Date of Award

8-2026

Document Type

Dissertation

Degree Name

Doctor of Engineering (DEng)

Department

Environmental Engineering and Earth Science

Committee Chair/Advisor

Andrew R. Metcalf

Committee Member

David A. Ladner

Committee Member

Cindy M. Lee

Committee Member

Mark Schlautman

Abstract

Black carbon (BC) is a light-absorbing aerosol emitted from incomplete combustion of fossil fuel, biofuel, and biomass burning. BC aerosol can strongly absorb solar radiation, modify atmospheric stability, and interact with cloud droplets. Because of this, it has important implications for air quality and regional climate forcing. These effects are especially important in coastal marine environments, where relatively clean marine air can mix with land-based emissions transported from nearby coastal and urban source regions. The Southern California Bight (SCB) provides a natural and complex environment for studying these processes. The persistent low-level marine clouds, strong temperature inversions, coastal transport, and island-wake circulations interact with emissions from the Los Angeles Basin, Long Beach port region, inland California, and transported biomass-burning plumes.

This study investigates refractory black carbon (rBC) aerosols and aerosol-cloud interactions in the SCB using aircraft observations from the Southern California Interactions of Low Cloud and Land Aerosol (SCILLA) campaign conducted in June 2023. Measurements were collected aboard the Naval Postgraduate School Twin Otter aircraft below, within, and above low-level marine clouds near San Clemente Island (SCI) and along the Southern California coastal corridor. A suite of aerosol, cloud, trace gas, meteorological, and chemical measurements was used to characterize rBC mass concentration, rBC size distribution and mixing state, aerosol size distributions, cloud droplet properties, and aerosol chemical composition. Back-trajectory analysis was used to evaluate air-mass transport pathways and source influences. The first part of this dissertation focuses on two contrasting aerosol-focused flights: a stratocumulus-topped case on June 16 and a cloud-free case on June 20, 2023. For both cases, marine regions in SCB showed cleaner background conditions, while Long Beach (LB) and Los Angeles-Santa Monica-Malibu (LA-SM-MAL) showed stronger influence from urban, port-related, traffic, and regional emissions. The June 16 stratocumulus-topped case reveals signs of cloud processing in aerosol size distributions, including bimodal structures and Hoppel-like features. In comparison, the June 20 cloud-free case shows stronger biomass-burning influence, higher rBC mass fractions, and enhanced ultrafine and Aitken-mode particles associated with fresh emissions or new particle formation. rBC mixing-state analysis shows that lower free-tropospheric rBC is often more thickly coated than rBC in boundary layer.

The second study in this dissertation examines six selected aerosol- and cloud-focused flights near San Clemente Island to investigate rBC and total aerosol properties. Below-cloud air masses in the sampling site were mainly clean marine air transported from the northwestern Pacific Ocean, whereas higher altitude air masses were more influenced by coastal and inland sources from the free troposphere. The boundary layer heights varied among flights, and most cases were characterized by shallow marine boundary layers capped by temperature inversions. Downwind clouds had smaller droplet modes than upwind clouds, consistent with enhanced aerosol loading. In-cloud residual size distributions shifted toward larger accumulation-mode particles, indicating preferential activation of larger particles and cloud processing. Size-resolved rBC comparisons show that larger and thickly coated rBC particles are preferentially incorporated into cloud residuals, with below-cloud rBC appearing to be the dominant source for droplet activation. Overall, this research provided new aircraft-based observational evidence that atmospheric processing over the SCB alters aerosol size distributions and rBC mixing state, as shown by the contrast between a stratocumulus-topped case and a cloud-free case. This study also presents unprecedented vertically resolved airborne measurements of rBC inside and outside stratocumulus clouds in the upwind and downwind regions of SCI, demonstrating that aerosol loading, particle size, and mixing state influence marine boundary-layer cloud microphysics in the SCB.

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Available for download on Tuesday, August 31, 2027

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