Accurate and Direct Measurements of Brown Carbon Aerosol Optical Properties During Formation and Atmospherically-Relevant Ageing Processes

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Aerosols are liquid or solid particles suspended in a gas and are pervasive in our atmosphere, with sources including anthropogenic emissions from burning of fossil fuels, and natural sources including from sea spray, desert dust and wildfire biomass burning. These aerosols have significant impacts on our atmosphere, affecting human health through, for example, smog events and global climate through interacting with Sun light and cloud droplets. Indeed, aerosols represent one of the largest uncertainties in predicting future climate change. The net aerosol cooling effect, provided by aerosol scattering sunlight back to space, partially offsets the warming impact of greenhouse gases. However, large uncertainties in this aerosol-light interaction degrade the confidence we have in models of future climate. Improvements to our understanding of aerosol-light interactions could lead to more effective risk mitigation strategies in managing climate change impacts.The important parameters to measure for constraining estimates of aerosol-light interactions are the magnitudes of light scattering and absorption by aerosol. In particular, light absorption is studied poorly for carbonaceous aerosol, with the optical properties of a class of aerosol called brown carbon aerosol (BrC) understood very poorly. BrC particles are formed readily in biomass burning regions where gaseous organic molecules emitted during burning rapidly condense onto liquid or solid particles, with these organic molecules reacting on particle surfaces or inside liquid particles to form light absorbing chromophores. The subsequent BrC particles possess strong wavelength-dependent absorption spectra, with stronger absorption at shorter (blue) optical wavelengths compared to longer (red) wavelengths giving a brown appearance. Also, atmospheric BrC consists of a variety of molecular species with differing light absorption spectra, while the compositions of these chromophores evolve significantly with atmospheric ageing. The uncertainties in BrC optical properties, and how they evolve with time and atmospheric processing, are understood so poorly that many climate models - including UK Met Office climate models - are devoid of any BrC representation. Thus, it is of paramount importance that our understanding of BrC optical properties is improved for better BrC representations in climate models.Traditional measurement approaches have shortcomings in measuring BrC optical properties accurately due to the relatively weak absorption by BrC. Moreover, common laboratory techniques for probing aerosol properties do not access the long ageing timescales of >50 hours that often pertain to atmospheric BrC. This work uses new state-of-the-art instruments available only in the UK to provide measurements of both light scattering and absorption by weakly absorbing aerosol with unrivalled accuracy, precision and sensitivity. Such tools include Single Particle Cavity Ring-Down Spectroscopy (SP-CRDS) and photoacoustic spectroscopy, with single particle trapping techniques such as SP-CRDS allowing measurements of aerosol optical properties on unlimited ageing timescales while particles are subjected to controlled ambient conditions. BrC optical properties will be measured during the BrC formation process and for subsequent ageing and atmospheric processing, such as changes in humidity, exposure to ultraviolet light and reaction with ozone. Furthermore, the proposal addresses both of the common BrC formation pathways, from reaction of gas precursors in aqueous droplets or from the heterogeneous reactions of gas precursors directly on particle surfaces. These results will be used to assess the sensitivity of aerosol-radiation models used at the UK Met Office to measured variations in BrC optical properties and to develop parameterisations of the ageing of BrC optical properties for comparison to recent field studies and future implementation in the next generation of climate models.