Abstract

Light-absorbing carbonaceous aerosols influence Earth's climate system by absorbing solar radiation, thereby heating the atmosphere and upon deposition, accelerating the melting of the cryosphere. While black carbon (BC) has traditionally been considered the primary driver of this radiative forcing, field observations in wildfire plumes in the past decade have revealed that brown carbon (BrC), specifically a highly absorbing subclass known as dark brown carbon (d-BrC), contributes three-quarters of the shortwave absorption. Despite its ubiquity and potent light-absorbing properties, the radiative impacts of d-BrC remain poorly constrained and are largely omitted from global climate models. This dissertation addresses these critical knowledge gaps in the atmosphere and cryosphere through a multi-pronged and multi-domain approach encompassing field campaign data, radiative transfer modeling, and the development of a novel laboratory set up to quantify the radiative impacts of d-BrC. The atmospheric prevalence and radiative impact of d-BrC beyond wildfire emissions has not yet been systematically investigated. Specifically, this includes the poor understanding of d-BrC within fossil fuel emission plumes in urban environments (hereafter, referred to as urban d-BrC). Study 1 utilizes single-particle scale optical properties derived from the TRacking Aerosol Convection interactions ExpeRiment (TRACER) campaign as a basis for radiative transfer simulations to quantify the surface dimming effect of urban d-BrC. The results demonstrate that urban d-BrC accounts for 40% of daytime-mean surface dimming and 50% of the total top-of-atmosphere radiative forcing attributable to light-absorbing carbon. The analysis also shows that more than 90% of the observed surface dimming due to urban d-BrC is attributable directly to absorption rather than scattering. These findings expose a systemic underestimation of lower atmosphere solar heating in urban areas and underscore the necessity of incorporating d-BrC into urban aerosol-radiation models. Study 2 evaluates the impact of d-BrC deposition on snow albedo and the resulting radiative forcing in the cryosphere. As wildfires increase in frequency and scale near glaciated regions, the deposition of biomass-burning aerosols accelerates snowmelt. Through comprehensive aerosol-snow radiative transfer calculations, this study quantifies the specific contribution of d-BrC to snow darkening. The results indicate that d-BrC deposition increases the annual mean snow radiative forcing by 0.6 to 17.9 W m$^{-2}$, representing a 1.6- to 2.1- fold enhancement over scenarios that only consider BC deposition. Furthermore, seasonal analyses for mid-latitude sites demonstrate that d-BrC-driven radiative forcing peaks during critical melt periods, with summer forcing reaching 70\% higher than the corresponding annual mean. These findings identify d-BrC as a critical, yet previously unrecognized, driver of snowmelt, particularly in mid-latitude mountain glaciers. Study 3 designs and develops a “Snowpack-on-a-Bench” apparatus to perform controlled dry deposition experiments on snow to precisely understand the reflectivity-reducing impacts of specific aerosols. These experiments overcome the limitations of outdoor field studies and resource-intensive cold rooms through introducing a modular, rapid and low-footprint laboratory set up. The set up utilizes a cryogenic chamber to flash-freeze atomized water droplets, mimicking natural snow accumulation, followed by a custom-built system for the controlled deposition of aerosols. To characterize the combined aerosol-snow samples, optical and microphysical measurements are performed using an integrating sphere spectrophotometer and a digital microscope to empirically validate the snow darkening. This bottom-up experimental approach elucidates the sensitivity of spectral albedo to aerosol type, concentration, and snow grain metamorphosis, allowing for ground-truthing of theoretical radiative forcing calculations. These studies collectively establish d-BrC as a potent radiative forcing agent across diverse environments. By bridging field observations, computational modeling, and laboratory experimentation, this work provides a comprehensive understanding of how d-BrC modulates energy balances in the atmosphere and accelerates cryospheric melting. These contributions emphasize the critical need to revise climate models to accurately represent the complex optical properties of the brown-black carbon continuum.

Committee Chair

Rajan Chakrabarty

Committee Members

Benjamin Kumfer; Jay Turner; Lu Xu; Roger Michaelides

Degree

Doctor of Philosophy (PhD)

Author's Department

Energy, Environmental & Chemical Engineering

Author's School

McKelvey School of Engineering

Document Type

Dissertation

Date of Award

8-17-2026

Language

English (en)

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Available for download on Monday, August 14, 2028

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