Iodine oxoacids in atmospheric aerosol formation - from chamber simulations to field observations

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Title: Iodine oxoacids in atmospheric aerosol formation - from chamber simulations to field observations
Author: He, Xu-Cheng
Contributor: University of Helsinki, Faculty of Science, Institute for Atmospheric and Earth System Research / Physics
Doctoral Programme in Atmospheric Sciences
Publisher: Helsingin yliopisto
Date: 2021-08-23
Language: en
Thesis level: Doctoral dissertation (article-based)
Abstract: New particle formation is estimated to contribute to around half of the cloud condensation nuclei (CCN) in the atmosphere which in turn have a cooling effect on Earth’s surface. Only a few gas species, including sulfuric acid, oxidized organic vapors and iodine species, are confirmed to contribute to new particle formation, which converts gases to aerosol particles. While new particle formation from sulfuric acid (with water or bases, such as ammonia and amines) is recognized globally, new particle formation solely induced by pure organic vapors only occurs under special conditions. Least is known about the coverage of iodine particle formation processes in the atmosphere. Iodine species have widely been measured in marine and polar environments. However, most ambient measurements concentrated on molecular iodine (I2 ), iodine monoxide (IO), iodine dioxide (OIO) and organic iodine precursors. These measurements constrained the effect of iodine species in catalytic ozone loss processes, but far from enough to understand the particle formation processes. In this thesis, I utilized a bromide chemical ionization method to nearly comprehensively measure inorganic iodine species, including I2 , iodine oxides and oxoacids. An unprecedented performance both in coverage and sensitivity is achieved. We further deployed the bromide chemical ionization method for ambient observations at the Mace Head observatory on the Atlantic coast of Ireland. First successful online measurements of hypoiodous acid (HOI), bromoiodide (IBr) and chloroiodide (ICl) confirmed the heterogeneous uptake of HOI at ambient conditions which enhanced iodine atom production rate by 32% and accelerated ozone (O3 ) loss by 12%. Comprehensive experiments were further carried out at the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber to understand the iodine particle formation mechanisms. We found that the ion-induced (charged) and neutral nucleation proceed via distinct mechanisms. The ion-induced nucleation proceeds primarily by sequential addition of iodic acid (HIO3 ) which was measured to proceed at the kinetic limit. However, in contrast to earlier expectations, neutral nucleation additionally involves iodous acid (HIO2 ) to stabilize HIO3 , replacing the role of the negative charge in the ion-induced nucleation. After passing the critical size of nucleation, the growth of iodine particles is essentially sustained by HIO3 , with minor contributions from other species, which are present at much lower concentrations. Additionally, iodine oxoacids have much faster particle formation rates than the sulfuric acid – ammonia mixture at the same acid concentrations (when the ammonia mixing ratio is 100 parts per trillion by volume). While sulfuric acid – ammonia new particle formation has been confirmed to be an important mechanism in polar regions, the role of iodine new particle formation is usually considered to have a limited global reach. We carried out iodic acid measurements at ten boundary layer sites, ranging from the cleanest polar regions to polluted urban environments. The existence of iodic acid is ubiquitously confirmed, with concentrations comparable to sulfuric acid. This indicates a greater importance of iodine oxoacid particle formation processes than just a coastal phenomenon.
Subject: atmospheric Sciences
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