Methanol Extracted Brown Carbon in PM2.5 Over Xi’an, China: Seasonal Variation of Optical Properties and Sources Identification

Abstract

Aerosol optical properties, including absorption coefficient (b abs ), absorption Å ngström exponent (AAE), and mass absorption coefficient (MAC), of methanol extracts in PM 2.5 samples were measured at an urban site in Xi'an, northwest China in a winter and summer season. Water soluble ions, organic carbon (OC), element carbon, eight carbon fractions, levoglucosan, and 13 polycyclic aromatic hydrocarbons were also determined to explore the sources of brown carbon (BrC) in PM 2.5 . b abs at 365 nm wavelength showed a clear seasonal variation with much higher value in winter than summer. High levoglucosan and K ? concentrations in winter and their good correlations with b abs inferred the important contribution of biomass burning to BrC. Good correlations between b abs and OC1, OC2, OC3, or OP indicated the mixed sources of biomass burning and coal combustion for BrC. Relatively poor correlations were found between b abs and indeno[1,2,3-cd]pyrene (IP) or Benzo[ghi]-perylene (BghiP), indicating limited contribution of vehicle exhaust to BrC. Seasonal distribution of AAE indicated that secondary organic carbon (SOC) BrC was mainly fresh SOC in winter but a mix of fresh and aged SOC in summer. MAC of BrC in winter was more than double of that in summer, suggesting that BrC from biomass burning and fresh SOC were stronger light absorbing in winter than the aged SOC in summer. A 7-wavelength Aethalometer model AE31 was used to evaluate the seasonal variations of b abs for BrC (370 nm) and black carbon (BC) (880 nm). BrC and BC were both important in light absorption in winter, but BC prevailed over BrC in summer.

Light absorption of brown carbon in eastern China based on 3-year multi-wavelength aerosol optical property observations and an improved absorption Ångström exponent segregation method

“…P BrC also presents a two-peak seasonal trend, with the high P BrC months of May-June and December. The mean P BrC in winter and summer is 19.6 and 14.4 %, respectively, which is lower than that in Xi'an but higher than the PRD region (Shen et al, 2017;Yuan et al, 2016). The 95th percentile of P BrC can reach to 32 % in December, which certainly cannot be ignored in the light absorption estimation in the YRD region.…”

“…The wavelength dependence of the aerosol absorption coefficient (b abs ) is normally represented by the absorption Ångström exponent (AAE), and its relationship with b abs is b abs (λ) ∝ λ −AAE (Moosmüller et al, 2011;Sun et al, 2007). Based on the difference of wavelength dependence for BC and BrC, previous studies segregated light absorption of BrC from multi-wavelength optical measurements (Lack and Langridge, 2013;Mohr et al, 2013;Shen et al, 2017), which is called the AAE method. Earlier, the similar concept was used to segregate carbonaceous aerosol fractions from different emission sources based on their difference in AAE value (e.g., wood burning and traffic emission) (Sandradewi et al, 2008;Healy et al, 2012).…”

“…Earlier, the similar concept was used to segregate carbonaceous aerosol fractions from different emission sources based on their difference in AAE value (e.g., wood burning and traffic emission) (Sandradewi et al, 2008;Healy et al, 2012). Usually, the AAE of BC (AAE BC ) was set to be 1.0 based on the properties of bulk BC by many previous studies, assuming the unity of AAE BC between any two wavelengths within the UV to the near-infrared (IR) range (Shen et al, 2017;Olson et al, 2015;Lack and Langridge, 2013). However, large uncertainty was mentioned by Lack and Langridge (2013) when using this assumption, and also this method can only be used when the proportion of BrC is high.…”

“…Yuan et al (2016) reported light absorption contributions of BrC in the Pearl River Delta (PRD) region to be 6.3 to 12.1 % at 405 nm in autumn and winter campaigns by using the AAE method. Shen et al (2017) conducted light absorption measurements in Xi'an in summer and winter, and reported the average b abs_BrC_370 of 6.4 and 43.0 Mm −1 in the two seasons, respectively, based on the AAE method. Yang et al (2009) calculated the mass absorption efficiency of BrC at 370 nm to be 2.2 m 2 g −1 and the AAE of BrC to be 3.5, respectively, during a campaign in March 2015 at Xianghe in northern China.…”

Sources and formation of carbonaceous aerosols in Xi'an, China: primary emissions and secondary formation constrained by radiocarbon

“…ECfossil values are on average 2-3 times higher than ECbb in winter and 5-8 times higher in other seasons. This implies that the winter-summer differences in biomass burning emissions is larger than fossil fuel combustion emissions, regardless of the fact that both biomass burning and coal combustion are expected to increase during wintertime for heating (T. Shen et al, 2017;Zhu et al, 2017). OC concentrations from fossil fuel combustion (OCfossil) range from around 1 to 20 μg m -3 , with an annual average of 6.8 ± 6.0 μg m -3 , which is comparable to non-fossil OC concentrations (range: 2-28 5 μg m -3 ; mean: 8.2 ± 8.2 μg m -3 ).…”