Measurement report : New particle formation characteristics at an urban and a mountain station in northern China

Atmospheric new particle formation (NPF) events have attracted increasing attention for their contribution to the global aerosol number budget and therefore their effects on climate, air quality and human health. NPF events are regarded as a regional phenomenon, occurring over a large area. Most observations of NPF events in Beijing and its vicinity were conducted in populated areas, whereas observations of NPF events on mountaintops with low anthropogenic emissions are still rare in China. The spatial variation of NPF event intensity has not been investigated in detail by incorporating both urban areas and mountain measurements in Beijing. Here, we provide NPF event characteristics in summer 2018 and 2019 at urban Beijing and a comparison of NPF event characteristics - NPF event frequency, formation rate and growth rate - by comparing an urban Beijing site and a background mountain site separated by similar to 80 km from 14 June to 14 July 2019, as well as giving insights into the connection between both locations. During parallel measurements at urban Beijing and mountain background areas, although the median condensation sink during the first 2 h of the common NPF events was around 0.015(-1) at both sites, there were notable differences in formation rates between the two locations (median of 5.42 cm(-3) s(-1) at the urban site and 1.13 cm(-3) s(-1) at the mountain site during the first 2 h of common NPF events). In addition, the growth rates in the 7-15 nm range for common NPF events at the urban site (median of 7.6 nm h(-1)) were slightly higher than those at the mountain site (median of 6.5 nm h(-1)). To understand whether the observed events were connected, we compared air mass trajectories as well as meteorological conditions at both stations. Favorable conditions for the occurrence of regional NPF events were largely affected by air mass transport. Overall, our results demonstrate a clear inhomogeneity of regional NPF within a distance of similar to 100 km, possibly due to the discretely distributed emission sources.Peer reviewe