Abstract

Aerosols are crucial to climate through their interactions with radiation and clouds. Aerosols can scatter and absorb solar radiation (the direct effect) and can act as cloud condensation nuclei (CCN), altering cloud microphysical properties and radiative forcing. New particle formation (NPF) is a major source of CCN in the marine atmosphere. In the free troposphere, newly formed particles associated with convective outflow can grow toward CCN sizes and be transported downward, contributing to CCN populations in the lower free troposphere and boundary layer. However, NPF in the tropical mid-free troposphere has been infrequently reported, and the respective roles of anthropogenic emissions and biomass burning remain incompletely understood. Near the ocean surface, NPF may also directly contribute to CCN in the marine boundary layer, but observational evidence has been scarce and sometimes ambiguous. In contrast to sources, in-cloud scavenging has been widely considered a dominant sink of CCN-sized particles, yet aerosol scavenging by mid-level convective clouds such as cumulus congestus has not been rigorously constrained using airborne observations together with large-eddy simulations (LES). This dissertation addresses these gaps through three objectives that combine airborne observations and observationally evaluated modeling. The first objective investigates how emission sources, convective activity, and synoptic conditions control NPF in the tropical free troposphere. Using airborne measurements from the NASA Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex), we identify NPF events and interpreted the favorable conditions with k-means clustering, statistical analysis and case studies. NPF occurs predominantly above ~5.5 km, coinciding with elevated relative humidity and reduced condensation sink. NPF frequency increases sharply with altitude, reaching ~50% above 8 km. Convectively detrained biomass-burning plumes and fresh urban emissions can enhance NPF, whereas aged urban plumes tend to suppress NPF when reactive precursors are depleted but sinks remain elevated. The second objective quantifies size-resolved aerosol in-cloud scavenging by a tropical cumulus congestus observed during CAMP2Ex and evaluates scavenging in an LES framework using two warm-rain microphysics schemes. Observation-derived scavenging efficiencies are 0.53 ± 0.09 (50–100 nm) and 0.79 ± 0.05 (100–600 nm), indicating efficient removal comparable to deeper convection. Secondary activation contributes to removal in both observations and simulations. The Seifert-Beheng (with a gamma DSD) simulation reproduces observed scavenging well, whereas the Khairoutdinov-Kogan (with an exponential DSD) simulation markedly overestimates scavenging potentially due to more aggressive accretion and stronger secondary activation in the mid-cloud layer. Neglecting outflow dilution by ambient air can bias scavenging low by up to ~50% when sampled outflow contains ~60% ambient air. The third objective examines NPF in the open-ocean Southern Ocean (SO) marine boundary layer (MBL) using the NASA Atmospheric Tomography Mission (ATom) in situ aerosol observations combined with HYSPLIT dispersion-chemistry simulations. We find that well-mixed SO MBL NPF was observed primarily during austral winter, when low temperature and a weak condensation sink favor elevated ultrafine particle signatures. Passive volcanic degassing, particularly from Mt. Erebus, can enhance SO MBL SO2 by roughly 10–15 pptv near the NPF location, supplying sulfuric acid (SA) on the magnitude of 106 molecules cm-3. Nucleation-rate calculations further indicate that nucleation rates yielded through traditional pathways such as SA-H2O and SA-NH3-H2O are too low to explain the observed NPF, whereas pathways involving iodic acid (IA) and dimethylamine (DMA) are likely required to achieve plausible nucleation rates given the simulated SO2 levels and SO MBL conditions. This dissertation advances the understanding of the key processes controlling marine aerosol populations. It also provides observational constraints and implications to help improve NPF and aerosol in-cloud scavenging representations in models and reduce uncertainty in aerosol-cloud radiative forcing.

Committee Chair

Jian Wang

Committee Members

Ann Fridlind; Jenna Ditto; Lu Xu; Randall Martin

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

3-17-2026

Language

English (en)

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Available for download on Tuesday, June 15, 2027

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Engineering Commons

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