Seasonal and diurnal variations of ultrafine particle concentration in urban Gwangju, Korea: Observation of ultrafine particle events

Ultrafine particle concentration (3–100 nm) was continuously measured in summer (6/7/2006–17/8/2006), fall (1/10/2006–31/10/2006), winter (2/2/2007–7/3/2007), and spring (7/4/2007–7/5/2007) in urban Gwangju, Korea. By considering the size range with elevated concentration and hypothesizing the source or formation pathways, we classified ultrafine particle events into: (1) “10–100 nm traffic event,” (2) “50–100 nm residential heating event,” and (3) “10–30 nm photochemical event.” Coincidence of NOx, CO, O3, and solar radiation with ultrafine particle concentration was used as an indicator for the traffic, residential heating, and photochemical contributions. We showed that particles due to traffic and residential heating led to enhanced ultrafine particle concentration in winter, while in summer particles formed by photochemical activity led to increased ultrafine particle concentration. The average ultrafine particle number concentrations were 8.8×103, 7.4×103, and 7.9×103 cm−3 with geometric mean diameters (GMDs) of 25, 48, and 53 nm in the photochemical, traffic, and residential heating events, respectively. Our ultrafine particle concentrations and other gas concentrations (NOx and CO) were found to be lower than observations in Atlanta and in Fresno, probably due to lower source strength such as traffic and residential heating in current site. We occasionally observed particle growth after formation of particles in the photochemical event with growth rates of 2.2–4.7 nm h−1. Morphological and elemental analysis for size-selected ultrafine particles showed that mixtures of sulfate and metals with the presence of semi-transparent part inside the particle were observed in the photochemical event (49 over 200 particles), while in the traffic event agglomerated carbonaceous particles were observed (66 over 130 particles), which were rarely detected in the photochemical event.