The Mechanisms Responsible for the Interactions among Oxalate, pH

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The mechanisms responsible for the interactions among oxalate, pH and Fe dissolution in PM 2.5

Ye Tao, and Jennifer Grace Murphy ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.9b00172 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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ACS Earth and Space Chemistry

The mechanisms responsible for the interactions among oxalate, pH and Fe dissolution in PM2.5 Ye Tao1, Jennifer G. Murphy*2 1Department

of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto, ON, M1C 1A4, Canada 2Department

of Chemistry, University of Toronto, Toronto, ON, M5S 3H6, Canada

*Corresponding author: [email protected] Abstract. In this study, 10-year records of PM2.5 oxalate concentrations in six urban and rural Canadian monitoring sites are analyzed along with the time series of other water-soluble ionic components, watersoluble metals and aerosol pH to identify the major factors influencing its abundance and speciation. Oxalate was found to have higher mass loadings relative to sulfate and nitrate when the temperature was higher and the aerosol was more acidic, which is counter to the gas/particle phase partitioning expected from the effective Henry’s Law constant. However, a strong correlation was found between oxalate and water-soluble Fe (WS-Fe), with a slope between 1.3 and 4.6, suggesting that the major forms of the particulate oxalate were in complexes with WS-Fe, whose concentration was also found to be significantly impacted by aerosol pH. When aerosol exhibits pH around 2, the proton-promoted dissolution of Fe leads to a strong relationship between WS-Fe and pH, indirectly supporting higher oxalate levels in more acidic particles. Meanwhile, higher aerosol pH in wintertime favors oxalate to fully partition into the particle phase, but higher WS-Fe concentration in aerosol liquid water was also often observed when aerosol had higher oxalate concentration. This phenomenon suggests that the chelating-promoted dissolution mechanism induced by oxalate can enhance Fe dissolution in wintertime. Our results reveal that the interactions among pH, oxalate and Fe exhibit strong seasonality, and an improved understanding of these mechanisms is important for the environmental fate studies of both organic acids and metals. Key words: PM2.5, pH, oxalate, water-soluble Fe, seasonal cycling 1 ACS Paragon Plus Environment

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1. Introduction Oxalate is one of the most widely studied organic acids in the atmosphere because it is the single most abundant dicarboxylic acid in the aerosol phase 1-3. Oxalate is estimated to contribute as much as 59 % of the global tropospheric burden of water-soluble organic carbon 4-6. Like sulfate, oxalate can form ammonium salts 7, and can also contribute to the charge balancing NH4+ in the particle aqueous phase 8. It is believed that the presence of particulate oxalate has the potential to alter aerosol acidity hygroscopicity

10.

9

and

Moderate to good correlations are commonly seen between particulate sulfate and

oxalate, which is often explained by the components sharing similar in-cloud formation pathways

11-13.

Thus, several studies use oxalate as the model compound to represent the interaction between organic acids and inorganic (sulfate, nitrate and ammonium) aerosol constituents 12, 14, 15. However, there are two distinct differences between sulfate and oxalate that could have significant impact on their environmental fate. The first pKa for oxalic acid is 1.3 at 25 oC 16, significantly higher than that of sulfuric acid (pKa=-3). Also, the vapor pressure of oxalic acid is about 107 times higher than that of sulfuric acid 17, 18. As a result, particulate oxalate concentrations are more strongly influenced by the gas/particle partitioning equilibrium: H2C2O4(g) ↔ H2C2O4(aq) ↔ HC2O4-(aq) + H+. When aerosol liquid water has a lower pH, this equilibrium will result in more oxalate evaporating into gas phase as oxalic acid. The theoretical calculations and ambient measurements by Nah, et al. 19 showed that about 40% of total oxalate (summation of particulate oxalate and gas phase oxalic acid) will partition into the gas phase at aerosol pH ~ 2 (at a temperature and liquid water content of 23.4 oC and 1.6 μg m-3 respectively). In comparison, under typical pH (>3) and liquid water content (0.1~1 g m-3) of cloud water 20, 21, oxalate will have almost complete (>99%) partitioning into cloud droplets. The second difference is that oxalate has a strong tendency to form stable complexes with metal ions 22-24.

Some studies have shown that oxalate mainly exists in the form of ligands to metals in fine particulate

matter 23, 25, 26, and this characteristic can be extended to other dicarboxylic acids 16, 27. Furthermore, the 2 ACS Paragon Plus Environment

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existence of oxalate in atmospheric water can also enhance the dissolution of Fe in mineral dust

16, 28.

After forming complexes, photodissociation through Fenton reaction will probably serve as the major sink for the ligand compounds 27, 29, 30. These factors suggest that the pH and water content of aerosol, along with the potential for organic-metal interactions need to be considered for the phase partitioning of oxalate and other similar organic acids such as malonate and tartrate 16. The NAPS (National Air Pollution Surveillance) Program database in Canada provides long-term measurements of gas phase and PM2.5 composition including water-soluble ionic components, watersoluble metals and total metal concentrations at many monitoring sites. In a previous study 31, we analyzed the 10-year (2007-2016) trends of aerosol pH and its major influencing factors at six sites located in Ontario and Quebec. In this study, we focus on the 10-year time-series of oxalate, aerosol pH and watersoluble metals concentrations at the same six sites to identify the major factors influencing particulate oxalate concentrations.

2. Method and data source 10-year (2007-2016) time series of PM2.5 composition and trace gas data at six Canadian sites, including Toronto, Ottawa, Windsor, Montreal, Simcoe and St Anicet, were obtained from NAPS program in Canada and used in this study. The relevant PM2.5 composition data and gas phase NH3 and HNO3 concentrations can be accessed from the NAPS database (http://maps-cartes.ec.gc.ca/rnspa-naps/data.aspx) and the detailed description of the protocols for sampling and chemical analysis can be found in DabekZlotorzynska, et al. 32. The gas phase NH3 and HNO3 were sampled by denuders coated with citric acid and sodium carbonate respectively and measured by Ion Chromatography (IC). The concentration of the major water-soluble inorganic ions and oxalate in PM2.5 were also measured by IC. Based on the equilibrium provided by Brandt and Vaneldik 33, we calculated that the alkaline environment in the eluent for anion IC (pH ≥ 11) will hydrolyze Fe(III) into species complexed with hydroxide anions, leaving the Fe-oxalate complexes negligible. As a result, the oxalate concentration measured by the IC reflect the sum of complexed and noncomplexed oxalate in the extract of PM2.5. Both the total metals and water-soluble 3 ACS Paragon Plus Environment


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metals in PM2.5 are measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The results at these six sites are very similar so the results from the Toronto site are shown in detail and a summary of the results from the other five sites is provided in section 3.3. The corresponding meteorological parameters were downloaded from the nearest monitoring sites at http://climate.weather.gc.ca/. The aerosol pH and water content are calculated with the E-AIM II

34

thermodynamic model, available at http://www.aim.env.uea.ac.uk/aim/aim.php. The influencing factors of aerosol pH have been discussed in detail in Tao and Murphy 31, which showed seasonal temperature variation to be the dominant factor. The number of valid measurements (days when all components are reported) for these six sites are all larger than 700, allowing for reliable statistical analysis.

3. Results and discussion 3.1 Oxalate and PM2.5 To study the phase partitioning of oxalate, the time series of oxalate and water-soluble inorganic ions, water-soluble metals and total metals in PM2.5 at the Toronto NAPS site from 2007-2016 were analyzed. The ratio of oxalate to the sum of sulfate and nitrate PM2.5 mass concentration is shown in Figure 1 to illustrate the variability and seasonality in the importance of oxalate relative to the major inorganic acidic components. The relative concentration of oxalate is shaded by aerosol pH. Ambient temperature is also plotted on the upper panel, showing that oxalate tended to have higher relative concentrations in summertime when the temperature was higher and the aerosol was more acidic (pH~2). In addition to being relatively more concentrated in summertime, oxalate also showed higher absolute concentrations in PM2.5 in summertime (shown in Figure S1).

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Figure 1. Time series of oxalate to sulfate and nitrate mass ratio in PM2.5 and ambient temperature from 2007 to 2016 in Toronto atmosphere. Oxalate to inorganic acids mass ratio was shaded by aerosol pH as calculated in Tao and Murphy 31.

While the overall production rate of oxalate and oxalic acid may be expected to maximize in the summer due to higher concentrations of precursors and faster photochemical production, higher particle phase concentrations are not necessarily expected

11, 35, 36.

This seasonality of the partitioning was

unexpected because high temperature and low pH should make oxalate less soluble in particles, if the partitioning is governed by the effective Henry’s law constant. Oxalate is semi-volatile and should establish equilibrium with gas phase oxalic acid, which is not measured in the NAPS program. The equilibrated gas phase oxalic acid concentrations corresponding to the aerosol pH and liquid water content were calculated through E-AIM II modelling, assuming that all the measured particle phase oxalate was in the form of non-complexed H2C2O4, HC2O4-, or C2O42-. Figure 2 shows the modelled gas phase fraction of oxalic acid calculated by the measured particulate oxalate and modelled gas phase concentrations as a function of aerosol pH with the color representing ambient relative humidity. The modelled gas phase fraction of oxalic acid to total oxalate indicates that more than half of total oxalate should partition into the gas phase under thermodynamic equilibrium when pH ≤ 2, whereas the gas fraction is less than 5% when aerosol pH ≥ 3. For a given pH, lower RH will promote more gas phase oxalic acid because of the influence on aerosol liquid water content.



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Figure 2. E-AIM modelled gas phase fraction of oxalate based on the measured particulate oxalate concentrations versus aerosol pH. Data points are color plotted by ambient relative humidity. The graph shows that total oxalate tends to have higher fraction partitioning into the gas phase when aerosol is more acidic.

The combination of high particle oxalate concentrations when particle pH is low leads to predictions of high gas oxalic acid concentrations during the summer. The modelled monthly average equilibrium mixing ratio of gas phase oxalic acid is shown in Figure 3, which suggests that the gas phase oxalic acid concentrations in summertime would have hypothetically reached 0.5 ppb in 2016. The typical mixing ratios in other summers since 2010 were consistently higher than 0.2 ppb. This range of values is unrealistically large compared with reports in the literature

14, 19, 37,

even for biomass burning plumes,

where the gas phase mixing ratios were reported to be less than 0.1 ppb 38. The average gas phase oxalic acid concentration we measured in the summer of 2017 at a nearby sampling site was only 8.7 ppt with a maximum value for a 28-hour period of 20.7 ppt

39.

This suggests that a thermodynamic modelling

framework that only accounts for the effective Henry’s law constant to predict the gas-particle partitioning of oxalic acid leads to a significant overestimation of the gas phase oxalic acid concentration in summertime by one to two orders of magnitudes. This strongly implies that some other mechanism(s) is responsible for governing the speciation of oxalate in atmospheric particles.


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Figure 3. Time series of modelled monthly average gas phase mixing ratio of oxalic acid with whiskers representing the ranges from maximum values to minimum values.

3.2 Oxalate and water-soluble metals Many dicarboxylic acids, including oxalic acid, have the potential to form complexes with metals 20, 40.

Thus, we investigated whether particulate water-soluble metals could significantly alter the phase

partitioning behavior of oxalate/oxalic acid. Among all the water-soluble metals in PM2.5 measured by the NAPS program, water-soluble Fe exhibited both the highest correlation with particle oxalate concentration (R2=0.68) and has the highest molar concentration in PM2.5 (the details are illustrated in Figure S2), suggesting the importance of WS-Fe in studying the phase partitioning of oxalate/oxalic. Further analysis of the relationship between oxalate and water-soluble Fe in PM2.5 is illustrated in Figure 4. The linear relationship and small intercept in Figure 4(a) shows that Fe-oxalate complexes were probably the major existing forms of oxalate, and the slope (2.89 mol/mol) suggests that the major forms of Fe-oxalate complexes were likely a combination of [Fe(C2O4)3]3- and [Fe(C2O4)2]-. Oxalate was poorly correlated with total Fe (R2

The Mechanisms Responsible for the Interactions among Oxalate, pH

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