Reaction Kinetics of Organic Aerosol Studied by Droplet Assisted Ionization: Enhanced Reactivity in Droplets Relative to Bulk Solution

Abstract

Droplet Assisted Ionization (DAI) is a relatively new method for online analysis of aerosol droplets that enables measurement of the rate of an aerosol reaction. Here, we used DAI to study the reaction of carbonyl functionalities in secondary organic aerosol (SOA) with Girard’s T (GT) reagent, a reaction that can potentially be used to enhance the detection of SOA in online measurements. SOA was produced by α-pinene ozonolysis. Particulate matter was collected on a filter, extracted, and mixed with GT reagent in water. While the reaction hardly proceeded at all in bulk solution, products were readily observed with DAI when the solution was atomized to produce micron-size droplets. Varying the droplet transit time between the atomizer and mass spectrometer allowed the reaction rate constant to be determined, which was found to be 4 orders of magnitude faster than what would be expected from bulk solution kinetics. Decreasing the water content of the droplets, either by heating the capillary inlet to the mass spectrometer or by decreasing the relative humidity of the air surrounding the droplets in the transit line from the atomizer to the mass spectrometer, enhanced product formation. The results suggest that reaction enhancement occurs at the droplet surface, which is consistent with previous reports of reaction acceleration during mass spectrometric analysis, where a bulk solution is analyzed with an ionization method that produces aerosol droplets.

Spontaneous Water Radical Cation Oxidation at Double Bonds in Microdroplets

“…Microdroplet chemistry is an emerging field in chemistry due to the remarkable reaction acceleration ( Girod et al, 2011 ; Müller et al, 2012 ; Yan et al, 2016 , 2017 ; Lee et al, 2017 ; Nam et al, 2017 ; Cooks and Yan, 2018 ; Sahota et al, 2019 ; Zhang et al, 2019 , Zhang et al, 2021 Y. ; Wei et al, 2020 , 2022 ; Cao et al, 2020 ; Kuai et al, 2021 ; Vannoy et al, 2021 ; Zheng et al, 2021 ), and the unique properties of the reaction environment ( Li et al, 2014 ; Lee et al, 2019a , 2019b ; Gao et al, 2019 ; Huang et al, 2021 ; Wang et al, 2021 ; Gong et al, 2022 ; Qiu et al, 2022 ).…”

Pre-deliquescent water uptake in deposited nanoparticles observed with in situ ambient pressure X-ray photoelectron spectroscopy

“…The surfaces of aerosol particles and droplets are distinct physical and chemical environments compared to their associated bulk phases. Reaction rates in micrometer-scale droplets have been measured (Jacobs et al, 2017;Marsh et al, 2019;Zhang et al, 2021) and modeled (Benjamin, 2019;Mallick and Kumar, 2020) to be higher than those in bulk water, with some reactions even proceeding spontaneously (Lee et al, 2019). For cloud and fog systems where the interfacial region makes up a significant fraction of the condensed aqueous phase, the reaction rate at the surface can be the rate-limiting step in multi-phase OH oxidation involving surface-active organic species such as pinonic acid (Huang et al, 2018).…”

“…For cloud and fog systems where the interfacial region makes up a significant fraction of the condensed aqueous phase, the reaction rate at the surface can be the rate-limiting step in multi-phase OH oxidation involving surface-active organic species such as pinonic acid (Huang et al, 2018). Interfacial water molecules can promote reactions between organic acids and SO 3 , which are distinct from those that occur in the gas phase and important for heterogeneous formation of H 2 SO 4 and subsequent new particle formation (Zhong et al, 2019;Lv and Sun, 2020). The bulk-surface partitioning of saccharide molecules in aqueous solution affects their ability to be oxidized by OH (Fan et al, 2019).…”

Droplet Flow Assisted Electrocatalytic Oxidation of Selected Alcohols under Ambient Condition

“…Droplets provide a confined, high-surface-area for chemical reactions, enhancing conversion and selectivity. Droplet-assisted reaction mechanisms are well-known, and several pieces of literature have already been discussed in detail [10,11]. Several reports have attributed this fast reaction rate to be due to the inherent interfacial properties that includes [9,12]: reagent (s) confinement [13,14], partial solvation [9,[15][16][17], pH extremes [18,19], and ordered molecule orientation [20][21][22][23][24].…”