10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Source Apportionment of Brown Carbon Absorption by Coupling Ultraviolet-Visible Spectroscopy with Aerosol Mass Spectrometry
VAIOS MOSCHOS, Nivedita Kumar, Kaspar Rudolf Dällenbach, Urs Baltensperger, Andre S.H. Prévôt, Imad El Haddad, Paul Scherrer Institute / ETH Zurich
Abstract Number: 819 Working Group: Source Apportionment
Abstract The optical properties and sources of atmospheric aerosols are of prime importance in the context of a changing climate.1 Primary and secondary organic aerosol (OA) emissions introduce light-absorbing compounds (brown carbon, BrC) in the atmosphere that affect tropospheric chemistry and might exert, along with soot carbon, a significant direct radiative forcing on the climate system.2 This effect has been largely acknowledged3 but remains uncertain, since either most climate models have been ignoring the contribution of BrC to absorption or the associated mixed emission sources4 and atmospheric lifetime5 are not accounted for.
In this work, we propose positive matrix factorization (PMF) as a framework to apportion the contributions of individual primary and secondary OA source components of BrC absorption, by combining long-term aerosol mass spectrometry (AMS) data with concurrent ultraviolet-visible spectroscopy measurements. The former feature time-dependent factor contributions to OA mass4 and the latter consist of wavelength-dependent absorption coefficients, determined using real-world mixed-source samples.
Using this approach for a full-year case study, we estimate for the first time the mass absorption efficiency (MAE) of major light-absorbing water-soluble OA components in the atmosphere. We show that secondary biogenic emissions, largely consisting of low molecular weight monoterpene oxidation products, contribute negligibly to absorption despite dominating the mass concentration in summer. In contrast, the MAE of strongly absorbing BrC from primary and aged wood burning emissions can be constrained within a confined range. For the primary emissions, this range is consistent with previous laboratory tests of open and residential burning of biomass or other fuels and near-source ambient samples, but lower than non-conventional BrC types (e.g., funeral pyres and humic-like substances). The MAE of the aged emissions follows that of common anthropogenic precursor (benzene, naphthalene, toluene) secondary OA formed under high NOx conditions. We note the reduced MAE of aged vs primary wood burning emissions at most wavelengths, in agreement with recent laboratory experiments and a post-biomass-burning event observation.
The MAE constrained here can be used to predict the impact of the identified BrC sources on climate, through Mie calculations and radiative transfer modeling. This novel platform may be applied to other datasets, including environments that are heavily polluted or largely represented by other sources (e.g. coal combustion in China). The approach may also be suited for online datasets, acquired using aerosol mass spectrometry and various online optical measurement techniques. The sensitivity of the PMF model results to the applied mass spectrometric analysis has to be assessed in similar future studies.
This work has been supported by funding from the European Union’s Horizon 2020 research and innovation programme under Grant 689443 via project ERA-PLANET (The European network for observing our changing planet) and the Swiss Federal Office for the Environment (FOEN). VM acknowledges the Onassis Foundation (Greece) for a graduate student scholarship.
1. Pöschl U. (2005). Atmospheric aerosols: Composition, transformation, climate and health effects. Angew. Chem. Int. Ed. 44: 7520-7540. 2. Feng Y, Ramanathan V, Kotamarthi VR. (2013). Brown carbon: a significant atmospheric absorber of solar radiation?. ACP. 13: 8607-8621. 3. Alexander DTL, Crozier PA, Anderson JR. (2008). Brown carbon spheres in East Asian outflow and their optical properties. Science. 321: 833-836. 4. Daellenbach KR, et al. (2017). Long-term chemical analysis and organic aerosol source apportionment at nine sites in central Europe: source identification and uncertainty assessment. ACP. 17: 13265-13282. 5. Sumlin BJ, et al. (2017). Atmospheric photooxidation diminishes light absorption by primary brown carbon aerosol from biomass burning. ESTL. 4: 540-545.