Assessing Spatial and Temporal Patterns of Observed Ground-level Ozone in China

Elevated ground-level ozone (O3), which is an important aspect of air quality related to public health, has been causing increasing concern. This study investigated the spatiotemporal distribution of ground-level O3 concentrations in China using a dataset from the Chinese national air quality monitoring network during 2013–2015. This research analyzed the diurnal, monthly and yearly variation of O3 concentrations in both sparsely and densely populated regions. In particular, 6 major Chinese cities were selected to allow a discussion of variations in O3 levels in detail, Beijing, Chengdu, Guangzhou, Lanzhou, Shanghai, and Urumchi, located on both sides of the Heihe-Tengchong line. Data showed that the nationwide 3-year MDA8 of ground-level O3 was 80.26 μg/m3. Ground-level O3 concentrations exhibited monthly variability peaking in summer and reaching the lowest levels in winter. The diurnal cycle reached a minimum in morning and peaked in the afternoon. Yearly average O3 MDA8 concentrations in Beijing, Chengdu, Lanzhou, and Shanghai in 2015 increased 12%, 25%, 34%, 22%, respectively, when compared with those in 2013. Compared with World Health Organization O3 guidelines, Beijing, Chengdu, Guangzhou, and Shanghai suffered O3 pollution in excess of the 8-hour O3 standard for more than 30% of the days in 2013 to 2015.

Ozone (O 3 ) at the ground level creates a major air pollutant that affects human health 1 . High concentrations of ground-level O 3 can cause cardiovascular and respiratory dysfunction [2][3][4][5] , and contribute to increased levels of mortality, especially for elderly people 6 . An increase of 21.3 μg/m 3 in the mean 3-day running concentration of O 3 resulted in a 6.6% increase in daily deaths in the warm season caused by respiratory diseases (95% CI:1. 8, 11.8) 7 in Montreal, Quebec. In China, a 10 μg/m 3 increase of the maximum 8-h average concentration of O 3 , was reflected in increases in non-accidental mortality, cardiovascular mortality, and respiratory mortality by 0.42% (95% CI, 0.32-0.52), 0.44% (95% CI, 0.17-0.70), and 0.50% (95% CI, 0.22-0.77), respectively 8 . The World Health Organization (WHO) set a guideline of 100 μg/m 3 for a maximum daily 8-hour average (MDA8) exposure to ground-level O 3 . Keeping air pollution below this concentration will provide adequate protection for public health, although some health effects may occur below this level 9 . The Review of Evidence on Health Aspects of Air Pollution summarized newly accumulated scientific evidence related to the adverse effects of O 3 on human health at levels below the WHO guideline. Additionally, the Review of Evidence on Health Aspects of Air Pollution points out that O 3 is involved in the formation of secondary inorganic and organic particulate matter (PM) in the outdoor environment. In addition, it shows that the reaction of O 3 with common indoor volatile organic compounds (VOCs) generates a plethora of various compounds, many of which have been proposed to be respiratory irritants 10 .
Ground-level O 3 is mainly produced during chemical reactions when mixtures of organic precursors (CH 4 and non-methane volatile organic carbon, NMVOC), CO, and nitrogen oxides (NO x = NO + NO 2 ) are exposed to the UV radiation in the troposphere 11 . The most important interactions that drive the production of O 3 concentrations in the troposphere and some of the related feedback mechanisms have been discussed thoroughly 12,13 .
In the last few decades, the burning of biomass has been recognized as an important source of O 3 precursors [14][15][16] . Because terrestrial vegetation is the dominant source of atmospheric VOCs, vegetation can have a large effect on the distribution of O 3 and its precursors 17 . In 2008, China was recently determined to be the largest contributor to Asian emissions of CO, NO x , NMVOC, and CH 4 ; the growth rates of these emissions from China were also the largest in Asia because of the current continuous increase in energy consumption, economic activity, and infrastructural development 18 . A complex coupling of primary emissions, chemical transformation, and dynamic transport at different scales has created the O 3 problem 19 . In addition, the chemical transformation for O 3 has nonlinear chemistry with respect to its precursors and the contributions from both local and regional sources 20 . The effect of VOCs and NO x on O 3 formation can be described by VOC-limited or NO x -limited regimes [21][22][23] . At elevated NO x levels, which is typical of the polluted urban environment, O 3 levels can be severely depleted locally because O 3 reacts directly with emitted NO in a reaction known as the 'NO x titration effect. ' The rate of the process of O 3 scavenging in the urban environment by titration with NO x gradually declines as NO x urban emissions are reduced when emissions are controlled. O 3 concentrations in urban areas have increased as emissions of NO have declined. This will have an important effect on control measures and will result in an increase in the exposure of urban populations to O 3 in the coming decade 24 .
Because of the importance of O 3 as it relates to air quality and public health, O 3 has received continuous attention from both the scientific and regulatory communities 25 . Numerous long-term monitoring sites have been established worldwide to observe the spatial and temporal features of ground-level O 3 . The Air Quality System of the U.S. Environmental Protection Agency contains data related to ambient air pollution collected from thousands of monitors. Regions with large urban atmospheres with poor ventilation in the Americas, such as the Los Angeles, Mexico City and Santiago de Chile metropolitan areas, have experienced O 3 in excess of 400 μg/ m 3 for short-term ground-level O 3 concentrations 26 . A trend analysis in Europe with the O 3 -monitoring sites data covering the 12 years from 1993 to 2005 showed that some Mediterranean cities recorded 1-hour average ground-level O 3 concentrations exceeding 300 μg/m 3 27 . The satellite remote sensing is another widely used and provides useful way to investigate the ground-level distribution of O 3 . The spatial coverage of the new generation of nadir-looking instruments onboard polar-orbiting satellites, such as the Global Ozone Monitoring Experiment, Infrared Atmospheric Sounding Interferometer and Ozone Monitoring Instrument, makes them interesting tools that can be used to monitor tropospheric O 3 over large regions, helping researchers to assess any problems related to air quality and transport [28][29][30][31] . Nevertheless, differences still remain between a tropospheric column O 3 derived from satellite observation and ground monitor data [32][33][34] .
Rapid industrialization and urbanization in China have led to high concentrations of ground-level O 3 35 , which often cause concerns related to public health in this populous country. Although numerous studies have been conducted O 3 epidemiology 36,37 , such studies are less commonly available in China [38][39][40][41][42][43] . High O 3 concentrations exceeding the national ambient air quality standards have been frequently observed in large cities of China [44][45][46][47][48] . Recent studies have also indicated increasing O 3 trends exist in several highly urbanized regions of China 49,50 . Meanwhile, few types of research have focused on the nationwide spatial and temporal variability of ground-level O 3 concentrations in China. The Chinese government at various levels began to establish a national air quality monitoring network in 2012, which released real-time ground-level O 3 monitoring data to the public. With the establishment of a national air quality monitoring network, large-scale real-time ground-level O 3 monitoring data become available.
The spatial and temporal variability of ground-level O 3 concentrations in China has been studied using a dataset from the national air quality monitoring network covering 2013-2015. The present paper investigates and demonstrates the spatial and temporal distribution of ground-level O 3 on a nationwide scale, including its yearly, monthly and diurnally patterns of ground-level O 3 concentration. In order to provide further insight into the variations between densely and sparsely populated regions, 6 major cities lied on both sides of Heihe-Tengchong line were selected to discuss in detail: Beijing, Chengdu, Guangzhou, Lanzhou, Shanghai, and Urumchi, their locations showed in Fig. 1.  Table 1). The T-test for equality of means showed that the O 3 level and variations between the densely and sparsely populated region were significantly different (Sig. <0.001 with α = 0.05).

Spatial distribution of ground-level O 3 .
Note that a strong positive correlation between exists between surface O 3 level and elevation [51][52][53]      were 91.43 and 33.35 μg/m 3 , and were observed at 15:00 and 7:00, respectively. In the sparsely populated region, the maximum and minimum values were 84.44 and 31.67 μg/m 3 , observed at 16:00 and 8:00, respectively. The hourly mean concentrations of ground-level O 3 in the densely populated region were higher than those measured in the sparsely populated region. In addition, a small peak existed at 4:00 for diurnal concentrations in the densely populated region, which was not found in the sparsely populated region; this can perhaps be explained by the accumulation of O 3 precursors because of the relatively low boundary layer in this region 46  Ground-level O 3 is subject to in situ chemical reactions and physical processes that are directly affected by precursor emissions, solar radiation and other meteorological factors 54 . NO x and VOCs play important roles in O 3 formation, while NO and VOCs concentrations were not included in the data collected by the national air quality monitoring network. Therefore, yearly NO 2 concentrations variations were measured from 2013 to 2015.
The yearly averaged NO 2 concentrations decreased year by year from 2013 to 2015 (Fig. 5). The national NO 2 mean was 31.1 μg/m 3 in 2015, and had declined by nearly 30% when compared with 2013. The Chinese State Council released the ' Atmospheric Pollution Prevention and Control Action Plan' on September 2013, in which they decided to implement critical strategies designed to control the burning of coal and vehicle exhaust, as well as for the management of power plants and so on 55 . After a 2-year effort, as one of the O 3 precursors, the NO 2 concentrations have indeed decreased since 2013 56 . In contrast, the national yearly average O 3 concentration in 2015 was higher than that in 2013, and the increase in the ground-level O 3 concentrations in the sparsely populated region was greater than that in the densely populated region.   54 reported that the change of VOCs emissions might have played a more important role in the O 3 increase than the effect of NO x in the northern part of eastern China 54 . Observations have shown that Beijing's efforts to control air pollution were somehow effective in cutting O 3 precursors, but still left a relatively high amount of ground-level O 3 ; researchers surmised that this resulted from potential contributions from VOCs and regional transport near Beijing 59 . Analysis using a smog production algorithm proved that the reduction in VOC is generally useful in reducing the photochemical production of O 3 while the combined reduction of NO x and VOC would be important to efforts to reduce the appearance of O 3 episodes in the PRD 60 . The computation of the production rate of total oxidants (O 3 + NO 2 ) indicated that the trends of ambient oxidant levels largely depended on the ratio of VOCs/NO x 61 , and that a more rapid reduction in VOC reactivity would be very effective for decreasing total oxidants 59 .           Figure 12 shows the hourly mean O 3 concentrations from 2013 to 2015 in 6 cities. As a typical product of photochemical reactions, the ground-level O 3 concentrations was closely related to the intensity of solar radiation. The production of O 3 began after sunrise and it accumulated until reaching peak concentrations in the  afternoon. With sunset, the photochemical reactions declined rapidly to near zero without solar radiation, so that O 3 reduction reactions occurred because of NO x , CO, NMHC and other O 3 precursors, resulting in low levels of O 3 concentrations in the night.
In Beijing, Guangzhou, and Shanghai ground-level O 3 concentrations started to increase at almost exactly 7:00, and peaked at about 15:00. In Lanzhou and Chengdu, it began to increase at 8:00 and reached peak concentrations at around 16:00. In Urumchi, the latitude and time difference caused O 3 concentrations to begin to increase at 9:00 and they reached peak values at 17:00. Note that all of China uses a single time zone, but spans 5 time zones of other countries, which is part of the cause of this difference. Figure 13 shows

Discussion
The results from the present analysis could improve our understanding of ground-level O 3 at a fine spatiotemporal resolution in China, given the lack of historic long-term monitoring.
The nationwide and regional three-year ground-level MDA8 O 3 concentrations were provided above. Ground-level O 3 concentrations showed a monthly variability peaking in summer and reaching their lowest in winter, while the diurnal cycle exhibited a minimum in the morning and peaked in the afternoon. Unlike the decrease in NO 2  With our understanding that the complexity of the air pollution mixture in China has improved, the need for specific strategies designed to limit and control air pollution for individual pollutants has become increasingly apparent. VOCs and NO x all play important roles in the formation of O 3 at ground-level, and their effect on O 3 is nonlinear. As a result, simply reducing levels of NO x may be ineffective in managing the O 3 problem. This, however, risks further increases in O 3 because a VOCs/NO x ratio more favorable to O 3 production may be reached. Few long-term and nationwide observational data are available for VOCs, which only serves to limit our understanding of O 3 production and control. It is time for the Chinese government to begin monitoring VOCs and to conduct related environmental monitoring. More specifics related to the relationships between various types of atmospheric pollution should be studied and considered during the formulation of revised management strategies.

Methods
O 3 monitoring data. The Department of the Environment continuously operates and maintains the national air quality monitoring network of China, an effort that began in 2012. At each monitoring site, the concentration of O 3 was measured using the ultraviolet absorption spectrometry method and differential optical absorption spectroscopy. The instrumental operation, maintenance, data assurance and quality control were properly conducted based on the most recent revisions of China Environmental Protection Standards 62 . The network was comprised of nearly 950 monitoring stations in 2013, which was extended to approximately 1500 stations by the end of 2015. The present study employed data from Jan. 2013 to Dec. 2015.
Maximum daily 8-hour average O 3 . When considering the affects associated with controlled O 3 exposures on health outcomes [63][64][65][66] , WHO set a guideline value for O 3 exposure of 100 µg/m 3 for a maximum period of 8 hours per day 9 . Therefore, we calculated an MDA8 for O 3 concentration of each station. MDA8 O 3 concentrations were calculated using greater than 5 hourly averages that were available (not zero) every 8 hours. Generally, if fewer than 6 hours of O 3 concentration data were available for a certain 8-hour period, then this 8-hour average was assigned as the 'missing' value 67 . In the present study, the 'missing' values (zero) was not considered in the next analysis. Finally, the maximum value of the daily 8-hour averages were used as the valid MDA8.