Sources and fluxes of rare earth elements in wet deposition at a Chinese coastal city downstream of the Asian continental outflow

Abstract

Atmospheric deposition is an important source of dissolved rare earth elements (REEs) to the oceans, but the flux estimating from various sources of REEs in atmospheric wet deposition (AWD) have received little attention to date. To quantify the contributions of the fluxes of REEs in AWD, 38 wet deposition samples were collected for a year in Qingdao, China, a coastal city on the transport path of the continental aerosols heading toward the Pacific Ocean as they are carried on the westerlies. According to the results from Post-Archean Australian Sedimentary rock (PAAS) normalization, REEs in wet deposition showed four types of patterns during observation. Types A (47% of a total of 38 AWD samples) and B (34%) were the main REE patterns, showing a convex-up pattern and Heavy REE enrichment, respectively. Both Types C and D were characterized by higher Light REE with significant La anomalies, while Type C tended to be relatively flat and Type D transitioned downward from Tb to Lu. Meanwhile, the optimum multiparameter method was used to quantify the contribution from each source. Consequently, loess-dust (46% ± 19%) and coal fly ash (38% ± 17%) were the main contributors in Type A, while loess-dust (45% ± 15%) and sea salt (44% ± 14%) account for high proportions in Type B. Fluidized-bed catalytic cracking contributes extra La in types C and D. Annual total REE flux was 55.3 μg m−2 y−1, of which loess-dust contributed 63%, coal fly ash 20%, sea salt 13%, and the catalyst of fluidized-bed catalytic cracking 5% of annual La deposition flux. Furthermore, since aerosols can be transported over long distances, human activities could significantly increase the input fluxes of REEs to the ocean's surface, especially for Middle REE and La.