The Chemical Composition Of Snowfall And Its Temporal Variation In Tianjin Northern China

In recent years, fog and haze occurred more and more frequently. It not only caused the lower atmospheric visibility and the change of air environmental quality, but also brought a lot of unfavorable influence to People’s Daily life and work. The deterioration of air environmental quality also caused high public attention. Based on the atmospheric wet deposition (rain and snowfall) of pollutants in the study, we can gain a better understanding the pollution status of heavy metals and inorganic ions in atmospheric environmental under adverse weather conditions, such as fog and haze.The snow samples of this study are collected from seven major snowfalls in two winter heating period from2012-2013and2013-2014in Tianjin. We measured and analyzed the chemical composition of snow using methods such as ICP-MS, ICP-AES, FAAS and IC. Analysis project included the pH of the snow, water-soluble ions (F", Cl,Br,PO43-,and NO2-,SO42-and NO3-,K+, Ca2+, Na+and Mg2+) and metals (Hg, Cd, Pb, As, Se, Cr, Co, Ni, Cu, Zn, V, Al, Mn and Fe, etc.). First, we analyze the pollution level of the snow through comparing the concentration of major pollutants in snow and surface water quality standard (GB3838-2002); Then, we discussed the snow acidity, ionic composition, spatial and temporal variation characteristics of pollutant concentration levels in the past two years and in the longer scale range by compared with the collected historical data analysis; Finally, we discussed the impact of air masses on concentration levels and temporal evolution of the various elements in snowfall by online simulation of backward trajectory of air masses which may came from different sources and transport scale. The backward trajectory model is provided by the U.S. NOAA official website. The results showed as follows:(1) There are few acid snowfall during the study period,83%of the snow has not reached the levels of acid rain. This may be due to a greater impact of the marine environment on snow because of the Bohai Sea near Tianjin; Different sources of ammonium and nitrates, and other alkaline substances in soil volatile into the atmosphere. This caused the acid-base neutralization between the acidic substance and a basic substance in the atmosphere. It might be one of the main reasons that Tianjin snowfall generally did not reach the levels of acid rain.(2) SO42-and NO3-are the main anion and acid ions, and they accounted for about 48.9%and25.8%respectively in anionic total concentration. The content of Cl" also has a large proportion because of the volatilization of the Bohai sea salt. The value of SO42-/NO3-in Tianjin snow is2.005. Tianjin is a mixed acid pollution according to the classification of acid rain. It reflects the shifting in the type of air pollution in Tianjin by the trend of the concentration of SO42-increasing and the value of SO42-/NO3-decreasing. The range of cationic total concentration in Snow is5.46-32.47mg/L, Na+and Ca2+are the major cations, and they accounted for about48.9%and25.8%respectively in cation total concentration. Compared with other cities, the concentration level of cation is higher, and Ca2+, Na+and Mg2+ions affected the Bohai Sea region environment. The cation, such as Ca2+, Na+and Mg2+, affected by the environment of Bohai sea area.(3) The contents of Zn, Fe, Al and Mn are higher in these seven snowfall, accounted for77.9%-97.9%in total metal elements. Hg pollution is most severe in all heavy metals, and the content of each snowfall exceeded surface water quality standard IV(GB3838-2002). Pollution severity followed by zinc, then Pb and Cd had the lowest levels; Tianjin snowfall has not been polluted by Cu, Se, As, Cr and other metal elements. Compared with some cities in our country and abroad, the concentration of Cd was the lowest; the concentration of As is higher than Beijing and Tangshan; the concentration of Pb is higher than other cities except Beijing, Singapore and Mexico City.(4) In all snowfall the content of Co and Mn showed the same change trend, this shows that the source of Co and Mn may be the same or similar; One possible source of metal in soil, water, sediment in Tianjin area and Bohai region is Atmospheric deposition which contain high concentration of Hg and Cd removed by snowfall.(5) In this paper, it did not show obvious correlation between water vapor sources of different direction and snow acidity, only reflected that the pH value of snow from the northeast air mass is higher than others. The airflow trajectory of same water vapor sources of air masses only showed that it had roughly similar water vapor source direction and similar transmission path. It is not very good for analysis the diffusion of the air in Tianjin region. Besides, meteorological factors such as wind direction, wind speed and relative humidity and so on should also be considered to analysis the characteristics of Tianjin snowfall. Comprehensive analysis shows that the snow anion and cation are greatly influenced by ocean, and metallic elements are mainly affected by terrigenous input. It is more conducive to the accumulation of heavy metals under weather conditions that the relative humidity is greater and the wind speed lower.