Vertical characterization of aerosol optical properties and brown carbon in winter in urban Beijing, China

Conghui Xie,Yingjie Zhang,Weiqi Xu,Qingyun Bian,Yele Sun,Hugh Coe,Junfeng Wang,Jian Zhao,Ping Chen,Tingting Han,Wei Du,Pingqing Fu,Dantong Liu,Zifa Wang,Xinlei Ge,Wei Zhou,Jie Li,Guiqian Tang,Qingqing Wang,J D Allan

doi:10.5194/acp-19-165-2019

Abstract

Abstract. Aerosol particles are of importance in the Earth's radiation budget since they scatter and absorb sunlight. While extensive studies of aerosol optical properties have been conducted at ground sites, vertical measurements and characterization are very limited in megacities. In this work, we present simultaneous real-time online measurements of aerosol optical properties at ground level and at 260 m on a meteorological tower from 16 November to 13 December in 2016 in Beijing along with measurements of continuous vertical profiles during two haze episodes. The average (±1σ) scattering and absorption coefficients (bsca and babs; λ=630 nm) were 337.6 (±356.0) and 36.6 (±33.9) Mm−1 at 260 m, which were 26.5 % and 22.5 % lower than those at ground level. Single scattering albedo (SSA), however, was comparable between the two heights, with slightly higher values at ground level (0.89±0.04). Although bsca and babs showed overall similar temporal variations between ground level and 260 m, the ratios of 260 m to ground varied substantially from less than 0.4 during the clean stages of haze episodes to > 0.8 in the late afternoon. A more detailed analysis indicates that vertical profiles of bsca, babs, and SSA in the low atmosphere were closely related to the changes in meteorological conditions and mixing layer height. The mass absorption cross section (MAC) of equivalent black carbon (eBC, λ=630 nm) varied substantially from 9.5 to 13.2 m2 g−1 in winter in Beijing, and it was strongly associated with the mass ratio of coating materials on refractory BC (rBC) to rBC (MR), and also the oxidation degree of organics in rBC-containing particles. Our results show that the increases in MAC of eBC in winter were mainly caused by photochemically produced secondary materials. Light absorption of organic carbon (brown carbon, BrC) was also important in winter, which on average accounted for 46 (±8.5) % and 48 (±9.3) % of the total absorption at 370 nm at ground level and 260 m, respectively. A linear regression model combined with positive matrix factorization analysis was used to show that coal combustion was the dominant source contribution of BrC (48 %–55 %) followed by biomass burning (17 %) and photochemically processed secondary organic aerosol (∼20 %) in winter in Beijing.

Highlights

Light scattering and absorption of aerosols reduce visibility and affect the radiation and energy budget of the Earth (Rosenfeld, 1999; Kim and Ramanathan, 2008)
Six organic aerosol (OA) factors were identified from High-resolution time-of-flight aerosol mass spectrometer (HR-AMS) measurements including fossil-fuelrelated OA (FFOA) predominantly from coal combustion, cooking OA (COA), biomass burning OA (BBOA), oxidized primary organic aerosol (POA) (OPOA), oxygenated OA (OOA), and aqueous-phase OOA (Xu et al, 2018)
Since the Single scattering albedo (SSA) is the ratio of bsca and bext, the variations in SSA at ground level were similar to those at 260 m for most of the time, several periods with much lower SSA at ground level were observed due to large emissions of black carbon (BC) at midnight, e.g., 17 November and 1 December

Summary

Introduction

Light scattering and absorption of aerosols reduce visibility and affect the radiation and energy budget of the Earth (Rosenfeld, 1999; Kim and Ramanathan, 2008). Scattering aerosols cool the atmosphere and exert a negative forcing while light-absorbing materials warm the atmosphere and, if they exist over a brighter underlying surface, contribute a positive forcing (Haywood and Boucher, 2000). Numerous studies have been conducted to investigate aerosol optical properties, accurate quantification of aerosol radiative forcing still remains challenging (Stocker et al, 2013). A recent study found that BC near the capping inversion is more effective in suppressing planetary boundary layer height and weakening turbulent mixing A recent study found that BC near the capping inversion is more effective in suppressing planetary boundary layer height and weakening turbulent mixing (Z. Wang et al, 2018; Ding et al, 2016)

Methods

Results

Conclusion