One-year observations of organic carbon, elemental carbon, water-soluble organic carbon, and major ions in marine aerosols from Okinawa Island, Japan—an outflow region of Asian dust

Organic carbon (OC) and elemental carbon (EC) are important components of the atmospheric aerosols. They have strong impacts on radiative forcing of climate, human health, pollution transport, and visibility degradation. Large amounts of organic compounds are present in the tropospheric aerosols from urban, rural, marine, Arctic and Antarctic regions, supporting that they are important chemical constituents in the atmosphere. The main purpose of this study is to obtain the atmospheric situation in Okinawa, which is located on the outflow region of Asian countries including China, in order to better understand the origin and transformation of OC, EC, water soluble organic carbon (WSOC) and major ions in their transport process. Marine aerosol samples were collected in October 2009 to October 2010 at Cape Hedo station of National Institute of Environmental Studies, Okinawa (128.28°E, 26.87°N), an outflow region of Chinese aerosols, using high-volume air sampler and pre-combusted quartz fiber filters for 7 days at a flow rate of 60 m3 h-1. The aerosols were analysed for OC, EC, WSOC and major ions. The concentration of OC and EC were measured using a Sunset Laboratory carbon (OC/EC) analyzer following Integragency Monitoring Protected Visual Environments (IMPROVE) thermal/optical evolution protocol. WSOC and major ions were measured by using a Shimadzu carbon analyzer (TOC-VCSH) and ion chromatography (761 Compact IC, Metrohm, Switzerland), respectively. Concentrations of OC, EC, and WSOC ranged from 0.42 to 3.51µg m-3 (ave, 1.12±0.58 µg m-3), 0.07-0.96 µg m-3 (0.28±0.19 µg m-3) and 0.27-1.9 µg m-3 (0.73±0.38 µg m-3) (n=50), respectively. The highest concentration of OC was found during winter season (1.35 ìg m-3) followed by spring (1.28 ìg m-3) and summer (0.84 ìg m-3) whereas the lowest concentration was observed during autumn (0.75 ìg m-3). In contrast, the highest concentrations of EC and WSOC were found during spring (ave, 0.41 ìg m-3 and 0.95 ìg m-3, respectively) followed by winter (0.37 and 0.90 ìg m-3) whereas the lowest concentrations were found during summer (0.19 and 0.52 ìg m-3, respectively). The highest concentration of EC was found during spring, followed by winter, suggesting that the marine aerosols are more influenced from incomplete combustion during spring than winter. The average OC/EC ratios during winter, spring, summer and autumn were 3.7, 3.07, 5.7 and 5.6, respectively, suggesting that organic aerosols are formed by secondary processes. The average WSOC/OC ratios in our sampling site were found 67%, suggesting the major fractions of organic aerosols were derived from significant atmospheric oxidation. The dominant ions were Na+, Cl-, Mg2+, SO42-, Ca2+, and NO3-. Ca2+, which is a tracer of crustal dust, maximized during spring. During whole campaign we found significant Cl- loss. We found very strong correlation between NO3- and biomass burning tracer (K+) during winter, suggesting that the NO3- is associated with the biomass burning. We found that Ca2+ plays major role in the neutralization of acidic species. The dominant ionic compositions in our sampling site are NH4HSO4 and (NH4)2SO4.