| 其他摘要 | Study on the composition of precipitation is the key approach to assess air quality and acid precipitation pollution. Studies on the low-molecular carboxylic acids help shed light on the biogeochemical cycles of carbon, hydrogen and oxygen, as well as on the formation of acid precipitation. As a city with severe acid precipitation, the city Zunyi in northern Guizhou, provides a typical background for gaining insight into the organic geochemistry in the heavily polluted atmospheric environment. We collected 76 precipitation samples in the downtown city for a whole year (May 2006~ April 2007) on the event basis, and measured inorganic and organic anions with ion chromatograph and cations with atomic absorption spectrum.
(1) The annual average pH and electrical conductivity of precipitation were 4.11 with frequency of 93.2% and 62.10 μs•cm-1 respectively, a clear indication of the acid atmosphere.
(2) The concentrations in the precipitation of Zunyi showed in this order: SO42-> Ca2+> H+> NH4+> NO3-> Cl-> F->HCOO-> Mg2+> K+> CH3COO-> Na+> (COO)22-> PO43-> NO2-. SO42-, Ca2+, H+, NH4+ and NO3- are the main constituents. Their concentrations were 148.15 μmol•L-1, 81.89 μmol•L-1, 77.74 μmol•L-1, 43.80 μmol•L-1and 31.50 μmol•L-1, accounting for 31.97%, 17.67%, 16.78%, 9.45% and 6.54% of the total anions respectively. It demonstrates SO42- and NO3- are important contributors of acidity. Ca2+ and NH4+ buffer the precipitation acidity. The pollution of precipitation in Zunyi belongs to calcium sulfate type. Fractional acidity and neutralization factor showed that only 77% of precipitation acidity was neutralized by alkaline, especially Ca2+ and NH4+. Factor analysis demonstrated the high loadings of H+, NO3-, nss-SO42- and NH4+, suggesting the importance of SO42- and NO3- to acidity. Enrichment factor showed the sources of inorganic ions. Burning coal used by human activity is responsible for the sources of SO42-, while soil and lime dust claim the sources of Ca2+. NO3- mainly originated from the NOx emitted by factories. Mg2+ is mainly from terrestrial input with limited amount from marine. NH4+ has several sources such as nitrogen fertilizer production and consumption, and biological emission in Zunyi. As the typical sea-salt species, Cl- in precipitation of Zunyi is primarily contributed by anthropogenic sources including the emission of HCl和Cl2 from plants. Those resultes showed the government in Zunyi should change the energy structure in Zunyi, especially in the Winter, to control the burning coal and improve the environment.
(3) Formic (HCOOH), acetic (CH3COOH), and oxalic ((HCOO)2) acids are ubiquitous in precipitation of Zunyi with volume weighted average (VWA) concentration of 9.29 μmol•l-1, 6.47 μmol•l-1 and 5.06 μmol•L-1 respectively. The total concentration of organic acids was 22.28 μmol•L-1, accounting for 9.39 % of the total anions. It demostrates organic acids are important in precipitation. The concentration of organic acids in seasons followed in the order: Spring> Winter> Autumn> Summer, showing a seasonality. Furthermore, the acids often decrease with time in a precipitation event, with occasional increases in the middle and last stages, indicating that organic acids are primarily scavenged from blow cloud, with limited amount from the long distance transportation.
(4) With the high coefficient of 0.86, formic is significantly correlated with acetic acid, suggesting that both acids share common sources. Correlation analysis showed median correlationship between organic and NO3-, nss-SO42-, K+, Na+, Ca2+, indicating human activities sources of organic acids. Based on the Henry’s Law, we proposed the ratio of formic and acetic acids in the precipitation (F/A)aq as the indicator of the sources, and thus found that the anthropogenic sources are responsible for the organic compounds in the Spring and Winter, while vegetation emissions claim the sources in the Summer and Autumn. This close range provenance of organic acids is facilitated by the local environmental condition, which is characterized by the mountain-enclosed valley with high humidity, low wind speed, and high atmospheric dust content and organic acid with short lifetime in atmosphere. The long distance transportation is induced by air mass.
(5) Based on the thermodynamic equilibrium calculation method, the contributions of formic and acetic acids to free acidity in samples with pH≤5 were 14.79% (ranged 0.42~91.14%) and 3.66% (0.02~31.55%), respectively. Those values, lower than that in remote regions, are attributed to low pH value and high inorganic acidity. Organic contributions in seasons followed in the order: Spring (31.95%) > Summer (26.16%) > Winter (11.17%) > Autumn (8.02%). This seasonality indicates the importance of organic to precipitation acidity. The highest organic contribution in the Spring is influenced by abundant organic acids, low temperature and high pH value. The second was that in the Summer due to the high pH value. The contributions in the Autumn and Winter were lowest because of the low pH in precipitation.
(6) The annual flux of wet deposition for inorganic acids accounted for 94% of the total wet deposition, with a significant seasonality. The main contributors are SO42-, Ca2+, H+, NH4+, NO3-, with 90.1 mmol•m-2•yr-1, 49.8 mmol•m-2•yr-1, 47.3 mmol•m-2•yr-1, 26.0 mmol•m-2•yr-1 and 19.2 mmol•m-2•yr-1, respectively. There are high deposition levels of SO42-, Ca2+, H+, and low level of NO3- in Zunyi compared with other regions in China. TIN (Total Inorganic Nitrogen including NH4+ -N, NO3- -N, and NO2- -N) wet deposition was 45.7 mmol•m-2•yr-1, with NH4+ -N and NO3- -N contributing 57.0% and 41.9% respectively. The high TIN mainly is attributed to the adequate application of nitrogen fertilizer and the emission of nitrogen oxides by factories. The wet deposition of nutrition P was low with 1.97 mmol•m-2•yr-1. The wet deposition of organic acid accounted for 6%, thus the contribution of organic acids to acid deposition can’t be ignored. Based on Henry’s Law, we estimated the total wet deposition of the volatile organic acids including formic, acetic, and propionic acids accounted for 52.8% of the total deposition, suggesting the importance of wet and dry deposition in Zunyi. Consequently, not only precipitation samples, but also gas samples should be collected when we research the impact of organic pollutants on environment. |
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