Elsevier

Atmospheric Environment

Volume 144, November 2016, Pages 249-256
Atmospheric Environment

Light absorption of organic aerosol from pyrolysis of corn stalk

https://doi.org/10.1016/j.atmosenv.2016.09.006Get rights and content

Highlights

  • We improve the extraction method and extended the spectral range of OC absorption.

  • We examine light absorption of OA from pyrolysis of corn stalk at three temperature.

  • Absorption of OA has strong spectral dependence and is not be negligible.

  • Higher pyrolysis temperature produced OA with higher absorption.

  • Absorption of OA from pyrolysis of corn stalk and wood appears to some consistency.

Abstract

Organic aerosol (OA) can absorb solar radiation in the low-visible and ultra-violet wavelengths thereby modifying radiative forcing. Agricultural waste burning emits a large quantity of organic carbon in many developing countries. In this work, we improved the extraction and analysis method developed by Chen and Bond, and extended the spectral range of OC absorption. We examined light absorbing properties of primary OA from pyrolysis of corn stalk, which is a major type of agricultural wastes. Light absorption of bulk liquid extracts of OA was measured using a UV–vis recording spectrophotometer. OA can be extracted by methanol at 95%, close to full extent, and shows polar character. Light absorption of organic aerosol has strong spectral dependence (Absorption Ångström exponent = 7.7) and is not negligible at ultra-violet and low-visible regions. Higher pyrolysis temperature produced OA with higher absorption. Imaginary refractive index of organic aerosol (kOA) is 0.041 at 400 nm wavelength and 0.005 at 550 nm wavelength, respectively.

Introduction

Organic aerosol (OA) is ubiquitous, comprising a major fraction (20–90%) of the atmospheric fine aerosol mass (Kanakidou et al., 2005, Zhang et al., 2007) around the world, and has important effects on regional and global climate, air quality visibility, and human health. OA mainly scatters incoming solar radiation and many climate models treat it as a purely cooling agent (Chung and Seinfeld, 2002, Ming et al., 2005, Myhre et al., 2007). However, there is a growing body of evidence that OA can absorb radiation in the low-visible (blue) and ultra-violet (UV) wavelengths thereby modifying radiative forcing (Kirchstetter et al., 2004, Alexander et al., 2008, Chen and Bond, 2010, Chakrabarty et al., 2010, Lack et al., 2012, Saleh et al., 2013, Saleh et al., 2014). Such OA is referred to as brown carbon (BrC) (Andreae and Gelencser, 2006). A few recent climate modelling studies demonstrated that BrC was an important atmospheric absorber, globally and regionally (Park et al., 2010, Feng et al., 2013, Lin et al., 2014, Lu et al., 2015). Due to lack of available measurement data on BrC's light absorbing properties, its absorbing effect was simplified in models, which couldn't reflect its absorptive behavior varieties from different sources.

The complex refractive index (m = nik) is the fundamental parameter to determine the particle's optical properties. BrC's light absorption is often quantified by mass absorption cross section (MAC, m2g−1), and its spectral dependence is described by Absorption Ångström Exponent (AAE). MAC can be calculated using Mie theory if m and particle size are known.

Because BrC is a complex mixture containing a large group of organic compounds with various absorptivities and lacking a formal analytical definition, many methods have been used to determine its light absorbing properties. Kirchstetter et al. (2004) extracted organic carbon (OC) with acetone from the loaded filters and used optical transmission method to determine the difference in the light transmission of the filters before and after extraction, and then estimated optical constants for OC from biomass smoke and motor vehicle. Chen and Bond (2010) extracted OC with different solvents and measured light absorption of bulk liquid extract, and then derived light absorbing properties of OC from wood pyrolysis. Alexander et al. (2008) applied the electron energy-loss spectrum in the transmission electron microscope and quantified the optical properties of individual amorphous carbon spherical particles in East Asia outflow. Yang et al. (2009) used a multi-wavelength aethalometer to measure atmospheric aerosol light absorption, assumed that black carbon (BC) followed an inverse-wavelength relationship from 370 to 950 nm, BC was the only significant light absorber at 950 nm, i.e. light absorption effect of BrC and dust was negligible at this wavelength, and then derived BrC light absorbing properties. Favez et al. (2009) applied the same method to quantify light absorption of organic aerosol from agricultural waste burning. Several studies employed optical closure methods (Chakrabarty et al., 2010, Lack et al., 2012, Saleh et al., 2013, Saleh et al., 2014), namely, the complex refractive index of the particles were retrieved from Mie closure calculation based on particle size, scattering and absorption measurement. Lu et al. (2015) reviewed available measurements of the light-absorption properties of primary OA from various combustion sources.

Since biofuel combustion and biomass open burning contributes about 90% of the total primary OC (Bond et al., 2004), investigation of the light absorbing properties of primary OC has been focused on this type of source. However, most of them concern wood combustion, savanna and forest fire, duff combustion (Kirchstetter et al., 2004, Chen and Bond, 2010, Chakrabarty et al., 2010, Lack et al., 2012, Saleh et al., 2013, Saleh et al., 2014). Agricultural wastes are often burned in the field after harvesting and used as fuel for cooking and heating in developing countries (Li et al., 2007, Li et al., 2009). These emissions contribute 9% to global annual OC emission and 27.5% to its emission in China (Bond et al., 2004, Cao et al., 2006). Only two studies have examined light absorption of OA from burning of rice straw (Favez et al., 2009, Saleh et al., 2014), and it is not known whether OA from waste of other crops, which is structurally different, has similar properties.

In this work, we explored light absorbing properties of primary OA from combustion of corn stalk, which is a major type of agricultural waste. The experiments were designed to generate OA under the pyrolysis combustion phase only, since approximately 80% of OC is emitted in this combustion phase (Einfeld et al., 1991). Roden et al. (2006) used real-time emission measurements to demonstrate that solid-fuel combustion occurs in distinct phases, with black and brown carbon emitted separately. Real-time measurements (Chen et al., 2012) also show that about half of organic carbon mass is emitted in the rapid ejection that occurs after fuel addition. Therefore, separating combustion by phase, rather than separating particles physically, allows exploration of different realistic aerosol types.

We put the corn stalk into an electrically-heated and temperature-controlled pyrolysis reactor, with nitrogen (N2) as carrier gas, and organic aerosol was generated at three temperatures. We used the improved extraction method and extracted organic aerosol samples with methanol and employed a UV–vis spectrophotometer to measure light absorption of the extracts. We derived absorption per mass of organic aerosol (α/ρ), a parameter related to MAC of OC. We also reported imaginary refractive index of organic aerosol (kOA) and its absorption Ångström exponent (AAE).

Section snippets

Generation and collection of organic aerosol

Corn stalks were collected from a field, near University of Illinois at Urbana-Champaign (Illinois, USA), after the corn was harvested. The corn stalks were dried in the lab for over a week and then cut into pieces with about 4 cm in length.

Organic aerosol was generated by pyrolyzing pieces of corn stalks in a temperature-controlled pyrolysis reactor. The reactor consisted of a resistive heating cylinder and a heater with 120 V, 650 W (Watlow VC403A06A). A proportional integral differential

Fraction of total OC extracted by methanol

The fraction of OC extracted by methanol at three pyrolysis temperatures ranged from 90.6% to 98.2%. Average fractions are 95.3 ± 1.8%, 96.1 ± 1.4% and 93.7 ± 2.3% at 210 °C, 270 °C and 360 °C, respectively. Our results for aerosol from pyrolysis of corn stalk are consistent with previous results from wood pyrolysis (Chen and Bond, 2010). The high extraction fraction by methanol indicated that absorption by methanol extraction could represent the behavior of the total OC. No obvious difference

Summary and conclusions

With the improved the extraction method and extended the spectral range of OC absorption, we investigated light absorption of organic aerosol from pyrolysis of corn stalks at different temperatures. The absorption shows strong spectral dependence (AAE = 7.7) and is not be negligible at shorter wavelengths. Absorption per mass of organic carbon increases with increasing pyrolysis temperatures in the range of temperatures we investigated (210 °C–360 °C). Imaginary refractive index of organic

Acknowledgements

This work is supported by the U.S. DOE research grant: “Isolating Weakly and Strongly-Absorbing Classes of Carbonaceous Aerosol: Optical Properties, Abundance and Wet Removal” (Grant No.A0871 DOE DE-SC0006689). Xinghua Li was partly supported by National Natural Science Foundation of China (Grant No. 41275121 and 41575119) and China Scholarship Council.

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