Long-term measurements of carbonaceous aerosol at Cape Hedo, Okinawa, Japan: Effects of changes in emissions in East Asia

We investigated the changes in the carbonaceous-aerosol sources and their effects on the long-term elemental carbon (EC) and organic carbon (OC) concentration trends at the Cape Hedo Atmosphere and Aerosol Monitoring Station (CHAAMS) in Okinawa, Japan, during the period 2004– 2013. We obtained the EC and OC concentrations by conducting semi-real-time measurements using a carbon monitor, and performing an offline thermal/optical filter analysis according to the Interagency Monitoring of Protected Visual Environments (IMPROVE) protocol. The annual average concentration of the EC remained constant between 2004 and 2013, but that of the OC decreased at a rate of 0.11 µg C m^–3 y^–1 (α > 0.05). The secondary OC (SOC)/OC ratio showed an increasing trend from 2004 till 2011, which may have been caused by a reduction in primary emissions of OC and compositional changes in the organic compounds originating in China, from which air pollutants were frequently transported during spring and winter. Although the EC concentration did not change appreciably in either season, the OC concentration decreased at rates of 0.10 µg C m^–3 y^–1 and 0.11 µg C m^–3 y^–1 during spring and winter, respectively. We estimated the contributions from the various sources of carbonaceous aerosol, viz., biomass burning, fossil-fuel combustion, and air-pollutant transport from China, based on the OC/EC ratio, which decreased from 5.7 to 2.4 in terms of the annual average. The growing share from fossil-fuel combustion is responsible for the decline in biofuel-burning OC emissions transported from China to CHAAMS.