The important role of nitrate in iron and manganese dissolution and sulfate formation in fine particles at a coastal site in Northern China

Bioavailable transition trace elements, such as soluble iron (Fes) and soluble manganese (Mns) in aerosols, play a crucial role in atmospheric sulfate formation and marine ecosystems. In this study conducted during the spring of 2017 in Qingdao, a coastal city in Northern China, we applied a combined approach of multiple linear regression (MLR) incorporating the results of positive matrix factorization (PMF) to estimate the solubility of Fe and Mn from various sources. PMF analysis showed that dust was the largest contributor to total Fe (FeT) (45.5 %), followed by non-ferrous smelting (20.3 %) and secondary formation processes (17.8 %). However, secondary formation processes (33.2 %), vehicle exhaust (19.3 %) and aqueous-phase processes (19.0 %) were found to be the primary contributors to Fes. For total Mn (MnT) and Mns, dust (21.2 % ∼ 35.0 %), secondary formation processes (20.3 % ∼ 25.6 %) and industry (12.6 % ∼ 16.3 %) were identified as the dominant contributors. The solubilities of Fe and Mn varied significantly depending on their sources. Interestingly, nitrate played a more pronounced role than sulfate in facilitating the dissolution of Fe and Mn during the acid processing due to the high molar ratio of NO3-/2SO42- (1.72 ± 0.54) under the average RH of 56 % ± 15 %. This phenomenon suggested that the acid processing was primarily triggered by nitrate formation due to the low deliquescence relative humidity (DRH) of nitrate. Additionally, we discovered that the catalytic oxidation of SO2 in aerosol water was primarily driven by Fe rather than Mn, serving as a more significant pathway for sulfate formation within a pH range of 2.0 to 4.4. These findings provide valuable insights into the impact of acidification on the dissolution of Fe and Mn under conditions of moderate RH in the real ambient atmosphere with the increasing of NO3-/2SO42- molar ratio.

Atmospheric sulfate formation in the Seoul Metropolitan Area during spring/summer: Effect of trace metal ions

Despite the effort to control SO₂ emission, sulfate is still one of the major inorganic components of PM_2.5 in urban area. Moreover, there is still a lack of understanding of the sulfate formation mechanism via SO₂ oxidation under various ambient conditions. In this study, we focus on sulfate formation during a haze pollution episode in the spring/summertime of 2016 in Seoul Metropolitan Area (SMA). During the pollution episode, PM_2.5 mass concentration exceeded over 60 μg m^-3, and sulfate accounted for about 25% of the total PM_2.5 mass concentration. A sharp increase of sulfur oxidation ratio (SOR) values along with aerosol liquid water content (AWC) under humid conditions could be ascribed to an apparent contribution of aqueous-phase oxidation of SO₂ of sulfate formation during the pollution period. Comparisons of SOR values with four representative oxidants for the aqueous-phase oxidation (i.e., NO₂, H₂O₂, O₃, and TMIs) indicated that TMIs concentration, especially for Mn (II), showed the best positive correlation. Furthermore, for calculating the sulfate production rate, the contribution of TMIs concentration was found to be dominant within the pH range in SMA (2.1-3.0), which was determined by the chemical composition and derived AWC. These results imply that not only the SO₂ emission but also other chemical components (e.g., TMI and nitrate) would play a critical combined role in sulfate formation under urban haze condition.

The role of source emissions in sulfate formation pathways based on chemical thermodynamics and kinetics model

Sulfate is a major PM_2.5 constituent and poses a significant threat to ecosystems and human health, which has attracted lots of attention to the sulfate formation mechanism. In recent years, there has been great scientific interest in the multiphase oxidation of SO₂ in aqueous aerosol particles. Many factors are involved in the reaction process, including precursor SO₂, oxidants/catalysts, and aerosol acidity, which are three channels closely related to the source emission. The conjoint analysis of source emissions and sulfate aqueous formation can provide a scientific basis for designing effective strategies, though the related research is extremely limited. Here, we applied an improved solute strength-dependent chemical Thermodynamics & Kinetics model (for aqueous pathway contribution) and the Partial Target Transformation-Positive matrix factor model (for source apportionment) to explore the role of source emission in sulfate aqueous formation. The results indicated H₂O₂ aqueous oxidation was the dominant pathway (65.9 %), and secondary nitrate source may grow together with sulfate formation from H₂O₂ pathway. H₂O₂ and TMI pathways were related to higher SOR (sulfur oxidation rate). TMI pathway was significant in summer (54.6 %) and increased with secondary sources and vehicle exhaust. NO₂ pathway was more significant at low secondary source and high coal combustion (higher contribution of NO₂ pathway appeared in winter, 24.7 %). While high formation rate of the O₃ pathway always occurred at low source levels. Coal combustion and vehicle exhaust showed obvious effects on sulfate aqueous formation. Notably, aerosol acidity is a significant factor related to sources and plays a key role in sulfate formation. The result also suggested aerosol pH may be more important than the amounts of substances involved in the oxidation reaction. The findings in this work can provide useful information for better understanding sulfate aqueous formation and offer a scientific basis for designing strategies for air pollution control and sulfate mitigation.

The influence of relative humidity on the heterogeneous oxidation of sulfur dioxide by ozone on calcium carbonate particles

Heterogeneous reactions of SO₂ and O₃ with CaCO₃ particles were investigated at a series of relative humidity (RH, 1% to 90%) and 298K using a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The uptake coefficients of SO₂ on CaCO₃ at different RHs were obtained for the first time. Our results proved that high RH could substantially promote the formation of sulfate, for which the highest concentration (80% RH and reaction time of 200min) and highest formation rate in stable stage (85% RH) were 14 times and 43 times that at 1% RH, respectively. The surface products, increment of concentration and formation rate of sulfate changed with RH which were due to the surface adsorbed water (SAW) on the particles. SAW could increase the reactive sites on the particles and thus accelerate the conversion of SO₂ into sulfite, and sulfite could be oxidized rapidly. Liquid-like water layers formed on the particle surface could enhance the ion mobility and promote the aggregation of CaSO₄ hydrates, which could expose more reactive sites and result in additional adsorption of SO₂. Piecewise equations of uptake coefficient with RH were given and could be referred by model simulation. The results are of importance in understanding the explosive growth of sulfate during severe haze episodes accompanied with high RH.