Sources, Transport and Chemical Characterization of Atmospheric Aerosols over the Dongsha Island

Two hundred and seventy nine aerosol samples were collected from January to December in 2012 at Dongsha Island in northern South China Sea. The water-soluble ions (Cl-, NO3-, SO42- and NH4+) and metals (Al, Fe, Na, Mg, K, Ca, Sr, Ba, Ti, Mn, Co, Ni, Zn, Mo, Ag, Cd, Sn, Sb, Pb, V, Cr, As, Se and P) were analyzed by ion chromatography (IC) and inductively couple plasma mass spectrometry (ICPMS), respectively. The year-long monitoring results indicate strong variation of elemental concentrations in aerosol samples over the South China Sea. The major chemical components in aerosol samples at the Dongsha Island were composed by Na, Cl-, NO3-, nss-SO42- and NH4+ and their average concentrations were higher than 1 μg/m3. A large proportion of elements concentration in aerosols displayed a similar trend with higher concentration in March, October and November, and lower concentration in April to September. The highest concentration of element P is in August. Correlation coefficient and enrichment factor calculations reveal that the sources of elements can be divided into three categories: (1) Fe, Al, Ti, Co, Mn, Cr, Ca and Ba were mainly derived from crustal sources, (2) Na, Mg, Sr, K and Ca were from seawater sources, and (3) V, Ni, Zn, Pb, Cd, As, Sn, Se, Sb and P were from anthropogenic sources. Among them, K, Ca, Sr and Mn were multisources. Principal component analysis (PCA) was used to clarify sources and associated processed and it shows the most predominant sources of atmospheric trace species to the South China Sea were mineral dust and anthropogenic materials from mainland China and it occupied more than 60% atmospheric chemical composition. PCA result also indicates the variability of chemical composition of aerosols is greatly influenced by the dust storm during the northeast monsoon season and delivered crustal and anthropogenic materials at Dongsha Island. In contrary, the impact materials derived from Indochina and South China Sea during the southwest monsoon season is relatively small. The Cl-/Na+ ratios in aerosol samples indicate strong chloride deficient and it suggests the atmosphere over the study area suffered severely acidic ion species pollution. The calculation results of molar ratio between NH4+ and nss-SO42-, NH4+ and NO3- reveal the capability of NH4+ to neutralize the acidic ion species, like nss-SO42- and NO3-, is strong. In this study, half of the aerosol component was composed by sea salt particles, and land source materials only occupied 30%. Moreover, in land source materials, the contribution from anthropogenic pollution was two times higher than crustal source. The atmospheric dry deposition fluxes of NO3-, nss-SO42-, Ca, K, NH4+, Al and Fe to the Dongsha area were higher than 100 mg/m2/yr which accounts for most of the total atmospheric flux and being the most important input to the Dongsha Island. The estimated fluxes of F, P and N to the Dongsha area were 81.9, 33.8, and 283 mg/m2/yr, respectively. The fraction of total dry deposition fluxes of N were 48% for NO3- and 52% for NH4+.