Effect of PM2.5 on AQI in Taiwan

Transcription

1 Environmental Modelling & Software 17 (2002) Effect of PM2.5 on AQI in Taiwan Chung-Ming Liu * Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan Abstract US EPA has included sub-indices of 8-hour average O 3 concentration and 24-hour average PM 2.5 level into a newly-modified Air Quality Index (AQI). Calculations at air-quality monitoring stations in Taiwan show that the updated AQI system is associated with two to three times higher occurring frequency of days with AQI larger than 100 than that of the widely used Pollution Standard Index (PSI) larger than 100. On these unhealthy days, more than 50% are dominated by PM 2.5 sub-index, followed by 8-hour average O 3 sub-indices. By analyzing the PM 2.5 data measured at four different stations, we note that PM2.5 tends to occupy more than 50% of the PM 10 level. These results indicate that the importance of fine particles in urban smog has been magnified in the AQI system. With a new category between , AQI does pose as a better descriptor than PSI to warn of an unhealthy environment for the sensitive group Elsevier Science Ltd. All rights reserved. Keywords: Air quality index; PM Introduction The air quality index is a useful index for reporting daily air quality. It informs the public about the current air quality in respect to its health effects. In Taiwan, the local Environmental Pollution Administration (EPA) has followed the US EPA to establish the Pollutant Standards Index (PSI), which includes sub-indices for O 3, PM 10 (particulates with diameter smaller than 10 µm), CO, SO 2 and NO 2. These sub-indices relate ambient pollutant concentrations to index values on a scale from 0 through 500. The index is normalized across pollutants by defining an index value of 100 as the numerical level of the 24-hours pollutant standard and an index value of 500 as the significant harm level. Table 1 outlines the breakpoints for pollutants sub-indices. These index values are divided into five categories with each serving to convey the health effect of the air quality. In each day, the pollutant with the largest sub-index value is taken as the daily major pollutant while its sub-index value assumes the daily representative PSI. With this PSI system, the local EPA has set up a task of improving the air quality to the status that the occurring frequency of days with PSI larger than 100 will be down to 3% in * Tel.: ; fax: address: liucm@ccms.ntu.edu.tw (C.-M. Liu) By the end of 1999, the percentage was 5.13%, that is down from 8.18% in 1993 and 6.59% in 1994 (EPA, 2000). In the USA, the national ambient air quality standards (NAAQS) were modified for O 3 and PM in 1997, based on the scientific studies that have linked ground-level ozone and particulate matter, especially fine particles (alone or in combination with other air pollutants), with a series of significant health problems, including: premature death (Schwartz et al., 1996); respiratory related hospital admissions and emergency room visits (Thurston et al., 1992); aggravated asthma (Anderson et al., 1992); acute respiratory symptoms, including aggravated coughing and difficult or painful breathing; chronic bronchitis; decreased lung function that can be experienced as shortness of breath; etc. The 1-hour O 3 NAAQS was replaced with an 8-hour O 3 NAAQS and supplemented the PM NAAQS with 24-hour and annual standards for fine particulate matter (PM 2.5, particulates with diameter smaller than 2.5 µm). Further, the US EPA adopted the Air Quality Index (AQI) for daily air quality reporting to the general public in 1999 (USEPA, 1999). The AQI is modified from the previous PSI to include sub-indices for 8-hours O 3 and 24-hour PM 2.5 and exclude the sub-index for NO 2. Also, it includes a new category described as unhealthy for sensitive groups for index values between (Table 2). In Taiwan, a steady decrease of PSI has been /02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S (01)

2 30 C.-M. Liu / Environmental Modelling & Software 17 (2002) Table 1 Breakpoints for pollutant sub-indices of PSI PSI value O 3 1-h (ppb) PM h (µg/m 3 ) SO 2 24-h (ppb) CO 8-h (ppm) NO 2 1-h (ppb) Category Good Moderate Unhealthy Very unhealthy Hazardous Table 2 Breakpoints for pollutant sub-indices of AQI O 3 PM 10 PM 2.5 SO 2 CO AQI value 1-h (ppb) 8-h (ppb) 24-h (µg/m 3 ) 24-h (µg/m 3 ) 24-h (ppb) 8-h (ppm) Category Good Moderate ( ) Unhealthy for sensitive groups; ( ) unhealthy Very unhealthy Hazardous accompanied with a general increase of the ozone concentration and a steady decrease of the PM 10 level. Hence, it will be a useful task to analyze the usage of AQI in replacement of PSI, and to understand the effect of ozone and fine particulate matter on the general air quality. Currently, five air-quality monitoring stations are monitoring the PM 2.5 concentration level, but one of them is to monitor air quality near traffic interaction. In this paper, an empirical relationship between the 24-hour mean of PM 2.5 and PM 10 is established based on the four ambient stations data, which is then applied in the calculation of the daily AQI at all island-wide fifty-seven ambient stations during the period of Analyses are done based on these datasets. 2. Empirical relationship between PM 2.5 and PM 10 The four air-quality monitoring stations that have been monitoring PM 2.5 and PM 10 simultaneously since July 1997 are located at KuTin, ChungMing, FonSam and LinYuan (shown in Fig. 1). At these sites, a positive correlation exists between these two datasets with the correlation coefficient being around Higher concentration levels occur during September April when the northeasterly prevails over Taiwan. This phenomenon appears most clearly at stations in southern Taiwan where a stagnant airflow with low air moisture, both favorites for enhancing the level of particulate matters, appears due to the blocking of the northeasterly by the Fig. 1. Ambient air-quality monitoring stations in Taiwan. Central Mountain Range. The annual-mean 24-h PM 10 levels in the southern area are about µg/m 3, which are much larger than those in the northern area, i.e. about µg/m 3 at KuTin. The ratios between 24-h PM 2.5 and 24-h PM 10 at these four stations are in general larger than 0.5 and have a mean value about 0.6 (Fig. 2). Such results are similar to those obtained locally by Mao (1997) and Yang (1997) etc., and indicate that fine particles released from

3 C.-M. Liu / Environmental Modelling & Software 17 (2002) Fig. 2. The occurring frequency of the ratio between PM 2.5 and PM 10 during July 1997 December 1999 at (a) KuTin, (b) ChungMing, (c) Fonsam and (d) LinYuan. fuel combustion from motor vehicles, power generation, and industrial facilities, and formed in the atmosphere by the transformation of gaseous emissions such as SO 2, NO x, and VOCs, are crucial to the accumulation of particles less than 10 micrometers in diameter in urban areas. Since Taiwan has the world second-highest population density, and has been developed quite extensively island-wide, it is reasonable to assume that all ambient air-quality monitoring stations bear similar characteristics between PM 2.5 and PM 10. To obtain an empirical relationship between 24-h PM 2.5 and 24-h PM 10, we have tried three approaches. Firstly, the long-term mean of the daily ratio between these two values is calculated. Secondly, linear regression between these two datasets is exercised. Lastly, the intercept is set to be zero when regression analysis is applied. The results are as follows: In northern Taiwan, the three empirical relationships at KuTin are: (1) PM 2.5 =0.60 PM 10, rms (root mean square error)=8.4; (2) PM 2.5 =0.49 PM 10 rms=8.1; +4.48, (3) PM 2.5 =0.57 PM 10, rms=8.3 (used in this study). In central Taiwan, the three empirical relationships at ChungMing are: (1) PM 2.5 =0.59 PM 10, rms=12.3; (2) PM 2.5 =0.37 PM 10 rms=8.2; , (3) PM 2.5 =0.49 PM 10, rms=9.3 (used in this study). In southern Taiwan, the three empirical relationships at FonSam and LinYuan are:

4 32 C.-M. Liu / Environmental Modelling & Software 17 (2002) (1) PM 2.5 =0.60 PM 10, rms=14.3; (2) PM 2.5 =0.49 PM 10 rms=13.7; +7.92, (3) PM 2.5 =0.56 PM 10, rms=14.4 (used in this study). Further analyses show that the usage of the mean ratio tends to overestimate the 24-h PM 2.5 level when the 24- hpm 10 level is high. Meanwhile, the linear equation obtained after regression analysis gives an estimate of 24-h PM 2.5 with a least error, but with an unreasonable high level when the concentration of 24-h PM 10 is near to zero. Therefore, in this paper we have decided to use the empirical equation obtained by the third approach to estimate the level of 24-h PM 2.5 at all ambient air-quality monitoring stations. 3. Differences between PSI and AQI Three questions are most interesting to us. Is the occurring frequency of the air quality index larger than 100 in each year different between the PSI and AQI system? Is the dominant pollutant different considerably between the PSI and the AQI system? Is the long-term trend changed when switching the PSI system to AQI? Selecting the data at LinYuan for a preliminary analysis, the comparison of Fig. 3(a) with Fig. 3(b) shows that the occurring frequency of PSI larger than 100 has been decreasing from 27 33% during to 10 17% during , while a similar decreasing trend is maintained when the AQI is applied but with a much larger frequency, i.e % during and 23 33% during Days that are very unhealthy (i.e. index 200) to near-by residents appear with the AQI system. The dominant pollutant that causes the PSI to be larger than 100 has been primarily the PM 10 during and then both the 1-h ozone and 24-h PM 10 during (Fig. 3(c)). The signal of ozone pollution associated with the increasing emission of precursors from moving vehicles is clearly getting stronger. In the AQI system, we note that the PM 2.5 dominates during and then the 8-h ozone during (Fig. 3(d)). It is clear that the decreasing trend of PSI is preserved in the AQI, while the switching of dominant pollutants from fine particles to ozone has been most clearly presented by the AQI system. Also, it is noted that the 8-h ozone standard is more stringent than the Fig. 3. At LinYuan, the occurring frequency of (a) PSI and (b) AQI larger than 100, and the dominant pollutants associated with the (c) PSI and (d) AQI systems.

5 C.-M. Liu / Environmental Modelling & Software 17 (2002) h ozone standard as the latter has less opportunity to be dominant. However, at KuTin, which is located in the northern area, a different situation appears. The occurring frequency of PSI larger than 100 has been low during , i.e. below 5% (Fig. 4(a)), but it has changed to show a significant increasing trend from 7% in 1994 to 34% in 1999 when the AQI is applied (Fig. 4(b)). Days characterized with very unhealthy air quality also appear. The dominant pollutant is 1-h ozone for PSI (Fig. 4(c)), but it has changed to PM 2.5 for AQI (Fig. 4(d)). It is clear that photochemical products such as ozone and PM 2.5 are the primary pollutants in metropolitan Taipei. But fine particles are more dominant than ozone in affecting residents health and can affect the long-term trend of AQI to be different from that of PSI. 4. Spatial distribution Since we are more concerned with the air quality at a larger area rather than at a specific site, the spatial distribution of the occurring frequency of PSI and AQI larger than 100 in each year during has been plotted and is shown in Fig. 5. With the PSI system, the area with the occurring frequency larger than 5% appears along the western coast from central Taiwan to the southern region. Pollution is most serious over the southern area, where the occurring frequency larger than 25% dominates during After 1997, the area with the occurring frequency larger than 5% shrinks visibly. In comparison, with the AQI system, the area with the occurring frequency larger than 5% appears from northern to southern regions along the western coast, though it shrinks in 1997 and Major pollution is still over the southern area where the occurring frequency larger than 45% dominates. Clearly, the AQI system maintains a similar spatial and temporal variation of the air pollution status as shown with the PSI system, but has a much larger chance to observe the index being larger than 100 and hence a larger area with the occurring frequency larger than 5%. To find out the major pollutant, the spatial distribution of the occurring frequency of PM 10, 1-h O 3,PM 2.5 and 8-h O 3 as the dominant pollutant when PSI or AQI is larger than 100, are plotted in Fig. 6 for year 1994 and in Fig. 7 for year With the PSI system, PM 10 appears to be the dominant pollutant at most monitoring Fig. 4. At KuTin, the occurring frequency of (a) PSI and (b) AQI larger than 100, and the dominant pollutants associated with the (c) PSI and (d) AQI systems.

6 34 C.-M. Liu / Environmental Modelling & Software 17 (2002) Fig. 5. Spatial distribution of the occurring frequency of PSI and AQI larger than 100 during Black marks in each figure are the air quality monitoring sites. stations in 1994, while 1-h O 3 dominates over the northern area. Other pollutants such as CO and SO 2 have hardly been causing any pollution episode. Then in 1999, the role of PM 10 has become less important, while 1-h O 3 is getting more dominant over northern, middle and southern areas. It appears that the anthropogenic emission of ozone precursors by the increasing number of automobiles is becoming the major pollution source in the Taiwan region. In comparison, with the AQI system, PM 2.5 appears to be the dominant pollutant at most monitoring stations in 1994, while 8-h O 3 is ranked second with a higher frequency over the northeastern region. Then in 1999, the role of PM 2.5 became less important along the western coast but became dominant over the eastern coast, while the occurring frequency of 8-h O 3 as the dominant pollutant increased over the western coast from middle Taiwan to the southern area. With the AQI system, PM 10 or 1-h O 3 has a minor chance to be a dominant pollutant. Furthermore, we have organized the data into six different areas of Taiwan (Fig. 1). In the northern area (area I), the occurring frequency of PSI larger than 100 has been below 4% during , with 1-h ozone being the dominant pollutant (Table 3). With the AQI system, the frequency has been doubled and a moderate decreasing trend is maintained, while PM 2.5 and ozone are dominant pollutants. In the northwestern area (area II), the occurring frequency of PSI larger than 100 has been below 2% during , with 1-h ozone and PM 10 both the dominant pollutants. With the AQI system, the frequency has been tripled and a moderate decreasing trend is maintained, while PM 2.5 and 8-h ozone are dominant pollutants. In the central area (area III), the occurring frequency of PSI larger than 100 has been below 5% during , with PM 10 being the dominant pollutant. With the AQI system, the frequency has been more than doubled and a moderate decreasing trend is maintained, while PM 2.5 and 8-h ozone are dominant pollutants. In the southwestern area (area IV), the occurring frequency of PSI larger than 100 has been below 6% during , with PM 10 being the dominant pollutant. With the AQI system, the frequency has been tripled and there is no obvious decreasing or increasing trend, while PM 2.5 is the dominant pollutant. In the southern area (area V), the occurring frequency of PSI larger than 100 has been below 17% during ,

7 C.-M. Liu / Environmental Modelling & Software 17 (2002) Fig. 6. (a) Spatial distribution of the occurring frequency of PM10 and 1-h O 3 as the dominant pollutant when PSI is larger than 100, and (b) spatial distribution of the occurring frequency of PM10, PM2.5, 1- ho 3 and 8-h O 3 as the dominant pollutant when AQI is larger than 100, in Fig. 7. (a) Spatial distribution of the occurring frequency of PM10 and 1-h O 3 as the dominant pollutant when PSI is larger than 100, and (b) spatial distribution of the occurring frequency of PM10, PM2.5, 1- ho 3 and 8-h O 3 as the dominant pollutant when AQI is larger than 100, in Discussion and conclusion with PM 10 and 1-h ozone both the dominant pollutants. With the AQI system, the frequency has been more than doubled and a moderate decreasing trend is maintained, while PM 2.5 is the dominant pollutant during and then 8-h ozone and PM 2.5 are both dominant. As to the eastern area (area VI), the occurring frequency of PSI larger than 100 has been around 0.2% during , with PM 10 and 1-h ozone both the dominant pollutants. With the AQI system, the frequency has been doubled and there is no obvious decreasing or increasing trend, while PM 2.5 and 8-h ozone are both dominant pollutants. In all, the best air quality appears in the eastern area, then the northwestern area. The worst air quality is in the southern area, then the southwestern area. For both PSI and AQI systems, a moderate decreasing trend is maintained, but the occurring frequency of the index value larger than 100 has been at least doubled or tripled when the PSI system is shifted to AQI. With the PSI system, 1-h ozone is the dominant pollutant in the northern and northwestern areas, while PM 10 is the dominant pollutant in the southern and southwestern areas. But with the AQI system, both fine particle and 8-h ozone are dominant pollutants in most areas. According to EIIP (1999), particles with aerodynamic diameter smaller than 2.5 µm have three primary origins: primary solid particulate matter that is emitted directly in the solid phase such as from the combustion of fossil fuels or biomass, primary condensable particulate that can be emitted at high temperature in the gas phase but which condenses into the solid phase upon dilution and cooling in the plume, and secondary particulate that is formed through atmospheric reactions of gaseous SO 2 and NO precursor emissions. Wang et al. (2000) analyze data collected in Taipei and find that on average, fine particles are composed of elemental carbon (9% by mass), metal elements (10%), water (20%), secondary organics (16%), sulphate (16%), nitrate (3%), other water-soluable ions (9%) and others (17%). High water content is associated with the humid environment in Taiwan. Wang et al. (2000) suggests that these particles originate from secondary photochemical reactions and dusts. Even though there are no other similar studies in other parts of Taiwan, it is quite reasonable to assume that secondary photochemical reactions and dusts are the main origins of fine particles in Taiwan. The question is: which source contributes more than the other. In Taipei, where the traffic emission is the main pollutant source,

8 36 C.-M. Liu / Environmental Modelling & Software 17 (2002) Table 3 The occurring frequency of PSI and AQI larger than 100 and that of the dominant pollutants in area I VI during PSI 100 AQI 100 Area Year PM10 O 3-1h PM10 O 3-1h O 3-8h PM2.5 I % 43% 58% 8% 0% 18% 9% 73% % 41% 59% 8% 0% 19% 16% 65% % 19% 81% 6% 0% 40% 12% 48% % 16% 84% 7% 0% 25% 24% 50% % 11% 89% 7% 0% 33% 13% 54% % 6% 94% 7% 0% 29% 20% 51% II % 57% 43% 6% 0% 5% 25% 70% % 61% 39% 5% 0% 5% 14% 81% % 26% 74% 5% 0% 9% 45% 46% % 37% 63% 4% 0% 3% 29% 68% % 68% 32% 3% 0% 1% 19% 79% % 11% 89% 5% 0% 8% 58% 34% III % 81% 19% 12% 0% 2% 27% 71% % 78% 22% 7% 0% 3% 24% 74% % 63% 37% 10% 0% 3% 49% 48% % 64% 36% 9% 4% 5% 41% 50% % 65% 35% 6% 5% 4% 37% 54% % 78% 22% 8% 3% 2% 34% 61% IV % 87% 13% 17% 0% 1% 24% 76% % 78% 21% 14% 0% 1% 17% 82% % 86% 14% 18% 0% 2% 13% 85% % 61% 39% 18% 0% 2% 33% 65% % 51% 49% 14% 0% 3% 32% 65% % 60% 40% 17% 0% 2% 29% 69% V % 82% 18% 36% 0% 4% 9% 86% % 77% 23% 36% 0% 4% 11% 85% % 64% 36% 34% 0% 4% 21% 75% % 44% 56% 31% 1% 6% 34% 59% % 45% 55% 28% 4% 4% 39% 53% % 48% 52% 29% 4% 5% 39% 52% VI % 100% 0% 0.4% 0% 0% 17% 83% % 0% 100% 0.5% 0% 29% 14% 57% % 100% 0% 0.4% 0% 0% 0% 100% % 0% 100% 0.1% 0% 0% 100% 0% % 33% 67% 0.5% 0% 29% 14% 57% % 67% 33% 0.5% 0% 13% 13% 75% and both the ozone and PM 2.5 concentration level is increasing, the majority of fine particles are from secondary pollutants. In other major cities in the North (and the South), it may be reasonable to suggest that the majority of particles also come from the secondary pollutants where ozone is a significant problem. This may not be true for the Central and Southwestern areas where particles are most dominant. Figure 8(a) illustrates a moderate decreasing trend of the occurring frequency of PSI larger than 100 for the whole Taiwan region, which is clearly maintained in Fig. 8(b) for the AQI system, except that the frequency has increased by about times from that of the PSI system. In 1999, the occurring frequency is 5.13% for PSI, but becomes 13.2% for AQI. As for the dominant pollutants, PM 10 and 1-h ozone are important in the PSI, with the latter gradually surpassing the former (Fig. 8(c)). Meanwhile, PM 2.5, 8-h and 1-h ozone are important in the PSI, with the first getting less dominant (Fig. 8(d)). With AQI, the effect of photochemical smog on Taiwan air quality is clearly presented. Acknowledgements The funding support by B.A.T. Services Limited, Taiwan Branch, is highly appreciated. Also, the Environmental Protection Administration of Executive Yuan of the R.O.C. Government is thanked for providing all necessary data. Certainly, the detailed analysis work by Ms Ariel Lin must be acknowledged here.

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