2025 Theses Doctoral

Brown Carbon in an Earth System Model: A Scheme to Study Biomass Burning Aerosols

DeLessio, Maegan Anne

Organic aerosols (OAs) are important, short-lived climate forcers that cool the atmosphere. However, there is significant uncertainty in the magnitude of their cooling effect, posing a large gap in aerosol science and modeling. Growing wildfire frequency and intensity, along with reduction of other aerosol sources via emission controls and cleaner technologies, are increasing the prominence of OAs in the atmosphere, making this a priority for model improvement.

This dissertation advances organic and biomass burning (BB) aerosol modeling by explicitly representing brown carbon (BrC) aerosols, OAs that absorb UV-to-visible light and are primarily emitted by BB, in an Earth system model (ESM). In the first part of this work, a BrC scheme was developed and implemented in the NASA GISS ModelE ESM (“ModelE”) through the definition of four key properties and processes: BB emissions of BrC, secondary formation of BrC as biogenic secondary organic aerosols (SOAs), optical properties of both primary and secondary BrC, and the chemical aging of primary BrC. The latter was simulated through a novel aging scheme, which utilizes local oxidant concentrations to increase (brown) then decrease (bleach) BrC light absorption.

The remainder of the dissertation details the evaluation of this BrC scheme. To start, model sensitivity tests were conducted to understand the overall impact of BrC in ModelE. Model simulation of total aerosol properties, specifically optical depth, with the addition of this scheme was also evaluated against Aerosol Robotic Network (AERONET) and Moderate Resolution Imaging Spectroradiometer (MODIS) retrieval data.

These initial assessments revealed that, on a global scale, while explicit representation of BrC, the inclusion of secondary BrC, and simulated bleaching had distinguishable effects in the model, varied optical properties and emission ratios did not. Further, model total optical depth performance was unchanged with the addition of BrC. This left several scheme parameters unconstrained, and necessitated evaluation against BrC-specific data. The next stage of evaluation constrained scheme parameters by harmonizing them with the aerosol property assumptions of an AERONET retrieval of BrC, resulting in a relative improvement in model performance.

This model-retrieval comparison, which focused on BB regions and seasons, created an alternative scheme configuration grounded in measured radiance fields represented by the retrieval. The alternative case was not, however, indicative of in-situ microphysical and chemical processes. As such, the final stage of work was an extensive evaluation of ModelE OAs and BrC against in-situ measurements from flight campaigns. Focusing on vertical profile comparisons, and looking at BB-influenced aerosols wherever possible, this revealed systematic underestimation in BrC absorption. Introducing variable OA-to-OC ratios and reducing the water-solubility of BrC improved model performance in an updated scheme, but there was still persistent model bias. The dissertation concludes with a discussion of further model improvements, as well as advances in measurement and satellite data, that could help address remaining bias and uncertainty.

In-depth analysis of the ModelE BrC scheme allowed for the exploration of the BrC parameter space and investigation of potential sources of biases, while at the same time highlighting the usefulness of different atmospheric science tools in model development and evaluation. The product of this dissertation is a scheme within an ESM that has several applications: estimating the radiative effect of OA absorption–between 0.03-0.04 W m-2 according to different scheme configurations–improving satellite retrieval algorithms, and, in general, furthering the study of biomass burning and organic aerosols.