Answer to Reviewer 1

Lubna Dada

doi:10.5194/acp-2021-266-ac1

Abstract

Abstract. 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 â NPF event frequency, formation rate and growth rate â by comparing an urban Beijing site and a background mountain site separated by â¼80âkm from 14Â June to 14Â JulyÂ 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 2âh of the common NPF events was around 0.01âsâ1 at both sites, there were notable differences in formation rates between the two locations (median of 5.42âcmâ3âsâ1 at the urban site and 1.13âcmâ3âsâ1 at the mountain site during the first 2âh of common NPF events). In addition, the growth rates in the 7â15ânm range for common NPF events at the urban site (median of 7.6ânmâhâ1) were slightly higher than those at the mountain site (median of 6.5ânmâhâ1). 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 â¼100âkm, possibly due to the discretely distributed emission sources.

Highlights

Atmospheric new particle formation (NPF) events resulting from the formation of clusters and stable aerosol particles from gas-phase precursors have been recognized as a major contributor to the global aerosol budget (Kulmala et al, 2004; Zhang et al, 2011)
This NPF event frequency was consistent with an earlier observation in summer in urban Beijing from 2004 to 2008, while it was smaller than during other seasons, especially winter during that observation, and another 1-year observation in urban site (UB) station (Wu et al, 2007; Wang et al, 2013; Deng et al, 2020)
We found that NPF events are most of the time a regional phenomenon occurring over the studied areas and connected closely with air mass source regions during our observation

Summary

Introduction

Atmospheric new particle formation (NPF) events resulting from the formation of clusters and stable aerosol particles from gas-phase precursors have been recognized as a major contributor to the global aerosol budget (Kulmala et al, 2004; Zhang et al, 2011). Komppula et al (2006) investigated the occurrence of NPF events at two forest stations in northern Finland during 2000–2003 Their results suggested that same air mass source regions, favorable weather conditions and clean air at both stations were necessary for NPF events occurring simultaneously at the two stations. Carnerero et al (2018) observed horizontal distribution and a regional impact of the NPF events with data from urban, urban background and suburban stations in the Madrid metropolitan area, Spain, in July 2016. Their results indicated that ultra-fine particles were detected quasihomogenously in an area spanning at least 17 km horizontally, and the NPF events extended over the full vertical extension of the mixed layer. The factors that influence the occurrence of NPF events at the two stations simultaneously were left undetermined

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