Impact of aerosol hygroscopic growth on the direct aerosol radiative effect in summer on North China Plain Y. Kuang a , C.S. Zhao a, * , J.C. Tao a , Y.X. Bian a , N. Ma b a Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China b Leibniz Institute for Tropospheric Research, Leipzig, Germany highlights  High frequency of high RH (>80%) proﬁles within the mixed layer during daytime.  Aerosol optical properties highly depend on RH proﬁles due to aerosol water uptake.  Aerosol hygroscopicity contributes to direct aerosol radiative effect signiﬁcantly. article info Article history: Received 8 April 2016 Received in revised form 3 October 2016 Accepted 7 October 2016 Available online 8 October 2016 Keywords: Relative humidity proﬁles Aerosol optical depth Single scattering albedo Asymmetry factor Aerosol optical properties abstract In this paper, relative humidity (RH) proﬁles and their impacts on the vertical variations of aerosol optical properties and the direct aerosol radiative effect (DARE) have been investigated based on surface measurements from the Haze in China campaign and sounding data from the North China Plain. Among the proﬁles obtained from July to September in 2008, about half have RHs greater than 80% within the mixed layer. The vertical variations in the aerosol optical properties at ambient RH, including the extinction coefﬁcient (s ext ), single scattering albedo (SSA) and asymmetry factor (g), are remarkably different from the variations in the dry aerosols and are highly dependent on the RH proﬁles. Increases of the aerosol optical depth and column-averaged SSA and g due to aerosol water uptake can reach up to 64%, 0.052 and 0.079, respectively. The fractional contribution to the instantaneous DARE at the top of the atmosphere due to aerosol hygroscopic growth reaches 60% in high RH proﬁles. DARE estimates can be signiﬁcantly biased if the RH dependence of SSA or g is not considered. We suggest that if their vertical proﬁles or column-averaged values are absent, then the ambient values of SSA and g at the surface should be used rather than the values of SSA and g obtained from dry aerosols when estimating DAREs. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Tropospheric aerosols affect the energy balance on the Earth by directly scattering and absorbing solar radiation and indirectly changing the properties and lifespans of clouds (Boucher et al., 2013). The changes in irradiance due to the interaction of aerosol and radiation are commonly described in terms of the direct aerosol radiative effect (DARE), and their estimates remain largely uncertain (Boucher et al., 2013). These uncertainties are mainly caused by the high temporal, spatial and compositional variability of aerosols as well as RH ﬁelds and an incomplete understanding of related, integral aerosol optical properties. Because many components of aerosol particles are hygroscopic, the particles can take up water and change in size depending on their chemical composition and the ambient relative humidity (RH). Hygroscopic growth becomes particularly important when the RH is greater than 60% and water often comprises more than 50% of the ﬁne particle mass when the RH exceeds 70e80% (Bian et al., 2014; McMurry, 2000). Therefore, RH is an important parameter that governs the optical properties of aerosols and DARE. Previously, numerous studies have investigated the dependence of aerosol optical properties on RH by considering the aerosol scat- tering coefﬁcient (s sp )(Zieger et al., 2013), single scattering albedo (SSA) (Tao et al., 2014; Yoon and Kim, 2006), hemispheric scattering coefﬁcient (Titos et al., 2014) and asymmetry factor (g)(Yoon and Kim, 2006). However, these studies were mainly based on surface measurements and only considered the RH at the surface. Few studies have investigated the inﬂuence of RH proﬁles on the * Corresponding author. E-mail address: zcs@pku.edu.cn (C.S. Zhao). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv http://dx.doi.org/10.1016/j.atmosenv.2016.10.013 1352-2310/© 2016 Elsevier Ltd. All rights reserved. Atmospheric Environment 147 (2016) 224e233