Multiple oxygen and sulfur isotope compositions of secondary atmospheric sulfate in a mega-city in central China
Introduction
Secondary atmospheric sulfate (SAS) is the ultimate oxidation product of, and sink for, sulfur gases such as SO2, H2S, and dimethyl sulfide (DMS) emitted from fossil fuel combustion, volcanism, and biological activities. Once formed, SAS falls to the Earth surface via wet and dry atmospheric deposition. SAS plays an important role in tropospheric and lower stratospheric chemical and physical processes, since it is one of the main atmospheric aerosol particles, a major component of cloud condensation nuclei (CCN), and a major contributor to acid rain. Sulfate aerosols have a cooling effect on the Earth surface by increasing Earth's albedo via direct scattering solar radiation and modifying reflective properties of clouds (Charlson et al., 1992). The role of SAS in scattering of solar radiation and in cloud microphysics represents one of the largest uncertainties in current assessments of climate change (Solomon et al., 2007).
The formation pathway of SAS determines the nucleation potential and optical properties of sulfate aerosols. The oxidation of atmospheric SO2 can take place in gas phase (Stockwell and Calvert, 1983) and in aqueous phase (Schwartz, 1987). In the gas phase, SO2 mainly reacts with OH radicals to produce H2SO4(g) that can nucleate new particles and increase aerosol density and population of CCN (Kulmala et al., 2000). Stabilized Criegee intermediates (CIs) have recently been recognized as being capable of oxidizing SO2 in the gas phase as well (Mauldin et al., 2012, Sarwar et al., 2013). However, the impact of CIs on SAS budget, cloud condensation nuclei, and aerosol indirect effect is believed to be limited to regions with alkene sources or anthropogenic VOC emissions and is insignificant in a global scale (Pierce et al., 2013). In the aqueous phase, SO2 can be oxidized by multiple oxidants, mainly H2O2 and O3 (Calvert et al., 1985), or O2 catalyzed by Fe3+–Mn2+ (Jacob and Hoffmann, 1983), and less significantly NO2 (Lee and Schwartz, 1982), NO3 (Feingold et al., 2002), HNO4 (Dentener et al., 2002), hypohalouacids (HOCl and HOBr) (Vogt et al., 1996), among others. Oxidation by H2O2 is not significantly dependent on pH within normal atmospheric pH ranges (pH = 2–7), while oxidation by O3 and transition metal catalysis becomes faster as pH increases (pH > 5–6) (Seinfeld and Pandis, 1998). The other oxidants are known to contribute to aqueous S(IV) oxidation in regionally polluted environments (Dentener et al., 2002, Feingold et al., 2002, Hoffmann and Jacob, 1984, Lee and Schwartz, 1982). Sulfate produced in the aqueous phases occurs on the surface of particles or in cloud droplets and does not lead to nucleation of new particles. The CCN activity and lifetime of heterogeneously produced sulfate can change through growth and increased hygroscopicity (Bower and Choularton, 1993).
The competing oxidation pathways are affected by atmospheric oxidant concentrations, cloud liquid water content, cloud-water pH, atmospheric transition metal concentrations, aerosol surface area, and other factors (Sofen et al., 2011). Geographically distinct sites have different meteorological conditions. Apportionment among competing oxidation pathways can be predicted by chemical reaction and transport models such as the GEOS-Chem global, three-dimensional atmospheric chemical transport model (Alexander et al., 2005, Park et al., 2004, Sofen et al., 2011) or the U.S. EPA Models-3/Community Multiscale Air Quality (CMAQ) model (Byun and Schere, 2006) with the Sulfate Tracking Model option (McHenry and Dennis, 1994, Bao et al., 2010). The modeling results can be examined and constrained by stable isotope compositions, especially the triple oxygen isotope composition of the SAS, because different oxidants (e.g., O3, H2O2, OH, and O2) transfer their unique oxygen isotope signatures to the oxidation product, sulfate (Savarino et al., 2000). In particular, the Δ17O values of SAS, defined as Δ17O = δ17O − 1000 × [(δ18O/1000)0.52 − 1], wherein δ17,18O = [(17,18O/16O)sample/(17,18O/16O)standard − 1] × 1000, offers direct assessment of the involvement of O3 relative to other oxidants during SAS formation. The δ34S values of SAS, defined as δ34S = [(34S/32S)sample/(34S/32S)standard − 1] × 1000, can provide information complementary to Δ17O concerning the source(s) of sulfur gases.
The oxygen isotopic constraints on SAS formation pathways have been discussed in detail previously (Alexander et al., 2012, Alexander et al., 2009, Alexander et al., 2005, Bao et al., 2010, Savarino et al., 2000, Sofen et al., 2011). The Δ17O value of SAS is dependent on the oxidation pathway of SO2 to sulfate. In aqueous phase, dissolved SO2 or SO32− readily exchanges its oxygen atoms with water whose Δ17O = 0‰. Given the average Δ17O (H2O2) and Δ17O (O3) value at 1.7‰ and 35‰, the Δ17O (SO42−) for product sulfate would be at 0.9‰ and 8.7‰, respectively. It should be noted that there are discrepancies in the literature regarding these values, which is partly related to the use of the bulk oxygen versus the transferable oxygen in ozone (Vicars et al., 2012). However, the ±0.05‰ analytical resolution for Δ17O can easily pick up a wide range of SAS Δ17O values. SASs formed via other pathways such as ·OH in gas-phase, transition-metal-catalyzed O2, halogen acids, or high-temperature production (“primary sulfate”) are expected to have their Δ17O at ∼0‰ (Lee et al., 2002, McCabe et al., 2006, Savarino et al., 2000). Once formed, sulfate in the atmosphere does not undergo further oxygen isotopic exchange.
SAS Δ17O data has been collected on the polar continents (Alexander et al., 2002, Alexander et al., 2004, Alexander et al., 2003, Alexander et al., 2009, Kunasek et al., 2010, McCabe et al., 2006) and on ocean cruises (Alexander et al., 2012, Alexander et al., 2005, Dominguez et al., 2008). However, data from mid-latitude continental sites is scarce. Lee and Thiemens (2001) conducted a pioneering study of the Δ17O of rainwater and aerosol SAS at La Jolla and White Mountain Research Station, California, a coastal and a high alpine region, respectively. Jenkins and Bao (2006) reported multi-year bulk atmospheric data from Baton Rouge, Louisiana (USA), a set of multiple stable isotope compositions (δ18O, Δ17O and δ34S) for SAS collected in a mid-latitude continental site. The available SAS Δ17O data appear to indicate that much of the Earth's mid-latitude continents may have a narrow range of annual mean SAS Δ17O values from ∼0.6 to 0.8‰, which is in a sharp contrast to polar continents where the SAS has much more positive Δ17O values (∼1.8–3.3‰). Recent Δ17O measurement of size-segregated sulfate aerosols in La Jolla, California supports the observation of a narrow range of fine-particle SAS Δ17O values at mid-latitudes during normal atmospheric conditions (Dominguez et al., 2008, Hill-Falkenthal et al., 2012). A global three-dimensional chemical transport model study suggests that much of the mid-latitude continents today should have lower Δ17O for SAS (<1.0‰) (Sofen et al., 2011), consistent with the observation so far. However, long-term observational data like those of Baton Rouge are missing from localities where the meteorological condition can be quite different from Baton Rouge.
Δ17O(SO42−) observation and modeling have demonstrated the importance of Fe3+–Mn2+-catalyzed oxidation by O2 at high northern latitudes in winter (Alexander et al., 2009, McCabe et al., 2006). Modeling and observational results indicate that this oxidation pathway may be of global-scale and regional importance with the highest fraction (up to 58%) observed over rural northern Eurasia (Alexander et al., 2009, Harris et al., 2013). Therefore, to examine these model predictions and to compare to previous observation in Baton Rouge, we conducted a 950-day sampling campaign on atmospheric bulk deposition in a heavily industrialized and urbanized mega-city, Wuhan, in central China, located in a mid-latitude continental interior, where both anthropogenic emissions and particulate matter contents are higher than those in Baton Rouge. Multiple oxygen (Δ17O and δ18O) and sulfur isotope compositions (δ34S) of bulk atmospheric sulfate, together with soluble ion concentrations, rainwater pH, and meteorological conditions, were measured and collected.
Section snippets
Study area
Wuhan, the capital of Hubei Province, central China, is an industrial metropolis located at 30°37′ N and 114°20′ E, with an urban area of 863 km2 and a population of 10.2 million. Wuhan is far away from ocean influence and is characterized by an Asian monsoon climate, with a distinct seasonality. Mean annual rainfall reaches 1200 mm with a relatively rainy period from April to August, during which the relative humidity is at an average of 70%. Wuhan is the home of prominent steel,
Results
No visible BaSO4 were precipitated from solutions after rinsing the dry buckets during two dry periods. However, rain or snow events always brought in large quantities of sulfate. Seasonal mean concentrations of major ions and pH values in rainwater in the city of Wuhan during the sampling duration are shown in Table 1. The pH ranges from 5.22 to 5.61, with an average of 5.47. The concentration of major ions of Wuhan is in the order of SO42− > NO3− > Ca2+ > NH4+> Cl− > Na+ > K+ > Mg2+. SO42−
Rainwater chemistry
The average seasonal mean concentration of major ion of rainwater in Baton Rouge are also displayed in Table 1, as recorded in the 2003–2005 National Trends Network (NTN) data from US National Atmospheric Deposition Program (NADP) stations LA12 and LA 30. The average rainwater [SO42−] of 7.54 mg L−1 in Wuhan is nearly eight times that in Baton Rouge (1.01 mg L−1). The elevated [SO42−] in Wuhan is consistent with its more polluted atmosphere, with major anthropogenic sulfur sources of coal
Conclusions
We have presented a set of long-term (950 days) stable isotopic data (Δ17O, δ18O and δ34S) for bulk atmospheric sulfate collected in Wuhan, an industrial, mid-latitude, and continental interior mega-city in central China. The data supports the prediction from atmospheric chemical and transport models on sulfur oxidation chemistry as well as an earlier observation that the long-term average Δ17O value for SAS in the mid-latitude sites falls within a narrow range, a range that is ∼1–2‰ lower than
Acknowledgment
This project was financially supported by the National Natural Science Foundation of China (NSFC 41202169 to X.Q. Li and NSFC 41072179 to Y.Q. Gan), China Postdoctoral Science Foundation (to X.Q. Li), and Charles L. Jones Professorship (to H.M. Bao).
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