Long-term trends of aerosol carbon and nitrogen, their stable isotopic compositions, and water-soluble organic carbon in the western North Pacific

The western North Pacific is an outflow region of Asian dusts and pollutants. A rapid industrial development in China and East Asian countries for last two decades may have seriously altered the air quality of the North Pacific. However, long-term changes of atmospheric composition have not been studied in this region. To better understand long-term atmospheric changes in the western North Pacific, we collected marine aerosols on weekly basis in 2001-2009 at a remote island, Chichijima (27°04'E; 142°13'N) using high volume air sampler and pre-combusted quartz filter. The island is located in the boundary of westerly (winter and spring) and trade wind (summer and autumn) regimes. Here, the aerosol samples were analyzed for total carbon (TC) and nitrogen (TN) and their stable isotopic compositions as well as water-soluble organic carbon (WSOC). Concentrations of aerosol TC and TN were in a range of 0.21-4.58 (average 1.07) ug/m3 and 0.01-3.43 (av. 0.29) ng/m3. These values are ca. 20 times lower than the concentrations in urban Tokyo. C/N weight ratios ranged from 0.50 to 41 (av. 6.6) with summer maxima. Both TC and TN showed a gradual increase with winter/spring maxima probably due to the enhanced anthropogenic activities such as fossil fuel combustion and fertilizer usage in East Asia. On the other hand, stable carbon isotopic ratios (d13C) of TC ranged from -27.3 to -15.1‰; however, no systematic trend was detected. Nitrogen isotopic ratios of TN (d15N) ranged from +1.56 to +25.2‰ with lower values in winter and higher values in summer. Interestingly, d15N values showed a long-term trend of increase by 4‰ from 2001 to 2009. This may be caused by an increased emission of nitrogen species (NH3, NOx) from biomass burning sources in East Asia and other regions. WSOC (0.05-2.46 ug/m3, av. 0.40 ug/m3) also showed a long-term increase probably due to the increased emissions of organic aerosols and their precursors and subsequent oxidations in the atmosphere during long-range transport. WSOC was found to comprise 10-74% (av. 38%) of aerosol TC.