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

• Main Authors: S. Jiang, F. Zhang, J. Ren, L. Chen, X. Yan, J. Liu, Y. Sun, Z. Li
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-09-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/14293/2021/acp-21-14293-2021.pdf

<p>The effect of new particle formation (NPF) on cloud condensation nuclei (CCN) varies widely in diverse environments. CCN or cloud droplets from NPF sources remain highly uncertain in the urban atmosphere; they are greatly affected by the high background aerosols and frequent local emissions. In this study, we quantified the effect of NPF on cloud droplet number concentration (CDNC, or <span class="inline-formula"><i>N</i>_d</span>) at typical updraft velocities (<span class="inline-formula"><i>V</i></span>) in clouds based on field observations on 25 May–18 June 2017 in urban Beijing. We show that NPF increases the <span class="inline-formula"><i>N</i>_d</span> by 32 %–40 % at <span class="inline-formula"><i>V</i>=0.3</span>–3 m s<span class="inline-formula">⁻¹</span> during the studied period. The <span class="inline-formula"><i>N</i>_d</span> is reduced by 11.8 <span class="inline-formula">±</span> 5.0 % at <span class="inline-formula"><i>V</i>=3</span> m s<span class="inline-formula">⁻¹</span> and 19.0 <span class="inline-formula">±</span> 4.5 % at <span class="inline-formula"><i>V</i>=0.3</span> m s<span class="inline-formula">⁻¹</span> compared to that calculated from constant supersaturations due to the water vapor competition effect, which suppresses the cloud droplet formation by decreasing the environmental maximum supersaturation (<span class="inline-formula"><i>S</i>_max</span>). The effect of water vapor competition becomes smaller at larger <span class="inline-formula"><i>V</i></span> that can provide more sufficient water vapor. However, under extremely high aerosol particle number concentrations, the effect of water vapor competition becomes more pronounced. As a result, although a larger increase of CCN-sized particles by NPF events is derived on clean NPF days when the number concentration of preexisting background aerosol particles is very low, no large discrepancy is presented in the enhancement of <span class="inline-formula"><i>N</i>_d</span> by NPF between clean and polluted NPF days. We finally reveal a considerable impact of the primary sources on the evaluation of the contribution of NPF to CCN number concentration (<span class="inline-formula"><i>N</i>_CCN</span>) and <span class="inline-formula"><i>N</i>_d</span> based on a case study. Our study highlights the importance of full consideration of both the environmental meteorological conditions and multiple sources (i.e., secondary and primary) to evaluate the effect of NPF on clouds and the associated climate effects in polluted regions.</p>