dc.description.abstract |
The definition of background air quality (AQ) concentration is the sum of local natural emissions, foreign natural and trace anthropogenic pollutants advected into an area. In other words, those AQ level cannot be affected by local anthropogenic activities. Through the estimation of background level of AQ concentration, one can quantify the contributions originate from other sources (e.g. local emission or long-range transport). In the literatures, background concentration of gas pollutants is often obtained by direct observations at natural background sites. However, Taiwan is located in the center of East Asia and populated region. It is scarce to find a standard background site that meets the strict limitations. To quantify the background PM2.5 concentration in Taiwan seems to be even difficult due to PM2.5 concentration strongly varied with weather, long range transport, chemical reactions, and so on.
In this study, we attempt to quantify PM2.5 background concentration level in Taiwan by using 11-year PM2.5 monitoring data obtained from EPA air quality stations in Wanli, Taitung and Hengchun. These sites are chosen as background sites since there are no large PM2.5 emission source adjacently. Modified AGAGE (Advanced Global Atmospheric Gases Experiment) method is used to flag the background condition for calculating PM2.5 background concentration level. In addition to the modified AGAGE method, the HMM (Hidden Markov Model) statistical method and literature reviews are also performed in this study to confirm the reliability of background PM¬2.5 concentrations.
The results show that the PM2.5 background concentration in Taiwan is about 4.4 μg m-3. Low correlation coefficients (r < 0.1) with at least 95 % significant level between background-flagged data and meteorological parameters indicates that the low concentration of background value is weakly related to the influences resulting from meteorological condition. The main factor of affecting background concentration is seasonal variation. It could result in a change in background value of around 50 %. Precipitation and diurnal pattern could contribute for 10 % variation of background PM2.5 value. According to the linear trend analysis, it shows that the decreasing trend of background concentration is -0.1 to -0.2 μg m-3 yr-1 in the past 11 years in corresponding to the decreasing trend of air pollution emissions in East Asia in recent years. This study suggested that the background trend may be related to the trend of surrounding environment or the influences caused by Asian continental sources.
Based on the result of background PM2.5 concentration level, we attempt to further investigate the local PM2.5 characteristics in Taiwan. Assuming that when all of hourly PM2.5 concentration measured at background sites (i.e., Wanli, Taitung and Hengchun) less than averaged background PM2.5 concentration (i.e., 4.4 μg m-3), the situation could be considered as no influence of long-range transport. Under this scenario, the PM2.5 concentrations gradually increase from North to South, and averaged PM2.5 concentrations are 5 – 35 μg m-3 due to local emission and topographic effect. The highest increment appears happened at Southwestern region, and it could be 5 times larger than background concentration level. Following the aforementioned dataset, we further analyze the PM2.5 spatial distribution under Northeast wind and Southwest wind, respectively. We found that the transport amount of PM2.5 across counties could reach 5 μg m-3 and it shows the accumulation effect at specific sites. Results of scenario assumptions show that there are about 10 μg m-3 of PM2.5 due to non-local pollutions (i.e., sum of background value and transported PM2.5 across counties). The 10 μg m-3 represents the lower bound of PM2.5 reduction limit as current emission condition. The results from this study provide a useful information to policy makers on making an achievable and reasonable PM2.5 deduction goal. | en_US |