Chemical composition of atmospheric precipitation in Lanzhou, Northwest China

To understand the chemical composition and temporal variations of atmospheric precipitation, the precipitation samples were collected and analyzed continuously from March 2007 to October 2007 in Lanzhou, a city located in the semiarid zone of Northwest China. All samples were analyzed for pH, electric conductivity and major ions (F
^-
, Cl
^-
, NO
₂
^-
, Br
^-
, NO
₃
^-
, SO
₄
^2-
, Na
⁺
, NH
₄
⁺
, K
⁺
, Ca
²⁺
and Mg
²⁺
). The results showed that the mean pH value observed in Lanzhou city was 6.81 and precipitation present neutralization, the mean electric conductivity was 72.4μS·cm
^-1
and shows that the air pollution was remarkable. In 26 precipitation samples, only one sample was acidic. It was observed that Ca
²⁺
and NH
₄
⁺
were the most abundant cations, accounting for 68.5% and 16.0% of the total cations, respectively. SO
₄
^2-
and NO
₃
^-
were the most abundant anions, accounting for 70.0% and 23.4% of the total anions, respectively. The concentrations of SO
₄
^2-
and Ca
²⁺
were higher than the other cities in China, which indicated the air over Lanzhou has been a severe polluted city over the world. The predominant energy source in Lanzhou is coal combustion resulting in the high emission of SO
₂
, which were the precursor of the high concentration of acidic ions SO
₄
^2-
. The high concentrations of alkaline ions (mainly Ca
²⁺
, NH
₄
⁺
) in the atmosphere over Lanzhou city have played an important role to neutralize the acidity of rainwater. Annual variation of the concentrations of ions shows higher levels in spring than in summer and fall. The SO
₄
^2-
/NO
₃
^-
equivalents concentration ratios is 5.4 in precipitation samples from the Lanzhou districts, which are high relative to the other cities of China, indicating that the air pollution is caused by coal consumption (burning) there. Observed precipitation scavenging is the removal of pollutants from the atmosphere, especially to particulate matter.