Experimental study of the sampling artifact of chloride depletion from collected sea salt aerosols

Transport route-based cluster analysis of chemical fingerprints and source origins of marine fine particles (PM2.5) in South China Sea

The fingerprints and source origins of marine PM_2.5 at two background islands in the South China Sea were clustered via trajectory analysis and positive matrix factorization. High PM_2.5 concentrations at the Dongsha Islands occurred for the north routes, while Nansha Islands had similar PM_2.5 concentrations amongst the transport routes. However, the chemical characteristics of PM_2.5 varied with the transport routes. Secondary inorganic aerosols (NO₃^-, SO₄^2-, and NH₄⁺) were abundant in water-soluble ions which dominated PM_2.5. Crustal metals were the abundant metals in PM_2.5, while trace metals were primarily originated from man-made sources. Organic carbon was superior to elemental carbon, and high concentrations of levoglucosan and organic acids were observed for the north routes. Overall, marine PM_2.5 at the Dongsha Islands was highly influenced by long-range transport of Asian continental outflows, while particulate air quality at the Nansha Islands was mainly governed by clean air parcels blown from the SCS.

Effects of NH3 and alkaline metals on the formation of particulate sulfate and nitrate in wintertime Beijing

Sulfate and nitrate from secondary reactions remain as the most abundant inorganic species in atmospheric particle matter (PM). Their formation is initiated by oxidation (either in gas phase or particle phase), followed by neutralization reaction primarily by NH₃, or by other alkaline species such as alkaline metal ions if available. The different roles of NH₃ and metal ions in neutralizing H₂SO₄ or HNO₃, however, are seldom investigated. Here we conducted semi-continuous measurements of SO₄^2-, NO₃^-, NH₄⁺, and their gaseous precursors, as well as alkaline metal ions (Na⁺, K⁺, Ca²⁺, and Mg²⁺) in wintertime Beijing. Analysis of aerosol acidity (estimated from a thermodynamic model) indicated that preferable sulfate formation was related to low pH conditions, while high pH conditions promote nitrate formation. Data in different mass fraction ranges of alkaline metal ions showed that in some ranges the role of NH₃ was replaced by alkaline metal ions in the neutralization reaction of H₂SO₄ and HNO₃ to form particulate SO₄^2- and NO₃^-. The relationships between mass fractions of SO₄^2- and NO₃^- in those ranges of different alkaline metal ion content also suggested that alkaline metal ions participate in the competing neutralization reaction of sulfate and nitrate. The implication of the current study is that in some regions the chemistry to incorporate sulfur and nitrogen into particle phase might be largely affected by desert/fugitive dust and sea salt, besides NH₃. This implication is particularly relevant in coastal China and those areas with strong influence of dust storm in the North China Plain (NCP), both of which host a number of megacities with deteriorating air quality.

Impact of Wildfire Emissions on Chloride and Bromide Depletion in Marine Aerosol Particles

This work examines particulate chloride (Cl^-) and bromide (Br^-) depletion in marine aerosol particles influenced by wildfires at a coastal California site in the summers of 2013 and 2016. Chloride exhibited a dominant coarse mode due to sea salt influence, with substantially diminished concentrations during fire periods as compared to nonfire periods. Bromide exhibited a peak in the submicrometer range during fire and nonfire periods, with an additional supermicrometer peak in the latter periods. Chloride and Br^- depletions were enhanced during fire periods as compared to nonfire periods. The highest observed %Cl^- depletion occurred in the submicrometer range, with maximum values of 98.9% (0.32-0.56 μm) and 85.6% (0.56-1 μm) during fire and nonfire periods, respectively. The highest %Br^- depletion occurred in the supermicrometer range during fire and nonfire periods with peak depletion between 1.8-3.2 μm (78.8% and 58.6%, respectively). When accounting for the neutralization of sulfate by ammonium, organic acid particles showed the greatest influence on Cl^- depletion in the submicrometer range. These results have implications for aerosol hygroscopicity and radiative forcing in areas with wildfire influence owing to depletion effects on composition.

Laboratory study on the hygroscopic behavior of external and internal C2-C4 dicarboxylic acid-NaCl mixtures

Atmospheric aerosol is usually found to be a mixture of various inorganic and organic components in field measurements, whereas the effect of this mixing state on the hygroscopicity of aerosol particles has remained unknown. In this study, the hygroscopic behavior of mixtures of C2-C4 dicarboxylic acids and NaCl was investigated. For both externally and internally mixed malonic acid-NaCl and succinic acid-NaCl particles, correlation between water content and chemical composition was observed and the water content of these mixtures at relative humidity (RH) above 80% can be well predicted by the Zdanovskii-Stokes-Robinson (ZSR) method. In contrast, a nonlinear relation between the total water content of the mixtures and the water content of each chemical composition separately was found for oxalic acid-NaCl mixtures. Compared to the values predicted by the ZSR method, the dissolution of oxalic acid in external mixtures resulted in an increase in the total water content, whereas the formation of less hygroscopic disodium oxalate in internal mixtures led to a significant decrease in the total water content. Furthermore, we found that the hygroscopicity of the sodium dicarboxylate plays a critical role in determining the aqueous chemistry of dicarboxylic acid-NaCl mixtures during the humidifying and dehumidifying process. It was also found that the hydration of oxalic acid and the deliquescence of NaCl did not change in external oxalic acid-NaCl mixtures. The deliquescence relative humidity (DRHs) for both malonic acid and NaCl decreased in both external and internal mixtures. These results could help in understanding the conversion processes of dicarboxylic acids to dicarboxylate salts, as well as the substitution of Cl by oxalate in the atmosphere. It was demonstrated that the effect of coexisting components on the hygroscopic behavior of mixed aerosols should not be neglected.

Comparative analysis of urban atmospheric aerosol by particle-induced X-ray emission (PIXE), proton elastic scattering analysis (PESA), and aerosol mass spectrometry (AMS)

A multifaceted approach to atmospheric aerosol analysis is often desirable in field studies where an understanding of technical comparability among different measurement techniques is essential. Herein, we report quantitative intercomparisons of particle-induced X-ray emission (PIXE) and proton elastic scattering analysis (PESA), performed of fline under a vacuum, with analysis by aerosol mass spectrometry (AMS) carried out in real-time during the MCMA-2003 Field Campaign in the Mexico City Metropolitan Area. Good agreement was observed for mass concentrations of PIXE-measured sulfur (assuming it was dominated by SO4(2-)) and AMS-measured sulfate during most of the campaign. PESA-measured hydrogen mass was separated into sulfate H and organic H mass fractions, assuming the only major contributions were (NH4)2SO4 and organic compounds. Comparison of the organic H mass with AMS organic aerosol measurements indicates that about 75% of the mass of these species evaporated under a vacuum. However approximately 25% of the organics does remain under a vacuum, which is only possible with low-vapor-pressure compounds, and which supports the presence of high-molecular-weight or highly oxidized organics consistent with atmospheric aging. Approximately 10% of the chloride detected by AMS was measured by PIXE, possibly in the form of metal-chloride complexes, while the majority of Cl was likely present as more volatile species including NH4Cl. This is the first comparison of PIXE/PESA and AMS and, to our knowledge, also the first report of PESA hydrogen measurements for urban organic aerosols.

Distribution of atmospheric marine salt depositions over Continental Western Europe

This contribution describes the distribution of marine salt aerosols in Belgium, France and Spain, as obtained from applying a kriging model to a set of data. The data was collected over a period of nine years and included wet as well as dry deposition results. It was found that the concentration of the salt particles decreased the fastest in Spain and the slowest in Belgium with increasing inland distance from the sea. These findings have implications for the degradation of monuments and historical buildings as a result of salt attack and ingress that often accompany degradation due to air pollution.

Reactions at interfaces as a source of sulfate formation in sea-salt particles

Understanding the formation of sulfate particles in the troposphere is critical because of their health effects and their direct and indirect effects on radiative forcing, and hence on climate. Laboratory studies of the chemical and physical changes in sodium chloride, the major component of sea-salt particles, show that sodium hydroxide is generated upon reaction of deliquesced sodium chloride particles with gas-phase hydroxide. The increase in alkalinity will lead to an increase in the uptake and oxidation of sulfur dioxide to sulfate in sea-salt particles. This chemistry is missing from current models but is consistent with a number of previously unexplained field study observations.