Characterization of the light absorbing properties, chromophores composition and sources of brown carbon aerosol in Xi'an, Northwest China

Abstract

<p><strong>Abstract.</strong> The impact of brown carbon aerosol (BrC) on the Earth's radiative forcing balance has been widely recognized but remains uncertain, mainly because the relationships among BrC sources, chromophores, and optical properties of aerosol are poorly understood. In this work, the light absorption properties and chromophore composition of BrC were investigated for samples collected in Xi'an, Northwest China from 2015 to 2016. Both absorption Ångström exponent and mass absorption efficiency show distinct seasonal differences, which could be attributed to the differences in sources and chromophore composition of BrC. Three groups of light-absorbing organics were found to be important BrC chromophores, including those show multiple absorption peaks at wavelength > 350 nm (12 polycyclic aromatic hydrocarbons and their derivatives) and those show single absorption peak at wavelength < 350 nm (10 nitrophenols and nitrosalicylic acids and 3 methoxyphenols). These measured BrC chromophores show distinct seasonal differences and contribute on average about 1.1 % and 3.3 % of light absorption of methanol-soluble BrC at 365 nm in summer and winter, respectively, about 7 and 5 times higher than the corresponding carbon mass fractions in total organic carbon. The sources of BrC were resolved by positive matrix factorization (PMF) using these chromophores instead of commonly used non-light absorbing organic markers as model inputs. Our results show that in spring vehicular emissions and secondary formation are major sources of BrC (~ 70 %), in fall coal combustion and vehicular emissions are major sources (~ 70 %), in winter biomass burning and coal combustion become major sources (~ 80 %), while in summer secondary BrC dominates (~ 60 %).</p>

Substantial decreases of light absorption, concentrations and relative contributions of fossil fuel to light-absorbing carbonaceous aerosols attributed to the COVID-19 lockdown in east China

“…Previous numerous studies have shown that combustion of fossil fuel and biomass burning (BB) were major sources of BC aerosols ( Zhao et al, 2012 ; Cheng et al, 2013 ; Zhang et al, 2017 ; Ni et al, 2018 ). Although the origins of BrC are not well characterized, biomass burning, coal combustion and vehicle emissions along with biogenic release of fungi, plant debris and humic matter are considered important sources of primary BrC ( Andreae and Crutzen, 1997 ; Rizo et al, 2011 , 2013 ; Yan et al, 2017 ; Zhang et al, 2017 ; Yuan et al, 2019 ). Secondary BrC can be produced by multi-phase reactions between gas-phase, particles and in-cloud processes ( Harrison et al, 2005 ; Nozière et al, 2007 ; De Hann et al, 2009 ).…”