Characterization of carbonaceous aerosols during TRACER-CAT

Abstract

Through their ability to absorb sunlight, absorbing aerosols have important impacts on the global radiation budget and cloud properties. Absorbing aerosol is a general term; the composition and properties of absorbing aerosols can vary substantially throughout the atmosphere, depending on their source and the extent to which they are transformed by chemical reactions. While the understanding of absorbing aerosol properties and effects has improved tremendously over the past decades, notable uncertainties remain. A key contributor to these uncertainties is incomplete understanding of the extent to which mixing with or coating of absorbing aerosols by other aerosol components—and by water especially—alters absorbing aerosol absorption.

To improve understanding of absorbing aerosol properties and behavior in the atmosphere, we will deploy new and existing experimental tools during the Tracking Aerosol Convection Interactions Experiment (TRACER) campaign in Houston, Texas as part of the Carbonaceous Aerosols Thrust (CAT). Using these tools, we will characterize a wide range of aerosol optical, chemical, and physical properties in detail. Our proposed TRACER-CAT measurements complement and expand on the planned TRACER instrumentation, allowing for more detailed characterization of aerosol properties of relevance to cloud development (a core focus of TRACER), such as the composition of particles that can act as cloud condensation nuclei, than would otherwise be available. Our measurements will also allow for assessment of the relationship(s) between absorbing aerosol optical properties (with a focus on absorption) and the chemical and physical characteristics (including the mixing state of black carbon (BC) containing particles). Finally, we will establish the response of absorption to relative humidity increases and mixing of absorbing aerosols with other aerosol components.

These field observations will occur in collaboration with Los Alamos National Laboratory in summer 2021 during the TRACER intensive operating period. The instrumentation we co-deploy will provide for measurement of many aerosol characteristics, including the following:

(i) multi-wavelength dry aerosol absorption, scattering, and extinction

(ii) the size-dependent composition and abundance of sub-micron aerosol, differentiating between those particles that do and do not contain black carbon

(iii)black carbon-specific concentrations and size distributions and total sub-micron particle concentrations and size distributions, and

(iv)the first field measurements at an ARM site of the influence of relative humidity on multi-wavelength absorption by ambient absorbing aerosols.

We will use the natural variability of the atmosphere and of aerosol sources in the Houston region to:

(i) specifically disentangle contributions to light absorption from black carbon, absorbing organic carbon (brown carbon), and coatings on black carbon

(ii) characterize the mixing state of black carbon—where mixing state refers to compositional differences between different particles—and assess the factors that give rise to compositional differences between black carbon-containing and black carbon-free aerosol, and

(iii) establish how water uptake influences absorption and how any such effect depends on particle composition and black carbon mixing state.

Overall, our study will contribute to the mission of the Atmospheric System Research program in multiple ways. Through the deployment of complementary, advanced instrumentation for characterization of a wide range of aerosol properties our proposed work will help to maximize the scientific impact of the TRACER campaign. Our work will also allow for development of new insights into the relationship(s) between aerosol composition, hygroscopicity, and the mixing state of BC with aerosol optical properties. Through this, our work will provide knowledge that can improve understanding and model representation of aerosol processes as they affect the Earth’s radiation budget.