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Design and Laboratory Evaluation of a New Flow Reactor to Study Aerosol Production and Processing through Gas- and Aqueous-phase Chemistry
NINGJIN XU, Don Collins, University of California, Riverside
Abstract Number: 522
Working Group: Instrumentation and Methods
Abstract
Cloud droplets serve as an important reaction medium for the formation and evolution of secondary aerosol formed through aqueous-phase reactions (aqSA). Large uncertainties remain in estimates of the production and chemical evolution of aqSA in dilute cloud droplets, which is partly due to the lack of available measurement tools and techniques. Recently, more attention has been directed to understand aqSA production in order to explain discrepancies between measured and modeled aerosol composition and concentrations. Unlike reported studies of secondary aerosol formed through gas-phase reactions (gasSA), laboratory investigations of aqSA mechanisms, products, and yields are usually performed in bulk aqueous solutions with high oxidant and precursor concentrations. The experimental concentrations and conditions often differ from those in the atmosphere and introduce uncertainty when results are implemented into multiphase models. Oxidation flow reactors (OFRs) are frequently used to study the formation and evolution of gasSA in the atmosphere, but few studies have reported the amount and properties of aqSA formed in aqueous aerosol and cloud droplets using an OFR.
Here we will describe a new OFR, the Accelerated Production and Processing of Aerosols (APPA) reactor, for measuring secondary aerosol formed from gas-phase chemistry and from aqueous-phase chemistry in aqueous aerosol and cloud droplets in both the laboratory and the field. For use in simulating in-cloud processes, droplets formed on monodisperse particles are introduced into the top of the flow cell, in which the relative humidity is controlled to 100%. We will describe the design and laboratory characterization of the APPA reactor, including the UV intensity distribution, OH exposure level, residence time distributions (RTD) for gases and particles, and transmission efficiencies of gases (SO2 and CO2) and particles. The droplet size distribution and transmission efficiency will also be presented. Aqueous-phase sulfate production from dissolution and reaction of SO2 and O3 in droplets was used as a test system. Results from those tests indicate that the observed aerosol growth is consistent with that predicted using first-order rate constants for the S(IV)-O3 reaction for a dilute solution. We will present preliminary measurements of SA production from organic precursors and from ambient air.