Modeling the Novel Heterogeneous Pathways of Aerosol Formation from Nitrate Photolysis and Iodine Chemistry in Marine Atmosphere

Abstract

The marine atmosphere is a complicated environment and a dynamic interface between land, sea, and sky. It is shaped by multifaceted factors including biogenic emissions from the marine ecosystem, injection of sea salt aerosols (SSA) from the ocean, thermodynamic exchanges between ocean and atmosphere, and biogeochemical cycling and deposition. In coastal and harbor regions, the marine air is especially influenced by human activities including vehicle transportation. Marine aerosols play a critical role in the intricate processes of marine atmospheric chemistry, affecting chemical formation and transportation, human health, and global climate change. In this thesis, two novel pathways of marine aerosol particle formation were implemented to the GEOS-Chem global 3-D atmospheric chemical transport model based on recent experimental evidences to evaluate their impacts on the marine air.

Recent studies have suggested that particulate nitrate (NO₃¯) photolysis could be an important source of atmospheric oxidants, and one of its main products, N(III), contributes to the heterogenous formation of sulfate (SO₄²¯) in aerosol water – a potential missing source of SO₄²¯ in the atmosphere. However, its effects on SO₄²¯ and air quality over different regions remain unexplored. In this thesis, a detailed model representation of SO₄²¯ formation via NO₃¯ photolysis is implemented into GEOS-Chem. The results find considerable impacts of NO₃¯ photolysis on surface SO₄²¯, especially over India and other coastal regions (up to 15% increment of annual average concentrations), which were previously unaccounted for in most models. The effects are majorly due to S(IV) oxidation by OH, H₂O₂, and O₃ following renoxification, which outcompetes aqueous oxidation by N(III), contrary to previous laboratory and modeling studies. Further analysis suggests that past studies might have underestimated the particle-to-gas transfer of N(III) in ambient aerosols.

Iodine affects tropospheric chemistry in a few ways, such as depleting oxidants and driving new particle formation (NPF). Iodine oxides (I_xO_y) have been proposed theoretically as a potential NPF source for years, until recent laboratory and field studies show that iodic acid (HIO₃) can act as a main driver of NPF in marine atmosphere. However, HIO₃ chemistry was rarely included in models thus the mechanisms controlling its global burden and its role in iodine cycling are still not well understood. Here, the thesis presents the most recent iodine mechanism in GEOS-Chem to describe the formation and transformation of HIO₃, and the consequential NPF. The model simulation results are compared with a series of observations of HIO₃ to constrain the uncertain parameters in the involved chemical processes. Impacts of HIO₃ chemistry on tropospheric iodine cycling and oxidants are analyzed using the model estimate. The simulation provides a global estimate of potential particle formation driven by HIO₃, which could be a useful reference for future field studies.

Date of Award	10 Dec 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Xuan WANG (Supervisor)