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Chapter 15
Modeling Heterogeneous Oxidation of NOx, SO2 and Hydrocarbons in the Presence of Mineral Dust Particles under Various Atmospheric Environments Myoseon Jang* and Zechen Yu Department of Environmental Engineering Sciences, University of Florida, Gainesville, Florida 32611, United States *E-mail:
[email protected] Airborne mineral dust particles are known to significantly promote atmospheric oxidation of SO2, NOx, and hydrocarbons via the heterogeneously photocatalytic process under ambient sunlight. However, simulation of this process is not fully taken into account by current models. This study streamlines the process of developing an atmospheric mineral dust chemistry model and simulates the influence of air–suspended mineral dust particles on the formation of sulfate, nitrate, and secondary organic aerosol under various environments. The model encompasses partitioning processes and reactions in multiple phases including gas phase, inorganic–salted aqueous phase (non–dust phase), and dust phase. The reaction of adsorbed chemical species occurs via two major paths: autoxidation in the open air and photocatalytic mechanisms in the presence of UV light. Photocatalytic rate constants of tracers on dust surfaces are derived from the integration of the combinational product of the dust absorbance spectrum and wave–dependent actinic flux for the full range of wavelengths from the light source. The photocatalytic ability of dust particles is diversified according to sources and metal compositions in the model. The buffering capacity of dust particles to react to acids is also used in the model to predict the dynamic chemical characteristics of dust particles. The hygroscopicity and buffering capacities of both the fresh dust particles and the aged particles are © 2018 American Chemical Society Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
applied in the model to predict the influence of humidity on partitioning processes and heterogeneous chemistry. The model predicted concentrations of sulfate and nitrate are then compared with outdoor chamber data obtained under natural sunlight. Systematic, in-depth dust chemistry can provide a platform for predicting the formation of nitrate, sulfate, and secondary organic aerosol at regional or global scales during dust events.
1. Introduction Despite numerous studies regarding the increased oxidation of SO2 and NO2 due to mineral dust particles, the underlying heterogeneous chemical processes remain uncertain, hampering the model prediction of sulfate and nitrate formation on regional and global scales. Figure 1 illustrates the fate of airborne dust particles and their atmospheric aging. This section will discuss where the airborne dust particles come from, what their compositions are, and how they impact the environment.
Figure 1. The schematic of heterogeneous oxidation of SO2 and NOx in the presence of mineral dust particles. 1.1. Dust Composition Mineral dust usually contains aluminosilicates, calcium species, and miscellaneous metal oxides. The typical Scanning Electron Microscope (SEM)/Energy Dispersive X-ray (EDX) analysis of particles has been used to obtain the elemental compositions of mineral dust (1–6). Figure 2 shows the elemental composition of Arizona Test Dust (ATD) (4), Gobi Desert Dust (GDD) (4), Sahara Dust (SD) (1) and Australia Dust (AD) (2). The types of elements in the four different mineral dust particles are similar, though the fraction of alkaline ions and metals varies with sources. For example, alkaline ions such as Ca and Mg are present in higher concentration in GDD particles than those in other types 302 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
of dust. Additionally, the percent composition of the element Ti in GDD is higher than in ATD but lower than in SD. The surface area of dust particles also varies with sources and influences their heterogeneous chemistry. For example, the BET surface areas of GDD and ATD particles in Figure 2 were respectively 39.6 and 47.4 m2 g-1.
Figure 2. Element fractions in various dust sources. The composition of dust particles affects their ability to modulate heterogeneous chemistry of tracers. For example, it is recognized that clays in mineral dust aerosols actively affect heterogeneously nucleating ice, even at temperatures warmer than required for homogeneous nucleation (7). Clays, whose major components are phyllosilicates (i.e. montmorillonite, kaolinite, illite, hectorite, and talc), are very efficient in this respect. The quantity and composition of conducting metal oxides also influence the photocatalytic activities of dust particles and consequently affect the heterogeneous chemistry of tracers. For example, the high content of titanium oxides in dust may increase the photocatalytic uptake coefficients of tracers (8). In addition, dust’s capacity to buffer acidic species is closely related to the content of alkaline carbonates. Dust compositions also vary depending upon geological environments. For example, coastal dust particles contain a large amount of Na compared with those of other source regions due to the mixing with sea salts (9). 1.2. Impact of Mineral Dust Particles Mineral dust particles affect the climate by directly modulating the radiative budget due to the absorption and scattering of both short and long-wave radiation (10, 11). They also have an important indirect effect on the climate due to their microphysical interactions with clouds. Hansell, et al. (12) reported that dust’s longwave warming effect counters more than 50% of dust’s shortwave cooling effect. In general, the warming effect of greenhouse gases is global but the radiative impact of dust is regional. For example, dust’s radiative impact, which is associated with its warming effects, measures from 2.3 to 20 watts m-2 of radiative forcing in Zhangye, China near the Gobi Desert area; in comparison, the warming effect by greenhouse gases measures around 2 watts m-2. Thus, the 303 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
influence of dust on the longwave radiation associated with climate effects is significant. Additionally, dust particles have been shown to act as efficient ice nuclei (IN) (13–15) and cloud condensation nuclei (CCN) (16, 17). Mineral dust deposited on snow can also influence snow’s ability to reflect sunlight, causing it to melt faster (18). Mineral dust particles can fertilize ocean surfaces because they provide nutrients to ocean plankton through long-range transport and deposition processes. For example, in the Mediterranean region, Saharan dust is an important source of nutrients for phytoplankton and other aquatic biosystems. However, Saharan dust adversely affects some ecosystems because it carries the fungus Aspergillus sydowii as well as others (19). Since 1970, dust outbreaks have worsened due to periods of drought in Africa (20), leading to a decline in the health of coral reefs across the Caribbean and Florida. Additionally, dust particles can carry infectious diseases such as bacterial meningitis and the diseases associated with micro-organisms transported in desert dust (i.e. Coccidioidomycosis) (21). The health impact of heavy metal dust exposure is also a global concern (22–25) due to the long-range transport of dust particles. In addition to the effect of dust itself, airborne sand dust in East Asia contains chemicals, metals, microorganisms, and ions from urban or industrial pollutant emissions across many regions (26–28). Watanabe, et al. (29) reported that the decrease in pulmonary function may be more severe when the levels of both sand dust particles and air pollution aerosols are high. In the future, it may be necessary to study the synergistic health effects of sand dust particles and air pollution aerosols in East Asia.
2. Heterogeneous Chemistry of Atmospheric Tracers on Mineral Dust Particle Surfaces With an average lifetime of up to several weeks, mineral particles can be transported over large distances downwind from the source. Dust particles can act as an important sink for atmospheric trace gases such as NOx, SO2, O3, and organics. Most laboratory studies (30–32) have been limited to certain metal oxides (i.e. TiO2 and Fe2O3) and the uptake of NO2 and SO2 without the presence of radiation. As noted in the recent chamber study by Park and Jang (33) the of SO2 in the presence of dry ATD particles reactive uptake coefficient increased by one order of magnitude (1.16 × 10-6) in an indoor chamber with light integrated from UV–A and UV–B lamps compared to that from autoxidation (1.15 × 10-7) without a light source. Dupart, et al. (34) also observed a significant enhancement of the uptake rate of NO2 on ATD dust particles using UV–A irradiation. The field study by Dupart, et al. (35) observed an increase in conversion of SO2 to sulfate by photooxidation chemistry during dust events. In the following sections, the mechanisms underlying the heterogeneous chemistry of atmospherically important tracers will be discussed. Figure 3 illustrates the simplified mechanisms of heterogeneous oxidation of major pollutants (O3, SO2, NO2, and hydrocarbons). The reaction of adsorbed tracers occurs via two major pathways: autoxidation in the open air (no need of a light source) and photocatalytic mechanisms under UV light. 304 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 3. Mechanisms for heterogeneous oxidation of SO2 and NOx on airborne dust particles. 2.1. Autoxidation Autoxidation of tracers on dust is an oxidation process via the reaction of an absorbed tracer molecule with an oxygen molecule. For example, the autoxidation of SO2 on dust particles (denoted as “d”) can be represented as the first order reaction (assuming a constant concentration of oxygen at 2 × 105 ppm).
In the dark condition, the formation of sulfate is primarily from autoxidation of SO2. For comparison with other studies, the uptake coefficient of tracer Xi on dust is estimated. The relationship between kauto and is written as (36)
where is the mean molecular velocity (m s-1) and Sdust is the surface area (cm2 m-3) of dust particles. It should be noted that the calculation of the uptake coefficient using BET surface area will be different from the geometric surface area. 2.2. Photoactivation Semiconducting metal oxides (i.e. TiO2, Fe2O3, and their mixture with nonsemiconducting metal oxides such as Al2O3 (37)) in dust particles could act as photocatalysts that yield electron (e-cb from the conduction band, cb)–hole (h+vb from the valence band, vb) pairs and produce strong oxidants (i.e. superoxide radical anions (O2−) and OH radicals) (34, 38–43). For example, oxygen molecules adsorbed on metal oxides receive an electron and produce highly reactive O2− 305 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
radicals. Water molecules adsorbed on dust also react with h+vb forming a hydroxyl radical (OH). The reactions are shown as follows:
OH radicals enable rapid oxidation of tracers, such as SO2 and NO2, on the surface of dust particles (33, 34, 44, 45). However, most studies on the photochemistry of dust have been limited to qualitative analyses and lack kinetic mechanisms that can be linked to a predictive model. 2.3. Heterogeneous Chemistry of Ozone The heterogeneous uptake of ozone on the surface of metal oxides results in O3 decomposition, forming a surface-bound atomic oxygen and an oxygen molecule. Usher, et al. (46) observed that O3 increased the conversion of sulfite/bisulfite to sulfate/bisulfate on the surface of metal oxides (Al2O3) using FTIR spectroscopy. In the presence of O3, the oxidation of NO2 to form nitric acid is also enhanced on dust particles via the pathway of the formation of N2O5 followed by its hydrolysis.
In the presence of UV light, O3 is efficiently decomposed on the surface of dust due to the interaction of O3 with e-cb, and is thereby able to form an OH radical.
306 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
This OH radical can oxidize tracers adsorbed or absorbed on dust surfaces. Using a global three-dimensional model, Dentener et al. (47) estimated that O3 levels close to a dust source are reduced by more than 10% due to this heterogeneous uptake process. Similarly, Wang et al. (48) reported that mixing ratios of O3 were reduced by up to 3.8 ppb (~9%) in the presence of dust. 2.4. Heterogeneous Chemistry of Nitrogen Oxides In the absence of a light source, the heterogeneous oxidation of NO2 still occurs but is much weaker. Underwood et al. showed that NO2 adsorption on the surfaces of metal oxides involved subsequent formation of surface nitrite (NO2-) ions and the production of nitrate ions via the reaction of two NO2- or the reaction of NO2- and gas phase NO2 under dark conditions (49). As discussed in section 2.3, nitrate can form from the hydrolysis of N2O5. Liu et al. (50) and Ma & Liu (51) reported the formation of dinitrogen tetroxide (N2O4), a critical oxidant, on the surface of metal oxides showing that NOx promotes SO2 oxidation but that the formation of nitrite ion was inhibited by SO2. In the presence of UV light, NO2 oxidation is enhanced on the surface of mineral dust particles. Ndour, et al. (8) found the production of HONO from NO2 autoxidation was negligible, but HONO formation from photooxidation of NO2 was significant in the presence of dust particles such as mixed TiO2-SiO2, Saharan dust, or Arizona Test Dust.
The OH radical produced from the photolysis of HONO can oxidize NO2 and increase nitrate formation. When Ndour, et al. (8) applied dust heterogeneous chemistry to three-dimensional modeling, they found that the photochemistry of dust may reduce the NO2 level up to 37% and ozone up to 5% during a dust event in the free troposphere. 2.5. Heterogeneous Chemistry of Sulfur Dioxide The laboratory studies characterizing the heterogeneous chemistry of SO2 have focused mostly on autoxidation. Zhang et al. found that SO2 was heterogeneously oxidized by active oxygen (O2-) or hydroxyl ions (OH-), which were produced from O2 or H2O adsorbed on the surface of metal oxides (i.e. Al2O3) (32). The oxidation of SO2 can also be promoted by the heterogeneous chemistry of atmospheric oxidants like O3 (46, 52–55), NOx (50, 51, 56, 57), and H2O2 (58, 59). For example, sulfite (SO32-) produced on the surfaces of dust particles due to the solubility of SO2 in the dust aqueous phase is heterogeneously oxidized to sulfate by exposure to ozone (53, 54). H2O2 adsorbed on the dust 307 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
particle can produce OH radicals and oxidize adsorbed SO2 (59). As shown in section 2.2, some metal oxides (i.e. photoactive semiconductor materials) in mineral dust particles are known to generate e-cb-h+vb pairs under UV light and create powerful surface oxidants (i.e. OH radicals) (35, 60, 61). The heterogeneous photooxidation of SO2 occurs by oxidation with these OH radicals forming SO3. The resulting SO3 is hydrated and forms H2SO4 (4, 42, 62, 63).
3. Chemical and Physical Parameters To Model Dust’s Heterogeneous Chemistry Both dust’s characteristics and meteorological parameters can significantly influence the heterogeneous chemistry of tracers. In this section, we will discuss how these properties are linked to the model parameters in the Atmospheric Mineral Aerosol Reaction (AMAR) model. 3.1. Buffering Capacity of Dust Particles Calcium-rich dust particles can buffer an atmospheric acid, which condenses or heterogeneously forms on dust surfaces, and increase pH values of precipitation (4, 64–68). Dust’s capacity to buffer acids is complex due to various reactions occurring on the dust surface:
In general, calcium nitrate (Ca(NO3)2) produced in Eq. 22 is much more hygroscopic than calcium carbonate. The buffering capacity of dust particles is closely related to the maximum nitrate concentration in the absence of sulfuric acid and semivolatile carboxylic acids (RCOOH), which can deplete nitrate.
A recent study by Yu and Jang (68) reported that the measured buffering capacity of GDD (0.02 µg µg-1) was significantly higher than that of ATD (0.011 µg µg-1). In general, the dust originating from Chinese loess contains a high fraction of CaCO3 (28). The inclusion of the buffering capacity of mineral dust into the model is essential in order to predict the formation of alkaline nitrate salts and the water content in dust phases. 308 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
3.2. Hygroscopicity of Dust Hygroscopicity of mineral dust is another important parameter because the dust water content influences gas-dust partitioning of tracers, the production of oxidants via photocatalytic heterogeneous chemistry, and reactions of tracers adsorbed on dust particles. Hygroscopicity of mineral dust varies with dust sources and the status of dust aging. The inorganic salts and metal oxides in dust particles absorb water via a thermodynamic equilibrium process and form a thin layer of a water film on the dust surface (67, 69–71). Figure 4 illustrates the hygroscopicity of two different mineral dust: ATD and GDD. The water content of dust particles was measured by the FTIR for %RH levels ranging from 10 to 85. No clear phase transitions nor obvious differences between the hydration and dehydration processes were observed because the hygroscopicity of dust particles is influenced by a variety of chemical species.
Figure 4. The measured water mass normalized by the dry dust mass using FTIR data for fresh ATD and fresh GDD. (Adapted with permission from reference (5). Copyright 2017, ACS Publications.)
In the recent study by Yu and Jang (68) a mathematical equation for the dust-phase water content (Fwater, µg µg-1), which is defined as the water mass normalized by the dry dust mass, was semiempirically derived using FTIR data. In their approach, Fwater was calculated by the three major contributors (Figure 5): (1) The low amount of water in authentic dust mostly originates from metal oxides and alkaline carbonates. Additionally, sulfate salts formed in dust are generally not hygroscopic and treated in the same way as alkaline carbonates. (2) The hygroscopicity of nitrate salts (Eq. 22) is much higher than that in fresh dust particles. (3) An excess amount of acidic sulfate can be neutralized by ammonia on dust surfaces, forming the ammonium sulfate system, and the acidic sulfate’s hygroscopicity can be estimated using the conventional inorganic thermodynamic model (72–74). The difference in model parameters for hygroscopicity between ATD and GDD is insignificant. The most influential variable that impacts the 309 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
hygroscopicity of aged dust is the amount of nitrate, which is closely related to the buffering capacity of dust.
Figure 5. The calculation of the water mass (Fwater, µg µg-1) normalized by the dry dust mass. The hygroscopic parameters of GDD are sourced from work by Yu and Jang (68)
The formation of nitric acid on dust is faster than that of sulfuric acid because NOx concentrations are generally higher than SO2 in ambient air, and the reaction of NO2 with OH radicals is also faster than that of SO2 (by one order of magnitude). Thus, mineral dust in the ambient air is typically aged by NOx first forming nitrate salts. In the presence of SO2, carbonate in fresh dust and nitrate in aged dust are further depleted by the sulfate formation. Figure 6 illustrates how the hygroscopicity of mineral dust dynamically modulates as dust ages.
Figure 6. The hygroscopicity of fresh dust; the aged dust with nitric acid; the aged dust with sulfuric acid; and the dust coated with sulfuric acid. 310 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
The upper boundary of Fwater at a given dust type is higher in the fully titrated mineral dust with nitric acid and is significantly affected by buffering capacity. For example, the water content in the aged GDD particles with nitric acid is almost 40% higher than in aged ATD because of the higher buffering capacity of GDD. Under the dust event, both nitrate and sulfate could possibly coexist on the dust surface, and dust particles can be partially or fully titrated by both inorganic acids and organic acids. 3.3. Photoactivation of Dust Particles The value of the photolysis rate constant (ji) of compound i is typically determined by actinic flux (I(λ), quanta cm-2 s-1 nm-1), the absorption cross section (σ(λ), cm2), and the quantum yield (ɸ(λ)) at each wavelength range (λ, nm):
In order to use the preexisting structure of the photolysis rate constant, the integration of wavelength–dependent actinic flux with the photocatalytic activity of mineral dust is also needed. In the AMAR model recently derived by Yu et al. (67) the photoactivation rate constant on dust particles (j[ATD] (s-1)) was introduced to produce an e-cb−h+vb pair, which was also dependent on both the actinic flux originating from the light source and the photocatalytic property of dust particles. In order to deal with σ(λ)×ɸ(λ), the mass absorption cross section of dust particles (MACATD, m2 g-1) was estimated in the model. MACATD was determined using the absorption coefficient (bATD, m-1) of Arizona Test Dust, a reference dust, and the particle concentration (mATD, g m-3):
bATD is calculated from the absorbance of a dust filter sample (AbsATD) measured using a reflective UV–visible spectrometer,
where Afilter (7.85 × 10-5, m2) is the area of the filter sample and V (m3) is the total volume of air passing through the filter during the sampling period. To eliminate the absorbance caused by filter material scattering, a correction factor (f = 1.4845) is coupled into Eq. (28) (75). Both σ(λ) and φ(λ) cannot be directly measured because of complexity in the amount of photoactive conducting matter in dust particles and the irradiation processes of the e-cb–h+vb pairs. The MACATD of dust particles originates from light–absorbing matter (i.e. metal oxides and metal sulfate). The MACATD spectrum is adjusted using the TiO2 absorption spectrum in order to account for the conducting matter in dust particles (76) and applied to σ(λ) × φ(λ). Figure 7 illustrates the activation process of mineral dust by UV light with the MACATD spectrum of Arizona Test Dust and the σ(λ) × φ(λ) spectrum applied to the model. 311 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 7. Photoactivation of mineral dust particles to produce electron-hole pairs. As shown in the composition of dust sourced from different locations (Figure 2 in section 1.1), the amount of conducting matter in Gobi Desert Dust is lower than that of Saharan dust but higher than that of Arizona dust. Collectively, heterogeneous chemistry under UV light is affected by the metal compositions of mineral dust. The recent study by Park et al. (4) observed that the reactive uptake coefficient of SO2 in the presence of the Gobi Desert dust was 2 to 2.5 times greater than in the presence of Arizona Test Dust. 3.4. Humidity and Temperature Effects on Heterogeneous Chemistry The Henry’s law constant of a compound can be applied to calculate the gas-particle partitioning coefficient of tracers. When temperature increases, the Henry’s law constant generally decreases. Moreover, temperature influences the mobility of the compound in particles. Lower temperature leads to slower movement of a molecule due to the high viscosity of the medium. Thus, the temperature may alter the reaction rate constant of tracers in the dust phase, but this is not well studied yet. Humidity influences the amount of dust phase water, and subsequently affects gas-particle partitioning of tracers and chemistry. As discussed in section 3.2, the dust phase water content is determined by the hygroscopicity of dust particles under varying humidity. Humidity modulates the production of the surface OH radical because the electron-hole pairs created in the photoactivation process reacts with a water molecule to generate surface oxidants such as OH radicals.
4. Modeling Heterogeneous Chemistry of Tracers on Dust Heterogeneous chemistry of tracers on dust surfaces has been traditionally approached using an uptake coefficient based on laboratory data (8, 30, 31, 57, 77–79). However, this uptake coefficient is limited to the heterogeneous reaction of tracers under dark conditions or a simple mechanism controlled by a pseudo 1st-order reaction. In the ambient environment, the reactive uptake of tracers via the heterogeneous photocatalytic process on dust particles cannot be treated 312 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
by a single uptake coefficient because it originates from multi-step reactions including 2nd order reactions. Furthermore, dust particles dynamically age, influencing partitioning and reaction rates. Therefore, an explicit kinetic model, which benefits from accurately simulating heterogeneous chemistry, can predict the change in atmospheric compositions on regional and global scales during the atmospheric processes of dust events. In the following section, the AMAR model, which is derived using explicit kinetic mechanisms (67), will be described and demonstrated to predict the heterogeneous oxidation of tracers.
4.1. Kinetic Mechanisms of Heterogeneous Chemistry in the Dust Model Figure 8 illustrates the model skeleton for the heterogeneous chemistry of tracers in the presence of mineral dust. In order to predict the atmospheric fate of tracers in the presence of dust, the model needs to include chemistry in the three phases: gas phase, aqueous aerosol phase, and dust phase. The reaction of SO2 with an OH radical in the gas phase ultimately produces sulfuric-acid vapor. The sulfuric acid vapor, with volatility as low as 10-9 mmHg in ambient humidity, is involved in nucleation to form sulfuric-acid seeds, hygroscopic aerosols (80–82). Except in the large presence of dust particles, the formation of sulfuric acid seeded aqueous aerosol is unavoidable. The explicit mechanisms of the aqueous–phase chemistry that occurs in inorganic salted aqueous aerosol (SO42-–NH4+–H2O and can be accounted for in the system without dust) to form preexisting model constructed by Liang and Jacobson (83)
Figure 8. The schematic of explicit kinetic mechanisms for heterogeneous chemistry in mineral dust particles. 313 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Under urban environments (relative humidity higher than 20%), dust particles are typically coated with multiple layers of water (69, 70, 84). Thus, the assumption of absorptive partitioning of tracers onto the water layer is reasonable and allows us to use the Henry’s law constant for the partitioning process between the gas and dust phase. For example, the gas-dust partitioning of tracer Xi is defined as, constant
where Adust (m2 m-3) is the concentration of geometric surface for dust particles. The partitioning process of tracer Xi is kinetically calculated using absorption and desorption processes as follows,
where (m3 m-2 s-1) and (s-1) are the absorption rate constant and the desorption rate constant, respectively. At equilibrium, the absorption rate (Eq. 30) equals the desorption rate (Eq. 31). Thus, is expressed as:
The characteristic time to reach equilibrium is as short as the order of 10-6 s. Typically, the adsorption and desorption rates are larger and faster than the reaction rates of tracers with oxidants in particle phase. In the model, the absorption rate constants are set at large numbers to simulate a fast partitioning process. The gas-dust partitioning constant is estimated first and then applied to Eq. 32 to determine the desorption rate constant. The desorption rate constant is dependent on temperature and humidity (Fwater, Figure 5). The temperature dependence of is estimated with an analog of the Henry’s law constant,
When the chemical species is dissociable in aqueous phase, the desorption rate constant is also affected by the acidity of particles. For example, the desorption rate constant of SO2 that forms H2SO3 is equilibrated with HSO3- and SO32-, and can be affected by acidity. The desorption rate constant is rewritten as shown below,
The dust phase reactions are divided into two major aspects: autoxidation and photooxidation. As shown in Figure 8, e-cb−h+vb pairs are produced via the photoactivation of dust particles. These e-cbh+vb pairs can be deactivated through a recombination process, or react with a water molecule or an oxygen molecule to form a strong oxidant, an OH radical, on the surface of dust as expressed below:
314 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
kOH (cm3 molecules-1 s-1) is the reaction rate constant to form the OH radical on dust and is a function of humidity. The tracer absorbed on the surface of mineral dust can be oxidized via the reaction with this OH radical. In the model, is the rate constant for autoxidation of species Xi and is the reaction rate constant for the photooxidation of Xi which is processed via the mediated OH radical (Figure 8). The production of e-cb−h+vb pairs is estimated using the fraction of the mass of effective conductive metals to total dust mass ([M*], ug ug-1):
In the model, the [M*] value of ATD is selected as reference dust and applied to scale [M*] for different types of dust particles based on laboratory data. For example, the photo-degradation rate of an organic dye (i.e. malachite green) impregnated on different dust filter samples was measured using the online reflective UV-Visible spectrometer. The [M*] value of GDD is 2.55 times higher than that of ATD. This finding is in accord with the results of Park, et al. (4). 4.2. Simulation of Heterogeneous Oxidation of SO2 and NOx Figure 9 shows that the AMAR model simulation agrees with chamber observations in a recent study. Figure 10 represents the simulation of nitrate and sulfate formation in the presence of the fresh GDD using the explicit heterogeneous chemistry mechanisms (AMAR) integrated with a box model given a typical urban environment. Several important findings that resulted from this simulation are summarized as follows: (1) In the presence of fresh dust particles, the formation of nitrate occurs earlier than that of sulfate because the oxidation rate of NOx is faster than that of SO2 in both gas phase and particle phase. (2) The nitrate concentration on dust particles at high humidity (wet aerosol) is slightly higher than that estimated from the buffering capacity of dust particles (section 3.1) because nitric acid can partition onto the water layer associated with wet alkaline nitrate salt on dust. (3) Dust particles are completely neutralized mostly by nitric acid in the morning under the simulation conditions shown in Figure 10. (4) In the morning when humidity is high (before 8 AM in Figure 10(a)), the formation of hygroscopic alkaline nitrate salts rapidly increases and thus, the water fraction (Fwater) in dust particles also increases. (5) With 20 ppb of SO2, nitrate is partially depleted in the simulation above (Figure 10). If more SO2 is introduced into the system, nitrate will be further depleted. (6) The water content of dust particles drops in the late morning because of the decrease in humidity as well as the formation of alkaline sulfate, which is less hygroscopic than nitrate salts. (7) Overall, heterogeneous photooxidation is the most important mechanism (57%) for forming sulfate in the presence GDD in this simulation. Autoxidation is less significant than photooxidation but it is still 315 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
more significant (31%) than non-dust chemistry (gas phase oxidation + aqueous chemistry) (12%) in the daytime. In the dark period, autoxidation continuously adds sulfate to the system.
Figure 9. AMAR-model simulation of sulfate and nitrate formation against chamber data obtained on April 25, 2017 in the presence of ATD particles using the Atmospheric PHotochemical Outdoor Reaction (UF-APHOR) chamber located in the University of Florida.
Figure 10. (a) Simulation of sulfate and nitrate formation in the presence of GDD particles under the typical urban environmental condition. (b) The ambient data for sunlight, temperature, and humidity are obtained on November 23, 2017 at Gainesville, Florida (latitude/longitude: 29.64185°/–82.347883°). Figure 11 shows model results for the impact of dust characteristics on the heterogeneous chemistry of SO2 in the presence of three different dust particles: ATD, GDD, and the GDD pre-aged with the oxidation of NO2. GDD has both a 316 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
higher photoactivation ability and greater buffering capacity than ATD, and thus yields higher sulfate formation. As shown in Figure 11 (a)-(c), sulfate formation increases from ATD to GDD, but the sulfate formation with GDD is similar to that with pre-aged GDD with NO2. Considering the insensitivity of SO2 oxidation to buffering capacity, the most critical factor in increased sulfate formation is the photoactivation capacity of dust particles. Under the ambient conditions within our simulation (see Figure 10 (b)), %RH drops to about 40 during the daytime and significantly reduces dust phase water content (Figure 4). In order to simulate the impact of humidity on sulfate formation, the prediction under ambient humidity (Figure 11 (b)) is compared to that at RH=55% (Figure 11 (d)). Humidity considerably affects sulfate formation (Figure 11 (b) vs (d)) but the impact of aging is small (Figure 11 (b) vs. (c) or Figure 11 (d) vs. (e)). Although not shown here, both the preexisting chamber studies (4, 33) and the previous model simulations (67) showed that the oxidation of SO2 was suppressed by NOx because SO2 and NO2 competed for the consumption of OH radicals.
Figure 11. The formation of sulfate from the photooxidation of SO2 in the presence of different dust particles (i.e. ATD, GDD and pre-aged GDD particles with nitrate) under the two different metrological environments (ambient conditions of Figure 10 (b) and fixed humidity at 55%). The photoactivation capability and the buffering capacity of GDD and aged GDD are scaled to reference dust, ATD. 4.3. Heterogeneous Chemistry of Hydrocarbons The impact of mineral dust particles on hydrocarbon oxidation is not understood well yet. Lederer, et al. (85) reported that the oxidation of d-limonene on the surface of mineral dust formed d-limonene epoxide and dihydroxy derivatives via hydrolysis. However, the uptake coefficient of d-limonene via autoxidation was insignificant after consideration of terpene concentrations in ambient air. The most important aspect of the heterogeneous chemistry of hydrocarbons is the accommodation of semivolatile oxygenated products on preexisting dust particles. Semivolatile organics are produced from the atmospheric oxidation of some hydrocarbons in the gas phase. These semivolatile organics on the dust surface can compete with inorganic tracers (i.e. SO2 and NO2) for the consumption of surface OH radicals. Thus, the organic coating on dust particles may modify the production of sulfate and nitrate. The coating of secondary organic aerosol (SOA) on mineral dust particles could vary with the consumption of hydrocarbon and the concentration of dust particles (86, 87). 317 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 12 illustrates our recent data that show the formation of the SOA produced from the photooxidation of α-pinene in the presence of NOx (VOC ppbC/NOx ppb = 6.9) and GDD using the outdoor chamber. Compared to the organic matter (OM) formed in the presence of silica particles, the OM produced with GDD was significantly greater, suggesting that authentic dust particles can be effectively coated by SOA. In Figure 12, the maximum nitrate concentration appears in the early stage but never reaches the maximum buffering capacity (7.8×10-4 gm-2). One possible explanation for such nitrate depletion is the formation of carboxylate salts via the reaction of carboxylic acids with carbonate or nitrate salts. The enrichment of carboxylate salt was also observed in the formation of SOA in the presence of sea salt aerosol (71).
Figure 12. The formation of organic matter (OM normalized by the surface area) and nitrate from the photooxidation of α-pinene in the presence of NOx and GDD particles (or SiO2 particles) using the UF-APHOR chamber. The nitrate on SiO2 is negligible. Carboxylate salts can also influence the hygroscopicity of mineral dust particles. Additionally, adsorbed organic compounds can react with dust-phase OH radicals and influence the oxidation rate of NO2 and SO2. Currently, little research has been conducted regarding the interaction of mineral dust with organic compounds. The incorporation of the chemical interactions of mineral dust with hydrocarbons and inorganic tracers into the dust-phase heterogeneous chemistry model should be conducted in the future.
5. Conclusions The primary components which are necessary to simulate the heterogeneous chemistry of atmospheric tracers (SO2 and NO2) in the presence of mineral dust were discussed in this chapter. The heterogeneous oxidation of tracers on dust surfaces is affected by both meteorological variables (i.e. temperature, humidity, and sunlight (section 3.4) and dust characteristics (i.e. buffering capacity, photoactivation ability, and hygroscopicity (sections 3.1-3.3). For NO2 oxidation, the maximum concentration of nitrate is limited by the buffering capacity of dust particles. However, buffering capacity cannot limit the formation of sulfate from 318 Hunt et al.; Multiphase Environmental Chemistry in the Atmosphere ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
the heterogeneous oxidation of SO2. Across different authentic mineral dust particles, the most important parameters that enhance tracers’ photooxidation is the dust’s photoactivation ability, which is catalyzed by conductive metal oxides. Hence, it is necessary to scale the photocatalytic ability of different dust particles. In our study, the photoactivation ability of different dust types (ATD and GDD) was scaled using the kinetic rate constant of a photo-degradation of the surrogate compound (section 3.3) on dust particles. In the future, the photoactivation ability of diverse dust particles should be connected to dust metal compositions. Through atmospheric processes, the hygroscopicity of mineral dust particles dynamically changes. The dust particles aged with nitrate become hygroscopic and can also become deliquesced at low humidity (
