New Particle Formation and Diurnal Variations in Number Concentrations at a Rural Site Downwind of Seoul, Korea

We investigated the characteristics of new particle formation (NPF) at a rural site downwind of Seoul, Korea, using scanning mobility particle sizer (SMPS) measurements in the range of 10.3-290.8 nm for the period February 2015 through June 2016. The study site is located in a horseshoe-shaped forested valley, over which biogenic emissions likely prevail, and is affected by vehicle emissions coming from the valley entrance. We classified the NPF events as Classes 1, 2, and undefined (U) each day, and the days without particle formation and growth as non-event (NE). Class 1, with clear and strong particle formation and growth, exhibited an evident early-afternoon peak of the nucleation mode, followed by the Aitken mode at its maximum. On Class U days, despite a higher concentration of the nucleation mode than on Class 2 days, the afternoon increase in the Aitken mode seemed to be limited, because of the dominant effects of vehicle emissions. The event days tended to have higher solar radiation, temperature, and wind speed and lower relative humidity than did NE days. During the Korea-US Air Quality (KORUS-AQ) field campaign, the NPF occurrence rate was 63%, higher than the seasonal means, probably because of strong solar radiation. We selected two characteristic periods each consisting of only Class 1 and NE days. We confirmed the possibility of nucleation by sulfate associated with high concentrations of SO2 and strong solar radiation, and an adverse effect of PM1 on the nucleation, for each period.