Source-specific light absorption by carbonaceous components in the complex aerosol matrix from yearly filter-based measurements

Abstract

Abstract. Understanding the sources of light-absorbing organic (brown) carbon (BrC) and its interaction with black carbon (BC) and other non-refractory particulate matter (NR-PM) fractions is important for reducing uncertainties in the aerosol direct radiative forcing. In this study, we combine multiple filter-based techniques to achieve long-term, spectrally resolved, source- and species-specific atmospheric absorption closure. We determine the mass absorption efficiency (MAE) in dilute bulk solutions at 370 nm to be equal to 1.4 m2 g−1 for fresh biomass smoke, 0.7 m2 g−1 for winter-oxygenated organic aerosol (OA), and 0.13 m2 g−1 for other less absorbing OA. We apply Mie calculations to estimate the contributions of these fractions to total aerosol absorption. While enhanced absorption in the near-UV has been traditionally attributed to primary biomass smoke, here we show that anthropogenic oxygenated OA may be equally important for BrC absorption during winter, especially at an urban background site. We demonstrate that insoluble tar balls are negligible in residential biomass burning atmospheric samples of this study and thus could attribute the totality of the NR-PM absorption at shorter wavelengths to methanol-extractable BrC. As for BC, we show that the mass absorption cross-section (MAC) of this fraction is independent of its source, while we observe evidence for a filter-based lensing effect associated with the presence of NR-PM components. We find that bare BC has a MAC of 6.3 m2 g−1 at 660 nm and an absorption Ångström exponent of 0.93 ± 0.16, while in the presence of coatings its absorption is enhanced by a factor of ∼ 1.4. Based on Mie calculations of closure between observed and predicted total light absorption, we provide an indication for a suppression of the filter-based lensing effect by BrC. The total absorption reduction remains modest, ∼ 10 %–20 % at 370 nm, and is restricted to shorter wavelengths, where BrC absorption is significant. Overall, our results allow an assessment of the relative importance of the different aerosol fractions to the total absorption for aerosols from a wide range of sources and atmospheric ages. When integrated with the solar spectrum at 300–900 nm, bare BC is found to contribute around two-thirds of the solar radiation absorption by total carbonaceous aerosols, amplified by the filter-based lensing effect (with an interquartile range, IQR, of 8 %–27 %), while the IQR of the contributions by particulate BrC is 6 %–13 % (13 %–20 % at the rural site during winter). Future studies that will directly benefit from these results include (a) optical modelling aiming at understanding the absorption profiles of a complex aerosol composed of BrC, BC and lensing-inducing coatings; (b) source apportionment aiming at understanding the sources of BC and BrC from the aerosol absorption profiles; (c) global modelling aiming at quantifying the most important aerosol absorbers.

Photodegradation of α-Pinene Secondary Organic Aerosol Dominated by Moderately Oxidized Molecules

“…40 For this calculation, we assumed an average molecular weight of 270 g mol −1 and accounted for enhanced absorption in the particle phase due to scattering using Mie calculations. 47 We note that the latter effect is trivial for the particle diameter of 120 nm under our conditions and implies an enhancement factor of the MAC values measured in diluted solutions of less than 1.2. The J SOA,theor value obtained is 3.3 × 10 −4 s −1 , a factor of ∼2 lower than the measured values.…”

“…We compared the J SOA,chamber values obtained under our conditions at high mass concentration with theoretical values (denoted J SOA,theor ), which are calculated based on the effective mass absorption cross section (MAC) of the bulk α-pinene SOA measured by Romonosky et al These values should be considered as upper limits as we assume effective quantum yields of unity, similar to small aldehydes . For this calculation, we assumed an average molecular weight of 270 g mol –1 and accounted for enhanced absorption in the particle phase due to scattering using Mie calculations . We note that the latter effect is trivial for the particle diameter of 120 nm under our conditions and implies an enhancement factor of the MAC values measured in diluted solutions of less than 1.2.…”

Characterization of soot produced by the mini inverted soot generator with an atmospheric simulation chamber

“…Prior to EC / OC determination, particle-loaded filters were analysed by the multi-wavelength absorbance analyzer (MWAA; Massabò et al, 2013Massabò et al, , 2015, a laboratory instrument for the offline direct quantification of the aerosol absorption coefficients at five different wavelengths (λ equal to 850, 635, 532, 405, and 375 nm). Such features have previously been exploited in the frame of several field campaigns in urban and rural sites (Scerri et al, 2018;Massabo et al, 2019Massabo et al, , 2020Moschos et al, 2021) and in remote sites (Massabò et al, 2016;Saturno et al, 2017;Baccolo et al, 2020).…”

Reply on RC4

“…CC BY 4.0 License. (Scerri et al, 2018;Massabo et al, 2019;Massabo et al, 2020;Moschos et al, 2021), as well as in peculiar and remote sites (Massabo et al, 2016;Saturno et al, 2017;Baccolo et al, 2021).…”