Ozone Measurements Research Articles

Мониторинг приземного озона в природном заповеднике «Карадагский» в период 2017–2021 гг.

The background environmental monitoring station in the Karadag Nature Reserve is the only point for routine measurements (since 2010) of surface ozone on the Black Sea coast of the Russian Federation. Surface ozone observations at the station over the past five years are discussed. An insignificant interannual variability of surface ozone is shown as compared to the previous years. Seasonal and diurnal variations in surface ozone averaged over the period of 2017–2021 generally coincide with the previous period and are comparable with the typical variations at foreign Mediterranean stations. The surface ozone regime at the analyzed station is characterized by an insignificant diurnal amplitude: it is about 5 μg/m3 in winter and increases by almost 3–4 times in the warm season. However, due to increased nighttime ozone, it remains significantly smaller than at lowland stations. In the dark time of the day, surface ozone is 50–55 µg/m3 in winter and autumn and about 70 µg/m3 in spring and summer. The surface ozone regime is significantly affected by the breeze circulation and, at night, by slope winds providing the influx of ozone from the free troposphere. At the background monitoring station, surface ozone values approaching the maximum single permissible concentration of ozone (MPCm.s.) are occasionally recorded. In spring, such events are associated with the movement of the upper-level frontal zone over the region. The registered cases of exceeding MPCm.s. were observed in the plumes of air polluted in remote areas and spreading to Crimea. Keywords: surface ozone, Karadag Nature Reserve, seasonal and diurnal variation, maximum permissible concentration of ozone

Ozone Monitoring Instrument (OMI) collection 4: establishing a 17-year-long series of detrended level-1b data

Abstract. The Ozone Monitoring Instrument (OMI) was launched on 15 July 2004, with an expected mission lifetime of 5 years. After more than 17 years in orbit the instrument is still functioning satisfactorily and in principle can continue doing so until the expected decommissioning of its platform Aura in 2025. In order to continue the datasets acquired by OMI and the Microwave Limb Sounder, the mission was extended up to at least 2023. Actions have been taken to ensure the proper functioning of the OMI operations, the data processing, and the calibration monitoring system until the eventual end of the mission. For the data processing a new level-0 (L0) to level-1b (L1b) data processor was built based on the recent developments for the TROPOspheric Monitoring Instrument (TROPOMI). With corrections for the degradation of the instrument now included, it is feasible to generate a new data collection to supersede the current collection-3 data products and reprocess the data of the entire mission up to now. This paper describes the differences between the collection-3 and collection-4 data. It will be shown that the collection-4 L1b data comprise a clear improvement with respect to the previous collections. By correcting for the gentle optical and electronic aging that has occurred over the past 17 years, OMI's ability to make trend-quality ozone measurements has further improved.

Global, high-resolution mapping of tropospheric ozone – explainable machine learning and impact of uncertainties

Abstract. Tropospheric ozone is a toxic greenhouse gas with a highly variable spatial distribution which is challenging to map on a global scale. Here, we present a data-driven ozone-mapping workflow generating a transparent and reliable product. We map the global distribution of tropospheric ozone from sparse, irregularly placed measurement stations to a high-resolution regular grid using machine learning methods. The produced map contains the average tropospheric ozone concentration of the years 2010–2014 with a resolution of 0.1∘ × 0.1∘. The machine learning model is trained on AQ-Bench (“air quality benchmark dataset”), a pre-compiled benchmark dataset consisting of multi-year ground-based ozone measurements combined with an abundance of high-resolution geospatial data. Going beyond standard mapping methods, this work focuses on two key aspects to increase the integrity of the produced map. Using explainable machine learning methods, we ensure that the trained machine learning model is consistent with commonly accepted knowledge about tropospheric ozone. To assess the impact of data and model uncertainties on our ozone map, we show that the machine learning model is robust against typical fluctuations in ozone values and geospatial data. By inspecting the input features, we ensure that the model is only applied in regions where it is reliable. We provide a rationale for the tools we use to conduct a thorough global analysis. The methods presented here can thus be easily transferred to other mapping applications to ensure the transparency and reliability of the maps produced.

СОДЕРЖАНИЕ ОЗОНА НАД ТЕРРИТОРИЕЙ РОССИЙСКОЙ ФЕДЕРАЦИИ В ПЕРВОМ КВАРТАЛЕ 2022 Г

The review is based on the results of operation of the total ozone (TO) monitoring system over the CIS and Baltic countries, operating in the operational mode at the Central Aerological Observatory (CAO). The monitoring system uses data from the domestic network of M-124 filter ozonometers, operating under the methodological guidance of the Main Geophysical Observatory; the quality of the entire system is operatively controlled in the Central Administrative District by comparison with the observational data made with the help of the OMI satellite equipment (NASA, USA). The main TO observational data for each month of the first quarter of 2022 and for the quarter as a whole are summarized. The results of regular measurements of ground-level ozone carried out in the Moscow region are also summarized.

NASA GEOS Composition Forecast Modeling System GEOS-CF v1.0: Stratospheric Composition.

The NASA Goddard Earth Observing System (GEOS) Composition Forecast (GEOS‐CF) provides recent estimates and 5‐day forecasts of atmospheric composition to the public in near‐real time. To do this, the GEOS Earth system model is coupled with the GEOS‐Chem tropospheric‐stratospheric unified chemistry extension (UCX) to represent composition from the surface to the top of the GEOS atmosphere (0.01 hPa). The GEOS‐CF system is described, including updates made to the GEOS‐Chem UCX mechanism within GEOS‐CF for improved representation of stratospheric chemistry. Comparisons are made against balloon, lidar, and satellite observations for stratospheric composition, including measurements of ozone (O3) and important nitrogen and chlorine species related to stratospheric O3 recovery. The GEOS‐CF nudges the stratospheric O3 toward the GEOS Forward Processing (GEOS FP) assimilated O3 product; as a result the stratospheric O3 in the GEOS‐CF historical estimate agrees well with observations. During abnormal dynamical and chemical environments such as the 2020 polar vortexes, the GEOS‐CF O3 forecasts are more realistic than GEOS FP O3 forecasts because of the inclusion of the complex GEOS‐Chem UCX stratospheric chemistry. Overall, the spatial patterns of the GEOS‐CF simulated concentrations of stratospheric composition agree well with satellite observations. However, there are notable biases—such as low NO x and HNO3 in the polar regions and generally low HCl throughout the stratosphere—and future improvements to the chemistry mechanism and emissions are discussed. GEOS‐CF is a new tool for the research community and instrument teams observing trace gases in the stratosphere and troposphere, providing near‐real‐time three‐dimensional gridded information on atmospheric composition.

Comment on “Comparison of ozone measurement methods in biomass burning smoke: an evaluation under field and laboratory conditions” by Long et al. (2021)

Abstract. Long et al. (2021) conducted a detailed study of possible interferences in measurements of surface O3 by UV spectroscopy, which measures the UV transmission in ambient and O3-scrubbed air. While we appreciate the careful work done in this analysis, there were several omissions, and in one case, the type of scrubber used was misidentified as manganese dioxide (MnO2) when in fact it was manganese chloride (MnCl2). This misidentification led to the erroneous conclusion that all UV-based O3 instruments employing solid-phase catalytic scrubbers exhibit significant positive artifacts, whereas previous research found this not to be the case when employing MnO2 scrubber types. While the Long et al. (2021) study, and our results, confirm the substantial bias in instruments employing an MnCl2 scrubber, a replication of the earlier work with an MnO2 scrubber type and no humidity correction is needed.

Chemical reactivity of volatile organic compounds and their effects on ozone formation in a petrochemical industrial area of Lanzhou, Western China

Measurements of ozone (O3) and its precursors were performed in the summer of 2019 in Lanzhou, a petrochemical industrial city, to better understand the reactivity of volatile organic compounds (VOCs) and their effects on O3 production. During the campaign, the daily maximum 8-hour average (MDA8) O3, NO2, and total VOC (TVOC) concentrations reached 72.2 ± 19.9 ppb, 24.9 ± 10.8 ppb, and 50.8 ± 46.1 ppb, respectively. Alkanes, alkenes, halocarbons, aromatics, and alkynes contributed 45.3%, 24.0%, 16.5%, 10.0%, and 4.2% to TVOCs, respectively. The OH reactivity and relative incremental reactivity (RIR) of VOCs at different times were calculated. The results indicated that alkenes played a predominant role, accounting for an average of 68.5% of the initial VOC reactivity. Compared to other regions, alkenes are relatively more important for O3 formation in the petrochemical industry area of Lanzhou, while aromatics are relatively less important. Generally, O3 formation occurred in a VOC-limited regime in the morning and in a transitional regime in the afternoon. The response surface methodology (RSM) combined with a chemical box model was applied to obtain relationships between O3 and its precursors and determine the most effective way to reduce the O3 concentration. Reduction in the non-alkene concentration slightly affected the O3 concentration. In contrast, the effect of nitrogen oxides (NOx) was closely related to the alkene concentration, and NOx concentration reduction could lead to an increase in the O3 concentration when alkenes were abated to less than 80% of the present concentration. To mitigate O3 pollution near the petrochemical industrial area of Lanzhou, reducing the alkene concentration, especially the C4 alkene concentration (1,3-butadiene, cis-2-butene, and trans-2-butene), was the fastest and most effective control strategy. The results of this study serve as a reference for O3 pollution control in petrochemical industrial areas.

Tropospheric ozone measurements at a rural town in New South Wales, Australia

In Australia, tropospheric ozone measurements in rural locations are scarce with measurements mostly made in cities. This limits the ability to estimate background ozone levels that inform policy development. The few studies that have assessed rural ozone in Australia have been associated with short campaign monitoring or specific, short-term research programs. Recognising this deficit of information, the New South Wales Government has established long-term ozone monitoring at two rural locations. This paper presents results from the first three years of monitoring at Gunnedah. We assess seasonal, diurnal and sectoral patterns of ozone. Several events are analysed, including high ozone associated with the 2019/20 Australian Bushfire Emergency and an extreme heatwave event. We find that ozone levels at Gunnedah exceed the screening standards set by Australia's National Environmental Protection (Ambient Air Quality) Measure, emphasising the need for additional ozone monitoring in rural and regional Australia. Our early results indicate that in NSW, background ozone mixing ratios for airmasses of continental origin is likely in the range of 36–39 ppb, higher than the 14–30 ppb associated with air masses of marine origin and greater than the 30 ppb background mixing ratio used for monitoring design and standard setting in Australia. Maximum 8-hourly ozone in non-bushfire impacted events is as high as 64 ppb, demonstrating the challenges that rural/regional communities may face in always meeting the new Australian 8-h ozone standard of 65 ppb. These results add to our understanding of rural background ozone within Australia and in the southern hemisphere.

Overview: On the transport and transformation of pollutants in the outflow of major population centres – observational data from the EMeRGe European intensive operational period in summer 2017

Abstract. Megacities and other major population centres (MPCs) worldwide are major sources of air pollution, both locally as well as downwind. The overall assessment and prediction of the impact of MPC pollution on tropospheric chemistry are challenging. The present work provides an overview of the highlights of a major new contribution to the understanding of this issue based on the data and analysis of the EMeRGe (Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales) international project. EMeRGe focuses on atmospheric chemistry, dynamics, and transport of local and regional pollution originating in MPCs. Airborne measurements, taking advantage of the long range capabilities of the High Altitude and LOng Range Research Aircraft (HALO, https://www.halo-spp.de, last access: 22 March 2022), are a central part of the project. The synergistic use and consistent interpretation of observational data sets of different spatial and temporal resolution (e.g. from ground-based networks, airborne campaigns, and satellite measurements) supported by modelling within EMeRGe provide unique insight to test the current understanding of MPC pollution outflows. In order to obtain an adequate set of measurements at different spatial scales, two field experiments were positioned in time and space to contrast situations when the photochemical transformation of plumes emerging from MPCs is large. These experiments were conducted in summer 2017 over Europe and in the inter-monsoon period over Asia in spring 2018. The intensive observational periods (IOPs) involved HALO airborne measurements of ozone and its precursors, volatile organic compounds, aerosol particles, and related species as well as coordinated ground-based ancillary observations at different sites. Perfluorocarbon (PFC) tracer releases and model forecasts supported the flight planning, the identification of pollution plumes, and the analysis of chemical transformations during transport. This paper describes the experimental deployment and scientific questions of the IOP in Europe. The MPC targets – London (United Kingdom; UK), the Benelux/Ruhr area (Belgium, the Netherlands, Luxembourg and Germany), Paris (France), Rome and the Po Valley (Italy), and Madrid and Barcelona (Spain) – were investigated during seven HALO research flights with an aircraft base in Germany for a total of 53 flight hours. An in-flight comparison of HALO with the collaborating UK-airborne platform Facility for Airborne Atmospheric Measurements (FAAM) took place to assure accuracy and comparability of the instrumentation on board. Overall, EMeRGe unites measurements of near- and far-field emissions and hence deals with complex air masses of local and distant sources. Regional transport of several European MPC outflows was successfully identified and measured. Chemical processing of the MPC emissions was inferred from airborne observations of primary and secondary pollutants and the ratios between species having different chemical lifetimes. Photochemical processing of aerosol and secondary formation or organic acids was evident during the transport of MPC plumes. Urban plumes mix efficiently with natural sources as mineral dust and with biomass burning emissions from vegetation and forest fires. This confirms the importance of wildland fire emissions in Europe and indicates an important but discontinuous contribution to the European emission budget that might be of relevance in the design of efficient mitigation strategies. The present work provides an overview of the most salient results in the European context, with these being addressed in more detail within additional dedicated EMeRGe studies. The deployment and results obtained in Asia will be the subject of separate publications.

Microwave and Lidar Measurements of Ozone and Temperature Variations during Stratospheric Warmings of 2012–2015

Microwave and Lidar Measurements of Ozone and Temperature Variations during Stratospheric Warmings of 2012–2015

Electrochemical gas sensing module combined with Unmanned Aerial Vehicles for air quality monitoring

The issue of air pollution and the monitoring of air quality is a very serious problem in today's society. This problem is basically caused by human in activities related to industry, construction, transport, etc. High pollution values have become an important risk factor for the health of citizens. Measuring pollution for air quality control is not an easy task, as it requires highly specialized equipment, working under special conditions, which makes reference systems bulky and costly installations. To this must be added the disadvantages of measuring in areas with little or no accessibility. The aim of this work is to develop an unmanned aerial vehicle (UAV) equipped with low-cost sensors to monitor air quality both on the move and stationary in a position that is difficult to access. It is equipped with sensors for carbon monoxide (CO), ozone (O3), nitrogen monoxide (NO) and nitrogen dioxide (NO2) measurement. The electronic instrumentation has been developed in the laboratory for this application and it is optimized to minimize interference between the different devices. It also incorporates a pneumatic pump and a tube for air sampling outside the influence of the propellers. In this study, a series of measurements of the UAV were carried out at a stationary location next to a reference station in order to perform the calibration of the sensors. Once calibrated, a series of measurements have been made in motion. The experimental results show the accuracy of the measurements of the proposed prototype as well as its robustness in-flight measurements.

Importance of ozone precursors information in modelling urban surface ozone variability using machine learning algorithm

Surface ozone (O_3) is primarily formed through complex photo-chemical reactions in the atmosphere, which are non-linearly dependent on precursors. Even though, there have been many recent studies exploring the potential of machine learning (ML) in modeling surface ozone, the inclusion of limited available ozone precursors information has received little attention. The ML algorithm with in-situ NO information and meteorology explains 87% (R^{2} = 0.87) of the ozone variability over Munich, a German metropolitan area, which is 15% higher than a ML algorithm that considers only meteorology. The ML algorithm trained for the urban measurement station in Munich can also explain the ozone variability of the other three stations in the same city, with R^{2} = 0.88, 0.91, 0.63. While the same model robustly explains the ozone variability of two other German cities’ (Berlin and Hamburg) measurement stations, with R^{2} ranges from 0.72 to 0.84, giving confidence to use the ML algorithm trained for one location to other locations with sparse ozone measurements. The inclusion of satellite O_3 precursors information has little effect on the ML model’s performance.

Changes in US background ozone associated with the 2011 turnaround in Chinese NOx emissions

Assessing the effectiveness of surface ozone air quality regulations is complicated by non-local sources and inhibited by sparse in situ observational networks. Here, we harness satellite measurements of ozone and its precursors over Asia to identify the Asian contribution to background ozone over the United States (US) from 2006-2016 using a state-of-the-art chemical data assimilation system. Our results indicate that Chinese NOx emissions reached an apex in 2011, increasing daily average ozone concentrations over Los Angeles by up to 3.8 ppb at 750 hPa and 0.7 ppb at the surface in 2012 relative to 2006, with somewhat larger impacts in the high altitude regions of the Western US (assuming constant VOC emissions). Decreasing Chinese NOx emissions after 2011 contributed to a reduction of similar magnitude in Western US background ozone from 2012-2016. We also assess the impact of direct assimilation of satellite-based tropospheric ozone profile measurements over Asia and find that it substantially modifies global ozone, including US surface concentrations. These results suggest that measurements of both ozone and its precursors over Asia play a substantial role in evaluating not only local but also global air quality. The simultaneous assimilation of ozone and its precursors proffers a powerful way to constrain the vertical profile of ozone and improve understanding of ozone variations. In the future, a more extensive satellite observing system has great potential to better constrain both local pollution and background ozone globally when employed in conjunction with chemical data assimilation.

The influence of the summer tropospheric circulation on the observed ozone mixing ratios at a coastal site in the Eastern Mediterranean

This study combines surface ozone (O3) measurements at the Finokalia site (Eastern Mediterranean) with backward trajectory modeling results, reanalysis, and satellite data to investigate how the summer tropospheric circulation affects the O3 mixing ratios in the Eastern Mediterranean. The whole dataset covers the summer months (June, July, August) of 2011–2018. Our findings show that north-northeastern flow, known as the Etesians, is a high frequent wind system over the Eastern Mediterranean during the summer period. Under this flow regime, polluted air masses from the continental central and Eastern Europe reach the monitoring site, increasing the O3 mixing ratios of the area. Contrary, western flows carrying maritime air masses from the Mediterranean Sea, though less frequent, have a central role in controlling the annual O3 mixing ratios in the Eastern Mediterranean. Furthermore, the vertical O3 profile analysis over the Finokalia site indicates enhanced O3 mixing ratios in the middle troposphere when northern flows prevail, confirming the significant relationship between the low-level summer atmospheric circulation and the downward transport of O3-rich air masses to the lower troposphere over Finokalia station, Eastern Mediterranean.

Analysing the Pollutants Dispersion in Erbil City-Kurdistan with Support of Statistical Analysis.

Analysing the Pollutants Dispersion in Erbil City-Kurdistan with Support of Statistical Analysis.

Influence of Absorption Cross-Sections on Retrieving the Ozone Vertical Distribution at the Siberian Lidar Station

The purpose of this paper is to study how the application of different sets of absorption cross-sections influence the ozone vertical distribution (OVD) in the upper layers of the troposphere—stratosphere in the altitude range ~(5–45) km, using a differential absorption lidar (DIAL), operating at the sensing wavelengths 299/341 nm and 308/353 nm. We analyzed the results of lidar measurements of OVD obtained in 2021 using meteorological data from the IASI/MetOp satellite at the Siberian Lidar Station (SLS). The retrieval was performed using the data of four groups concerning the absorption cross-sections: Gorshelev et al., Malicet et al., SCIAMACHY, and GOME. To estimate how the absorption cross-sections influence the OVD retrieval from lidar measurements, we calculated the average deviations between the profiles retrieved using different sets both in a particular case on 2 January 2021 and throughout 2021. Our study showed that, out of the four absorption cross-section sets, the data of Gorshelev et al. should be used for long-term lidar monitoring of the ozone. These data show a more discrete dependence of the absorption cross-sections on the temperature values, which is more urgent for tropospheric and stratospheric ozone measurements.

Ozone Formation and Key VOCs of a Continuous Summertime O3 Pollution Event in Ji'nan

In the summer of 2019, field measurements of ozone (O3) and its precursors[volatile organic compounds (VOCs) and nitrogen oxides (NOx)] were carried out at an urban site in Ji'nan. We found that the daily maximum 8-hour averages φ(O3) were (103.0±14.5)×10-9. The average φ(NOx) and φ(VOCs), which are ozone precursors, were (16.7±11.3)×10-9and (22.4±9.4)×10-9, respectively. The ·OH reactivity of VOCs was determined (9.6±3.8) s-1. Ji'nan suffered from serious O3 pollution. An observation-constrained chemical box model was deployed to evaluate in situ photochemical O3 production, which indicated that chemical reactions made positive contributions to O3 production rates between 07:00 and 19:00 LT, with the average hourly O3 production rate of 35.6×10-9 h-1. To evaluate the effectiveness of various ozone precursor control strategies in reducing ozone pollution, we combined the observation-based model (OBM) with the relative incremental reactivity (RIR) method. The key indicators that affect the local ozone production rate were identified. Ji'nan was under VOC-limited conditions and the key VOC precursors were alkenes. The O3 formation mechanism changed from the VOC-limited regime in the morning to the transitional regime in the afternoon. Correspondingly, the simulated local O3 production rate was increased from 18.3×10-9 h-1 to 29.6×10-9 h-1. To further explore the role of anthropogenic emissions in ozone pollution, we used the positive matrix factorization (PMF) model to identify the major sources contributing to VOCs. The major sources in Ji'nan were vehicular exhaust and gasoline evaporation, accounting for more than 50% of the observed VOCs. Therefore, constraints on vehicular emissions is the most effective strategy to control O3 pollution in Ji'nan.

Validation and Utility of Satellite Retrievals of Atmospheric Profiles in Detecting and Monitoring Significant Weather Events

Abstract Infrared and microwave sounder measurements from polar-orbiting satellites are used to retrieve profiles of temperature, water vapor, and trace gases utilizing a suite of algorithms called the National Oceanic and Atmospheric Administration (NOAA) Unique Combined Atmospheric Processing System (NUCAPS). Meteorologists operationally use the retrievals similar to radiosonde measurements to assess atmospheric stability and aid them in issuing forecasts and severe weather warnings. Measurements of trace gases by NUCAPS enable detection, tracking, and monitoring of greenhouse gases and emissions from fires that impact air quality. During the polar winters, when ultraviolet measurements of ozone are not possible, absorption features in the infrared spectrum of the sounders enable the assessment of ozone concentration in the stratosphere. These retrievals are used as inputs to monitor the ozone hole over Antarctica. This article illustrates the utility of NUCAPS atmospheric profile retrievals in assessing meteorological events using several examples of severe thunderstorms, tropical cyclones, fires, and ozone maps.

A temperature dependent extreme value analysis of UK surface ozone, 1980–2019

Elevated surface ozone during heatwaves and recent hot summers raises concerns over the potential for climate change to exacerbate ozone air pollution in the UK. In this paper, we perform a robust statistical analysis of four decades worth of daily maximum 8-h (MDA8) ozone measurements from the UK's Automated Urban and Rural Network. A temperature dependent extreme value model is developed to characterise the magnitude and frequency of extreme ozone events and to determine probabilities for ozone exceeding health thresholds, as defined in the UK's air quality index. Our model is found to describe the tails of the MDA8 ozone distributions well at all 119 monitoring sites considered. For the decade 2010–2019, we estimate that >90% of sites have a 1-year MDA8 ozone return level greater than the ‘moderate’ ozone threshold of 100 μg/m3. We also find that 33% of sites are currently expected to breach the UK government's national air quality objective that MDA8 ozone should not exceed 100 μg/m3 more than ten times per year. We estimate the present overall probability of MDA8 ozone exceeding 100 μg/m3 on a given day to be between <0.1% and 5.4%, depending on site, with averages of 2.7% (rural) and 1.6% (urban background locations). Our analysis reveals a significant decline over time in the likelihood of the UK experiencing extreme ozone episodes, with 1-year return levels in the 1980s now roughly comparable to 10-year return levels in the present. Similarly, probabilities of MDA8 ozone exceeding 100 μg/m3 have decreased by a factor of ∼2–6 since the 1980s in some locations. However, our results also highlight a strong positive temperature dependence to the risk of ozone exceedances. In consequence, increasingly hot summers due to climate change may offset some of these gains.

Long-term measurements of ground-level ozone in Windsor, Canada and surrounding areas

This study investigated temporal variability of ground-level ozone (O3) during smog season in Windsor, Canada and surrounding states of the US during 1996–2015. Cluster analysis classified six sites into two groups with similar features of O3 concentrations. The first group consists of four urban/suburban sites, Windsor, Allen Park and Lansing in Michigan, and Erie in Ohio, and the second group includes two rural sites, Delaware and National Trail School in Ohio. The similarities among all six sites include (1) diurnal and seasonal variability of O3 concentrations owing to similar weather conditions, and (2) decreasing peak O3 (95th percentile) concentrations due to reduced emissions of precursors, thus less photochemical O3 formation. However, how O3 levels changed with reduced NOX emissions during the study period and on weekends differed between the two groups. Lower O3 concentrations were recorded at urban/suburban sites (30 ppb) than at rural sites (34 ppb). At urban sites, annual smog season O3 concentrations increased by 0.12–3.2 ppb/year. The increasing trends occurred at all percentile levels except for 95th percentile and in most months, due to weakened NO titration effect. At the rural sites, smog-season O3 concentrations decreased by 0.01–2.71 ppb/year. The decreasing trends were observed at 50-95th percentile levels and in most months. Between the two groups, the urban/suburban group had a greater increment in weekend O3 concentrations (3.3 vs. 1.6 ppb) due to a greater reduction of local NO emissions on weekend, thus, weakened NO titration effect. Overall, O3 formation was more sensitive to VOCs during the study period; however, the O3 formation regime gradually shifted toward more sensitive to NOX during 1996–2007 then became more sensitive to VOCs during 2008–2015. Therefore, controlling anthropogenic VOC emissions is needed to effectively mitigate O3 pollution in the study region.