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

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.

Atmospheric aerosols represent one of the largest uncertainties in climate models. Policy makers need reliable climate forecasts to manage and mitigate impacts of climate change on society, such as rising temperatures and sea levels and extended periods of flooding and drought. Improving the predictive skill of climate models requires a better understanding of atmospheric aerosol. Aerosols directly affect atmospheric temperatures via light scattering and absorption, with aerosol light absorption known poorly in particular. The light scattering and absorption of a range of brown carbon aerosol (BrC), representing an ill studied class of light absorbing aerosol, will be characterised in this work using state-of-the-art techniques available only in the UK. This project will provide the critically needed intrinsic optical properties for implementing BrC representations in climate models. The outcomes of this proposal will benefit society, improving predictive skill in climate models and thereby society's preparedness for global change. Public funded institutes and researchers will benefit from this work, and international researchers will benefit from reduced aerosol uncertainties in Intergovernmental Panel on Climate Change assessments.

This work will benefit the Met Office (MO), a UK public funded research institute and world leading climate modelling centre. This project involves a significant collaboration with the MO, using and developing their state-of-the-art E-CRD-PAS instrument for measurement of aerosol optical properties in combination with University of Bristol (UoB) SP-CRDS instruments. The E-CRD-PAS is a valuable UK research asset designed to operate on the UK research aircraft, while this laboratory-based project enhances the value and utility of the E-CRD-PAS. Ultimately, developments made to the E-CRD-PAS during this project (e.g. improving measurement sensitivity and accuracy in aerosol optical properties) will benefit the whole UK airborne research community that includes many NERC-funded researchers from Universities including those of Manchester, York, Leeds and Exeter. Furthermore, a key MO science objective is to understand and develop parameterisations for BrC optical properties. While the MO is approaching this objective from an airborne observations perspective, this project addresses this MO science goal though a series of carefully controlled laboratory studies to elucidate evolving BrC optical properties on atmospherically relevant timescales and processing. This work develops parameterisations of BrC optical properties with evolving age for implementation in the next generation of climate models.

This work benefits the wider aerosol science community. The proposal studies the impacts of changes in initial particle and gas phase composition on reaction rates for forming light absorbing BrC chromophores. These measurements will represent the most accurate and systematic study of in situ particle light absorption for BrC formed from the reaction of ammonium- and amine- containing particles with alpha-dicarbonyl gas, and the impacts of ambient humidity and exposure to UV light and ozone. Importantly, using UoB instruments, ageing of laboratory-generated BrC on extended >50 hour timescales relevant to atmospheric aerosol, will be performed for the first time. These studies allow changes in aerosol composition to be related directly to changes in aerosol optical properties that are of most relevance to climate models.

Dr Cotterell will benefit from this outstanding opportunity to build his independent research profile and establish a reputation in state-of-the-art research on aerosol optics. Dr Cotterell will use access to the NERC GW4+ Doctoral Training Partnership to provide potential PhD recruitment and build his research group. He will supervise final year undergraduate project students at UoB, who will mutually benefit from exposure to leading atmospheric science research outside of UoB.