Impact of particle number and mass size distributions of major chemical components on particle mass scattering efficiency in urban Guangzhou in southern China

Peng Cheng,Jun Tao,Renjian Zhang,Zhisheng Zhang,Laiguo Chen,Junji Cao,Zhijun Wu,Mei Li,Yunfei Wu,Leiming Zhang

doi:10.5194/acp-19-8471-2019

Abstract

Abstract. To grasp the key factors affecting particle mass scattering efficiency (MSE), particle mass and number size distribution, PM2.5 and PM10 and their major chemical compositions, and the particle scattering coefficient (bsp) under dry conditions were measured at an urban site in Guangzhou, southern China, during 2015–2016. On an annual average, 10±2 %, 48±7 % and 42±8 % of PM10 mass were in the condensation, droplet and coarse modes, respectively, with mass mean aerodynamic diameters (MMADs) of 0.78±0.07 in the droplet mode and 4.57±0.42 µm in the coarse mode. The identified chemical species mass concentrations can explain 79±3 %, 82±6 % and 57±6 % of the total particle mass in the condensation, droplet and coarse mode, respectively. Organic matter (OM) and elemental carbon (EC) in the condensation mode, OM, (NH4)2SO4, NH4NO3, and crustal element oxides in the droplet mode, and crustal element oxides, OM, and CaSO4 in the coarse mode, were the dominant chemical species in their respective modes. The measured bsp can be reconstructed to the level of 91±10 % using Mie theory with input of the estimated chemically resolved number concentrations of NaCl, NaNO3, Na2SO4, NH4NO3, (NH4)2SO4, K2SO4, CaSO4, Ca(NO3)2, OM, EC, crustal element oxides and unidentified fraction. MSEs of particle and individual chemical species were underestimated by less than 13 % in any season based on the estimated bsp and chemical species mass concentrations. Seasonal average MSEs varied in the range of 3.5±0.1 to 3.9±0.2 m2 g−1 for fine particles (aerodynamic diameter smaller than 2.1 µm), which was mainly caused by seasonal variations in the mass fractions and MSEs of the dominant chemical species (OM, NH4NO3, (NH4)2SO4) in the droplet mode. MSEs of the dominant chemical species were determined by their lognormal size-distribution parameters, including MMADs and standard deviation (σ) in the droplet mode.

Highlights

Light extinction coefficients of atmospheric particles, which are the sum of their scattering and absorption coefficients, is a key index of haze weather (Hand and Malm, 2007)
10 ± 2 %, 48 ± 7 % and 42 ± 8 % of total mass in the size-segregated samples were in the condensation, droplet and coarse modes, respectively, with the average mass mean aerodynamic diameters (MMADs) being 0.78±0.07 μm in the droplet mode and 4.57±0.42 μm in the coarse mode
SO24− and NH+4 were mainly distributed in the droplet mode; elemental carbon (EC) was distributed in both condensation and droplet Na+, Ca2+, and Cl−

Summary

Introduction

Light extinction coefficients (bext) of atmospheric particles, which are the sum of their scattering (bsp) and absorption (bap) coefficients, is a key index of haze weather (Hand and Malm, 2007). Bsp accounted for more than 90 % of bext (Takemura et al, 2002; Tao et al, 2017a). J. Tao et al.: Factors affecting particle mass scattering efficiency and Hand, 2007; Sisler and Latimer, 1993; Sisler et al, 1996; Sisler and Malm, 2000; Wang et al, 2014; Zhao et al, 2013). Knowledge of the dominant chemical species in PM2.5 (e.g., (NH4)2SO4, NH4NO3 and OM) and their contributions to bsp is crucial for making feasible policies for alleviating haze (Watson, 2002)

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Conclusion