Number size distribution of aerosols at Mt. Huang and Nanjing in the Yangtze River Delta, China: Effects of air masses and characteristics of new particle formation

Aerosol number spectra in the range of 10nm–10μm were observed at Mt. Huang (Aug. 15–Sep. 15) and Nanjing (Oct. 13–Nov. 15) by a wide-range particle spectrometer (WPS) in 2011. Based on the backward trajectories obtained using the HYSPLIT model, the transport pathways of observed air masses during the study periods were classified into the following four groups: maritime air mass, continental air mass, marine–continental mixed air mass and local air mass. The variations in the aerosol number spectrum and the new particle formation (NPF) events for various types of air masses were discussed, along with meteorological data. The results showed that the average number concentration was 12,540cm−3 at Nanjing and only 2791cm−3 at Mt. Huang. The aerosol number concentration in Nanjing was 3–7 times higher than that in Mt. Huang; the large discrepancy was in the range of 10–100nm. Different types of air masses had different effects on number concentration distribution. The number concentration of aerosols was higher in marine air masses, continental air masses and continental–marine mixed air masses at 10–50nm, 100–500nm and 50–200nm, respectively. Under the four types of air masses, the aerosol size spectra had bimodal distributions in Nanjing and unimodal distributions in Mt. Huang (except under continental air masses: HT1). The effects of the diverse air masses on aerosol size segments of the concentration peak in Mt. Huang were stronger than those in Nanjing. The local air masses were dominant at these two sites and accounted for 44% of the total air masses. However, the aerosol number concentration was the lowest in Mt. Huang and the highest in Nanjing when local air masses were present. The number concentrations for foreign air masses increased at Mt. Huang and decreased at Nanjing. Different types of air masses had greater effects on the aerosol spectrum distribution at Mt. Huang than at Nanjing. During the NPF events, the particle growth rates at Mt. Huang (6.5–9.0nmh−1) were faster than those at Nanjing (4.8–5.6nmh−1). The relative humidity at Mt. Huang (36–65%) was higher than that at Nanjing (30–47%), but the wind speed trend was the opposite. The air masses during the NPF events were clean, i.e., they were mainly from over the ocean or districts with low ultrafine particle concentrations.