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Modeling Secondary Organic Aerosol Production from Photosensitized Humic-like Substances (HULIS) William Gang Tsui, and V. Faye McNeill Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.8b00101 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018
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Modeling Secondary Organic Aerosol Production from Photosensitized Humic-like
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Substances (HULIS)
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William G. Tsui, V. Faye McNeill*
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Department of Chemical Engineering, Columbia University, New York, NY 10027
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*Corresponding author. E-mail:
[email protected], Telephone: (212)-854-2869
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Abstract Humic-like substances (HULIS) are ubiquitous in atmospheric aerosols. Despite
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experimental evidence that HULIS can catalyze secondary organic aerosol (SOA) formation
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through photosensitizer chemistry, the potential contribution of this pathway to ambient SOA has
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not been quantified. In this study, GAMMA, a photochemical box model, was used to analyze
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the experimental data of Monge et al. (2012) in order to quantify the uptake kinetics of limonene
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to particles containing humic acid, a laboratory proxy for HULIS. The results indicate that
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limonene is taken up to irradiated particles containing humic acid efficiently, with a reactive
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uptake coefficient of 1.6×10-4. Consequently, simulations of limonene-HULIS photosensitizer
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chemistry under ambient conditions, simultaneously with other aqueous SOA formation
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processes, show that this pathway could contribute up to 65% of total aqueous SOA at pH 4. The
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potential importance of this pathway warrants further laboratory studies and representation of
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this SOA source in atmospheric models.
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Introduction
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Atmospheric aerosols affect air quality, health, climate, and visibility. Organic material
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and liquid water make up a significant portion of total atmospheric aerosol mass, and much of
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the organic portion of atmospheric aerosols is secondary in origin.1–4 Thus, an understanding of
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the formation and evolution of secondary organic aerosols (SOA) is necessary for modeling air
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quality. However, many models are currently unable to accurately represent organic aerosols due
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to uncertainties in SOA production.3,5,6 Recent studies suggest that heterogeneous or multiphase
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reactions of volatile compounds in aqueous aerosols can be important pathways of SOA
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formation.7 These processes may contribute to the discrepancy between model predictions and
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observed atmospheric aerosol content.
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Humic-like substances (HULIS) are a class of macromolecular water-soluble organic
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species typically several hundred daltons in molecular weight.8 They can comprise 10 – 35% of
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fine organic material in atmospheric aerosols, and have been observed to account for as high as
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72% of water-soluble organic carbon in some ambient aerosol samples.9,10 They are likely
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formed through biomass burning and secondary aqueous reactions.8–17 HULIS are named as such
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due to apparent similarities to humic acids, such as their polyacidic and amphiphilic nature,
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aromaticity, and light absorption. As a result of these characteristics, HULIS can affect aerosol
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properties, including optical properties and CCN activity.
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Some light-absorbing organic aerosol species can participate in SOA formation by acting
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as photosensitizers. A photosensitizer is excited to a triplet state when irradiated. In the excited
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triplet state, the photosensitizer can then reduce O2 to form oxidizing radicals or directly oxidize
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a VOC to form SOA, as shown in Figure 1 with HULIS as a photosensitizer in the presence of
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limonene and isoprene. Trace aerosol species such as 2-imidazole carboxaldehyde18–22 (IC) and
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non-phenolic aromatic carbonyls23,24 have been shown to contribute to SOA formation via
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photosensitizer chemistry.
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Figure 1 – HULIS reaction pathways as a photosensitizer with VOCs limonene and isoprene
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In aerosol flow tube experiments performed by Monge and coworkers, particles
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containing humic acid, a proxy for HULIS, were observed to grow when exposed to limonene
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under UV-A irradiation.25 Mass spectrometry analysis indicated the presence of higher molecular
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weight limonene reaction products, and control experiments suggested that O2 was a source of
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oxidizing radicals in the photosensitizer pathway. They also observed isoprene uptake to thin
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films containing humic acid.
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Due to the high abundance of HULIS in atmospheric aerosols, the potential contribution of HULIS photosensitizer chemistry to ambient SOA formation is larger than for trace species
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such as IC.9,10,22,26 Here, we apply GAMMA (Gas-Aerosol Model for Mechanism Analysis), a
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photochemical box model of multiphase atmospheric chemistry developed by the McNeill
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group,27 to quantify the kinetics of limonene SOA formation due to the HULIS photosensitizer
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pathway in the laboratory experiments of Monge et al (2012).25 We then use the calculated
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reactive uptake coefficient to compare SOA formation from this pathway to other important
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aqueous SOA formation pathways under ambient conditions.
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Materials and Methods
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In this study, we performed two sets of model simulations. All simulations use GAMMA 4.0.22
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First, we modeled the laboratory experiments of Monge et al.25 Then, we used the results of
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those simulations to model aerosol growth through photosensitizers and other atmospherically
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relevant processes under ambient conditions.
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GAMMA 4.0. GAMMA is a photochemical box model developed by the McNeill group that
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couples detailed gas and aqueous aerosol phase chemistry of organic species and includes mass
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transfer between the gas and aqueous phases. The time-dependent evolution of the partial
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pressure of a gas-phase species i, Pi, and the concentration of an aqueous phase species i, Ci, in
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GAMMA follows Schwartz28 and is given by the following equations:
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, ∗
,
,
∗
,
,
,
1
2
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where kmt,i is the gas-aerosol mass transfer coefficient, aL is the aqueous aerosol liquid volume
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fraction, Hi* is the effective Henry’s Law constant, Ei is the emission rate, Di is the deposition
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rate, R is the gas constant, T is temperature, and rij,gas and rij,aq are the gas and aqueous phase
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reactions of species i respectively, which may be positive or negative corresponding to formation
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or consumption of species i.
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Aqueous phase chemistry of organic material in GAMMA includes oxidative and dark
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formation of SOA and organosulfate species by water soluble VOC precursors such as glyoxal
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and methylglyoxal. Aqueous processing of isoprene epoxydiols (IEPOX) to form 2-methyltetrols
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(tetrol) and IEPOX organosulfate (IEPOXOS) is expressed in GAMMA as a branching reaction
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→ 1
3
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with branching ratio β = 0.4 being used based on the work of Eddingsaas et al.29 Imidazole
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photosensitizer reactions were added in the recent 4.0 update.22 McNeill et al. (2012) and Tsui et
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al. (2017) provide a more detailed discussion of GAMMA and the 4.0 update, respectively.22,27
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Reactive uptake of limonene. Photosensitizer SOA formation can be modeled using a reactive
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uptake formulation, which represents the mass transfer and particle-phase chemical kinetics of
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the process.22 The reactive uptake of limonene to humic acid-containing particles was added to
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GAMMA using the following equations for the change in partial pressure of limonene, where
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reactive uptake replaces the partitioning terms from Equation 1.27,28
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,
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where 1
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100
,
,
2
1
,
,
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and
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,
4
3
1
0.38 1
6
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In the equations above, species i is gas-phase limonene, R is the aerosol particle radius, Dg,i is the
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gas phase diffusivity, γi is the reactive uptake coefficient, Sa is the aerosol surface area per
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volume, ωi is the thermal velocity, and Kn is the Knudsen number, which is the ratio of the mean
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free path of air to the particle radius. Emissions and depositions are only considered for ambient
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simulations. The amount of SOA formed is assumed to be equal to the amount of limonene taken
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up. Although it is possible that the SOA products may be (semi)volatile and repartition to the gas
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phase, here any loss to the gas phase is already accounted for in the reactive uptake coefficient
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since it was calculated based on the net observed particle growth.30
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Experimental simulation conditions. Full details of the simulation initial conditions can be
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found in the Supporting Information. Simulations of laboratory experiments of humic acid-
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containing particle growth followed the conditions used by Monge et al.25 In these experiments,
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ammonium nitrate aerosols containing humic acid and succinic acid, a proxy for other organic
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species in ambient aerosols, were used. The initial aerosol composition for aerosols containing
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humic acid, succinic acid, and ammonium nitrate was determined by using the relative humidity
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and the weight ratio of these species in the aqueous solution (1:10:1) prior to aerosolizing to
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calculate equilibrium concentrations of each species with E-AIM Model II, a thermodynamic
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aerosol model which includes inorganic species H+, NH4+, SO42-, and NO3- and H2O in the liquid
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phase and HNO3, NH3, H2SO4 and H2O in the gas phase.31,32 Organic compounds, such as
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succinic acid, can be added to the E-AIM model by accessing multiple publicly available
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libraries via the E-AIM interface. Since they are not well-defined molecules, thermodynamic
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calculations for HULIS or humic acid are not available in E-AIM. Therefore, succinic acid was
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used as a proxy for humic acid in our E-AIM calculations. In order to account for the
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concentration effect associated with bringing the aerosols to
