Characterization of the Air Quality Index in Southwestern Taiwan

This study provides an investigation of atmospheric PM2.5, PM10, SO2, NO2, CO, and O3 in the period from 2015–2017 in the southwestern and central part of Taiwan. In addition, the seasonal distribution of six AQI classes and corresponding primary pollutants were further analyzed. The daily AQIs were 15–194 in 2015, 17–213 in 2016, and 16–184 in 2017, respectively. During the three year period, in the studied area, the mean proportions of levels with Grade I, II, III, IV, V, and VI were 3%–31%, 30%–57%, 17%–46%, 0%–12%, 0%–1% and 0% in spring; were 38%–76%, 21%–57%, 0%–10%, 0%–2%, 0% and 0% in summer; were 2%–45%, 34%–77%, 13%–36%, 0%–18%, 0% and 0% in fall, and were 0%–12%, 38%–67%, 24%–41%, 1%–22%, 0% and 0% in winter. Generally, it was found that summer has the best air quality and that winter has the worst, where spring and fall are similar in terms of air quality. PM2.5 was the typical primary air pollutant when the AQI classes were 51–200, followed by O3 in summer, spring and fall, and PM10 in winter. As for class 201–300, the primary air pollutant was O3 and the 300–500 class did not occur.


INTRODUCTION
In recent years, air pollution has become a particularly serious issue in many cities in the world (Ran et al., 2011;Li et al., 2012;Tang et al., 2012;Li et al., 2014).Previous investigation demonstrated the close relationship between air pollutants and human health (Pope and Dochery, 2006;Cao et al., 2012;Heal et al., 2012;Pope and Dochery, 2013;Jin et al., 2017).Air pollutants in the atmospheric environment, including ozone (O 3 ) and particle matter (PM), have had serious impacts on human health (Liu et al., 2002), such as premature death (Schwartz et al., 1996), respiratory-related hospital admissions and emergency room visits (Thurston et al., 1992), acute respiratory symptoms with aggravated coughing and difficult or painful breathing, chronic bronchitis, aggravated asthma (Anderson et al., 1992), lung function experienced as shortness of breath, and so on.It is important to inform the public about the current air quality and to take corresponding health protection measures.
Initially, a local Environment Pollution Administration (EPA) established the Pollutant Standards Index (PSI) system to estimate air quality in Taiwan.Base on the US EPA, this PSI system contains sub-indices for ozone (O 3 ), coarse particulate matter (PM 10 ), carbon monoxide (CO), sulfur dioxide (SO 2 ), and nitrogen dioxide (NO 2 ).These sub-PSI indices were associated with ambient pollutant concentrations scaled from 0 to 500, where an index value of 100 can be defined as normal, and an index value of 500 is associated with significant harm to human health (Liu et al., 2002).A daily major pollutant is defining as a pollutant with the largest daily sub-index value.The PSI system was based on general increases in O 3 concentrations and decreases in PM 10 levels in a steadily increasing PSI system.It was replaced with the Air Quality Index (AQI), which can lead to a better understanding of the effects of O 3 and fine particulate matter (PM 2.5 ) on the general air quality.
In Taiwan, the air quality in several areas has worsened along with growth in population density and the rapid increases in the use of coal-fired power plants and motor vehicles.Since 1955, the control of air pollution began to gain attention, and Taipei began to implement controls on the burning of coal (Lee et al. 2018).Previous studies have demonstrated that the pollutant standards index (PSI) in Taiwan is often between 50 and 100, and some places even reach more than 100 (unhealthy air quality) (Bureau of Air Quality Protection and Noise Control, 1991;Fang and Chen, 1996).These factors, including geographic conditions, emission sources, and meteorological factors are all important contributors to the air quality problem in Taiwan (Zhang et al., 2009;Liu et al., 2012;Liu and Wang, 2014;Zhang and Cao, 2015;Li et al., 2017a;Shen et al., 2017).
The object of this study was to examine air quality in the southwestern and central part of Taiwan through a discussion of the characteristics of atmospheric PM 2.5 , PM 10 , SO 2 , NO 2 , CO, and O 3 , after which the calculated Air Quality Index (AQI) was used as an important basis for judging the air quality.In addition, the seasonal variations in AQI are further discussed, as well as the corresponding primary pollutants.This research is intended to provide valuable information and better understanding of the air pollution characteristics in seven specific areas of Taiwan.

METHODS
The air quality of seven cities or townships in Taiwan, Puzih, Singang, and Chiayi in Chiayi City, and Douliou, Taisi, Lunbei, and Mailiao in Yunlin County, were analyzed in this study.From January 2015 to December 2017 in these districts, meteorological data, including PM 2.5 , PM 10 , SO 2 , NO x , CO, and O 3 levels, were collected from local air quality stations.
Chiayi City is located in the southwestern part of Taiwan, which is crossed by the Tropic of Cancer and has a subtropical humid climate, with an annual average temperature of 22.0°C and an annual rainfall capacity of 1,500 mm.Yunlin County is a central area, located in the center of Taiwan.It has a south subtropical humid climate, where the annual average temperature is about 22.6°C, and the precipitation is approximately 1,697 mm.

Air Quality Index (AQI)
The sub-AQI of the six criteria pollutants was first calculated with the observation concentrations, as shown in Eq. (1) (She et al., 2017;Shen 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) (She et al., 2017;Shen 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 criteria air pollutants have acute effects on human health.The daily AQIs were calculated using 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 .Based on the United States Environmental Protection Agency (U.S. EPA) AQI, the ranges of the AQI values related to air quality can be classified into six classes: Grade I: 0-50 (Good, Green); Grade II: 51-100 (Moderate, Yellow); Grade III: 101-150 (Unhealthy for Sensitive Groups; Orange); Grade IV: 151-200 (Unhealthy; Red); Grade V: 201-300 (Very unhealthy; Purple), and Grade VI: 300-500 (Hazardous; Maroon) (Hu et al., 2015;Lanzafame et al., 2015;She et al., 2017;Zhao et al., 2018).

PM 2.5 Concentration
PM 2.5 is suspended in the atmosphere for long periods and transports over long distances, which has a detrimental effect on both human health and air quality.During the period from 2015-2017, the atmospheric monthly PM 2.5 concentrations in Puzih City, Singang Township, and Chiayi County are presented in Figs.1(a)-1, 1(b)-1, 1(c)-1, and those in Douliou City, Taisi Township, Lunbei Township, and Mailiao Township are presented in Figs.1(a)-2, 1(b)-2, 1(c)-2, respectively.
Among the seven cities and townships under consideration, the highest annual mean PM 2.5 concentration occurred in Lunbei Township (31.1 µg m -3 ), which was in the range of 15.8-47.1 µg m -3 , and the lowest level was found in Puzih City (24.0 µg m -3 ), with a range of 10.0-39.1 µg m -3 in 2015.In 2016, the highest annual concentration of PM 2.5 occurred in Lunbei Township (29.3 µg m -3 ), with a range of 15.5-41.8µg m -3 , and the lowest level was found in Taisi Township (24.7 µg m -3 ) and was 11.1-35.9µg m -3 .In 2017, the highest annual average PM 2.5 concentration was found in Mailiao Township (27.9 µg m -3 ), which ranged from 14.8-39.9µg m -3 , and the lowest values occurred in Puzih City (21.9 µg m -3 ), with a range of 10.0-32.0 µg m -3 .Overall, the three-year mean PM 2.5 concentrations in Puzih City,Singang Township,Chiayi County,Douliou City,Taisi Township,Lunbei Township,and Mailiao Township were 24.1,24.9,27.2,27.5,25.0,29.2 and 25.7 µg m -3 , respectively.It was found that the highest levels occurred in Lunbei Township and the lowest in Puzih City.These investigated districts were all above the WHO air quality regulated standard (10 µg m -3 ).Furthermore, with the exception of Douliou City, they exhibited annual declines in PM 2.5 concentration (29.1, 26.7 and 26.6 µg m -3 in  2015, 2016, and 2017, respectively).The fluctuations did not have obvious downward trends in the other six cities and townships.PM 2.5 is the primary air pollutant in these districts; thus more efforts to reduce the PM 2.5 concentration would be meaningful.
The above study indicated that the PM 2.5 concentrations in the seven counties and townships generally occurred in the following order: winter > spring > fall > summer.This result is similar to that obtained locally by Lee (2018), who studied atmospheric PM 2.5 concentrations in 22 cities and counties in Taiwan and pointed out that the mean PM 2.5 concentrations were 23.2, 12.1, 19.1, and 25.0 µg m -3 in spring, summer, fall and winter, respectively.This may have been caused by the various air temperatures in different seasons, where temperature inversions easily occur under low ground temperature conditions, which will impede the dispersion of air pollutants and lead to elevations in atmospheric PM 2.5 in cold seasons (Tang et al., 2017;Xing et al., 2017;Lee et al., 2018;Wang et al., 2018a).The primary sources of fuel combustion, including industrial facilities, vehicle emissions, and power plants can release fine particles, and secondary atmospheric gaseous emissions, such as VOCs, NO x , and SO 2 , also transform PM 2.5 (Mao et al., 1997; Yang et al., 1997; Liu et al., 2002).This universal source of PM 2.5 determines its crucial effect on regional air quality.Therefore, it is of great significance to reduce the level of PM 2.5 by reducing the number of automobiles and promoting clean energy in order to improve overall air quality.

PM 10 Concentration
PM 10 comprises inhalable particles.These coarse particles are usually derived from motor vehicles running on unpaved asphalt and cement roads, the crushing and grinding of materials, as well as dust from the wind.PM 10 can be inhaled by humans and is deposited in the upper respiratory tract and pulmonary alveoli and causes related diseases (Song et al., 2008; Tao et al., 2009; Matus et al., 2012; Sun  et al., 2014; Liang et al., 2016).PM 10 can affect residents' health and the air quality of a region (Liu et al., 2002).For the period from 2015-2017, the monthly average PM 10 concentrations in the ambient air in Puzih City, Singang Township, and Chiayi County are presented in Tables 1(a)-1, 1(b)-1, and 1(c)-1, and those in Douliou City, Taisi Township, Lunbei Township, and Mailiao Township are presented in Tables 1(a)-2, 1(b)-2, 1(c)-2, respectively.
It can be seen in Tables 1(a)-1 and 1(a)-2 that Mailiao Township (72 µg m -3 ) has the maximum levels of PM 10 , ranging from 36-149 µg m -3 , while Taisi Township (47 µg m -3 ) has the minimum values, ranging from 20-94 µg m -3 in 2015.Table 1(b)-1 and 1(b)-2 shows that the highest PM 10 concentration occurred in Mailiao Township (66 µg m -3 ) and ranged from 31-125 µg m -3 , while the lowest PM 10 concentration occurred in Taisi Township (45 µg m -3 ) and ranged from 19-85 µg m -3 in 2016.For 2017, Tables 1(b)-1 and 1(b)-2 demonstrate that the highest annual average PM 10 concentration was found in Mailiao Township (71 µg m -3 ), which ranged from 36-144 µg m -3 , and that the lowest values occurred in Taisi Township (49 µg m -3 ), with a range of 24-104 µg m -3 .Among the seven observation areas, due to the establishment of some factories and an industrial zone in Mailiao Township, it has gradually become a major industrial township in Taiwan and has been accompanied with the highest PM 10 levels.Taisi Township is located in the western part of Taiwan, which is close to the sea, where the strong winds and rain from the sea favor the dispersion and deposition of PM 10 , thus it has the lowest PM 10 levels.The average PM 10 concentrations for the three years under observation were 65, 58, 57, 52, 47, 54, and 70 µg m -3 in Puzih City, Singang Township, Chiayi County, Douliou City, Taisi Township, Lunbei Township, and Mailiao Township, respectively.Although the observed PM 10 levels in western Taiwan were lower than those in Wuhu (82 µg m -3 ) and Bengbu (95 µg m -3 ), two central cities in China, they were still approximately 2.35-3.50times higher than the WHO standard (20 µg m -3 ).A previous study indicated that PM 10 is an important major atmospheric air pollutant that has a large regional impact on AQI (Liu et al., 2002).Thus, further control of PM 10 is of great significance to improve the air quality in western Taiwan.
An examination of the seasonal PM 10 concentration characteristics indicated that the values were between 44 (Taisi Township) and 70 µg m -3 (Mailiao Township) in spring, were between 29 (Taisi Township) and 46 µg m -3 (Mailiao Township) in summer, ranged from 52 (Taisi Township) to 78 µg m -3 (Mailiao Township) in fall, and were between 65 (Taisi Township) and 96 µg m -3 (Mailiao Township) in winter, respectively, in 2015.These values ranged between 51 (Taisi Township) and 66 µg m -3 (Mailiao Township) in spring, were between 31 (Taisi Township) and 47 µg m -3 (Mailiao Township) in summer, ranged from 45 (Taisi Township) to 70 µg m -3 (Mailiao Township) in fall, and were between 54 (Taisi Township) and 80 µg m -3 (Mailiao Township) in winter, respectively, in 2016.These values ranged between 53 (Taisi Township) and 72 µg m -3 (Mailiao Township) in spring, were between 29 (Taisi Township) and 41 µg m -3 (Mailiao Township) in summer, ranged from 52 (Douliou City) to 77 µg m -3 (Mailiao Township) in fall, and were between 59 (Taisi Township) and 93 µg m -3 (Mailiao Township) in winter, respectively, in 2017.PM 10 levels exhibit notable seasonal distinctions.The average values in summer with the lowest levels (36 µg m -3 ) were 52.0% in magnitude lower than those in winter, when the highest levels (75 µg m -3 ) were observed.Spring had similar levels to those in the fall, and both were in the middle levels (60 µg m -3 in spring and 61 µg m -3 in fall, respectively).In Taiwan, the airflow with low temperature and moisture from the north in winter favors enhancement of the level of PM 10 .This result was consistent with the seasonal distribution of PM 10 found in Wuhu and Bengbu (Shang et al., 2018;Wang et al., 2018b).Taiwan is surrounded by the sea and has abundant rainfall, which is very beneficial to the removal of PM 10 from the atmosphere by wet deposition; thus its overall PM 10 levels were lower than those for Wuhu and Bengbu.Coarse particles (PM 10 ) are mainly due to natural and anthropogenic processes, including dust storms, local soil re-suspension, road dust, and various industrial processes (Querol et al., 2004; Xu et al., 2017).The PM 2.5 /PM 10 ratio is a useful index by which to determine the most likely atmospheric processes leading to accumulation due to the diverse sources of PM 2.5 and PM 10 .It can thus be used to reflect the status of air quality (Speranza et al., 2014; Blancobecerra et al., 2015; Xu et al., 2017; Wang et al., 2018b).Previous studies have found that the PM 2.5 /PM 10 ratio varies from seasons to season, where the dry season is lower than the wet season, which is mainly due to the contribution of dust re-suspension (Akinlade et al., 2014;  Speranza et al., 2014; Wang et al., 2018a).Some studies have also reported that due to the effect of re-suspended coarse road dust, there is negative correlation between the number of vehicles and the PM 2.5 /PM 10 ratio (Querol et al., 2001;Evagelopoulos et al., 2006).

SO 2 Concentration
Anthropogenic activities (including the burning of biomass, fossil fuel combustion, and metal smelting) and natural processes (including volcanic eruption, soil oxidation, and hydrogen sulfide oxidation) generate and emit sulfur dioxide (SO 2 ) into the atmosphere (Kettle and Andreae, 2000;Dong et al., 2002;Halmer et al., 2002;Vijay et al., 2004;Dentener et al., 2006;Lee et al., 2008;Lin et al., 2010;Khattak et al., 2013).Thus, atmospheric SO 2 irritates the respiratory tract and injures the cardiovascular system of living things (Lovati et al., 1996).SO 2 in the atmosphere is absorbed by water aerosol droplets, such as clouds, fog, and rains, where acid rain forms in this process and increases the risk of harm to ecological systems (Ferrari and Salisbury, 1999;Chen et al., 2001;Goncalves et al., 2002;Chen et al., 2011).SO 2 is a noticeable air pollutant that will damage human health and the environment (Aneja et al., 2001;Chen et al., 2011).Figs.2(a The highest annual average SO 2 concentration among the seven cities and townships occurred in Taisi Township (9.89 ppb), ranging from 4.44-14.4ppb, and the lowest level occurred in Douliou City (5.54 ppb), ranging from 4.19-6.58ppb in 2015.In 2016, the maximum annual SO 2 value was found in Taisi Township (8.49 ppb), ranging from 4.60-12.2ppb, and the lowest value was found in Douliou City (5.25 ppb), ranging from 4.00-5.92ppb.In 2017, the maximum annual SO 2 level was found in Taisi Township (7.28 ppb), ranging from 4.30-10.0ppb, and the lowest value was found in Douliou City (4.94 ppb), ranging from 3.98-5.96ppb.During the period 2015-2017, the highest concentration of SO 2 always was in Taisi Township, and the lowest values were always in Douliou City.The three-year mean SO 2 concentrations in Puzih City, Singang Township, Chiayi County, Douliou City, Taisi Township, Lunbei Township and Mailiao Township were 5. 51, 5.66, 6.20, 5.24, 8.55, 5.85, and 6.69 ppb, respectively.With the exception of Taisi Township, the levels of SO 2 in other six observed regions were slightly lower than the WHO standard of 7.00 ppb.This result was similar to that found for Wuhu City (6.54 ppb) and Bengbu City (7.88 ppb) (Wang et al., 2018b).Some studies have indicated that volcanic eruptions are the major contributors to the natural sources of SO 2 in the atmosphere (Andres and Kasdnoc et al., 1998;Halmer et al., 2002;Kato et al., 2016).Although there are several volcanoes in Taiwan, including Datunshan volcano, Keelung volcano, and Penghu volcano, these are all extinct volcanoes and will not increase atmospheric SO 2 concentration in the region.
The seasonal variations in atmospheric SO 2 are discussed as follows: In 2015, the average SO 2 concentrations were 4.71 ppb (Puzih City)-8.16 ppb (Taisi Township), 4.23 ppb (Lunbei Township)-6.69ppb (Taisi Township), 5.79 ppb (Singang Township)-13.07ppb (Taisi Township), 6.44 ppb (Douliou City)-11.7 ppb (Taisi Township) in spring, summer, fall, and winter, respectively.In 2016, these values were 5.52 ppb (Douliou City)-10.2ppb (Taisi Township), 4.05 ppb (Lunbei Township)-5.26ppb (Singang Township), 5.48 ppb (Puzih City)-9.06 ppb (Taisi Township), 5.18 ppb (Douliou City)-8.73 ppb (Taisi Township) in spring, summer, fall, and winter, respectively.In 2017, these values were 5.46 ppb (Douliou City)-8.29 ppb (Taisi Township), 3.91 ppb (Lunbei Township)-6.18ppb (Taisi Township), 5.13 ppb (Douliou City)-8.01 ppb (Taisi Township), 4.75 ppb (Douliou City)-7.79 ppb (Chiayi County) in spring, summer, fall and winter, respectively.This indicated that summer had the minimum SO 2 values, followed by spring, fall and winter, when similar levels were observed.Generally, the average SO 2 concentration of these seven areas in summer (4.86 ppb) was 26.1% lower than that in other seasons (6.40, 6.58, and 6.75 ppb in spring, fall and winter, respectively).This may be attributed to the fact that Taiwan has a monsoon climate with strong rainfall intensity in summer, so some of the SO 2 was scavenged from the ambient air through wet deposition (Chen et al., 2011).This result was compared with those found for Wuhu and Bengbu, where the SO 2 concentrations in winter were slightly higher than in the other seasons, and where without the requirement of fuel combustion for heating and less effect of polluted airflow from north China in the winter in Taiwan lead to this distinction.

NO 2 Concentration
Anthropogenic activities of industrial facilities and traffic emissions are the main sources of atmospheric nitrogen dioxide (Cheng et al., 2018).NO 2 participates in a complex photochemical reaction, where as the corresponding oxidation products are incorporated into ambient aerosols, they have adverse effects on visibility (Anttila et al., 2011;  He et al., 2014).NO 2 is also the crucial precursor of ozone and acid rain, which can acidize ecosystems and impact the human respiratory system (Bowman et al., 1994;Plaisance et al., 2002;Cox et al., 2003;An et al., 2006;Khokhar et al., 2016).Based on air quality guidelines established by WHO,    when evaluating the air quality index of a region, NO 2 is classified as a monitored air pollutant (He et al., 2014).Comparing the annual mean NO 2 concentration in the seven observed areas during the period 2015-2017, Douliou was the city with the highest levels of NO 2 , which were 24.3, 24.7, and 22.6 ppb in 2015, 2016, and 2017, for which the corresponding ranges were 14. 6-34.3 ppb, 14.5-35.5As a whole, during these recent three years (2015-2017), the average NO 2 concentration in Puzih City, Singang Township, Chiayi County, Douliou City, Taisi Township, Lunbei Township, and Mailiao Township were 18.0, 18.4, 22.9, 23.9, 18.3, 18.2 and 18.2 ppb, respectively.It can be seen that the NO 2 levels in Chiayi County and Douliou City were slightly higher than the WHO standard of 19.6 ppb and that the others were slightly lower than the WHO standard.The overall levels in the surveyed regions were comparable to those of Wuhu (21.4 ppb) and Bengbu (18.0 ppb) (Wang et al., 2018b).Fu et al. (2000) and Cheng et al. (2018) pointed out that due to recent economic development and rapid urbanization, motor vehicles have become the major emission source of NO 2 .The human population of Chiayi County (274,051) and Douliou City (108,231) is far above that of Puzih City (44,234), Singang Township (35,347), Taisi Township (25,116), Lunbei Township (28,003), and Mailiao Township (34,014), where it can be inferred that the former urbanization rate was higher than that occurring later.Thus, a larger number of vehicles are emitting NO 2 into the ambient air, which results in the higher NO 2 concentration in Chiayi County and Douliou City.
In 2015, the seasonal NO 2 concentrations were 18.4 ppb (Taisi Township)-24.9ppb (Douliou City), 12.3 ppb (Lunbei Township)-15.2ppb (Douliou City), 18.2 ppb (Singang Township)-25.0ppb (Douliou City), 22.2 ppb (Taisi Township)-32.2ppb (Douliou City) in spring, summer, fall, and winter, respectively.In 2016, these values were 19.0 ppb (Mailiao Township)-28.0ppb (Douliou City), 12.7 ppb (Chiayi County)-15.7 ppb (Singang Township), 17.0 ppb (Lunbei Township)-24.3ppb (Douliou City), 20.3 ppb (Taisi Township)-32.0ppb (Chiayi County) in spring, summer, fall, and winter, respectively.In 2017, these values were 17.9 ppb (Puzih City)-24.7 ppb (Douliou City), 12.6 ppb (Lunbei Township)-14.0ppb (Douliou City), 15.8 ppb (Puzih City)-24.2ppb (Douliou City), 17.8 ppb (Taisi Township)-28.2ppb (Chiayi County) in spring, summer, fall, and winter, respectively.As for the spatial distribution, Douliou City usually had with the maximum NO 2 levels, which may be tightly related to vehicle emissions.In terms of the temporal distribution, the minimum NO 2 concentration always occurred in summer, and the maximum values occurred in winter, with three-year average values in summer (13.7 ppb) that were 43.2% lower than those in winter (24.1 ppb), while the NO 2 levels for spring and fall were between those of summer and winter, and that of spring (21.1 ppb) was slightly higher than fall (19.9ppb).This seasonal distribution is attributed to distinct meteorological conditions in different seasons, where there is a hot summer and a cold winter.The high ground temperature in summer and the severe winter are conducive to the transport of NO 2 , and the abundant rainfall favors the deposition of NO 2 (Wang et al., 2018b).Thus, the NO 2 levels are lowest in summer and highest in winter.
The above results indicate that atmospheric NO 2 concentrations are determined by both emissions and meteorological conditions.Therefore, when implementing control of industrial and vehicle emissions, it is necessary to pay attention to restricting NO 2 concentrations during seasons without favorable weather conditions (Cheng et  al., 2018).

CO Concentration
During the combustion process, carbon monoxide (CO) is emitted along with NO x and volatile organic compounds (VOCs) and into the ambient air.CO can exist longer than one month and remain at high concentrations after being transported for a long distance (Kato et al., 2016).Therefore, CO is an important indicator of air pollution to understand the air quality of a region.During the period 2015-2017, the monthly average CO concentrations in Puzih City, Singang Township, and Chiayi County are given in Tables 2(a)-1, 2(b)-1, 2(c)-1, respectively, and the monthly average CO concentrations in Douliou City, Taisi Township, Lunbei Township and Mailiao Township are given in Tables 2(a)-2, 2(b)-2, 2(c)-2, respectively.
From 2015-2017, comparing the annual mean CO concentration among the seven districts, the highest levels of CO were found in Chiayi County, which were 0.514, 0.506, and 0.478 ppm in 2015, 2016, and 2017, for which the corresponding ranges were 0.295-0.793ppm, 0.307-0.797ppm, and 0.293-0.899ppm, respectively.Taisi was the Township with the lowest CO levels, which were 0.332, 0.324, and 0.296 ppm in 2015, 2016, and 2017, for which corresponding ranges were 0.151-0.611ppm, 0.168-0.567ppm, and 0.145-0.498ppm, respectively.Generally, it can be inferred that the CO levels of these studied locations were significantly lower than those in Wuhu (0.532-1.21 ppm, with an average of 0.831 ppm) and Bengbu (0.465-1.16 ppm, with an average of 0.720 ppm), two middle latitude cities in China.Furthermore, a comparison the annual average CO concentration of the seven areas during the three years under observation (2015)(2016)(2017) indicates that these values all decreased annually.The levels in 2015 reduced by 1.10-7.15%as compared with those observed in 2016 and went down by 5.62-10.8%from 2016 to 2017.As a whole, during the three years under observation (2015-2017), the mean CO concentrations in Puzih City, Singang Township, Chiayi County, Douliou City, Taisi Township, Lunbei Township, and Mailiao Township were 0.384, 0.413, 0.499, 0.437, 0.317, 0.377, and 0.374 ppm, respectively.The WHO air quality eight-hour average regulated standard of CO is 8.00 ppm, while the CO concentrations in these seven  0.328-0.6680.480 22.5 0.183-0.6200.373 29.8 0.259-0.7510.446 24.9 0.220-0.6590.427 25.8 Annual 0.264-0.6000.397 20.9 0.145-0.4980.296 31.1 0.200-0.5490.353 25.5 0.192-0.5660.355 26.9 investigated regions were all significantly lower than WHO standard, which demonstrated that atmospheric CO does not have a serious impact on the air quality in these areas.
As for variations in atmospheric CO concentration by season, in 2015, these values were 0.316 ppm (Taisi Township)-0.493ppm (Chiayi County), 0.185 ppm (Taisi Township)-0.330ppm (Douliou City), 0.384 ppm (Taisi Township)-0.546ppm (Chiayi County), 0.443 ppm (Taisi Township)-0.733ppm (Chiayi County) in spring, summer, fall, and winter, respectively.In 2016, the values were 0.381 ppm (Taisi Township)-0.537ppm (Chiayi County), 0.201 ppm (Taisi Township)-0.305ppm (Douliou City), 0.324 ppm (Taisi Township)-0.506ppm (Chiayi County), 0.392 ppm (Taisi Township)-0.692ppm (Chiayi County) in spring, summer, fall, and winter, respectively.In 2017, the values were 0.361 ppm (Taisi Township)-0.523ppm (Chiayi County), 0.176 ppm (Taisi Township)-0.301ppm (Chiayi County), 0.280 ppm (Taisi Township)-0.468ppm (Chiayi County), 0.366 ppm (Taisi Township)-0.618ppm (Chiayi County) in spring, summer, fall, and winter, respectively.Generally, it can be seen that the hot summer had the minimum values and the cold winter had the maximum values, while spring and fall had levels in the middle, which indicated that the CO values in spring were slightly higher than those in fall, especially in 2016 and 2017.The average CO concentrations in the seven researched areas in spring were all higher than those in fall.Wang et  al. (2018b) reported the same result for Wuhu (0.867, 0. 651, 0.770, with 1.04 ppm in spring, summer, fall, and winter, respectively) and for Bengbu (0.723, 0.513, 0.670, and 0.960 ppm in spring, summer, fall, and winter, respectively).The seasonal variation may be a result of the distinct meteorological conditions occurring in different seasons, where a higher air temperature is conducive to the long-range transport of CO in the summer (Kato et al. 2016), and the lower ground air temperature in the winter can lead to an accumulation of CO, which concurs with the seasonal distribution of NO 2 .

O 3 Concentration
Ozone (O 3 ) is a secondary source of pollution that is generated through natural or anthropogenic photochemical reactions of oxides of nitrogen (NO x ), volatile organic compounds (VOCs), carbon monoxide (CO), methane (CH 4 ), and nonmethane hydrocarbons (NMHCs) (Seinfeld and Pandis, 1998;Tu et al., 2007;Lan et al., 2015;Wu et al., 2015;Gong et al., 2018).Atmospheric NO x and VOCs play a dramatic role in the formation of O 3 in polluted atmospheres, especially unsaturated VOCs (Wang et al., 2018b).From the Air Quality Guidelines of WHO 2005 , the simplified photochemical reactions can be summarized as follows: Nitrogen dioxide dissociates to form nitric oxide (NO) and an 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: The intensity of the sun and the concentration of NO x and VOCs are all key factors that influence ambient ozone concentrations.Some studies have also indicated that the ratio of VOCs to NO x also has an effect on the O 3 formation mechanism.These studies reported that ratios in a range of 4:1 to10:1 are most favorable for formation of O 3 .Furthermore, meteorological factors, including air temperature, humidity, and wind speed, play an important role in the process of dispersion, transport, and dilution of O 3 (Seaman, 2000; Tu et al., 2007; Jin et al., 2013; Cheng  et al., 2015).
Tropospheric O 3 is a crucial greenhouse gas and has a direct adverse impact on ecosystems and human health (Solomon et al., 2000; Silva et al., 2013; Lelieveld et al.,  2015; Kato et al., 2016).With the impact of anthropogenic activities, including industrial emissions and transportation and meteorological conditions, the dramatic increase O 3 has made it a concern, and it has become an important criteria air pollutant.From 2015 to 2017, the monthly average O 3 concentration in Puzih City, Singang Township, and Chiayi County are plotted in Figs.4(a In 2015, the highest annual mean O 3 level among the seven cities and townships was found in Taisi Township (50.3 ppb), and the lowest value was in Lunbei Township (42.2 ppb), which ranged from 29.4-63.3ppb and 27.8-55.9ppb, respectively.In 2016, the highest annual mean O 3 level among the seven cities and townships was found    and Mailiao Township were 47.7,47.5,48.0,50.0,48.7,45.0 and 46.1 ppb, respectively.The average O 3 levels in Lunbei Township and Mailiao Township met the WHO standard of 46.6 ppb, but the others were all slightly higher than WHO standard and were also higher than in Wuhu City (35.9 ppb) and Bengbu City (44.7 ppb) (Wang et al., 2018b).This result can be attributed to the sunshine intensity.The latitude of Taiwan is significantly lower than that of Wuhu and Bengbu, and with a higher solar altitude angle, low latitude areas have stronger UV radiation, which leads to the formation of O 3 .
As for the seasonal variations in O 3 , the average O 3 concentrations were 38.8 ppb (Puzih City)-51.6ppb (Taisi Township), 32.5 ppb (Mailiao Township)-40.7 ppb (Chiayi County), 52.2 ppb (Lunbei Township)-63.3ppb (Chiayi County), 40.3 ppb (Lunbei Township)-51.6ppb (Taisi Township) in spring, summer, fall, and winter, respectively, in 2015.In 2016, these values were 47.3 ppb (Chiayi County)-52.2 ppb (Taisi Township), 38.3 ppb (Mailiao Township)-46.2ppb (Douliou City), 47.3 ppb (Mailiao Township)-53.9ppb (Douliou City), 38.9 ppb (Lunbei Township)-48.4ppb (Taisi Township) in spring, summer, fall, and winter, respectively.In 2017, these values were 55.0 ppb (Chiayi County)-60.1 ppb (Puzih City), 33.6 ppb (Mailiao Township)-43.5 ppb (Douliou City), 47.6 ppb (Mailiao Township)-58.8ppb (Douliou City), 44.1 ppb (Chiayi County)-50.1 ppb (Taisi Township) in spring, summer, fall, and winter, respectively.Overall, the seasonal O 3 concentration exhibited different patterns in the three years under observation.Firstly, all the observed cities achieved their maximum levels in fall and their minimum in summer, while spring and winter exhibited intermediate levels in 2015.Previous studies reported there to be a negative correlation between the atmospheric relative humidity and O 3 concentration (Kato et al., 2016; Li et al., 2017b; Gong  et al., 2018; Wang et al., 2018b), where during a dry fall season, the lower relative humidity results in higher O 3 levels than those in summer and spring, and the stronger sun radiation leads to higher O 3 levels than those in winter.During the summer, the high atmospheric relative humidity and low precursor concentrations are not favorable to the formation of O 3 .Thus, the summer in these observed areas exhibited the lowest O 3 levels.Secondly, as for 2016, the maximum O 3 levels typically occurred in spring or fall, and the minimum O 3 levels typically occurred in summer or winter.This may be attributed to the fact that during spring and fall, the UV radiation strength in the monitored areas is stronger than in winter, and the concentrations of precursors (including NO x and VOCs) are higher than in summer in monitored areas, so the strong UV radiation and increasing precursors enhance the production of O 3 .Thirdly, as seen in 2017, the O 3 concentrations in both spring and fall were at high levels, and those in spring were higher than those in fall, followed by winter, with the lowest in summer.This may be analyzed from the perspective of air temperature, sunshine intensity, and precursor concentrations.The temperature and sun intensity in spring and fall were higher than those in winter, which directly increases O 3 production rates and thus leads to higher O 3 levels.While high temperatures and wind speeds in summer are favorable to the dispersion and dilution of air pollutants, a significant lack of precursors will hinder the formation of O 3 .Therefore, the summer had the minimum O 3 level among the four seasons.The differences in seasonal ozone distribution over the three years under observation may have been related to the different contributions of various influencing factors at different ozone levels, including the concentration of NO x and VOCs, air temperature, humidity, and wind speed.

AQI Analysis
The Air Quality Index (AQI) is a useful index for evaluating daily air quality and its health effects, which can be used to inform the government and the public about the current air quality and thus help them take proper health protection measures when necessary.From 2015-2017, the day fractions of the six AQI categories in different seasons for Puzih City, Singang Township, Chiayi County, Douliou City, Taisi Township, Lunbei Township, and Mailiao Township are plotted in Figs.5(a)-1 to 5(g)-3, respectively, and the corresponding cumulative number of days of primary pollutants are shown in Tables 3(a)-3(g).
In general, PM 2.5 was the most frequent primary air pollutant during the three year study period, but the levels declined annually.This was followed by O 3 , where the days with this gaseous species as the primary air pollutant increased in various AQI classes.In 2016, Grade V levels, with O 3 as the primary air pollutant, was observed in all seven areas, which means that the impact of O 3 on spring pollution is increasing.Among the other four air pollutants, PM 10 was an occasional primary pollutant, and none of the other pollutants were primary pollutants in spring in these regions.
PM 2.5 was still the most important primary air pollutant in fall, followed by O 3 , and the number days it was the primary pollutant was significantly higher in fall than in spring and summer.It is worth noting that the number of days in which PM 10 was the primary air pollutant at of Grade II and III was also significantly higher than in spring and summer.This means PM 10 had a more significant influence on autumn pollution.
Generally, PM 2.5 was the most important primary air pollutant, followed by PM 10 and O 3 occasionally being the primary air pollutants in winter.This indicates that particulate matter played a decisive role in the winter AQI in these seven districts.Moreover, the most frequent primary pollutant lead to a Grade IV was PM 2.5 in all the study areas, which means PM 2.5 has a huge impact on unhealthy winter air quality.
Similar to the results of previous studies (Shen et al., 2017;Wang et al., 2018b), when observing the seasonal variations in air quality in these areas, summer had the best air quality, with a marked Good (Grade I) condition, and winter had the worst, with typically Moderate (Grade II) and Unhealthy for Sensitive Groups (Grade III) rankings, while the spring was similar to fall at the Moderate (Grade II) level.As for the primary air pollutants, PM 2.5 was the typical primary air pollutant in all four seasons, while the frequency of O 3 as the primary air pollutant rose annually in spring, and the fraction of O 3 as the primary air pollutant in summer was higher than that in spring.It should be noted that O 3 had a significant impact on the occurrence of pollution, which were Unhealthy (Grade V) in spring and Unhealthy for Sensitive Groups in summer (Grade IV).The number of days of both O 3 and PM 10 as the primary pollutant in autumn was significantly higher than that in spring and summer.Finally, in winter, PM 2.5 was the most important primary air pollutant, followed by PM 10 , and the frequency of O 3 as the primary air pollutant was extremely low.

CONCLUSION
The results of this study on air quality index in southwestern Taiwan can be summarized as follows: 1.The PM 2.5 concentrations fluctuate significantly in the study areas, where the focal three-year mean PM 2.5 concentrations in Puzih City, Singang Township, Chiayi County, Douliou City, Taisi Township, Lunbei Township and Mailiao Township were 24.1, 24.9, 27.2, 27.5, 25.0, 29.2 and 25.7 µg m -3 , respectively.It can be seen that Lunbei Township had the PM 2.5 highest values, and Puzih City had the lowest.The seasonal characteristic of PM 2.5 generally are in the following order: winter > spring > fall > summer.2. The three-year mean PM 10 concentrations in Puzih City, Singang Township, Chiayi County, Douliou City, Taisi Township, Lunbei Township, and Mailiao Township were 65, 58, 57, 52, 47, 54 and 70 µg m -3 , respectively, which were lower than those in Wuhu (82 µg m -3 ) and Bengbu (95 µg m -3 ).As a whole, of these seven observed districts, the average PM 10 values in summer (36 µg m -3 ) were 52.0%lower than those in winter (75 µg m -3 ), and spring (60 µg m -3 ) had the similar levels to those observed in fall (61 µg m -3 ). 3. From 2015-2017, Taisi Township always had the highest concentration of SO 2 , and Douliou City had the lowest values, and the three-year average SO 2 concentrations in Puzih City, Singang Township, Chiayi County, Douliou City, Taisi Township, Lunbei Township, and Mailiao Township were 5. 51, 5.66, 6.20, 5.24, 8.55, 5.85 and 6.69 ppb, respectively.In summer (4.86 ppb) the minimum SO 2 levels were observed, which were 26.1% lower than that found for the other seasons (6.40, 6.58 and 6.75 ppb in spring, fall and winter, respectively).4. The average NO 2 concentration in Puzih City, Singang Township, Chiayi County, Douliou City, Taisi Township, Lunbei Township, and Mailiao Township were 18.0, 18.4, 22.9, 23.9, 18.3, 18.2 and 18.2 ppb, respectively, during the three-year study period.These values were comparable to those of Wuhu (21.4 ppb) and Bengbu (18.0 ppb).In terms of seasonal variations, Douliou City usually had the maximum NO 2 levels in terms of spatial distribution, while summer always had the minimum values.Winter had the maximum values in terms of temporal distribution, for which the three-year average values in summer (13.7 ppb) were 43.2% lower than those in winter (24.1 ppb). 5.The mean CO concentrations in the three year study period in Puzih City, Singang Township, Chiayi County, Douliou City, Taisi Township, Lunbei Township, and Mailiao Township were 0.384, 0.413, 0.499, 0.437, 0.317, 0.377 and 0.374 ppm, respectively, which were significantly lower than the WHO standard (8.00 ppm), while the seasonal CO characteristics generally occurred in the following order: winter > spring > fall > summer.6.The O 3 concentration in all observed cities rose by 0.43-7.93%.This increasing tendency poses a serious challenge in terms of air quality.The three-year average O 3 concentrations in Puzih City, Singang Township, Chiayi County, Douliou City, Taisi Township, Lunbei Township, and Mailiao Township were 47.7, 47.5, 48.0, 50.0, 48.7, 45.0 and 46.1 ppb, respectively.The O 3 concentrations in most of these places were slightly higher than the WHO standard (46.6 ppb).7. Due to meteorological conditions, there were three O 3 seasonal distribution patterns in the study period.Firstly, all the observed cities had their maximum levels in fall and their minimum levels in summer, while spring and winter were at intermediate levels in 2015.Secondly, the maximum O 3 level normally occurred in spring or fall, and the minimum O 3 level normally occurred in summer or winter in 2016.Thirdly, the O 3 concentrations in spring and fall were both at high levels and those in spring were higher than those in fall, followed by winter, with the lowest in summer in 2017.8.The percentages of air quality at the Moderate level were greatest in spring in the study areas, after which they gradually deteriorated.PM 2.5 was the most frequent primary pollutant, followed by O 3 , which increased in different AQI classes.PM 10 was an occasional primary pollutant.9.The air quality exhibited a marked Good (Grade I) condition in the summer in the study areas.PM 2.5 was the most frequent primary air pollutant, and the fraction of O 3 as the primary air pollutant was significantly higher in summer than in spring.Furthermore, O 3 had a greater impact on unhealthy summer conditions.10.The air quality mostly remained at Moderate levels in the fall in the study areas, and Unhealthy conditions were the most serious in Duliou City and the least serious in Mailiao Township.PM 2.5 was still the most important primary air pollutant in fall, followed by O 3 , and the number of days of PM 10 as the primary air pollutant significantly increased in the fall as compared with spring and summer.11.Some of the studied districts had Good conditions in winter, but Moderate and Unhealthy for Sensitive Groups occurred frequently.PM 2.5 was the most important primary air pollutant, followed by PM 10 and occasionally O 3 as the primary air pollutants, and particulate matter played a decisive role in the winter AQI in these seven districts.12.The results of this study provide useful information for the establishment of air pollution control strategies and for future studies conducted by the scientific community.
the six AQI classes but had the lowest proportion in 2016, when the fraction of Grade II increased obviously.Grade III had the highest proportion in 2016, but the total fractions were relatively low in the different AQI classes.The levels of Grade IV, V, and VI, with the exception of Grade IV, had fractions from 1%-2% in 2016, and the other classes remained constant at 0% over the study period.Therefore, the air quality exhibited a marked Good (Grade I) condition, and the lowest Good level and highest Unhealthy for Sensitive Groups levels reflected the worst air quality in 2016 during the study period in the summer among all seven districts.
ppb, and 13.2-30.1 ppb, respectively.The lowest levels were Jan.Feb.Mar.Apr.May June July Aug. Sep.Oct. Nov. Dec.

Table 3 (
a). Cumulative number of days of major pollutants for Puzih City from 2015-2017.PM 10 SO 2 O 3 PM 2.5 PM 10 SO 2 O 3 PM 2.5 PM 10 SO 2 O 3 PM 2.5 PM 10 SO 2 O 3

Table 3 (
b). Cumulative number of days of major pollutants for Singang Township from 2015-2017.

Table 3 (c).
Cumulative number of days of major pollutants for Chiayi County from 2015-2017.

Table 3 (
d). Cumulative number of days of major pollutants for Douliou City from 2015-2017.PM 10 SO 2 O 3 PM 2.5 PM 10 SO 2 O 3 PM 2.5 PM 10 SO 2 O 3 PM 2.5 PM 10 SO 2 O 3

Table 3 (
e). Cumulative number of days of major pollutants for Taisi Township from 2015-2017.

Table 3 (f).
Cumulative number of days of major pollutants for Lunbei Township from 2015-2017.

Table 3 (
g). Cumulative number of days of major pollutants for Mailiao Township from 2015-2017.PM 10 SO 2 O 3 PM 2.5 PM 10 SO 2 O 3 PM 2.5 PM 10 SO 2 O 3 PM 2.5 PM 10 SO 2 O 3