Different Characteristics of New Particle Formation Events at Two Suburban Sites in Northern China

The formation of new atmospheric aerosol particles and their subsequent growth have been observed under different environmental conditions globally; such observations are few over northwest China. Here, we present an analysis of some case studies for new particle formation (NPF) events from two distinct suburban locations in northern China during May and June of two consecutive years, and provide more information to understand the characteristics of NPF events in North China. Particle number size distribution was measured at suburbs of Beijing (39.75◦ N, 116.96◦ E) during 1 June to 2 July 2013 and at suburbs of Xi’an (34.09◦ N, 108.55◦ E) during 1 to 25 May 2014. The average of total particle number concentration in the similar size range of 10–487 nm at the suburbs of Beijing (9.0 × 103 cm−3) was about two times higher than those observed at Xi’an (4.7 × 103 cm−3), and the mean particle mode diameter at Beijing was 1.4-fold higher than that at Xi’an. The estimated total condensation sink (CS) at Beijing (3.11 × 10−2 s−1) was also higher than at Xi’an (1.13 × 10−2 s−1). The frequency of NPF events at suburb of Beijing was 24%, lower than that in Xi’an (50%), and also lower than urban site of Beijing (35% in June) and another suburb of Beijing (over 50% in June). The NPF events with (Class I) or without (Class II) subsequent growth were both observed at the two suburb sites. The derived GR at the suburb of Beijing (range from 4.6 to 8.6 nm h−1) was a little higher than that at Xi’an (range from 3.3 to 6.7 nm h−1), which are generally comparable to typical values in mid-latitude reported in previous studies. The air masses coming from north or northwest China favor the occurrence of NPF event under low condensation sink and clear days. The number size distributions of freshly nucleated particles showed clear bimodal distributions on both sites. Additionally, Mode Dp of nucleated particles at the two sites was 17 ± 1 nm and 22 ± 4 nm, respectively during the periods with NPF events. The case study of NPF events at the two suburb sites shows that the surface area concentration and total scattering coefficient (SC) was significant decreased during the NPF events at both sites. High temperature, low condensation sink and low relative humidity furthered the occurrence of NPF events, and wind direction shifts were important for the subsequent growth of particles. NPF events in the suburbs of Beijing usually occurred when relative humidity (RH) < 55%, CS < 0.02 s−1, or 55% < RH < 68%, CS < 0.01 s−1. However, there is no clear range for Xi’an. Furthermore, we observed that some NPF events occurred at higher RH and very low CS in this study on both sites, which means that low CS may be more important than low RH for the particle formation on clear days.