【摘要】 报道了 1 998年 4月至 1 999年 3月广州市典型森林区降雨的化学组成中 SO2 - 4,NO- 3,Ca2 +浓度很高 ,并具有明显的季节性变化 ,春夏季酸雨较秋冬季严重。广州地区全年酸雨频率为 80 %,酸雨量占总降雨量的 95 %。结合气象资料并通过与其它地区酸雨化学组成的对比分析 ,认为广州地区酸雨形成机制具有局地冲刷和中远距离传输叠加的双重特征。酸雨中主要离子除来自于工业和交通污染源外 (SO2 - 4和 NO- 3) ,还受到陆地源尘埃 (Ca2 +)和海洋源 (Na+和 Cl- )的影响。广州酸雨中 SO2 - 4和 NO- 3的年沉降率分别为 3 2 0 0 eq/(hm2 · a)和 5 70 eq/(hm2 · a) ,远远超过了维持生态平衡的最低临界值 (2 0 0～ 3 0 0 eq/(hm2·a) ) ,因而已对本地区生态环境构成了威胁更多还原

【Abstract】 The precipitation has been collected during the period of April 1998 to March 1999 from two typical forest sites in the city of Guangzhou, south China The samples were analyzed for major element compositions (SO 2- 4, NO - 3, Cl -, Ca 2+ , Na +, K +, Mg 2+ , NH + 4). The total annual amout of precipitation is 1625mm during the monitoring period.pH values vary between 3 3 and 6 3,with an average of 4 45.The frequency of acid rain(pH<4 5)is 80%.The amount of acid rain represents about 95% of total precipitation,indicating the severe environmental status at Guangzhou.The rainwater has high concentrations of SO 2- 4, NO - 3 and Ca 2+ and shows a remarkable seasonal variation, with acid rain being more important in the spring and summer than in the autumn and winter. The rainwater at Guangzhou shows a chemical composition similar to that in southwest China. The high SO 2- 4 contents in precipitation from these two regions are largely related to coal combustion for the purpose of electricity generation and heating.Even if concentration of SO 2- 4 in precipitation is high,pH of precipitation is relatively high,probably due to the neutralization of base cations.Concentrations of Ca 2+ and other base cations are indeed very high in the precipitation.The exact origin of Ca is unclear at this stage but could be derived from road dust,cenment factories and long distance transported desert dust.Marine sources cannot be ignored for the precipitation chemistry at Guangzhou,especially for Na + and Cl -. It is interisting to note that the seasonal variation in chemistry of precipitation at Guangzhou is the reverse of what observed at Chongqing(SW China),where pH in rainwater being lower in winter than in summer.This may imply the different mechanism of generation of acid rain in two areas.In the SW China,large cities are commonly stiuated along valleys.Limited air circulation in these regions is not favorable for the transport and diffusion of the locally emitted SO 2.Consequently,these pollutants tend to accumulate in the lower atmospheric layer and enter the rainwater when precipitation is formed.This is supported by similar δ 34 S between atmospheric SO 2 aerosol and SO 2 particles issued during coal combustion in SW China.Therefore,the seasonal variation in precipitation chemistry in SW China may be directly related to the extent of local consumption of coal,which is greater in winter than in summer. The source of pollutants from which acid rain develops at Guangzhou seems to be more complex.In this area.δ 34 S of atmospheric SO 2 aerosol is significantly lower than(by about 12‰)that of SO 2 particles derived from coal combustion.Therefore in addition to the locally produced pollutants related to coal consumption,a source with low δ 34 S is required in order to account for the whole δ 34 S range of precipitation at Guangzhou.In fact,the air circulation in the Pearl Delta region is strong.Long middle distance transported pollutants may also contribute in the formation of acid precipitation. Ca 2+ contents in rainwater in winter and autumn are significantly higher than in spring and summer.This is particularly true for the Baiyunshan site.It is likely that high pH in precipitation in winter is due to the increasing neutralizing capacity of base cations.Assuming that Ca 2+ in rainwater was mainly contributed by alkaline dust,we attribute the seasonal variation in precipitation chemistry at Guangzhou to seasonal variation in weather and dry deposition.At Guangzhou,the majority of rainfall takes place in spring and summer and accounts for about 90% of annual precipitation.Soils are thus relatively wet and surface dusts are not easily incorporated into the air.Semi continuous washing by rainfall also efficiently lowered the contents of alkaline particles in the atmosphere.This leads to the decrease in concentration of base cations in rainwater and the lowering of acid neutralizing capacity.In contrast,a high neutralizing capacity is更多还原