Abstract
Abstract. Hygroscopic growth of aerosol particles can significantly affect their single-scattering albedo (ω), and consequently alters the aerosol effect on tropospheric photochemistry. In this study, the impact of aerosol hygroscopic growth on ω and its application to the NO2 photolysis rate coefficient (JNO2) are investigated for a typical aerosol particle population in the North China Plain (NCP). The variations of aerosol optical properties with relative humidity (RH) are calculated using a Mie theory aerosol optical model, on the basis of field measurements of number–size distribution and hygroscopic growth factor (at RH values above 90%) from the 2009 HaChi (Haze in China) project. Results demonstrate that ambient ω has pronouncedly different diurnal patterns from ω measured at dry state, and is highly sensitive to the ambient RHs. Ambient ω in the NCP can be described by a dry state ω value of 0.863, increasing with the RH following a characteristic RH dependence curve. A Monte Carlo simulation shows that the uncertainty of ω from the propagation of uncertainties in the input parameters decreases from 0.03 (at dry state) to 0.015 (RHs > 90%). The impact of hygroscopic growth on ω is further applied in the calculation of the radiative transfer process. Hygroscopic growth of the studied aerosol particle population generally inhibits the photolysis of NO2 at the ground level, whereas accelerates it above the moist planetary boundary layer. Compared with dry state, the calculated JNO2 at RH of 98% at the height of 1 km increases by 30.4%, because of the enhancement of ultraviolet radiation by the humidified scattering-dominant aerosol particles. The increase of JNO2 due to the aerosol hygroscopic growth above the upper boundary layer may affect the tropospheric photochemical processes and this needs to be taken into account in the atmospheric chemical models.
Highlights
Single-scattering albedo (ω) is one of the most important aerosol optical properties
Particle number–size distribution (PNSD) of the externally mixed light-absorbing carbonaceous (LAC) and core-shell mixed particles, and the diameters of the cores (Dcore) in different sizes of core-shell particles can be obtained from the parameter retrieved in the mixing state (Ma et al, 2012)
Under the assumption in Sect. 2.2.1, total PNSD of the aerosol population is comprised of subset PNSDs of the externally mixed LAC and the core-shell mixed particles, which are needed in the Mie calculation
Summary
Single-scattering albedo (ω) is one of the most important aerosol optical properties It influences the aerosol’s radiative effect and is a significantly uncertain factor. The growth factors at RHs of up to 98.5 % measured by a high humidity tandem differential mobility analyzer (HH-TDMA) indicated the existence of a dominant more-hygroscopic group of aerosols (Liu et al, 2011). This result agreed well with the retrieved values from the microbalance UMT-2 (Mettler Toledo, Switzerland) (Liu et al, 2014) and in combination contributed to enhancing extinction at high RHs and to the low visibility on hazy days (Chen et al, 2012).
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