Characterization of Air Quality Index for both Handan and Kaifeng Cities, China

In this study, the atmospheric PM10, SO2, NO2, CO, and O3concentration from 2015-2017, in Handan and Kaifeng were investigated. Besides, the seasonal variations of AQI values and their corresponding primary pollutants of six AQI grades were also discussed. In Handan, the daily AQI values ranged from 22 to 500 in 2015, from 19 to 500 in 2016, and from 27 to 500 in 2017, for which the corresponding average values were 143, 132 and 151, respectively. In Kaifeng, the daily AQI ranged from 25 to 496 in 2015, from 20 to 420 in 2016, and from 21 to 434 in 2017, for which the corresponding average values were 129, 124 and 131, respectively. During the three-year study, in Handan, the average proportion of grades I, II, III, IV, V and VI were 4.0%, 35.0%, 25.6%, 31.7%, 2.5% and 1.2% in spring; were 4.0%, 35.1%, 22.6%, 35.8%, 2.5% and 0% in summer; were 14.2%, 31.0%, 15.3%, 35.8%, 3.7% and 0% in fall, and were 3.3%, 26.1%, 20.0%, 37.8%, 8.1% and 4.7% in winter. In Kaifeng, the average proportion of grades I, II, III, IV, V and VI were 2.6%, 34.0%, 24.4%, 36.1%, 2.6% and 0.3% in spring; were 18.2%, 41.9%, 21.8%, 17.0%, 1.1% and and 0% in summer; were 8.6%, 34.3%, 17.4%, 34.8%, 4.9% and 0% in fall, and were 2.1%, 15.1%, 7.8%, 44.5%, 26.2% and 4.3% in winter. Overall, the air quality in the two cities were the worst in winter. The AQI values were between 101 and 150, where in Handan, PM2.5, PM10, and O3 were the primary air pollutants over the three years. In Kaifeng, the primary air pollutants were PM2.5, PM10, NO2, and O3 in 2015; were PM2.5, PM10, and O3 in 2016, and were PM2.5, PM10, and O3 in 2017. When AQIs ranged between 151 and 200, in Handan, the primary air pollutants were PM2.5, PM10, and O3 in both 2015 and 2016, and were PM2.5, PM10, NO2, and O3 in 2017. In Kaifeng, PM2.5, PM10, and O3 were the primary air pollutants from 2015-2017. When AQIs were between 201 and 300, in Handan, the primary air pollutants were PM2.5 and PM10 in 2015 and were PM2.5 and O3 in 2017. In Kaifeng, the primary air pollutants comprised PM10 in 2015, PM2.5 and PM10 in 2016, and PM2.5 and O3 at the same time. When the AQI values were between 301 and 500, which did not occur in Handan in 2015, the primary air pollutants comprised PM10 in 2016, PM2.5 and PM10 in 2017, and in Kaifeng from 2015-2017, PM2.5, and PM10 were the primary air pollutants. This study investigated the AQI values and corresponding primary pollutants in each season in more detail, the control strategies for these air pollutants will be more precisely.


INTRODUCTION
A high extent of economic development, urbanization and industrialization in the past three decades, China has faced serious air pollution problems (Lin et al., 2010;Hu et al., 2015a).According to the monthly report on urban air quality issued by the Ministry of Environmental Protection of China, the concentrations of PM 2.5 , PM 10 , NO 2 , CO, SO 2 , and O 3 in January 2013 exceeded the limit by 68.9%, 46.9%, 12.3%, 14.9% and 1.2%, respectively (http://www.zhb.gov.cn/).Therefore, a heavy loading of air pollutant emission (Chen et al., 2013;Brauer et al., 2016) has caused public concern world-wide (Wang et al., 2015a, b).
Recently, the term "air pollution and human health" is often being discussed.The Air Quality Index (AQI) is based on the ambient air quality standards and the impact of various pollutants on human health, ecology, and the environment.Several air pollutant concentrations that are routinely monitored are reduced to a single, conceptual index value.It classifies the degree of air pollution and air quality, and it is suitable for representing the short-term air quality status and trends of the city.The AQI is divided into six categories of health concern.
According to the World Health Organization (WHO), the AQI value is determined by the standard concentrations of six pollutants, including sulfur dioxide, nitrogen dioxide, carbon monoxide, ozone, and PM 2.5 and PM 10 .The WHO estimates that approximately 7 million people die each year from exposure to fine particles that can permeate the lungs and the cardiovascular system in polluted air, resulting in diseases such as stroke, heart disease, lung cancer, and chronic obstructive pulmonary disease, including pneumonia and respiratory tract infection.Coarse particles (PM 10 ) and fine particles (PM 2.5 ) have been relating to hospital admissions for respiratory (Brunekreef and Forsberg, 2005) and cardiovascular disease (Bell, 2012).Atmospheric carbon monoxide not only destroys the neurological function of the heart but also can affect the central nervous system and even cause suffocation leading to death (Yang et al., 2012).The main sources of SO 2 is the combustion of coal, a process highly common in China (Kurokawa et al., 2013).Long-term exposure to sulfur dioxide may cause respiratory symptoms and illness and may also aggravate asthma.Ozone (O 3 ) is an important greenhouse gas contributing to global warming (Johnson et al., 1992;Berntsen et al., 1997;Sitch et al., 2007) and has a direct adverse impact on ecosystems and human health (Solomon et al., 2000;Silva et al., 2013;Lelieveld et al., 2015).O 3 can be produced by photochemical reactions in the presence of NO x and volatile organic compounds (VOCs), which work as peroxy radical sources, especially in polluted areas.
CO is emitted along with NO x and VOCs during the process of combustion.The lifetime of CO is longer than one month, so its concentration generally remains high even after being transported to remote locations.Therefore, monitoring of O 3 and CO concentrations provides useful information leading to an understanding of the long-range transport of polluted air to remote areas (Kato et al., 2016).In recent years, China has taken many measures to control the concentration of atmospheric pollutants.September 3, 2015 was the 70 th anniversary of China's Second World War victory (V-Day).In commemoration of the anniversary celebration, China imposed strict controls on emissions from Beijing and its surrounding areas from August 20 to September 3, 2015.In order to improve Beijing's air quality during the annual parade, the number of cars in the streets has been reduced by half, and 10,000 companies have been asked to restrict production or to shut down construction activities (Li et al., 2016).Li et al. (2016) discussed the observed data of SO 2 , NO x , and PM and found that SO 2 , NO x , and PM 1 concentrations decreased by 61-77%, 48-57%, and 64% during V-Day, respectively.Compared with the same period of the previous year, the NO 2 concentration decreased by an average of 44.6%.The North China Plain is considered to be the hardest-hit area (Zhang et al., 2015) due to the combined results of high air pollutant emissions caused by heating and frequent stable meteorological conditions in the area (Wang et al., 2015b;Elser et al., 2016).It is located on the edge of the national atmosphere, adjacent to Henan, Hebei, Shandong, and Shanxi Provinces (Wei et al., 2010).Handan was one of the top ten most polluted cities in China from 2013 to 2017 (http://www.mep.gov.cn/), with the highest concentration of PM 2.5 (over 700 µg m -3 per hour average) and persistent haze (Wei et al., 2014a, b).From Zhao et al.'s (2017) study on Kaifeng PM 2.5 and dioxins, it can be seen that the pollution of ambient air also presents a very severe hazard.Therefore, this study mainly discusses the pollution from PM 10 , NO 2 , CO, SO 2 , and O 3 in Kaifeng and Handan, and an AQI analysis is conducted.The purpose of this study is to better understand the characteristics of air pollution in Handan and Kaifeng, providing better suggestions for air pollution control.

METHODS
Data for this study was obtained from the data bank for three-years from January 2015 to December 2017 in both Handan and Kaifeng cities.The PM mass concentration (including daily PM 2.5 and PM 10 ) and gaseous pollutants (including daily SO 2 , NO 2 , CO, and 8h -averaged O 3 ) were obtained from the China Air Quality Online Monitoring and Analysis Platform (http://www.aqistudy.cn/).

Air Quality Index (AQI)
As shown in Equation ( 1), the sub-AQIs of the six standard pollutants are first calculated using the observed concentrations (Shen et al., 2017;She et al., 2017).The overall AQI represents the maximum of sub-AQI for all contaminants.When the AQI is above 50, the highest contributor to AQI is defined as the primary contaminant of the day, as in Eq. ( 2).(Shen et al., 2017;She et al., 2017): (1) 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 standard air pollutants have a dramatic impact on health.The daily AQI value is calculated from the 24hour average concentration of SO 2 , NO 2 , PM 2.5 , PM 10 , CO and the daily maximum 8-hour concentration of O 3 .From the US Environmental Protection Agency (US EPA) AQI, according to the scope of the AQI value, air quality can be divided into six categories: Grade I: 0-50 (good, green); Grade II: 51-100 (moderate, yellow); Grade III: 101-150 (unhealthy for sensitive group ; orange); Grade IV: 151-200 (unhealthy; red); Grade 5: 201-300 (very unhealthy; purple); Grade VI: 300-500 (hazardous; maroon) (Hu et al., 2015b;Lanzafame et al., 2015;She et al., 2017;Zhao et al., 2018).

PM 10 Concentration
The rapid economic development and increase in the number of vehicles play a crucial role in the increase of PM 10 pollution (Han et al., 2005;Tang et al., 2005;Huang et al., 2010).Because PM 10 consists of multiple chemical components over a broad-sized spectrum (Hajek and Olej, 2015), the risks of PM 10 related to human health cannot be ignored  average annual value of three years was 167.2 µg m -3 , 150.6 µg m -3 , and 157.4 µg m -3 , respectively, with a threeyear average value of 158.4 µg m -3 , which was 7.9 times of magnitude higher than the WHO air quality regulated standard (20 µg m -3 ).Over the three years examined, the maximum daily average concentration occurred in December (886.0 µg m -3 ) of 2016, and the minimum also occurred in July (19.2 µg m -3 ) of the same year.This is mainly because in December, all northern regions of China entered a period requiring heating, and the concentration of PM 10 increased sharply due to the large amount of pollutants discharged during this period as well as adverse weather conditions.
As for Kaifeng, in 2015, the monthly average PM 10 concentration was in the range of 78.0 and 204.0 µg m -3 , with those in 2016 ranging between 65.2 and 197.1 µg m -3 , and those in 2017 ranging from 66.2 to 184.7 µg m -3 .Overall, the average annual values for the three years were 129.0 µg m -3 , 124.0 µg m -3 , and 116.6 µg m -3 , respectively, with a three-year average value of 123.2 µg m -3 , which was 6.2 times of magnitude higher than the WHO air quality regulated standard (20 µg m -3 ).The average annual concentration declined slightly over the three years, but the pollution is still very serious.The average daily maximum occurred in January (607.0 µg m -3 ) 2016, and the minimum occurred in October (16.0 µg m -3 ) 2017.This is also related to the large amount of coal production because Kaifeng entered into the period during which heating is required, and adverse weather conditions have also aggravated the pollution of PM 10 .
With regard to seasonal variations, in Handan during 2015, the average PM 10 concentrations in spring, summer, fall, and winter were 154.0, 135.5, 136.5, and 242.9 µg m -3 , respectively; and those in 2016 were 148.5, 90.2, 151.8, and 211.7 µg m -3 , respectively; while those in 2017 were 151.7, 116.8, 138.8, and 222.2 µg m -3 , respectively.On the three-year basis, we can see that the concentration of PM 10 basically was in the following order: winter > spring > fall > summer.The maximum PM 10 concentration occurred in the winter (242.9 µg m -3 ) of 2015, which was approximately 1.8 times of magnitude higher than the minimum occurring in the summer (135.5 µg m -3 ) of 2015.Similarly, in 2016, the maximum PM 10 concentration occurred in the winter (211.7 µg m -3 ), which was approximately 2.3 orders of magnitude higher than the minimum occurring in the summer (90.2 µg m -3 ).In 2017, the maximum PM 10 concentration occurred in the winter (222.2 µg m -3 ), which was approximately 1.9 orders of magnitude higher than the minimum occurring in the summer (116.8 µg m -3 ).
For Kaifeng in 2015, the average PM 10 concentrations in spring, summer, fall and winter were 131.1, 93.2, 114.4,and 177.3 µg m -3 , respectively, and those in 2016 were 137.1, 71.9, 111.9, and 175.0 µg m -3 , respectively; while those in 2017 were 127.0, 78.5, 103.5, and 157.5 µg m -3 , respectively.On the three-year basis, similar to the Handan, Kaifeng's PM 10 concentration was also in the following order: winter > spring > fall > summer.The maximum PM 10 concentration occurred in the winter (177.3 µg m -3 ) of 2015, which was approximately 1.9 times of magnitude higher than the minimum occurring in the summer (93.2 µg m -3 ) of 2015.Next, in 2016, the maximum PM 10 concentration occurred in the winter (175.0 µg m -3 ), which was approximately 2.4 times of magnitude higher than the minimum occurring in the summer (71.9 µg m -3 ).Finally, in 2017, the maximum PM 10 concentration occurred in the winter (157.5 µg m -3 ), which was approximately 2.0 times of magnitude higher than the minimum occurring in the summer (78.5 µg m -3 ).
The transportation, power plants, industry and domestic heating sources, agriculture, and biomass open burning were identified as the major local PM 10 sources (Wang et al., 2008).A comparison of the PM 10 pollution situation for Kaifeng and Handan indicates that the pollution in Handan was more serious than that in Kaifeng.This is related to the fact that Handan is an important heavy industry city in Hebei Province.In addition, the maximum value of PM 10 occurs in winter, and the minimum value occurs in summer.This is related to heating coal in winter in northern China.The burning of large amounts of coal and unfavorable weather conditions increase the concentration of PM 10 .Previous research has proven that most pollution incidents occur frequently in the heating season and continue for a long time (Zheng et al., 2015;Ma et al., 2017).
Fine particulate matter (PM 2.5 ) is considered as an important air pollutant with significant adverse effects on human health, including the effects on the heart, nervous system and vascular system (Chan et al., 2006;Zeller et al., 2006;Leiva G et al., 2013).Previous research has shown that the PM 2.5 -to-PM 10 ratio reveals that changes in the PM 2.5 concentration are similar to changes in the PM 2.5 /PM 10 ratio (Fang et al., 2016).In addition, it is benefit to clarify the characteristics of regional air quality of both PM 2.5 and PM 10 , by using the PM 2.5 /PM 10 ratios, in general, ranging from 0.5 to 0.9 (Wang et al., 2012).Therefore, the concentration of PM 2.5 can be controlled by reducing the PM 10 concentration.

SO 2 Concentration
SO 2 is absorbed by the aerosol moisture and oxidized by H 2 O 2 or O 3 to SO 4 2- (Seinfeld et al., 1998).In high humidity conditions (fog or wet haze) in many other cities, liquid chemicals in the pollution process play an important role in sulphate pollution.(Sun et al., 2006;Du et al., 2011;Sun et al., 2013).
The monthly SO 2 concentrations in Handan ranged between 8.6 and 34.8 ppb, with an average of 15.9 ppb in 2015; ranged between 5.6 and 31.6 ppb, with an average of 14.6 ppb in 2016; and ranged between 3.7 and 31.3 ppb, with an average of 12.9 ppb in 2017.From 2015 to 2017, the average annual value of SO 2 declined slightly with a decrease of 18.9% in 2017 compared to 2015.Over the threeyear period, the maximum 24-hour average concentration of SO 2 occurred in July (24.7 ppb) 2015, while the minimum occurred in October (0.25 ppb) 2017.It can be seen that the concentration of SO 2 in Handan met the national secondary standard (52.50 ppb).
As for Kaifeng, the monthly concentration of SO 2 ranged from 3.6-21.4ppb, with an average of 10.7 ppb in 2015.In 2016, the concentration ranged from 2.4-18.8ppb, with an average of 9.7 ppb; in 2017 the concentration was between 1.0 and 12.4 ppb, with an average of 7.1 ppb.The concentration of SO 2 in Kaifeng also showed an annual decreasing trend.The concentration in 2017 decreased by 34.0% compared to 2015.Over the three-year period, for the 24-hour average maximum concentration of SO 2 occurred in January (21.4 ppb) 2015, while the minimum occurred in October (0.2 ppb) in the same year.It can be seen that the concentration of SO 2 in Kaifeng met the national secondary standard (52.5 ppb).
The annual reduction in the SO 2 concentration may be related to the in Power 195 factory installation of flue gas desulfurization (FGD) system, the use of clean combustion technology in industrial boiler, applied in the transportation new vehicle standards, and use clean fuel in the residential sector (Wang et al., 2014).Total sulfur dioxide emissions from China's coal-fired power plants are estimated to have reduced by 54% between 2006 and 2010 due to the installation of the FGD system (Liu et al., 2015).In addition, Handan City took measures such as coal-to-gas conversion, coal-to-coal conversion, and non-dissolving coalification in the main city area.
As for Kaifeng, in 2015, the average SO 2 concentrations were 10.6, 5.0, 9.2, and 18.1 ppb in spring, summer, fall, and winter, respectively, and those in 2016 were 10.0, 3.3, 8.6, and 17.1 ppb, respectively; while those in 2017 were 7.9, 2.0, 6.5, and 12.1 ppb, respectively.It was obvious that the concentrations of SO 2 both in Handan and Kaifeng are in the following grade order: winter > spring > fall > summer.High concentrations of SO 2 occurred in winter, and the lowest concentrations occurred in summer.Similar conclusions have also been reached in a study by Zhao et al. (2017) on SO 2 in northern China cities. Sulfur dioxide pollution caused by human activities, such as burning of fossil fuels, metal smelting and biomass burning (Kettle and Andreae, 2000;Halmer et al., 2002;Vijay et al., 2004;Dentener et al., 2006;Lee et al., 2008), the burning of coal is a common source in China (Kurokawa et al., 2013;Kato et al., 2016).Therefore, increased coal burning in winter promotes an increase in SO 2 .In warm seasons, higher temperature favor the diffusion of pollutants, and stronger rainfall can help remove pollutants.

CO Concentration
If carbon monoxide in the atmosphere is prone to oxidative stress and inflammation, human can develop symptoms such as headaches, dizziness, and even nausea because they inhale carbon monoxide.Atmospheric carbon monoxide not only destroys the neurological functions of the heart but also affects the central nervous system, and even causes suffocation that may lead to death (Yang et al., 2012).However, few studies have investigated the CO concentration in northern cities.In Handan, the monthly mean CO concentration ranged from 0.74 to 2.72 ppm in 2015, was between 0.94 and 2.68 ppm in 2016, and ranged between 0.83 and 2.89 ppm in 2017, while the corresponding annual average values were 1.28, 1.44, and 1.29 ppm, respectively.The annual mean concentrations of CO in 2016 increased about 20.5% compared with those observed in 2015, and decreased approximately 10.4% from 2016 to 2017.Over the threeyear period, the 24-hour average maximum concentration of CO occurred in December (6.96 ppm) 2016, while the minimum occurred in April (0.24 ppm) 2015.It can be seen that the maximum value of CO exceeded the national secondary standard (3.20 ppm).
For Kaifeng, the monthly mean CO concentrations varied from 0.95 to 1.83 ppm in 2015, were 0.76 to 1.79 ppm in 2016, and were 0.67 to 1.90 ppm in 2017, while the corresponding annual average values were 1.26, 1.25 and 0.97 ppm, respectively.The mean CO concentrations have decreased continuously.During the observed three-year period, the 24-hour average maximum concentration of CO occurred in December (4.56 ppm) 2015, while the minimum occurred in July (0.4 ppm) 2017.It can be seen that the maximum value of CO exceeded the national secondary standard (3.20 ppm) by approximately 42.5%.A comparison of the concentration of CO in Handan City and Kaifeng City shows that the concentration in Handan City was slightly higher than that of Kaifeng City.
As for seasonal variation sin CO concentrations, in Handan, in 2015, the concentrations of CO were 0.91, 0.88, 1.22, and 2.11 ppm in spring, summer, fall, and winter, respectively, and those in 2016 were 1.05, 1.05, 1.38, and 2.30 ppm, respectively; while those in 2017 were 1.02, 0.94, 1.01, and 2.21 ppm, respectively.As for Kaifeng, in 2015, the concentrations of CO were 1.19, 1.02, 1.16, and 1.69 ppm in spring, summer, fall, and winter, respectively, and those in 2016 were 1.22, 0.92, 1.16, and 1.71 ppm, respectively; while those in 2017 were 0.86, 0.70, 0.87, and 1.45 ppm, respectively.The above results indicated that the CO concentration varied significantly from season to season, where the cold season always achieved the maximum values, and the warm season displayed the minimum one, while the values were very similar in spring and summer, and were both among the middle grades.In general, the three-year mean CO concentration in Handan in winter (2.21 ppm) was 2.3 times of magnitude higher than that in summer (0.96 ppm), and the values in Kaifeng in summer (0.88 ppm) were 45.7% times of magnitude lower than those in winter (1.62 ppm).Previous studies have shown that there is a strong correlation between PM 2.5 and CO (r = 0.745), which implies that the CO emission is accompanied by the emission of PM 2.5 .An increase in concentration is probably because of the burning of fossil fuels and biomass (Fang et al., 2016).Zhao et al. (2017) studied the seasonal variations in PM 2.5 in both Handan and Kaifeng cities from 2015-2017.It was found that CO and PM 2.5 were well-correlated.Therefore, the high concentration of CO in winter was still related to the increase in coal burning.In addition, the low concentration in summer was related to air temperature since a lower CO concentrations are always accompanied by higher air temperatures, which is in accordance with the report of Kato et al. (2016).

O 3 Concentration
As a result of urban expansion and industrial development, tropospheric ozone levels have been rising (Vingarzan, 2004).Since 1980, as China and low-latitude countries become more industrialized, emissions have begun to shift to equatorwrd.(Zhang et al., 2016).
O 3 precursors such as VOCs and NO x have dramatically increased in the past decade (Akimoto, 2003;Vingarzan, 2004;Richter et al., 2005;Monks et al., 2009) and will continue to increase (Ou et al., 2010;Wang et al., 2013;Yang et al., 2015;Wu et al., 2017).Transportation is one source of major anthropogenic emissions of NO x (Zhang et al., 2009;Liu et al., 2015Liu et al., , 2016;;Li et al., 2017a;Saikawa et al., 2017).In a recent evaluation of Chinese O 3 data, Wang et al. (2017) found that Beijing, Chengdu, Guangzhou, and Shanghai had O 3 that exceeded the WHO standard of 100 µg m -3 on more than 30% of the days in 2013-2015. In this study, Figs. 4(a), 4(b), and 4(c) showed the variations in the monthly average O 3 concentrations in Handan and Kaifeng in 2015, 2016, and 2017, respectively.In Handan, the monthly average concentrations of O 3 ranged between 14.32 and 54.55 ppb, with an average of 37.45 ppb in 2015; they ranged between 14.13 and 68.37 ppb, with an average of 42.15 ppb in 2016, and were between 17.02 and 88.41 ppb, with an average of 48.67 ppb in 2017.O 3 concentrations rose significantly from 2015 to 2017, by approximately 29.9%.As can be seen from the figure, the concentration of O 3 is in an inverted U-shaped distribution.On the basis of the three-year period, the maximums all occurred in June (the concentrations were 54.59, 68.35, and 88.44 ppb, respectively).In addition to the minimum value of 2017, which occurred in January (17.03 ppb), the other minimum occurred in December (the concentrations were 14.36 and 14.13 ppb).As a whole, the three-year average O 3 concentrations ranged between 14.16 and 88.46 ppb, and with an average of 42.74 ppb.The results show that the O 3 concentration in Handan met the WHO air quality regulated standard of 46.67 ppb for O 3 .The increase in O 3 concentration may be related to the rapid growth in the number of vehicles, which discharge a significant amount of VOCs.Because the increase in NO 2 concentration was caused by the growth in the number of vehicles in Handan from 83.56 million in 2015 to 132.83 million in 2017 that led to an increase in VOCs, resulting in the grade evaluation obtained for O 3 .
In Kaifeng, the monthly mean concentrations of O 3 ranged between 14.   of 2017, for which the value was 5.7 times higher than the minimum occurring in December (14.26 ppb) of 2015.On a whole, the three-year average O 3 concentration ranged between 14.22 and 80.42 ppb, with an average of 43.59 ppb.
The results show that the O 3 concentration in Kaifeng met the WHO air quality regulated standard of 46.6 ppb for O 3 .
It can be seen that the three-year average for Kaifeng City was similar to that of Handan City.
As to seasonal variability, in Handan, in 2015, the seasonal average concentrations of O 3 of spring, summer, fall, and winter were 42.73, 51.75, 35.01, and 20.13  From the data, it can be clearly seen that the three-year O 3 concentration due to seasonal changes in Handan City and Kaifeng City had obvious regularity.Overall, the threeyear mean O 3 concentration were 51.47, 60.74, 36.77, and 22.09 ppb in spring, summer, fall, and winter, respectively in Handan and were 57.23, 55.96, 35.97, and 25.23 ppb in spring, summer, fall and winter, respectively, in Kaifeng.This indicated that the grades for Handan was in the following order: summer> spring > fall > winter; while in Kaifeng they were: spring> summer> fall > winter.The results for the O 3 concentrations in 16 cities in China from 2014 to 2016 conducted by Gong et al. (2018) showed that O 3 grades in the four seasons of Handan were similar to those of Chengdu and Shijiazhuang, for which the concentrations were 53.85 ppb (spring), 62.44 ppb (summer), 30.63 ppb (fall), 22.77 ppb (winter), and were 48.02 ppb (spring), 57.81 ppb (summer), 30.86 ppb (fall), and 17.08 ppb (winter), respectively.In contrast, the grades for O 3 in Kaifeng were similar to those for Qingdao and Lhasa, for which the concentrations were 55.54 ppb (spring), 53.98 ppb (summer), 46.33 ppb (fall), and 29.50 ppb (winter), and 58.56 ppb (spring), 52.68 ppb (summer), 43.24 ppb (fall), and 39.11 ppb (winter), respectively.It can be seen that the low values of O 3 occur in the winter and that the highest values of O 3 occur in the warmer periods, especially summer.In addition, the study by Gao et al. (2017) also confirmed that 3/4 out of 16 cities had the same situation.This may be related to the haze phenomenon that frequently occurred in winter.Cold weather in winter increased the consumption of coal, thereby increasing the concentration of particulate matter in the atmosphere.It is known from previous studies that higher mass concentrations of particles reduce radiation and the decrease in radiation intensity, which will alter photochemical reactions and oxidant concentrations.The concentration of O 3 was reduced by 91.3%, dropping from 42.4 µg m -3 with a clean grade to 3.7 µg m -3 with a severely polluted grade from November 16, 2015 to March 14, 2016 (Zhang et al., 2017).On the contrary, accelerated radiation and high temperatures in summer accelerate the progress of photochemical reactions.The spring peak likely reflects the dominance of global background O 3 , which typically peaks in spring at remote sites in the Northern Hemisphere (Zhang and Jaffe, 2017).In addition, 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, so low relative humidity is conducive to the formation of O 3 during the dry fall period.

NO 2 Concentration
Transportation is one of the major anthropogenic sources of NO x emissions (Zhang et al., 2009;Liu et al., 2015Liu et al., , 2016;;Li et al., 2017a;Saikawa et al., 2017).Adame et al. (2014) found that a total of 211544 tons/year of NO x was emitted, with traffic and industrial activities as the main sources, at 36% and 34%, respectively in Andalusia in 2007.From 2015 to 2017, the monthly mean concentration of NO 2 in Handan and Kaifeng are shown in Figs.5(a), 5(b), and 5(c), respectively.
In Handan, in 2015, the monthly average NO 2 concentration ranged between 15.6 and 38.0 ppb, and the annual average value was 23.0 ppb; while in 2016, those values ranged between 16.0 and 42.2 ppb, and the annual average value was 26.4 ppb.In 2017, the values ranged from 14.1 to 39.0 ppb, and the annual average value was 25.0 ppb.Over the three-year period, the maximum average of NO 2 occurred in December (42.2 ppb) of 2016, where the value increased by 199% from the minimum value that occurred in July (14.1 ppb) of 2017.
In Kaifeng in 2015, the monthly average concentration of NO 2 ranged between 10.0 and 31.0 ppb, with an average of 19.6 ppb; while in 2016, those values ranged between 10.0 and 31.0 ppb, with an average of 19.5 ppb, and in 2017, they ranged between 10.0 and 29.2 ppb, with an average of 18.8 ppb.It can be seen that the maximum NO 2 concentrations during the three-year period were all around 31.0 ppb and the minimum values were all 10.0 ppb.The annual average NO 2 concentration decreased slowly over the three-year period.The 2016 value decreased by 0.5% as compared to that in 2015, and the 2017 value decreased by 3.6% as compared to 2016.
With regard to the seasonal variations, in Handan, the seasonal NO 2 concentrations during spring, summer, fall, and winter were 19.7, 18.0, 26.0 and 28.3 ppb in 2015.They were 25.0, 18.3, 27.3 and 35.0 ppb in 2016, and 23.7, and 35.0 ppb, 25.0 and 34.3 ppb in 2017, respectively.We can see that typically, the seasonal NO 2 exhibited U shape patterns, with lower values from May to August with higher values in November and December.The concentration of NO 2 was in the following order: winter > fall > spring > summer.During the three-year period, the maximum occurred in the winter (35.0 ppb) of 2016, for which the value was 2.1 times higher than the minimum that occurred in summer (17.0 ppb) in 2017.
For Kaifeng, the seasonal NO 2 concentrations during spring, summer, fall, and winter were 16.3, 12.3, 21.3 and28.4 ppb in 2015. They were 18.6, 11.0, 20.7 and27.7 ppb in 2016 and17.0, 10.3, 21.3 and26.3respectively.Similar to Handan, the concentraion of NO 2 in Kaifeng also exhibited U shape patterns.The maximum occurred in the winter (28.4 ppb) of 2015, for which the value was 2.8 times higher than the minimum that occurred in the summer (10.3 ppb) of 2017.
A comparison of the concentrations of NO 2 in Handan and Kaifeng shows that the NO 2 pollution in Handan was slightly worse than that in Kaifeng.This was mainly related to the pollution from vehicles.Previous research shows that the increase of NO 2 concentration in the atmosphere caused by the large amount of nitrogen oxides from the increase of vehicles in China will directly affect the ecological environment and adversely affect human health (Boersma et al., 2009;Sachin et al., 2009) The number of vehicles in Handan City was much higher than the number in Kaifeng.The correlation coefficient between the Beijing NO 2 concentration and the number of vehicles was been calculated to be -0.86 (95% confidence interval, two-tailed) during the 2000-2015, the correlation between vehicle NO x emissions and the concentration of NO 2 was 0.86 (95% confidence interval, two-tailed), reduce vehicle NO x emissions have significant influence on Beijing NO 2 concentration decline (Cheng et al., 2017).In terms of seasonal variations, during the three-year period, the average NO 2 concentrations in fall and winter were relatively higher than those in spring and summer, and the higher monthly average NO 2 concentration was significantly related to PM 2.5 pollution events (Yang et al., 2015).

AQI Analysis
For this reason of health effects, the Chinese government set the Technical Regulation on the Ambient Air Quality Index (AQI) in 2012, which included PM 2.5 concentration for the first time (Du et al., 2017).The AQI index is used to characterize the degree of air contamination and its associated health risks.In Handan, the daily AQI ranged from 22 to 500, with an annual value of 143 in 2015; these values ranged from 19 to 500, with an annual average value of 132 during 2016 and ranged from 27 to 500, with an annual average value of 151 during 2017.In Kaifeng, the daily AQI ranged from 25 to 496, with an annual value of 129 in 2015; it ranged from 20 to 420, with an annual value of 124 during 2016, and it ranged from 21 to 434, with the annual value of 131 in 2017.Handan City had a maximum of 500 AQI during the three years under consideration, where the values were higher than the maximum (496, 420, and 434) in Kaifeng City.This indicated that the air pollution situation in Handan was more serious than that in Kaifeng City.In addition, the values of the AQI in the two cities fluctuated significantly, which was related to the local pollution emission characteristics and meteorological conditions.
In this study, we analyzed the seasonal pollution of six categories in Handan and Kaifeng.were 2.2%, 33.3%, 23.7%, 40.8%, 0%, and 0% in 2015, respectively; were 7.6%, 32.6%, 29.3%, 27.2%, 2.2%, and 1.1% in 2016, respectively, and were 2.2%, 39.1%, 23.9%, 27.2%, 5.4%, and 2.2% in 2017, respectively.From 2015 to 2017, except for a slightly higher percentage of grade IV (40.8%) in 2015, the other grades were similar.The state of ambient air quality was mainly concentrated in grades II-IV, which showed that there was mild pollution in Handan in the spring.From Table 1(a), it was obvious that Handan was polluted by particulate matter (PM 2.5 and PM 10 ) in the spring.This was related to the wind-driven sand in the spring.
In summer, the distribution of the six AQI classes in Handan, as shown in Fig. 6(a)-(B), Fig. 6(c)-(B), and Fig. 6(e)-(B) with grades I, II, III, IV, V and VI were 3.3%, 41.1%, 15.6%, 40%, 0%, and 0% in 2015, respectively; were 8.7%, 35.9%, 27.2%, 28.2%, 0%, and 0% in 2016, respectively, and were 0, 28.3%, 25%, 39.1%, 7.6%, and 0% in 2017, respectively.From 2015-2016, most of the days that belonged to grades II-IV, for which the value was as high as 90% or more.In 2107, the number of days belonged to Grade IV increased significantly.It can be seen that the air in the summer was relatively good, which was related to summer conditions that favor the diffusion of pollutants.Table 1(a) shows that PM 2.5 remains the most frequently occurring primary pollutant, followed by PM 10 and O 3 .The frequency of occurrence of O 3 as the primary pollutant in the summer was significantly higher than in other seasons.Severe O 3 pollution in summer may be attributed to higher VOC emissions, which provide a high concentration of precursor for the formation of O 3 , during which high air temperature and intense solar radiation can promote the photochemical production of O 3. This result is consistent with previous research results (Atkinson and Arey, 2003;Zhang and Ying et al., 2011;Li et al., 2012;He et al., 2017;Shen et al., 2017).
In the fall of 2015, the distribution of the six AQI classes, as shown in Fig. 6 for grades I, II, III, IV, V and VI were 19.6%, 32.6%, 17.4%, 27.2%, 3.2%, and 0%, respectively; in 2016, these fractions were 18.7%, 33.0%, 16.5%, 24.2%, 6.6%, and 1.0%, respectively, and in 2017, these fractions were 4.4%, 23.1%, 12.1%, 56.0%, 4.4%, and 0%, respectively.Obviously, the number of days with each grade in 2017 was in the following order: Grade IV > Grade II > Grade III > Grade I = Grade V > Grade VI and others appeared Grade II > Grade IV > Grade I > Grade III > Grade V > Grade VI.In general, the number of days of Grade II and Grade IV were the highest, with the number of days with these two Grades accounting for more than 50% of the whole year, indicating that the air pollution grade in Handan City was categorized as moderately polluted in fall.Table 1 (a) shows that in 2015, with the exception that PM 10 and PM 2.5 were the primary pollutants most days, the number of days that NO 2 was the primary pollutant was significantly increased.However, in 2016 and 2017, the number of days with NO 2 as the primary pollutant became zero again, due to the dominant source of pollution being particulate matter.This indicated that the pollution from NO 2 in fall did not always occur.This may be related to the automobile exhaust gas and gas phase conditions at the time.
As for the distribution of six AQI categories of winter throughout the observed three-year period, in Handan, Fig. 6 ) show that during 2015, grades I, II, III, IV, V, and VI were 0%, 6.7%, 6.7%, 58.9%, 16.7%, and 11.0%, respectively; while in 2016, the proportions were 7.7%, 32.6%, 29.3%, 27.2%, 2.2%, and 1.0%, respectively, and in 2017, the proportions were 2.2%, 39.1%, 23.9%, 27.2%, 5.4%, and 2.2%, respectively.In 2015, Grade IV accounted for more than half of the spring, followed by grades V and VI.There were no good days in the winter in Handan City.The Grade IV index indicated an unhealthy environment.This mean that everyone may begin to experience negative health effects, and members of sensitive groups may face serious sick.Moreover, Table 1(a) shows that PM 2.5 was the most frequent primary air pollutant.This was mainly related to the massive amount of coal burning and unfavorable meteorological conditions in winter (Sun et al., 2014).
It is well known that PM 2.5 is an inhaled lung particle.Due to its large specific surface area, PM 2.5 is prone to the accumulation of a large amount of toxic and harmful substances, stays in the atmosphere for a long time, and is far from the transportation distance.It has a greater impact on human health and the atmospheric environment than other pollutants.In Kaifeng, Fig. 6 ) show that in spring, the proportion of grades I, II, III, IV, V and VI were 1.1%, 33.3%, 31.2%,34.4%, 0%, and 0% in 2015, respectively; they were 5.6%, 39.3%, 20.2%, 30.3%, 4.6%, and 0% in 2016, respectively, and were 1.1%, 29.3%, 21.7%, 43.5%, 3.3%, and 1.1% in 2017, respectively.From 2015 to 2017, each grade basically showed an increase from 2015 to 2016, but the proportion from 2016 to 2017 decreased.This showed that the environmental quality grade in the spring of 2016 was worse than that in the other two years.From Table 1(b), different from Handan City, apart from the frequent occurrence of PM 2.5 and PM 10 as primary pollutants, the proportion of gaseous pollutant O 3 also accounted for some proportion.The above analysis of O 3 shows that O 3 pollution often occurs in spring and summer.During this period, particulate matter is lighter, and the radiation is stronger, which is favorable for the occurrence of photochemical reactions.showed the proportion of grades I, II, III, IV, V, and VI in summer, which were 19.4%, 38.7%, 17.2%, 24.7%, 0%, and 0% in 2015, respectively.They were 20%, 45.6%, 24.4%, 10%, 0%, and 0% in 2016, respectively and were 15.2%, 41.3%, 23.9%, 16.3%, 3.3%, and 0% in 2017, respectively.Between 2015 and 2017, the number of days of Grade V and Grade VI were basically zero, and the number of days with Grade II indexes was the highest.In addition, compared to other seasons, there were more summer day ranked as Grade I. From Table 1(b), it can be seen that, unlike Handan, PM 10 was the most important pollutant in terms of number of days with this pollutant, and in 2017 , the proportion of grades I, II, III, IV, V, and VI in the fall were 11.2%, 34.8%, 12.4%, 38.2%, 3.4%, and 0%, respectively; while in 2016, these fractions were 12.5%, 34.1%, 17.0%, 28.4%, 8.0%, and 0%, respectively, and in 2017, they were 13.3%, 28.9%, 16.7%, 37.8%, 3.3%.and 0%, respectively.Comparing the proportions of different AQI categories from 2015 to 2017, Grade I days grew by18.8%.There was a slight reduction in Grade II, which decreased by 17.0% from 2015 to 2017.Grade III showed an increasing trend initially and then decreased, and Grade IV exhibited an opposite trend.Grade V maintained a very low occupancy rate for all three years.The number of days in Grade VI were all zero.From Table 1( shows that during 2015, the proportion of grades I, II, III, IV, V, and VI was 0%, 18.9%, 7.8%, 44.4%, 23.3%, and 5.6%, respectively; while in 2016, those proportions were 3.0%, 12.1%, 10.1%, 36.4%,33.3% and 5.1%, respectively, and in 2017, they were 3.3%, 14.3%, 5.5%, 52.8%, 22.0%, and 2.1%, respectively.From 2015-2017, the proportions of different AQI classes in Kaifeng show that AQI >150 days exceeded 70% each year mainly in terms of the VI and V grades.The pollution of the ambient air has a severe impact on humans.From Table 1(b), similar to Hnadan, it is shown that PM 2.5 is the most frequent primary air pollutant.

CONCLUSION
The results of this study on the atmospheric deposition in Handan and Kaifeng can be summarized as follows: As for seasonal variations of PM 10 basically were in the following order: winter > spring > fall > summer.In summer of Handan (114.2 µg m -3 ) was 49.4% in magnitude lower than that in winter (225.6 µg m -3 ); as for Kaifeng in summer (81.2 µg m -3 ) was 52.2% in magnitude lower than that in winter (169.9 µg m -3 ). 2. In Handan, the average concentrations of SO 2 for the three years were between 3.   6.In the spring of Handan, the state of ambient air quality was mainly concentrated on Grade II-IV, which showed that there was mild pollution in Handan in spring.PM 2.5 and PM 10 were the most frequent primary pollutants; different from Handan City, apart from the frequent occurrence of PM 2.5 and PM 10 as primary pollutants, the proportion of gaseous pollutants O 3 also accounts for the proportion.7.In the spring in Handan, the state of ambient air quality was mainly grades II-IV, which showed that there was mild pollution in Handan in spring.PM 2.5 and PM 10 were the most frequent primary pollutants; different from Handan City, apart from the frequent occurrence of PM 2.5 and PM 10 as the primary pollutants, the proportion of gaseous O 3 also accounted for a proportion of the pollution.8.In summer, the AQI was mainly grades I-II in both cities, which indicated that the quality of the ambient air was good or moderate.Different from other seasons, O 3 was as the highest primary pollutant.9.In fall, in general, the number of days in Grade II and Grade IV were the highest, with these two Grades accounting for more than 50% of the whole year both in two cities.The primary pollutants were PM 2.5 and PM 10 .The pollution of NO 2 and O 3 only occured occasionally for a few days.10.In the winter in Handan, the fraction of excellent air quality was zero, but the heavy pollution was improved.Similar to Handan, there were very few days with excellent conditions in Kaifeng City.PM 2.5 and PM 10 were the typical primary air pollutants.NO 2 appeared only once in the Kaifeng in 2015.
The monthly mean concentrations of CO in Handan and Kaifeng are shown in Figs.3(a), 3(b), and 3(c) from 2015 to 2017, respectively.
Fig. 3(a).Monthly average atmospheric CO concentration in Handan and Kaifeng in 2015.
. Mar. Apr.May June July Aug. Sep.Oct. Nov. Dec.
Fig. 5(a).Monthly average atmospheric NO 2 concentration in Handan and Kaifeng in 2015.
. Mar. Apr.May June July Aug. Sep.Oct. Nov. Dec.
Fig. 6(a).The number fractions of the six AQI categories for Handan in (A) Spring, (B) Summer, (C) Fall, and (D) Winter in 2015.
Fig. 6(a)-(A), Fig. 6(c)-(A), and Fig. 6(e)-(A) present the distribution of six AQI classes in Handan in the spring, for which the proportion of grades I, II, III, IV, V and VI
a), it can be seen that the city of Kaifeng was mainly dominated by particulate pollution.The pollution of NO 2 and O 3 only occurred occasionally for a few days.Finally, in winter, Fig. 6(b)-(D), Fig. 6(d)-(D), and Fig. 6(f)-(D)
7 and 34.8 ppb; and those values were in range of 1.0-21.4ppb in Kaifeng, with the corresponding average values were 14.5 and 9.2 ppb, respectively.The result indicated that the pollution of Handan by SO 2 was more serious than that of Kaifeng City.3.During the three-year period, the average concentrations of NO 2 were between 14.1 and 42.2 ppb, in Handan.In Kaifeng the concentration were from 10.0 to 31.0 ppb.The corresponding average values were 24.8 and 19.3 ppb, respectively.Comparing the concentration of NO 2 in Handan and Kaifeng, we can know that NO 2 pollution in Handan was slightly worse than Kaifeng.In addition, for annual seasonal variation, the concentration of NO 2 exhibited U shape patterns both in Handan and Kaifeng.4.During the observed three-year period, in Handan and Kaifeng the average CO concentration ranged 0.74-2.89ppm and 0.67-1.90ppm, respectively; the corresponding average values were 1.34 and 1.16 ppm, respectively.The high concentration of CO in winter was related to the increase of coal burning.Besides, the low concentration in summer was related to air temperature, a lower CO concentration was always accompanied with a higher air temperature.5.The three-year average O 3 concentration ranged from 14.13 to 88.41 ppb, and with an average of 42.76 ppb in Handan, and ranged between 14.20 and 80.45 ppb, and with an average of 43.56 ppb in Kaifeng.The monthly average concentration of O 3 both in Kaifeng and Handan showed inverted U-shaped.Besides, the maximum value of O 3 concentration always occurred in spring and summer, and the minimum value was in winter.
. It can be seen from the data provided by the statistics bureaus of Handan and Kaifeng (http://tj.hd.gov.cn;http://www.ha.stats.gov.cn) that the number of vehicles in Handan increased by 59%, from 83.49 million in 2015 to 132.76 million in 2017.In Kaifeng, the number of vehicles in Handan increased by 41.1%, from 38.4 million in 2015 to 54.2 million in 2017.

Table 1 (
a). Cumulative number of days of primary pollutants for Handan 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