| 摘要: | 揚沙是造成空氣品質下降的原因之一，本研究探討雲林街道、河沙、營建工地揚塵之粒徑特性，包括斗六、崙背、麥寮、台西四鄉市街道揚塵，濁水溪沖積平原上游(林內鄉近彰雲大橋附近河灘地)、中游(西螺鎮近西螺大橋附近河灘地)及下游(崙背鄉近自強大橋附近河灘地、二崙鄉近西濱大橋附近河灘地)之河沙揚塵與雲林縣內營建工地(建大工業廠房興建、湖山水庫下游自來水工程、客仔厝抽水排水站工程、四湖道路拓寬工程)之揚塵負荷量及化學組成，藉以瞭解揚塵不同粒徑的物理化學性質及比較大氣氣膠化學組成差異。斗六市之自行車專用道坋土(PM75)負荷為1.65 g/m2，可視為街塵負荷背景值，不包括自行車專用道的其他街道揚塵的平均坋土負荷為13.958.61 g/m2，非都市型地區(崙背鄉、麥寮鄉、台西鄉)的坋土負荷高於都市型地區(斗六市)，主因為非都市型地區道路維護不若都市良好且洗掃街頻率較少，導致滯留道路塵土較多，都市及非都市的街道揚塵平均PM10/PM75 ratio為0.1500.147 %，明顯較自行車專用道的0.0198 %為高，亦顯示一般街道存在較多的PM10揚塵潛量。濁水溪河沙揚塵的表面坋土負荷，在中游段的西螺大橋為48.33 g/m2，較下游段的西濱大橋的10.9 g/m2為高，推測因下游段塵土長期被風揚起，反倒坋土留置於河床表面的量較少，但PM2.5/PM10 ratio從上游段的31.1%至下游段的38.9 %，卻呈現上升趨勢，顯示雖然下游段的表面坋土負荷並非最高，但影響空氣品質的PM2.5佔PM10的比例逐漸增高，當秋冬季節東北季風吹襲著廣大乾燥的河灘地，將增加河川下游原生性河沙PM2.5揚塵飛散潛勢。四個營建工地揚塵平均表面坋土負荷為29.9±15.8 g/m2，亦顯示若無灑水、覆蓋或植披等基本防塵措施，營建工地具有高度揚塵逸散潛勢。總碳(total carbon, TC)佔三類揚塵PM10質量比例平均介於2.81~10.1 %，TC佔其PM2.5質量比例更高，平均介於7.57~28.7 %，均較水溶性無機鹽類組成佔PM質量比例高出甚多。街道揚塵PM10的水溶性無機鹽類負荷量以Ca2+最高，Cl-、SO42-次之，營建工地及河沙表面揚塵無機鹽類負荷量以SO42-為最高，Ca2+、Cl-次之，顯示三種離子為揚塵的主要特徵物種。Malonate, lactate及acetate為街道、河沙、營建工地揚塵之PM10及PM2.5的主要原生有機酸物種，與大氣氣膠因光化產物居多的oxalate不同。街道、河沙、營建工地類型揚塵之Acetate/Formate (A/F)及Malonate/Succinate (C3/C4)比率分別介於5~40及1~20，而對應大氣氣膠的A/F及C3/C4比率較低，顯示大氣氣膠之A/F、C3/C4比值指標並不適用解釋揚塵對大氣氣膠濃度的貢獻。此外，街道、河沙、營建揚塵坋土粒徑均以單峰分布型態呈現，主要位於粗粒峰(coarse mode)，這與典型大氣氣膠粒徑多為三峰分布(tri-mode distribution)型態截然不同，尤其揚塵的SO42-, NO3-, NH4+濃度負荷粒徑皆主要在單一coarse mode呈現，這與大氣氣膠主要呈現於累積峰(accumulation mode)或次微米液滴峰(submicron-droplet mode)有相當大差異。三類型揚塵坋土的中性化比率(neutralization ratio, NR)>1，但遠較NR plus Ca2+低，顯現揚塵主要受Ca2+貢獻而呈現鹼性，且從揚塵的各粒徑之NR及NR plus Ca2+發現，細微粒呈現酸性；粗微粒呈現鹼性。而街道揚塵之NR及NR plus Ca2+皆較河沙及營建工地揚塵為大，主要乃河沙及營建工地揚塵存在較街道揚塵更為豐富的硫酸鹽所致。由本研究對三類揚塵粒徑分布及其化學組成發現，揚塵所代表的原生性化學組成與大氣氣膠的化學組成具有極大的差異，且PM2.5佔坋土比例僅為ppm或sub-ppm級別，顯示大氣氣膠，特別是PM2.5的污染來源非以揚塵所顯著貢獻，但因各類裸露地的廣闊，若未有適當的揚塵防治與管制，其所揚起的微粒將對大氣PM10及PM2.5濃度造成區域性不可忽略的貢獻潛勢。
Dust emission is resulting in the decline of air quality. This study investigated the size of street dust in Douliu, Lunbei, Mailiao, and Taihsi Townships, river dust from the upstream (river flat in Linnei Township near the Zhangyun Bridge), midstream (river flat in Xiluo Township near the Xiluo Bridge), and downstream (river flats in Lunbei Township near the Ziqiang Bridge and in Erlun Township near the Xibin Bridge) of the alluvial plain of Zhuoshui River, and the load and chemical composition of dust emitted from the construction works in Yunlin County (factory construction of Kenda Rubber Industrial Company, water supply construction in the downstream area of Hushan Reservoir, Kotzutso pumping and drainage station construction, road expansion construction in Sihu Township). The objectives were to investigate the physiochemical properties of different sizes of dust and to compare the chemical compositions of aerosol. The load of silt (PM75) of bike lanes in Douliu City was 1.65 g/m2, which can be regarded as the load background value of street dust. Dust from streets other than the bike lanes had an average silt load of 13.95 ± 8.61 g/m2. The silt load in rural area (Lunbei, Mailiao, and Taihsi Townships) was higher than that in urban area (Douliu City), mainly because road maintenance and cleaning in rural area are less frequent than in urban area. Therefore, a large amount of dust remains on the roads. The average PM10/PM75 ratio of street dust in urban and rural areas was 0.150 ± 0.147%, which is considerably higher than that of bike lane dust (0.0198%), indicating that streets had a larger amount of PM10 dust than bike lanes. The surface load of silt of the midstream Zhuoshui River was 48.33 g/m2 at the Xiluo Bridge, which was higher than that in the downstream area, near Xibin Bridge (10.9 g/m2). This study inferred that the river dust in the downstream region has been constantly raised by the wind; therefore, only a small amount of silt remained on the riverbed. The PM2.5/PM10 ratio increased from 31.1% in the upstream region to 38.9% in the downstream region. This indicates that although the downstream region did not exhibit the highest surface load of silt, the primary factor that affects air quality, PM2.5, gradually increased along the river. In autumn and winter, the northeast monsoon increases the potential for the scattering of primary PM2.5 in the river dust at the downstream region as the wind blows over the vast dry river flats. The average surface load of silt at the four construction works was 29.9 ± 15.8 g/m2, indicating that the construction works would have a high potential of dust emission if lacking basic dust prevention measures such as watering, covering, and planting. The mass proportion of total carbon (TC) to the PM10 of the three types of dust was between 2.81% and 10.1%, and that of TC to PM2.5 was between 7.57% and 28.7%. Both were much higher than the mass proportion of water-soluble inorganic salt to particulate matter. Among the PM10 of street dust, Ca2+ had the highest load of water-soluble inorganic salt, followed by Cl- and SO42-. Regarding the construction works and river dust, SO42- had the highest load of water-soluble inorganic salt, followed by Ca2+ and Cl-, signifying that the three ions are the primary characteristic species of dust. Malonate, lactate, and acetate are primary organic acids of PM10 and PM2.5 of street, river, and construction works dust; they are different from oxalate, a group of salts commonly produced in the photochemical reaction of aerosol. The acetate/formate (A/F) and malonate/succinate (C3/C4) ratios of street, river, and construction works dust were 5–40 and 1–20, respectively. The A/F and C3/C4 for aerosol were relatively low, indicating that A/F and C3/C4 are not suitable for determining the contribution of dust to aerosol concentrations. In addition, the size of silt of street, river, and construction works dust exhibited unimodal distribution and concentrated at the coarse mode. This differs from the typical aerosol size that exhibits tri-mode distribution. In particular, the concentration load of SO42-, NO3-, and NH4+ of the dust primarily appeared at the coarse mode of size distribution, which considerably differs from that the concentration load of aerosol typically appear at the accumulation mode or submicron-droplet mode. The neutralization ratios (NR) of silt of the three types of dust were all larger than 1 but were still considerably lower than NR plus Ca2+. This signifies that dust is alkaline due to Ca2+. The NR and NR plus Ca2+ of dust size showed that fine particulate matter was acidic and coarse particulate matter was alkaline. The NR and NR plus Ca2+ of street dust were larger than those of river and construction works dust because river and construction works dust contain abundant sulfate in comparison with street dust. This study examined the size distribution and chemical composition of three types of dust and found that the primary chemical composition of dust and that of aerosol were substantially different. The proportion of PM2.5 to silt was only at the ppm or sub-ppm level, implicating that dust is not the primary source of aerosol, particularly not for PM2.5. However, considering the vast bare land, if no appropriate dust prevention and control measures are implemented, the raised particulate matter may have substantial influences on PM10 and PM2.5 concentrations in specific regions. |
