Characterization of the Air Quality Index for Wuhu and Bengbu Cities , China

From 2015–2017, the atmospheric PM10, SO2, NO2, CO, and O3 in Wuhu and Bengbu were investigated in this study. In addition, the AQI values and seasonal variations in six AQI classes and corresponding primary pollutants were also studied. In Wuhu, the daily AQI ranged from 23 to 298 in 2015, from 33 to 290 in 2016, and from 34 to 278 in 2017, and the corresponding mean values were 81, 80 and 90, respectively. In Bengbu, the daily AQI ranged from 23 to 288 in 2015, from 32 to 286 in 2016, and from 27 to 500 in 2017, and the corresponding mean values were 88, 89 and 97, respectively. During the three-year study, in Wuhu, the mean proportion of levels with Grade I, II, III, IV, V and VI were 9.33%, 69.3%, 18.3%, 3.00%, 0% and 0% in spring; were 35.0%, 55.0%, 7.00%, 3.00%, 0% and 0% in summer; were 13.6%, 65.0%, 18.0%, 3.33%, 0% and 0% in fall, and were 5.33%, 48.7%, 30.7%, 9.67%, 5.67% and 0% in winter. In Bengbu, the mean proportion of levels with Grade I, II, III, IV, V and VI were 3.00%, 64.0%, 30.3%, 2.67%, 0.333% and 0.333% in spring; were 19.3%, 68.7%, 11.3%, 0.667%, 0% and 0% in summer; were 20.7%, 56.3%, 17.3%, 4.67%, 1.00% and 0% in fall, and were 9.67%, 36.7%, 31.0%, 32.0%, 5.67% and 0% in winter. Generally, the air quality in the two cities were in the following order: summer > fall > spring > winter. AQI ranged between 101–150, where in Wuhu, the primary air pollutants were PM2.5 and NO2 in 2015; were PM2.5, NO2 and O3 in 2016, and were PM2.5, PM10, NO2 and O3 in 2017. In Bengbu, PM2.5, PM10 and O3 were the primary air pollutants during the three years. When AQIs ranged between 151 and 200, in Wuhu, the primary air pollutant was PM2.5 in 2015; were PM2.5 and PM10 in 2016, and were PM2.5, PM10, and O3 in 2017. In Bengbu, the primary air pollutant was PM2.5 in 2015 and 2016 and comprised PM2.5 and O3 in 2017. When AQIs were between 201 and 300, in Wuhu, PM2.5 was the primary air pollutant in 2015–2017. In Bengbu, the primary air pollutant was PM2.5 in 2015 and 2016 and comprised PM2.5 and PM10 in 2017. When the AQI ranged between 301–500, which did not occur in Wuhu from 2015–2017 or in Bengbu during 2015–2016, PM2.5 was as the primary air pollutant in Bengbu in 2017. When the AQI can be analyzed in more detail, the control strategies for air pollution will be more precise.


INTRODUCTION
Over past decades, along with the acceleration of economic growth and urbanization, air quality has been influenced directly or indirectly by industrial activities, biomass burning, and vehicle exhaust, which emit large amount of pollutants into the atmosphere (Liu et al., 2012;Liu and Wang, 2014;Li et al., 2017a), as well as meteorological factors (Zhang et al., 2009;Zhang and Cao, 2015;Shen et al., 2017).Large numbers of cities currently suffer from severe air quality degradation (Ran et al., 2011;Tang et al., 2012;Li et al., 2012b;Li et al., 2014).
Lots of previous studies have demonstrated clearly that air pollution poses a high risk to human health (Pope and Dochery, 2006;Cao et al., 2012;Heal et al., 2012;Pope and Dochery, 2013;Jin et al., 2017).Related studies have found that there is a positive correlation between the number of deaths caused by respiratory diseases and atmospheric particulate matter (PM) and that PM 2.5 present greater harm than PM 10 (Brook et al., 2013;Deng et al., 2013a, b).Sulfur dioxide in the atmosphere can stimulate the human respiratory tract and injure the cardiovascular system and liver.In particular, children and old people are more sensitive to this type of air pollutant (Lovati et al., 1996).Atmospheric nitrogen dioxide can affect the functioning of the human respiratory system and significantly reduce the lung function indicators in humans (Chen et al., 2011).Ozone can easily enter the deep part of the respiratory tract, causing an increase in airway inflammation, aggravating asthma, and decreasing lung function (Yang et al., 2012).Human will exhibit symptoms including headaches, dizziness, and even nausea if they inhale carbon monoxide because atmospheric carbon monoxide can easily induce oxidative stress and inflammatory reactions.Atmospheric carbon monoxide not only destroys the neurological function of the heart but also affects the central nervous system and even causes suffocation to the point of death (Yang et al., 2012).Previous studies have revealed that from 2010-2012, the number of premature deaths was caused by global air pollution increased from 0.22 million to 3.7 million.Therefore, the huge burden of air pollution (Chen et al., 2013;Brauer et al., 2016) and the frequent occurrence of severe air pollution events has triggered growing worldwide public awareness (Wang et al., 2015a, b).
The Air Quality Index (AQI) is a typical indicator to estimate environmental air quality, which can inform the public to take proper health protection measures and provide the government with guidance to formulate corresponding pollution regulations.According to the World Health Organization (WHO), the AQI value is determined by six criteria for the concentration of pollutants, including sulfur dioxide (SO 2 ), nitrogen dioxide (NO 2 ), carbon monoxide (CO), ozone (O 3 ), fine particulate matter (PM 2.5 ), and coarse particulate matter (PM 10 ).When the AQI value is used to reflect the air quality, better air quality is associated with a lower AQI.The largest contributor to air quality degradation among the six air pollutants is defined as the daily "primary pollutant," which determines the Air Quality Index for that day (She et al., 2017).
Since September 2013, China began to implement the Air Pollution Prevention and Control Action Plan, which is viewed as an unprecedentedly stringent air pollution regulation (Sheenhan et al., 2014;Shi et al., 2016;Li et al., 2017c), and the air quality has gradually improved (Xie et al., 2016).However, air pollution has spatial and temporal variation characteristics, so China is experimenting with various air pollution control policies, some of which were reflected in pollution control programs for the APEC Meeting and the Victory-day Parade in Beijing.Li et al. (2017d) show that the regulations resulted in short-term, substantial improvements in air quality, where the AQI decreased by 35.9% during the APEC Meeting period, and the daily average concentrations of PM 2.5 , PM 10 , SO 2 , NO 2 , and CO reduced by 41.3%, 48.2%, 56.5%, 38.9%, and 35.5%, respectively; the AQI was lowered by 37.4% during the Victory-day Parade period, and the daily average concentrations of PM 2.5 , PM 10 , O 3 , SO 2 , NO 2 , and CO were reduced by 55. 8%, 50.1%, 27.2%, 35.9%, 39.9%, and 28.8%, respectively.Moreover, the information about heavily polluted days also arouse heated discussion and debates among the general public.Based on this situation, this study focused on the air pollution characteristics in Wuhu and Bengbu through the AQI values, in the hope of offering a useful basis by which to formulate and experiment with tighter air pollution control strategies for local governments.
This research discusses the six criteria pollutants, including PM 2.5 , PM 10 , SO 2 , NO 2 , CO and O 3 (PM 2.5 was discussed in our previous study by Wang et al. (2018)).In addition, as for the section on the Air Quality Index (AQI) analyses, the variations in the six AQI classes for four seasons and the corresponding primary pollutants are further discussed.The objective of this study is providing essential information and a better understanding of the air pollution characteristics in both Wuhu and Bengbu, so as to propose better suggestions for the control of atmospheric pollutants.

METHODS
Two cities, Wuhu (31°33′N, 118°38′E) (south of the Huai River) and Bengbu (32°93′N, 117°34′E) (north of the Huai River) located in Anhui province, China, which are located across the basins of the Yangtze River and the Huai River, were selected and evaluated in this study.The main climate features in both Wuhu and Bengbu include being cold and dry in the winter and warm and humid in the summer.Over recent decades, these two cities underwent economic development with the total GDP rising to more than 0.13 and 0.26 trillion RMB in Wuhu and Bengbu, respectively.It was argued in a previous study that the increasing population and urbanization and rapid economic growth has been equaled with air quality deterioration (Che et al., 2009).Wang et al. (2018) showed that the annual average PM 2.5 concentration in both Wuhu (53.0 µg m -3 , 2015-2017) and Bengbu (61.4 µg m -3 , 2015-2017) have exceeded the limitation of the World Health organizational air quality standards (10 µg m -3 ) to characterize and gain more insight into the air pollution issue is of great importance for the protection of human health.
This study obtained data for three years from January 2015 to December 2017 in both Wuhu and Bengbu cities.The PM mass concentration (including daily PM 2.5 and PM 10 ) and gaseous pollutants (including daily SO 2 , NO 2 , CO, and 8 hr-averaged O 3 ) were obtained from the China air quality online monitoring and analysis platform (http://www.aqistudy.cn/).

Air Quality Index (AQI)
The sub-AQI of the six criteria pollutants were first calculated with the observation concentrations, as shown in Eq. (1) (Shen et al., 2017;She et al., 2017).The overall AQI represents the maximum of the sub-AQI of all pollutants, where when the AQI is higher than 50, the highest sub-AQI contributor is defined as the primary pollutant on that day, as shown in Eq. (2) (Shen et al., 2017;She et al., 2017): (1)  (Hu et al., 2015;Lanzafame et al., 2015;She et al., 2017;Zhao et al., 2018).

PM 10 Concentration
Atmospheric particulate matter (PM) carries acids and toxic species such as polycyclic aromatic hydrocarbons and heavy metals, which are presently recognized as affecting visibility and human health (Song et al., 2008;Tao et al., 2009;Matus et al., 2012;Sun et al., 2014;Liang et al., 2016).Increasing PM 10 pollution plays a crucial role in severe haze and degradation of air quality.For the period 2015-2017, the monthly average PM 10 concentration in the ambient air of Wuhu and Bengbu are shown in Tables 2(a), 2 (b) and 2(c), respectively.
For Wuhu, in 2015, the monthly average PM 10 concentration was in the range of 50-151 µg m -3 , and with an average of 85 µg m -3 ; while in 2016, it was 39-125 µg m -3 , with an average of 78 µg m -3 ; and the level in 2017 was in the range of 48-111 µg m -3 , with an average of 83 µg m -3 .Generally, based on the annual average values, the highest PM 10 concentration occurred in 2015 and was reduced by approximately 9.0% by 2016; but from 2016 to 2017, the PM 10 concentration increased by approximately 6.0%.As a whole, over the three years under examination, the PM 10 concentration ranged between 39 and 151 µg m -3 , with an average of 82 µg m -3 .The above results also indicate that the PM 10 concentration in Wuhu was 4.1 times higher than In Bengbu, the monthly average PM 10 concentration in 2015 was between 64 and 119 µg m -3 , with an average of 91 µg m -3 and that in 2016 ranged from 59 to 144 µg m -3 , with an average of 92 µg m -3 ; during 2017, it ranged from 55 to 144 µg m -3 , with an average of 101 µg m -3 .On the contrary, the annual average PM 10 concentration rose slowly during the three years, increasing by approximately 1.09% from 2015 to 2016, and approximately 8.91% from 2016 to 2017.This may be attributed to the poor control of industrial activity and the increasing number of vehicles (Han et al., 2005;Tang et al., 2005;Huang et al., 2010).As a whole, the PM 10 concentration for these three years ranged between 55 to 144 µg m -3 , with an average of 95 µg m -3 , so the PM 10 concentration was higher than that in Wuhu, which was 4.8 times higher than the WHO air quality regulated standard (20 µg m -3 ), and the increasing tendency for increases in PM 10 should be of concern and improved.
With regard to seasonal variations, in Wuhu, in 2015, the average PM 10 concentrations in spring, summer, fall, and winter were 73, 57, 88 and 120 µg m -3 , respectively, and those in 2016 were 80, 48, 78, and 107 µg m -3 , respectively.Those in 2017 were 104, 56, 74 and 99 µg m -3 , respectively.On a three-year basis, the average PM 10 concentration in winter (109 µg m -3 ) was approximately 2.0 orders of magnitude higher than that in summer (54 µg m -3 ).For Bengbu, during 2015, the average PM 10 concentrations in spring, summer, fall and winter were 88, 72, 99 and 105 µg m -3 , respectively, and those in 2016 were 94, 62, 87 and 124 µg m -3 , respectively.Those in 2017 were 118, 71, 86 and 131 µg m -3 , respectively.This indicates that the PM 10 concentrations varied from season to season, and that, generally, the highest values always occurred in winter and the lowest in summer, while the values for spring were very similar with those in fall and were both at intermediate levels.On the three-year basis, the average PM 10 concentration in winter (120 µg m -3 ) was approximately 1.8 orders of magnitude higher than that in summer (68 µg m -3 ).
Previous studies reported that the main source of coarse particles (PM 10 ) is natural processes such as dust storms and re-suspension of local soil, and anthropogenic processing is another important source, such as road dust and various industrial processes (Querol et al., 2004;Xu et al., 2017).In winter, a temperature inversion with a low ground temperature can hinder the dispersion of pollutants (Tang et al., 2017;Xing et al., 2017;Wang et al., 2018), and polluted air caused by heating activities blowing from northern China (Shang et al., 2018) can elevate the atmospheric PM 10 levels in both Wuhu and Bengbu.During summer, higher temperatures with both violent vertical transport of air current and stronger rainfall scavenging can reduce the PM 10 concentration significantly.
Traffic and power plants produce largely primary and secondary anthropogenic combustion products, which are a crucial source of fine particles (PM 2.5 ).Since the diverse sources of fine and coarse particles and different physicchemical properties reveal different particle pollution characteristics, the PM 2.5 /PM 10 ratio can be used to determine underlying atmospheric processes and thus evaluate the status of air quality (Speranza et al., 2014;Blanco-Becerra et al., 2015;Xu et al., 2017;Wang et al., 2018).Speranza et al. (2014) and Wang et al. (2018) found that the PM 2.5 /PM 10 ratio shows apparent seasonal variations, where the value in cold seasons is higher than that in warm seasons.Akinlade et al. (2015) also found that the PM 2.5 /PM 10 ratio was higher in the wet season and lower in the dry season as a consequence of the significant contribution of dust resuspension to PM 10 .It has also been found that due to resuspended coarse road dust, traffic hours correspond with the minimum PM 2.5 /PM 10 ratio (Querol et al., 2001;Evagelopoulos et al., 2006).This demonstrates that controlling dust is an important way to reduce PM 10 concentrations and improve air quality.

SO 2 Concentration
Atmospheric sulfur dioxide (SO 2 ) reacts with hydroxyl (OH) and other oxidizing agents and converts into sulfate particles and sulfuric acid in the troposphere.It is a key precursor of acid rain that presents a hazard to forests and fresh water ecosystems (Ferrari and Salisbury, 1999).Thus, SO 2 as one of the criteria pollutants can affect air quality and regional climate (Aneja et al., 2001;Khattak et al., 2013).The monthly average SO 2 concentrations in Wuhu and Bengbu from 2015-2017 are provided in Figs.1(a), 1(b) and 1(c), respectively.
The monthly SO 2 concentration in Wuhu ranged between 4.20 and 9.45 ppb, with an average of 6.80 ppb in 2015; it ranged between 5.25 and 10.9 ppb, with an average of  Wuhu Bengbu Wang et al., Aerosol and Air Quality Research, 18: 1198-1220, 20181203 7.47 ppb in 2016, and it ranged between 2.80 and 7.35 ppb, with an average of 5.34 ppb in 2017.The annual average SO 2 concentration increased by approximately 8.97% from 2015 to 2016 but reduced by approximately 28.5% from 2016 to 2017.In the case of Bengbu, the monthly concentration of SO 2 was in the range of 5.95-11.9ppb and with an average of 9.04 ppb in 2015; of 5.60-8.40ppb, with an average of 7.47 ppb in 2016 and was 4.20-9.80ppb, with an average of 7.12 ppb in 2017.By comparing the mean concentrations, SO 2 concentrations were shown to have decreased slowly during the three-year period, reducing by approximately 17.4% from 2015 to 2016, and by 4.69% from 2016 to 2017.The fluctuation in the average SO 2 concentrations in the three-year period was 2.80-10.9ppb in Wuhu and 4.20-11.9ppb in Bengbu, for which the corresponding mean values were 6.54 and 7.88 ppb, respectively.The results indicated that the SO 2 concentration in Wuhu was slightly lower than WHO air quality regulated standard of 7.00 ppb, but that of Bengbu were slightly higher than the WHO standard.
SO 2 results from anthropogenic activities, like fossil fuel combustion, metal smelting, and the burning of biomass (Kettle and Andreae, 2000;Halmer et al., 2002;Vijay et al., 2004;Dentener et al., 2006;Lee et al., 2008) and the combustion of coal are common major sources in China (Kurokawa et al., 2013;Kato et al., 2016).Another crucial emitted source of SO 2 is natural processes, such as volcanic activity, oxidation in the soil, and the oxidation of hydrogen sulfide.Previous studies have indicated that volcanic eruptions are sporadic primary contributors (Andres and Kasdnoc, 1998;Halmer et al., 2002;Kato et al., 2016).The natural sources of SO 2 were found to significantly limited by the absence of volcanic activity in both Wuhu and Bengbu.
In general, except for the fact that the SO 2 levels in winter were slightly higher than in the other seasons, there are no clear seasonal cycles in either city.This can be mainly attributed to the fact that Wuhu and Bengbu are located in the middle latitude area of China, the polluted air current with a higher SO 2 concentration from the northern cities of China would slightly increase SO 2 concentration in winter.

NO 2 Concentration
Nitrogen dioxide (NO 2 ) mainly results from anthropogenic activities, including industrial facilities and traffic emissions (Cheng et al., 2018).NO 2 is involved in complex thermal NO x formation and is photochemical, reacting with volatile organic compounds that can affect the oxidizing capacity of the atmosphere (Jacob et al., 1999;Seinfeld and Pandis, 2012).It is also an important precursor of ozone and acid rain (Bowman et al., 1994;An et al., 2006;Anttila et al., 2011;Khokhar et al., 2016).Due to increases in the number of vehicles in China, more NO 2 in atmosphere has adversely impacted human health and the ecological environment (Boersma et al., 2009;Sachin et al., 2009).The monthly mean concentration of NO 2 in Wuhu and Bengbu from 2015 to 2017 are shown in Figs 2(a), 2(b) and 2(c), respectively.
As for Wuhu, in 2015, the monthly average NO 2 concentration ranged between 9.30 and 29.7 ppb, and the annual mean value was 18.0 ppb.In 2016, those values ranged between 12.2 and 38.5 ppb, with an annual mean value of 21.9 ppb.In 2017, the values ranged from 15.6 to 32.6 ppb, with an annual mean value of 23.9 ppb.The mean NO 2 concentrations increased slowly during the threeyear period, rising by approximately 17.8% from 2015 to 2016 and also increasing by 8.37% from 2016 to 2017.For Bengbu, the monthly concentration of NO 2 ranged between 12.2 and 22.4 ppb, with an average of 16.6 ppb.In 2016, those values ranged between 13.1 and 27.8 ppb, with an average of 18.5 ppb, and in 2017, the values ranged between 11.2 and 30.2 ppb, with an average of 19.5 ppb.The mean NO 2 concentrations increased slowly during the three-year period, rising by approximately 10.3% from 2015 to 2016 and increasing by 5.13% from 2016 to 2017.As a whole, the average concentrations of NO 2 were 9.30-38.5 ppb in Wuhu and 11.2-30.2ppb in Bengbu during the three-year period, for which the corresponding mean values were 21.4 and 18.0 ppb, respectively.The results indicated that the NO 2 concentrations in Wuhu were slightly higher than those in Bengbu.Official statistics revealed that Wuhu has a larger human population (3.67 million) than Bengbu (3.33 million), and the urbanization rate (63.5%) in the former is also significantly higher than the latter (53.7%) (Anhui Statistical Yearbook 2000-2017).It can thus be inferred that Wuhu has more vehicles.Previous studies have reported that motor vehicles have become the primary source of NO 2 emissions with economic development and rapid urbanization during recent years (Fu et al., 2000;Cheng et al., 2018); thus, a larger amount of nitrogen oxide (NO x ) is being emitted by the vehicles in Wuhu, which results in the higher atmospheric NO 2 concentration as compared to that of Bengbu.It also can be seen that the annual NO 2 levels in Wuhu (21.4 ppb) and Bengbu (18.0 ppb) were near the WHO air quality regulated standard (19.5 ppb), but the highest monthly average values were approximately 2.0 and 1.5 times higher than the WHO air quality regulated standard in Wuhu and Bengbu, respectively.This means that NO 2 should be considered an important air pollutant.Efficient vehicle emission control measures should be issued to enhance the efforts aimed at improving the air quality, which is consistent with the conclusion that decreasing vehicle NO x emissions have a crucial effect on decreasing NO 2 in the ambient air (Cheng et al., 2018).
In Wuhu, the seasonal NO 2 concentrations during spring,  Wuhu Bengbu Wang et al., Aerosol and Air Quality Research, 18: 1198-1220, 20181205 Generally, the NO 2 concentrations in fall and winter were higher than the values in spring and summer in both Wuhu and Bengbu, which was similar to the observed NO 2 concentration seasonal variations in urban Beijing (Yang et al., 2015;Cheng et al., 2018).This phenomenon might be related to the various meteorological conditions in different seasons.The two cities have a subtropical monsoon climate with clear distinctions among the four seasons, where especially during fall and winter, the ground meteorological factors are not favorable for the dispersion of air pollutants, which can be mainly characterized as a stable atmosphere with low wind speeds and low rainfall intensity.
The highest monthly mean NO 2 concentration was always observed in winter.In Wuhu, the maximum values of 29.7, 38.5 and 32.6 ppb for 2015, 2016, and 2017 all occurred in December in the three-year period, respectively.In Bengbu, the highest monthly mean NO 2 concentration was observed in January, which was 22.4 ppb for 2015, and the maximum values were 27.8 and 30.2 ppb for 2016 and 2017, respectively, which were both observed in December.In order to control the atmospheric concentration of NO 2 more effectively, numerous emission reduction measures from industrial facilities should be implemented as soon as possible, and restricting vehicle emissions should become the most crucial issue, especially during seasons with unfavorable weather conditions.Based on a previous study, Beijing has obtained significant results from temporary traffic management during air quality assurance periods (Cheng et al., 2018).

CO Concentration
Carbon monoxide (CO) is mainly emitted from fossil fuel combustion and the burning of biomass.These combustion processes also emit NO x and volatile organic compounds (VOCs).CO is one important air pollutant that has a long lifetime of more than one month and has become a strong indicator of pollution (Kato et al., 2016).The monthly mean concentrations of CO in Wuhu and Bengbu are shown in Tables 2(a), 2(b) and 2(c) from 2015 to 2017, respectively.
In Wuhu, the monthly mean CO concentrations varied from 0.532 to 1.21 ppm in 2015; were between 0.574 and 1.15 ppm in 2016, and ranged between 0.606 and 1.15 ppm in 2017, while the corresponding annual average values were 0.817, 0.852, and 0.826 ppm, respectively.The annual mean concentrations of CO in 2015 increased by about  (2016) demonstrated that higher CO tends to appear with a higher water vapor content; thus the level of CO in Wuhu was found to be slightly higher than that in Bengbu.In Wuhu, in 2015, the three highest monthly mean CO concentration were observed in January (1.21 ppm), December (1.04 ppm), and February (1.01 ppm).In 2016, the three highest monthly mean values were observed in December (1.15 ppm), March (1.15 ppm), and November (1.06 ppm), and in 2017, the three highest monthly mean values were observed in January (1.15 ppm), February (0.889 ppm), and December (0.885 ppm).The three lowest monthly mean CO concentrations were observed in July (0.532 ppm), September (0.544 ppm), and August (0.565 ppm) in 2015 and were observed in June (0.574ppm), July (0.581 ppm), and August (0.596 ppm) in 2016 and in March (0.606 ppm), July (0.733 ppm), and October (0.748 ppm) in 2017.As for Bengbu, during 2015, the three highest monthly mean CO concentrations were observed in February (1.16 ppm), December (1.11 ppm), and January (1.08 ppm).In 2016, the three highest monthly mean values were observed in December (1.01 ppm), January (0.842 ppm), and February (0.800 ppm), and in 2017, the three highest monthly mean values were observed in January (0.973 ppm), December (0.875 ppm), and February (0.780 ppm).The three lowest monthly mean CO concentrations were observed in July (0.526 ppm), June (0.574ppm), and August (0.591 ppm) in 2015.They were observed in July (0.480 ppm), June (0.530ppm), and August (0.537 ppm) in 2016, and in July (0.465 ppm), August (0.522 ppm), and June (0.542ppm) in 2017.The dates indicated that the highest CO concentrations mainly occurred before or after the winter and that the lowest values mainly occurred before or after the summer.
As for seasonal variations in CO concentrations, in Wuhu, in 2015, the concentrations of CO were 0.874, 0.603, 0.704, and 1.09 ppm in spring, summer, fall, and winter, respectively, and those in 2016 were 0.970, 0.583, 0.805, and 1.05 ppm, respectively.Those in 2017 were 0.757, 0.768, 0.802, and 0.976 ppm, respectively.As for Bengbu, in 2015, the concentrations of CO were 0.898, 0.564, 0.751 and 1.12 ppm in spring, summer, fall, and winter, respectively; and those in 2016 were 0.637, 0.516, 0.634, and 0.885 ppm, respectively.Those in 2017 were 0.633, 0.510, 0.626, and 0.876 ppm, respectively.This indicated that the concentration of CO varied significantly from season to season and that the cold season always achieved the maximum levels, and the warm season achieved the minimum levels, while the values were very similar in spring and fall, where both were at intermediate levels.In general, the three-year mean CO concentration of Wuhu in summer (0.651 ppm) was 37.2% lower than that in winter (1.04 ppm), and the value of Bengbu in summer (0.530 ppm) were 44.7% lower than those in winter (0.958 ppm).The seasonal variations in CO may be related to the air temperature, where in summer, a lower CO concentration is always accompanied with a higher air temperature, which concurs with the report of Kato et al. (2016).While the meteorological factors during winter are not conducive to the long-range transport of CO, a higher concentration of CO in winter is mainly a result of the accumulation of CO.

O 3 Concentration
Ozone (O 3 ) is generated by photochemical reactions with precursors including oxides of nitrogen (NO x ), volatile organic compounds (VOCs), carbon monoxide (CO), methane (CH 4 ), and monmethane hydrocarbons (NMHC) (Tu et al., 2007;Lan et al., 2015;Wu et al., 2015;Gong et al., 2018), which work as peroxy radical sources (Kato et al., 2016).Tropospheric O 3 is an important greenhouse gas and a secondary air pollutant.High O 3 levels have an adverse impact on plant growth and directly create risks to human health (Solomon et al., 2000;Silva et al., 2013;Lelieveld et al., 2015;Kato et al., 2016).In polluted atmospheres, the greatest contributor of precursors to the formation of O 3 are NO x and VOCs, especially unsaturated VOCs.The simplified general equations that regulate atmospheric photochemistry may be summarized as follows: Nitrogen dioxide dissociates to form nitric oxide (NO) and atomic oxygen radical: Atomic oxygen radical combines with molecular oxygen to form ozone: Ozone is decomposed by reacting with nitric oxide, forming nitrogen dioxide and molecular oxygen: The reaction of nitric oxide with atmospheric peroxides (RO 2 ) is the main cause of disturbances in the photochemical equilibrium, as presented in reaction 6: Atmospheric peroxides are formed by the oxidation of VOCs as represented in the equations below, which describe the oxidation of an alkene: (10) (generation of organic nitrates) Ozone (O 3 ) formed through a series of complex reactions in the atmosphere are driven by the energy transferred to nitrogen dioxide (NO 2 ) molecules when they absorb light from solar radiation.There are several classes of VOCs in the atmosphere, mainly in emissions from large urban centers and industrial areas, where high solar radiation can increase the complexity of photochemical reactions.
The ambient concentration of ozone depends on the concentration of NO x and VOCs and the ratio of VOCs to NO x .The VOCs: NO x ratio most favorable to ozone formation lies in the range of 4:1 to10:1.Meteorological factors, such as air temperature, humidity, and wind speed are the major contributors to the dispersion, transport, and dilution of O 3 , while anthropogenic activities, including industrial emissions and transportation, also play an important role in the variations and process of O 3 .Figs. 3( In Wuhu, the monthly mean concentrations of O 3 were between 13.5 and 31.7 ppb, with an average of 22.8 ppb in 2015 and ranging between 29.8 and 57.3 ppb, with an average of 38.7 ppb in 2016.The concentrations were between 20.0 and 77.4 ppb, with an average of 46.1 ppb in 2017.O 3 concentrations rose significantly, rose by approximately 41.1% from 2015 to 2016, and approximately 50.5% from 2016 to 2017, which may be related to the rapid growth in the number of vehicles in recent years.Cheng et al. (2018) indicated that temporary traffic management, such as improving fuel quality, promoting clean energy and green energy vehicles, and the odd-and-even license plate rule, have achieved outstanding benefits on the reduction of NO x emissions during the air quality assurance efforts in Beijing.Without the effective implementation of vehicle emission control measures in Wuhu, the NO x emissions from vehicles increased considerably, which led to greater amounts of precursors for the formation of O 3 .As a whole, the three-year mean O 3 concentration ranged between 13.5 and 77.4 ppb, with an average of 35.9 ppb.The results show that O 3 concentrations in Wuhu can meet the WHO air quality regulated standard of 46.6 ppb for O 3 .On the other hand, following the increasing tendency of O 3 concentrations, the annual O 3 levels in Wuhu may easily exceed the WHO air quality regulated standard.Moreover, the number of days exceeding the WHO air quality regulated standard (zero day in 2015, 82 days in 2016, and 149 days in 2017) have increased substantially over the years.The change in the O 3 concentration poses serious threats to the air quality and human health.Therefore, it should be viewed as urgent and should receive more public attention.
In Bengbu, the monthly mean concentrations of O 3 were between 23.8 and 58.7 ppb, with an average of 40.5 ppb in 2015, were between 24.7 and 64.8 ppb, with an average of 43.3 ppb in 2016, and were between 26.6 and 74.1 ppb, with an average of 49.9 ppb in 2017.Comparing the annual mean O 3 concentrations, we can see that the levels of O 3 increased continuously from 2015 to 2017 and rose by approximately 6.47% from 2015 to 2016 and by approximately 13.2% from 2016 to 2017.As in Wuhu, this also may be associated with an increase in emissions of NO x and VOCs.As a whole, the three-year mean O 3 concentrations ranged between 23.8 and 74.1 ppb, with an average of 44.7 ppb.The observed dates show that O 3 concentrations in Bengbu were above or below the WHO air quality regulated standard of 46.6 ppb during the three    and winter were 18.6, 18.6, 30.8 and 23.3 ppb, respectively;while in 2016, the values were 38.2, 35.0, 47.1, and33.6 ppb, respectively. In 2017, the values were 55.9, 62.4, 39.6, and26.6 ppb, respectively.In Bengbu, the seasonal average O 3 concentrations during 2015 were 41.5, 45.2, 48.9, and26.6 ppb in spring, summer, fall, andwinter, respectively. In 2016, they were 54.5, 47.1, 41.9, and29.8 ppb in spring, summer, fall, andwinter, respectively, andin 2017, they were 61.5, 60.1, 45.2, and32.6 ppb in spring, summer, fall, and winter, respectively.Generally, the seasonal distribution of O 3 can be classified into three patterns for the two cities during 2015-2017.In Wuhu during 2015, the first pattern displays a relatively uniform distribution in the four seasons, with lower O 3 levels throughout the year of 22.8 ppb.This pattern was similar to the results for Xiamen, where the O 3 concentrations were 35.9, 35.0, 36.3 and 29.8 ppb in spring, summer, fall, and winter, respectively (Gong et al., 2018).The unclear seasonal variations may mainly be limited by the concentration of precursors (NO x and VOCS) in the atmosphere, where when O 3 is at a lower level, the meteorological conditions have less effect on its formation.As seen in Wuhu during 2016 and Bengbu during 2015, the second pattern is where the highest mean values are in the fall, and the lowest mean values in are the winter; while the spring and summer exhibit similar values, and both are at intermediate levels, with the annual mean O 3 concentrations ranging between 38.7 and 40.5 ppb.Previous studies (Kato et al., 2016;Li et al., 2017b;Gong et al., 2018) have indicated that the atmospheric relative humidity is negatively correlated with the O 3 concentration and that low relative humidity is conducive to the formation of O 3 during the dry fall season.When the air pollutant concentration is not very high, high temperatures and wind speeds are favorable to the dispersion and dilution of air pollutants during warm seasons (defined as late spring and summer), the decreasing precursors can hinder the formation of O 3 .Thus, the warm summer season exhibits a lower O 3 level than the fall.On the other hand, high temperatures and strong UV radiation will increase ozone production rates in warm seasons, which have higher O 3 levels than in the winter.As seen in Wuhu during 2017 and Bengbu during 2016-2017, the third pattern shows the highest mean values occurred both in the spring and summer, where the lowest mean values occurred in the winter, and in the fall were at intermediate levels, where the annual mean O 3 concentration ranged between 43.3 and 49.9 ppb.From Fig. 3(c), it can be seen that the high O 3 concentration in spring occurred in late spring, under the condition of a high VOC concentration, where higher air temperature and strong UV radiation in late spring and summer directly impact on the chemical reaction rates, which are conducive to O 3 formation.At lower temperatures and a less UV radiation, the processes are not favorable for photochemical reactions in winter.

AQI Analysis
The Air Quality Index (AQI) plays a crucial role in evaluation of air status and its associated health risks.Air pollution has become particularly severe issue due to the acceleration of economic growth and urbanization in China over recent decades.The AQI can provide guidance for the government to inform the public of the status of air quality and thus help them take proper health protection measures.In Wuhu, the daily AQI ranged from 23 to 298, with an annual value of 81 in 2015.The daily AQI ranged from 33 to 290, with an annual average value of 80 in 2016 and ranged from 34 to 278, with an annual average value of 90 in 2017.In Bengbu, the daily AQI ranged from 23 to 288, with an annual value of 88 during 2015.It ranged from 32 to 286, with an annual value of 89 in 2016 and ranged from 27 to 500, with an annual value of 97 in 2017.In general, based on the annual mean values, the air quality did not improve obviously during the three-year observation period Table 3( ) show that during 2015, the proportion of levels with Grade I, II, III, IV, V, and VI were 13%, 74%, 10%, 3%, 0% and 0%, respectively; while in 2016, these proportions were 13%, 72%, 14%, 1%, 0%, and 0%, respectively, and in 2017, they were 2%, 62%, 31%, 5%, 0%, and 0%, respectively.Comparing the proportions of the different AQI classes from 2015 to 2017, both grades I and II continuously decreased by about 84.6% and 16.2%, respectively, and grades III and IV continuously increased by approximately 67.7% and 40.0%, respectively.The decreasing proportion of good (Grade I) and moderate (Grade II) levels and increasing proportion of pollution (Grade III and IV) levels indicate that the air quality in Wuhu in spring has gradually deteriorated.Both Grade V and VI remain constant at 0%, indicating that there are no heavy air pollution events in the spring in Wuhu, which is likely due to the favorable meteorological conditions for pollutant dilution and dispersion, as well as increasing vegetation coverage, which absorbs pollutants in the spring.Moreover, Table 3(a) shows that PM 2.5 is the most frequent primary air pollutant and that its levels have declined, but O 3 as the primary pollutant rose significantly during the observed years.This result highlights that controlling O 3 pollution is essential to improving air quality in addition to reducing PM pollution in the spring.
In Bengbu, Fig. 4(b)-(A), Fig. 4(d)-(A), Fig. 4(f)-(A) present that during 2015, the proportion of levels with Grade I, II, III, IV, V and VI was 4%, 76%, 20%, 0%, 0%, and 0%, respectively; while in 2016, the proportions were 4%, 61%, 33%, 2%, 0%, and 0%, respectively, and in 2017, the proportions were 1%, 55%, 38%, 4%, 1%, and 1%, respectively.As in Wuhu, from 2015-2017, the proportions of different AQI classes in Bengbu show that Grade I and II continuously decreased by about 75.0% and 27.6%, respectively, and Grade III and IV continuously increased by about 47.4% and 100.0%, respectively, which also indicated that the air quality in Bengbu in spring has gradually deteriorated.This was the same in Wuhu, where Table 3(b) also shows that the most frequent primary air pollutant was PM 2.5 , which has declined annually, but O 3 the as primary pollutant rose significantly.Both Grade V and VI represented 1% in 2017, indicating that heavy air pollution events occurred in spring in 2017 in Bengbu and that the primary pollutant is PM 10 .Therefore, appropriate measures aiming to lessen PM 10 pollution should be taken to improve the heavy pollution status in spring in Bengbu.It can be seen that the proportions of AQI levels in grades I and II in Wuhu were higher than those in Bengbu, and the proportions AQI levels of grades III and IV in Wuhu were lower than those in Bengbu, in summary, Wuhu had better air quality in spring than Bengbu.
Secondly, in regard to the distribution of the six AQI classes of summer, in Wuhu, Fig. 4(a)-(B), Fig. 4(c)-(B), Fig. 4(e)-(B) illustrate that the proportions of grades I, II, III, IV, V and VI were 38%, 61%, 1%, 0%, 0%, and 0% in 2015, respectively, were 42%, 57%, 1%, 0%, 0%, and 0% in 2016, respectively, and were 25%, 47%, 19%, 9%, 0%, and 0% in 2017, respectively.From 2015-2016, the total fraction of grades I and II were extremely high at 99%, and the remaining AQI classes only represented 1%, which indicated that the air quality is good and moderate in general and that there are rarely pollution issues in the summer of Wuhu.However, in 2017, the total fraction of grades I and II reduced to 72%, and grades III and IV rose significantly to 28% although the levels of grades V and VI remained constant at 0%.Therefore, the air quality in the summer in Wuhu exhibited a marked trend of deterioration.
From Table 3(a), initially, PM 2.5 was the most frequent primary pollutant, followed by NO x and PM 10 , but O 3 gradually become significant primary pollutant in 2017 and dominated PM 2.5 , NO 2 and PM 10 .The serious O 3 pollution during summer might be attributed to the higher VOC emissions, which provide high precursor concentrations for the formation of O 3 , and the high air temperature and strong solar radiation can facilitate photochemical production of O 3 .This result is consistent with the findings of previous studies (Atkinson and Arey, 2003;Zhang and Ying et al., 2011;Li et al., 2012a;He et al., 2017;Shen et al., 2017).As for Bengbu, Fig.  (B) illustrate that the proportion of grades I, II, III, IV, V and VI were 21%, 68%, 10%, 1%, 0%, and 0% in 2015, respectively, were 19%, 77%, 4%, 0%, 0%, and 0% in 2016, respectively, and were 18%, 61%, 20%, 1%, 0%, and 0% in 2017, respectively.The fraction of Grade I slowly decreased by about 14.3% from 2015 to 2017, but the fraction of Grade II increased by about 11.7% from 2015 to 2016 and then decreased by about 20.8% from 2016 to 2017.The fraction of Grade III decreased by about 60.0% from 2015 to 2016, and then increased by about 80.0% from 2016 to 2017.With the exception of 2016, the AQI class of Grade IV occurred in both 2015 and 2017.In general, the air quality in Bengbu in summer was good and moderate, and the best air quality state occurred in 2016, but the air quality state in 2017 was worse than that in 2015.Similar to Wuhu,Table 3(b) shows that the primary pollutant changed from PM 2.5 to O 3 .O 3 pollution may be an important factor contributing to the deterioration of air quality in the summer.This result underscores that more efforts are urgently needed to improve the air quality through lessening O 3 pollution during the summer.
Thirdly, as for the distribution of six AQI classes in fall, Fig. 4   proportions in Wuhu, in 2015, where the fraction of grades I, II, III, IV, V, and VI were 4%, 69%, 21%, 6%, 0%, and 0%, respectively; in 2016, these fractions were 11%, 73%, 13%, 3%, 0%, and 0%, respectively, and in 2017, they were 26%, 53%, 20%, 1%, 0%, and 0%, respectively.It should be noted that the air quality level of Grade I obviously increased by an appropriate 84.6%, which means that the good air quality has been improved in the autumn in Wuhu in the three years under observation.Grade II is the dominant level.Table 3 (a) shows the most frequent primary air pollutant changed from particulate matter to gaseous species, which was PM 2.5 in 2015, O 3 in 2016, and NO 2 in 2017.The increasing pollution of gaseous species may reflect a change in a complex source of air pollution, such as increasing traffic emissions; the fraction of Grade III fluctuated from 2015 to 2017, and PM 2.5 was the most frequent primary pollutant; O 3 and NO 2 also were occasionally the primary pollutants.The fraction of Grade IV obviously decreased by 83.3%, and the primary pollutant was PM 2.5 .PM 2.5 was the typical primary pollutant during polluted days, which may be related to the fact that the low ground temperature led to the accumulation of PM 2.5 , as well as the high PM 2.5 concentration air current blowing from the northern cities of China during the late fall (Tang et al., 2017;Wang et al., 2018).The levels of grades V and VI remained constantly at 0%, indicating that no heavy air pollution events occurred in the fall in Wuhu.
In Bengbu, Fig. where the fractions of grades I, II, III, IV, V and VI were 5%, 63%, 19%, 11%, 2% and 0%, respectively; while in 2016, these fractions were 15%, 61%, 21%, 2%, 1%, and 0%, respectively, and in 2017, they were 42%, 45%, 12%, 1%, 0% and 0%, respectively.The calculated dates are remarkably similar to the fall in Wuhu, where the air quality of Grade I significantly rose by an appropriate 88.1%, and the level of Grade II accounted for the largest percentage among these six classes.  of Grade III fluctuated during the observed period, where the most frequent primary air pollutant was PM 2.5 in 2015 and 2017, O 3 in 2016, and where PM 10 also was occasionally the primary pollutant.The proportion of grades IV and V dramatically reduced by 90.9% and 100%, respectively, and PM 2.5 was also the primary pollutant during heavily polluted days.Overall, our study showed clearly that PM 2.5 is the most frequent primary pollutant, so it is of great significance to control PM 2.5 pollution to reduce the air pollution in the fall in both Wuhu and Bengbu.It is also important to note the controlling of gaseous pollutants, especially O 3 pollution, which may dominate over PM 2.5 and become the most important air contaminant in the fall.Moreover, these results also show that the heavy air pollution in the fall in Bengbu is worse than that in Wuhu.Finally, in winter, Fig. 4 where the proportion of levels with grades I, II, III, IV, V and VI were 9%, 40%, 30%, 12%, 9%, and 0% in 2015, respectively, were 6%, 48%, 30%, 11%, 5%, and 0% in 2016, respectively, and were 1%, 58%, 32%, 6%, 3%, and 0% in 2017, respectively.From 2015 to 2017, the fraction of Grade I decreased by about 88.9%, but the fraction of Grade II increased by about 31.0%, and from Table 3(a), the most frequent primary air pollutant was PM 2.5 in 2015 and 2016, after which NO 2 become the mainly primary air pollutant in 2017.PM 10 also was a primary pollutant occasionally; while Grade IV and V continuously decreased by about 50.0% and 66.7%, respectively, and the primary pollutant was PM 2.5 at these two pollution levels.This result suggests that the days with moderate air quality have increased and that the heavy pollution status has improved in the winter in Wuhu, which can be attributed to the effort to derive air pollution control strategies and control actions.The analyses of primary pollutants indicated that PM 2.5 was the typical primary air pollutant in the winter in Wuhu, followed by NO 2 , which can be attributed to unfavorable metrological conditions hindering the diffusion of PM 2.5 and the low vegetation coverage obstructing the absorption of NO 2 .Therefore, the controlling of PM 2.5 and NO 2 may   ) present the distribution of the six AQI classes in Bengbu, where the proportion of grades I, II, III, IV, V and VI were 8%, 44%, 30%, 13%, 5%, and 0% in 2015, respectively, were 7%, 35%, 27%, 22%, 9%, and 0% in 2016, respectively, and were 14%, 31%, 36%, 16%, 3%, and 0% in 2017, respectively.In contrast to Wuhu, from 2015 to 2017, the fraction of Grade I increased by about 42.9%, but the fraction of Grade II decreased by about 29.5%.Table 3 (b) shows that PM 2.5 was the main primary air pollutant, followed by PM 10 and NO 2 , which were primary pollutants very few days; while Grade III, IV and V fluctuated, with mean proportions of 31.0%,17.0% and 5.7%, respectively, of which PM 2.5 was the primary pollutant.It should be noted that the changes in the air quality in the winter in Bengbu are complex, but the days with good air quality have increased.The analyses of primary pollutants also suggest that PM 2.5 was the typical primary air pollutant, followed by PM 10 .It is important to note that there was little O 3 pollution in the winter in both Wuhu and Bengbu due to the low ground temperature and weak solar radiation during winter.Hence, taking effective actions to lessen PM pollution has important significance in guiding improving the air quality of Bengbu's winter.
With regard to the seasonal variations in the air quality statuses in the two cities, in the spring in Wuhu, the levels of good and moderate days have declined without heavy pollution events, and PM 2.5 is the most frequent primary pollutant, while the number of days that O 3 is a primary pollutant have increased significantly.To improve the air quality of Wuhu in the spring, it is necessary to control PM 2.5 and lessen O 3 pollution.The air quality worsens gradually in the spring in Bengbu, and its primary pollutant situation is similar to that of Wuhu.While the primary  pollutant is PM 10 during heavily polluted days, it may effective to improve the heavy pollution through reducing PM 10 in Bengbu in the spring.The air quality mainly is good and moderate in the summer in both Wuhu and Bengbu, but it has deteriorated annually, which is very significantly related to the increasing O 3 pollution.Hence, taking proper actions for controlling O 3 is very important.The fraction of good air condition has increased, and the level of moderate air quality is the greatest in the fall in Wuhu.The most frequent primary air pollutant changed from particulate matter (PM 2.5 ) to gaseous species (O 3 and NO 2 ) during moderate air quality days, and the primary air pollutant was PM 2.5 during polluted days.The air quality status in the fall in Bengbu is similar to that in Wuhu, with the difference being the occurrence of heavily polluted days with PM 2.5 as the primary air pollutant.In the winter in Wuhu, the fraction of good air quality days has decreased; those of moderate air quality have increased, and those of heavily polluted air quality have improved.PM 2.5 was the typical primary air pollutant, followed by NO 2 , and PM 10 occurred occasionally.The change in the air quality status in Bengbu winter was opposite to that in Wuhu, where the typical primary air pollutant was PM 2.5 , followed by PM 10 , and NO 2 , which occurred occasionally.Furthermore, the situation in which O 3 was the primary air pollutant, did not occur in either Wuhu and Bengbu in winter.

CONCLUSION
The results of this study on atmospheric deposition in Wuhu and Bengbu can be summarized as follows: 1.The PM 10 concentrations in the focal three years in Wuhu were between 39 and 151 µg m -3 , with an average of 82 µg m -3 .For Bengbu, the figures were 55 to 144 µg m -3 , with an average of 95 µg m -3 .The PM 10 level in Bengbu was higher than that in Wuhu.In general, the seasonal variations in the PM 10 concentration in the summer in Wuhu (54 µg m -3 ) were 50.5% lower than that in the
From 2015-2017, the day fractions of the six AQI categories in different seasons for Wuhu and Bengbu are shown in Figs.4(a)-4(f), and the cumulative number of days of primary pollutants are shown in Tables 3(a)-3(b).
a). Cumulative number of days of primary pollutants for Wuhu from 2015-2017.10 NO x O 3 PM 2.5 PM 10 NO x O 3 PM 2.5 PM 10 NO x O 3 PM 2.5 PM 10 NO x or Bengbu.The data also indicated that the AQI values in Bengbu were higher than those in Wuhu, which means that the air pollution of the former was more severe than that of the latter.In addition, every year, the two cities exhibited obvious fluctuations in daily AQI values.This can be attributed to the air pollutant emission characteristics and meteorological conditions in different seasons.Air quality can be classified into six classes based on the ranges of the AQI values: Grade I: 0-50; Grade II: 51-100; Grade III: 101-150; Grade IV: 151-200; Grade V: 201-300; Grade VI: 301-500.The proportions of different AQI classes in the different seasons for Wuhu and Bengbu from 2015-2017 are further discussed.Firstly, as for the distribution of six AQI classes of spring throughout the observed threeyear period, in Wuhu, Fig. 4(a)-(A), Fig. 4(c)-(A), Fig. 4(e)-(A Fig. 4 (a).The fractions of the six AQI categories for Wuhu in Spring (A), Summer (B), Fall (C), and Winter (D) in 2015.
Fig. 4 (e).The fractions of the six AQI categories for Wuhu in Spring (A), Summer (B), Fall (C), and Winter (D) in 2017.
Fig. 4(f).The number fractions of the six AQI categories for Bengbu in Spring (A), Summer (B), Fall (C), and Winter (D) in 2017.
IAQI p : the air quality sub index for air pollutant p; C P : the concentration of pollutant p; C low : the concentration breakpoint that is ≤ C P ; C high : the concentration breakpoint that is ≥ C P ; I low : the index breakpoint corresponding to C low ; I high : the index breakpoint corresponding to C high .The six criteria air pollutants have acute effects on human health.The daily AQIs were calculated by the 24-hour average concentrations of SO 2 , NO 2 , PM 2.5 , PM 10 , CO, and the daily average 8-hour maximum concentration of O 3 .

Table 1 (a).
Monthly average atmospheric PM 10 concentrations in Wuhu and Bengbu in 2015.

Table 1 (
b). Monthly average atmospheric PM 10 concentrations in Wuhu and Bengbu in 2016.

Table 1 (
c). Monthly average atmospheric PM 10 concentrations in Wuhu and Bengbu in 2017.

Table 2 (a).
Monthly average atmospheric CO concentrations in Wuhu and Bengbu in 2015.

Table 2 (
b). Monthly average atmospheric CO concentrations in Wuhu and Bengbu in 2016.

Table 2 (
c). Monthly average atmospheric CO concentrations in Wuhu and Bengbu in 2017.

Table 3 (
b). Cumulative number of days of primary pollutants for Bengbu from 2015-2017.PM 10 NO x O 3 PM 2.5 PM 10 NO x O 3 PM 2.5 PM 10 NO x O 3 PM 2.5 PM 10 NO x O 3 Table 3(b) shows that the most frequent primary air pollutant was PM 2.5 in 2015, which then changed into O 3 in 2016 and 2017.The fraction