Modeling of Carbonyl/Ammonium Sulfate Aqueous Brown Carbon Chemistry via UV/Vis Spectral Decomposition

Mengjie Fan,Ziwei Dai,Shiqing Ma,Nahin Ferdousi,Joseph L Woo

doi:10.3390/atmos11040358

Abstract

The proper characterization of aqueous brown carbon (BrC) species, their formation, and their light absorbance properties is critical to understanding the aggregate effect that they have on overall atmospheric aerosol climate forcing. The contribution of dark chemistry secondary organic aerosol (SOA) products from carbonyl-containing organic compounds (CVOCs) to overall aqueous aerosol optical properties is expected to be significant. However, the multiple, parallel pathways that take place within CVOC reaction systems and the differing chromophoricity of individual products complicates the ability to reliably model the chemical kinetics taking place. Here, we proposed an alternative method of representing UV-visible absorbance spectra as a composite of Gaussian lineshape functions to infer kinetic information. Multiple numbers of curves and different CVOC/ammonium reaction systems were compared. A model using three fitted Gaussian curves with magnitudes following first-order kinetics achieved an accuracy within 65.5% in the 205–300-nm range across multiple organic types and solution aging times. Asymmetrical peaks that occurred in low-200-nm wavelengths were decomposed into two overlapping Gaussian curves, which may have been attributable to different functional groups or families of reaction products. Component curves within overall spectra exhibited different dynamics, implying that the utilization of absorbance at a single reference wavelength to infer reaction rate constants may result in misrepresentative kinetics for these systems.

Highlights

Atmospheric aerosols are a major source of uncertainty in current climatological models, contributing both warming and cooling effects to the total energy balance of the planet
volatile organic compounds (VOCs) processing into lower volatility products that uptake into the condensed phase [3,4]
Aqueous aerosol mimic solutions were prepared in sets of at least eight replicates in flat-bottomed, 320-μL UV-transparent 96-well plates (Corning) by combining fixed quantities of organic-containing and ammonium sulfate stock solutions

Summary

Introduction

Atmospheric aerosols are a major source of uncertainty in current climatological models, contributing both warming and cooling effects to the total energy balance of the planet. The contributing effects of aerosols are dependent on their composition, which contain highly variable concentrations of both inorganic and organic compounds [1,2]. Much of the organic component in aerosols is secondary organic aerosol (SOA) mass, formed indirectly through the partitioning and reaction of gas-phase volatile organic compounds (VOCs). Literature attempting to elucidate mechanisms of SOA production emphasized the accumulation of SOA in ultrafine particles through gas-phase. VOC processing into lower volatility products that uptake into the condensed phase [3,4]. Additional relevant SOA formation mechanisms have been identified that contribute via reversible. VOC uptake followed by irreversible particle-phase processing [5,6].

Objectives

Methods

Results