The evolution of cloud microphysics upon aerosol interaction at the summit of Mt. Tai, China

Abstract. The influence of aerosols, both natural and anthropogenic, remains a major area of uncertainty when predicting the properties and behaviour of clouds and their influence on climate. In an attempt to understand better the microphysical properties of cloud droplets, the aerosol-cloud interactions, and the corresponding climate effect during cloud life cycles in the North China Plain, an intensive observation took place from 17 June to 30 July 2018 at the summit of Mt. Tai. Cloud microphysical parameters were monitored simultaneously with number concentrations of cloud condensation nuclei (NVcx) at different supersaturations, PM>« mass concentrations, particle size distributions and meteorological parameters. Number concentrations of cloud droplets (Nc), liquid water content (LWC) and effective radius of cloud droplets (tco) show large variations among 40 cloud events observed during the campaign. Perturbations of aerosols will significantly increase the N, of cloud droplets and shift cloud droplets toward smaller size ranges. Clouds in clean days are more susceptible to die change in concentrations of particle number (Np). LWC shows positive correlation with frfr. As Nc increases, rar changes from a trimodal distribution to a unimodal distribution. By assuming a cloud thickness of 100 m, we find mat the albedo can increase 36.4% if the cloud gets to be disturbed by aerosols. This may induce a cooling effect on the local climate system. Our results contribute more information about regional cloud microphysics and will help to reduce the uncertainties in climate models when predicting climate responses to cloud-aerosol interactions.

Introduction

Clouds represent a key process In (he atmospheric hydrological cycle, which plays an important role In the atmospheric energy budget and significantly influence the global and regional climate (Chang ctal..20l9;Zhangctal.,2004b). Clouds can be physically described by their liquid water contents (LWC). number concentrations of droplets (Ni land effective radius of droplets (r,ri>. These parameters may show small inter-annual variations for the same monitoring station (M oiler ct al. 1996), but they vary over a large range among different cloud types (Quantc. 2004), cloud altitudes (Padmakumari et a). 2017 /hao el al . 2018) and in different parts of a cloud (Deng ct al.. 2009) Anthropogenic aerosol emission increases the number of cloud condensation nuclei (CCN) and hence they increase the number of cloud droplets, which is called the "Twomcy Effect" (Twomcy, 1974).

The interactions between the clouds and the aerosols behave in complicated ways. Clouds efficiently remove aerosols by activating CCN to cloud droplets (Croft el al 2010,Zhang el al. 2004a). The cloud processes can increase particles sizes (Hereru ct al.. 2018) and alter the CCN compositions through homogeneous and heterogeneous reactions (Roth ct al.. 2016). A recent study found that new particle formation near the cloud edges is probably attributable to the I V irradiation enhanced by the cloud reflection (Wehncr ct a). 20151 In addition, the increase in aerosol concentrations alters the cloud microphysics. which has been investigated for cloud processes under clean and polluted conditions. Padmakumari ct al. (2017) found that the convcclivc clouds over land were characterized by lower LWC and higher N, due to the perturbation of pollution aerosol. Ground-based observations by radiometers during the summers of the U.S. Mid-Atlantic region revealed that cloud events withsmallcidroplcts<< 7 pm) occurred more frequently in the polluted years than in the clean years (Li ct al.,2017b) Ebmeicr et a) (2014) also found a strong anti-correlation between the aerosol optical depth (AOD) and r,i» of the local clouds downwind from degassing volcanoes In Brazil, the N< of cumulus clouds was little influenced by the aerosol particles under polluted conditions, and only r,» correlated well with LWC (Rcid et al. 1999). The influence of pollution aerosols on the cloud microphysics is evident but varies for different regions and for different cloud types.