• Understanding Processes involved in Atmospheric Aerosol Formation using Laboratory Chambers

    Laboratory Teflon chambers have been widely used for the study of atmospheric chemistry. Interpretation of data obtained in such chambers is complicated by the competition between the underlying chemistry and particle growth and the interaction of vapors and particles with the walls of the chamber itself. Organic vapors generated from the photochemistry of a given parent hydrocarbon in chamber experiments undergo 1) further oxidation yielding more functionalized products; 2) nucleation or partitioning onto existing particles forming secondary organic aerosol; 3) deposition onto the chamber walls. The deposition of SOA-forming vapors onto the chamber wall potentially leads to underestimation of the derived SOA yields. We define a parameter, Rwall (the ratio of SOA yield from a given precursor in the absence of vapor wall loss to that derived from actual chamber measurements), to quantitatively evaluate the impact of vapor wall losses on the SOA yields. As shown in the figure below, the vapor wall losses can be mitigated by increasing the existing seed particle surface area and by accelerating the photooxidation rates in the chamber. 

        vapor wall loss


  • Measurements of Labile Species in the Condensed Phase using Electrospray Ionization Technique

    Characterizaing organic peroxides at molecule level remians one of the great challenges in analytical chemistry. We found that organic peroxide functionalities are readily cationized by the attachment of sodium cation during electrospray ionization operated in the positive ion mode. The figure to the left shows the general working principle of electrospray ionization: a high voltage (>2kV) is applied to the tip of the capillary tip, where species (M) in the solution that carry opposite charges tend to escape from the solution and undergo a series of processes forming positive ions. Here species 'M+' can be cations (e.g., K+), protonated ions (e.g., [RNH2+H]+), and ion adducts (e.g., [ROOH+Na]+). The figure to the right shows the postive ESI mass spectrum for isoprene hydroperoxide (ISOPOOH) in methonal solution. Monomers, dimers, and trimers combining sodium are detected. 


 Figure source: Zhang et al., EST, (2017)


  • Understanding the Formation and Evolution of Molecular Components in Secondary Organic Aerosols 

    This figure shows the molecular composition of secondary organic aerosol (SOA) generated from the ozonolysis of alpha-pinene (when 99% of alpha-pinene is consumed by ozone) mapped on the carbon oxidation state (OSc) vs. carbon number (nC) space. These identified components in total account for 58.4 ~ 72.3% of the alpha-pinene SOA mass and are characterized as semi/low-volatility monomers and extremely low volatility dimers. The mean carbon oxidation state of the identified SOA molecular constituents (0.68agrees essentially identically with the average level derived from the Aerosol Mass Spectrometer (AMS) measurement (0.72). Also given here are their temporal profiles, by which the role of individual molecular constituents in the initial growth and aging of particles can be inferred. At first glance, SVOC products dominate the alpha-pinene SOA whereas the LVOC and ELVOC products in total account for less than 45% of the overall organic mass. The growth rates of ELVOCs are comparable to or even exceed those for SVOCs and LVOCs. As alpha-pinene oxidation proceeds, ELVOC production is inhibited, whereas SVOC and LVOC accumulation is favored.



  • A 2-D Mass-to-Charge Ratio vs. Collision Cross Section Space for Distribution of Atmosperic Compounds 

    The collision cross section is a quantity that is related to the structure and geometry of molecules and is derived from ion mobility measurements. By combination with the mass-to-charge ratio (m/z), a two-dimensional  space is developed to facilitate the comprehensive investigation of the complex organic mixtures.The figure below shows the distribution of three common classes of organic species of atmospheric interest, carboxylic acid, amine, and alcohol, on the collision cross section vs. mass-to-charge ratio 2-D space. Species of the same chemical class, despite variations in the molecular structures, tend to situate as a narrow band on the space and follow a trend line. These trend lines provide a useful tool for categorization of structurally related compounds. Mapping out the locations and distribution patterns for various functionalities on the 2-D space would therefore facilitate classification of chemical classes for unknown compounds. 






ACOM | Atmospheric Chemistry Observations & Modeling