary among seasons and pollution events. Super regional transport is the major contributor to average O3 concentration while local and upwind adjacent area contributions can dominate during high O3 concentration episodes. Source sector analysis indicates that industry and vehicle emissions are major anthropogenic sources. Within the industrial sector, power plants, boilers, and petroleum industry are the top three contributors. Reducing super-regional O3 transport is of the most importance to lowering the O3 background but local to regional precursor emission reductions are also of great significance to control O3 episodes with collaboration at the regional level offering potential for synergistic benefits to adjacent cities.

Source apportionment of biogenic contributions to ozone formation over the United States

Vegetation is the leading emitter of volatile organic compounds (VOC), a key ingredient for ozone formation. The contribution of biogenic VOC (BVOC) emissions to regional ozone formation needs better quantification so that air quality regulators can effectively design emission control strategies. One of the key uncertainties for modeling BVOC emissions comes from the estimation of photosynthetically active radiation (PAR) reaching canopy. Satellite insolation retrieval data provide an alternative to prognostic meteorological models for representing the spatial and temporal variations of PAR. In this study, biogenic emission estimates generated with the MEGAN and BEIS biogenic emissions models using satellite or prognostic PAR are used to examine the contribution of BVOC to ozone in the United States. The Comprehensive Air Quality Model with Extensions (CAMx) is applied with Ozone Source Apportionment Technology (OSAT) and brute force zero-out sensitivity runs to quantify the biogenic contributions to ozone formation during May through September 2011. The satellite PAR retrievals are on average lower than modeled PAR and exhibit better agreement with SCAN and SURFRAD network measurements. Using satellite retrievals instead of modeled PAR reduces BEIS and MEGAN estimates of isoprene by an average of 3%–4% and 9%–12%, respectively. The simulations still overestimate observed ground-level isoprene concentrations by a factor of 1.1 for BEIS and 2.6 for MEGAN. The spatial pattern of biogenic ozone contribution diagnosed from OSAT differs from the brute force zero-out sensitivity results, with the former more smoothly distributed and the latter exhibiting peak impacts near metropolitan regions with intense anthropogenic NOx emissions. OSAT tends to apportion less ozone to biogenics as BVOC emissions increase, since that shifts marginal ozone formation toward more NOx-limited conditions. By contrast, zero-out source apportionment of ozone to biogenics increases with BVOC emissions. OSAT simulations with BEIS show that BVOCs typically contribute 10–19% to regional ozone concentrations at nonattainment receptor sites during episode days.

Source apportionment of surface ozone in the Yangtze River Delta, China in the summer of 2013

We applied ozone source apportionment technology (OSAT) with tagged tracers coupled within the Comprehensive Air Quality Model with Extensions (CAMx) to study the region and source category contribution to surface ozone in the Yangtze River Delta area in summer of 2013. Results indicate that the daytime ozone concentrations in the YRD region are influenced by emissions both locally, regionally and super-regionally. At urban Shanghai, Hangzhou and Suzhou receptors, the ozone formation is mainly VOC-limited, precursor emissions form Zhejiang province dominate their O3 concentrations. At the junction area among two provinces and Shanghai city, the ozone is usually influenced by all the three areas. The daily max O3 at the Dianshan Lake in July are contributed by Zhejiang (48.5%), Jiangsu (11.7%), Anhui (11.6%) and Shanghai (7.4%), long-range transport constitutes around 20.9%. At Chongming site, the BVOC emissions rate is higher than urban region. Regional contribution results show that Shanghai constitutes 15.6%, Jiangsu contributes 16.2% and Zhejiang accounts for 25.5% of the daily max O3. The analysis of the source category contribution to high ozone in the Yangtze River Delta region indicates that the most significant anthropogenic emission source sectors contributing to O3 pollution include industry, vehicle exhaust, although the effects vary with source sector and selected pollution episodes. Emissions of NOx and VOCs emitted from the fuel combustion of industrial boilers and kilns, together with VOCs emissions from industrial process contribute a lot to the high concentrations in urban Hangzhou, Suzhou and Shanghai. The contribution from regional elevated power plants cannot be neglected, especially to Dianshan Lake. Fugitive emissions of volatile pollution sources also have certain contribution to regional O3. These results indicate that the regional collaboration is of most importance to reduce ambient ozone pollution, particularly during high ozone episodes.

Source apportionment and health effect of NOx over the Pearl River Delta region in southern China

As one of the most notorious atmospheric pollutants, NOx not only promotes the formation of ozone but also has adverse health effects on humans. It is therefore of great importance to study the sources of NOx and its effects on human health. The Comprehensive Air Quality Model (CAMx) modeling system and ozone source apportionment technology (OSAT) were used to study the contribution of NOx from different emission sources over southern China. The results indicate that heavy duty diesel vehicles (HDDVs) and industrial point sources are the two major local NOx sources, accounting for 30.8% and 18.5% of local NOx sources, respectively. In Hong Kong, marine emissions contributed around 43.4% of local NOx in 2011. Regional transport is another important source of this pollutant, especially in February and November, and it can contribute over 30% of ambient NOx on average. Power plant point emission is an significant regional source in Zhuhai, Zhongshan and Foshan. The total emission sources are estimated to cause 2119 (0–4405) respiratory deaths and 991 (0–2281) lung cancer deaths due to long-term exposure to NOx in the Pearl River Delta region. Our results suggest that local governments should combine their efforts and vigorously promote further reduction of NOx emissions, especially for those sources that make a substantial contribution to NOx emissions and affect human health: HDDV, LDGV, industrial point sources and marine sources.

An ozone episode over the Pearl River Delta in October 2008

The north and east Pearl River Delta (PRD) is usually a clean, upwind area in autumn. Serious ozone pollution there in mid-late October 2008 was first discovered and then analyzed. Trajectory analysis, process analysis, ozone source apportionment technology, and sensitivity analysis were used to study this episode. Under the influence of a weak south wind, the precursors emitted in Guangzhou and Foshan were transported to the north and northeast PRD and formed ozone there, which resulted in high ozone concentration (>100 ppb). As the wind direction later transited to northerly, the precursors in the northeast PRD that originated from the central and west PRD were transported to the south, and caused severe ozone pollution in the southeast PRD. The ozone contributed by chemical processes reached >20 ppb/h in Jinguowan. More than 40 ppb ozone was contributed by the precursor emission in the central and west PRD during the episode. The ozone concentration was highly sensitive to the precursor emission in the PRD region in the high-ozone situations. This episode showed the complexity of regional pollution in the PRD. When the PRD is controlled by a low air pressure system and then cold air moves from northern China to the south, the risk of ozone pollution in the north and southeast PRD increases.

Comprehensive study of emission source contributions for tropospheric ozone formation over East Asia

AbstractEmission source contributions of tropospheric ozone (O3) were comprehensively investigated by using the higher‐order decoupled direct method (HDDM) for sensitivity analysis and the ozone source apportionment technology (OSAT) for mass balance analysis in the comprehensive air‐quality model with extensions (CAMx). The response of O3 to emissions reductions at various levels in mainland China, Korea, and Japan were estimated and compared with results calculated by the brute force method (BFM) where one model parameter is varied at a time. Emissions were assessed at three receptor sites in Japan that experienced severe pollution events in May 2009. For emissions from China, HDDM assessed O3 response with a bias of only up to 3 ppbv (a relative error of 4.5%) even for a 50% reduction but failed to assess a more extreme reduction. OSAT was reasonably accurate at 100% reduction, with a −4 ppbv (−7%) bias, but was less accurate at moderate ranges of reduction (∼50–70%). For emissions from Korea and Japan, HDDM captured the nonlinear response at all receptor sites and at all reduction levels to within 1% in all but one case; however, the bias of OSAT increased with the increasing reduction of emissions. One possible reason for this is that OSAT does not account for NO titration. To address this, a term for potential ozone (PO; O3 and NO2 together) was introduced. Using of PO instead of O3 improved the performance of OSAT, especially for emissions reductions from Korea and Japan. The proposed approach with PO refined the OSAT results and did not degrade HDDM performance.

Ozone source attribution during a severe photochemical smog episode in Beijing, China

Beijing, the capital of China, frequently suffers from the high levels of ozone in summer. A 3-D regional chemical transport model, the Comprehensive Air Quality Model with extensions (CAMx), has been used to simulate a heavy O3 pollution episode in Beijing during June 26–July 2, 2000. Ozone Source Apportionment Technology (OSAT) and Geographic Ozone Assessment Technology (GOAT) were applied to quantify the contributions of the precursor emissions from different regions to O3 concentrations in Beijing, to identify the relative importance of different ways by which regional sources affected the O3 levels in Beijing urban areas, and to investigate the sensitivity of O3 formation to the precursors during the episode. The O3 pollution in Beijing showed a significant spatial distribution with strong regional contribution. The results suggested that the plume originating from Beijing urban areas greatly affected the O3 concentrations at the Dingling site, accounting for 55% of elevated O3 there, while O3 pollution in the Beijing urban areas resulted from both local emissions and those from Tianjin and the south of Hebei Province. Transport of O3 was responsible for about 70% of the regional O3 contribution to Beijing urban areas, while transport of O3 precursors accounted for the remainder. The formation of O3 was limited by volatile organic compounds (VOCs) in the urban areas of Beijing, while being more sensitive to NOx levels in the suburban and more remote areas. Therefore, it is necessary to consider a large number of factors, including impacts of emissions from different regions, the two modes of regional contribution as well as the sensitivity of O3 formation to precursors, in the design of emissions control strategies for O3 reduction in Beijing.