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Abstract. The effect of new particle formation (NPF) on cloud condensation nuclei (CCN)varies widely in diverse environments. CCN or cloud droplets from NPFsources remain highly uncertain in the urban atmosphere; they are greatlyaffected by the high background aerosols and frequent local emissions. Inthis study, we quantified the effect of NPF on cloud droplet numberconcentration (CDNC, or Nd) at typical updraft velocities (V) in cloudsbased on field observations on 25 May–18 June 2017 in urban Beijing. Weshow that NPF increases the Nd by 32 %–40 % at V=0.3–3 m s−1during the studied period. The Nd is reduced by 11.8 ± 5.0 % atV=3 m s−1 and 19.0 ± 4.5 % at V=0.3 m s−1 compared tothat calculated from constant supersaturations due to the water vaporcompetition effect, which suppresses the cloud droplet formation by decreasingthe environmental maximum supersaturation (Smax). The effect of watervapor competition becomes smaller at larger V that can provide moresufficient water vapor. However, under extremely high aerosol particlenumber concentrations, the effect of water vapor competition becomes morepronounced. As a result, although a larger increase of CCN-sized particles byNPF events is derived on clean NPF days when the number concentration ofpreexisting background aerosol particles is very low, no large discrepancyis presented in the enhancement of Nd by NPF between clean andpolluted NPF days. We finally reveal a considerable impact of the primarysources on the evaluation of the contribution of NPF to CCN numberconcentration (NCCN) and Ndbased on a case study. Our study highlights the importance of fullconsideration of both the environmental meteorological conditions andmultiple sources (i.e., secondary and primary) to evaluate the effect of NPF onclouds and the associated climate effects in polluted regions.

Evaluation of the contribution of new particle formation to cloud droplet number concentration in the urban atmosphere