Long-term (2001–2012) observation of the modeled hygroscopic growth factor of remote marine TSP aerosols over the western North Pacific: impact of long-range transport of pollutants and their mixing states
In order to assess the seasonal and annual variability of long-range transported anthropogenic pollutants from East Asia and their effect on the hygroscopicity and precipitation process over the western North Pacific, we conducted long-term calculations of bulk hygroscopicity, g(90%)ZSR, based on the ZSR model using chemical composition data from 2001–2012 at Chichijima Island. We found that sea-salts (Na+ and Cl−) are the major mass fraction (65%) of the total water-soluble matter followed by SO42− (20%) and WSOM (6%). The seasonal variation of g(90%)ZSR was high in summer to autumn and low in winter to spring months, probably due to the influence of the long-range transport of anthropogenic SO42−, dust, and organics from East Asia and their interaction with sea-salts through heterogeneous reactions. On the other hand, annual variations of g(90%)ZSR showed a decrease from 2001 to 2006 and then an increase from 2007 to 2012. Interestingly, the annual variations in SO42− mass fractions showed an increase from 2001 to 2006 and then a decrease from 2007 to 2012, demonstrating that SO42− seriously suppresses the hygroscopic growth of sea-salt particles over the western North Pacific. This is further supported by the strong negative correlation between SO42− and g(90%)ZSR. Based on the MODIS satellite data, the present study demonstrates that long-range transported anthropogenic pollutants from East Asia to the North Pacific can act as efficient cloud condensation nuclei but significantly suppress the precipitation by reducing the size of cloud droplets over the western North Pacific.