Development of Soft Ionization for Particulate Organic Detection with the Aerodyne Aerosol Mass Spectrometer

Abstract

In this DoE-supported SBIR Phase II project, we have developed several soft ionization techniques for use with the Aerodyne Aerosol Mass Spectrometer (AMS). The AMS allows quantitative, real-time, size and composition analysis of atmospheric aerosol particles . AMS measurements of aerosol chemistry are routine and are now standard in most field campaigns, including those supported by DoE as well as other agencies (worldwide) [Canagaratna et al., 2007;. This work has recently been highlighted in Science [Robinson et al., 2007] which used laboratory and field AMS results to estimate regional secondary organic aerosol (SOA) budgets. That work drew heavily on DoE-supported AMS results from Mexico City in 2003 [Volkamer et al., 2006] that showed that photochemical SOA production occurs daily on a time scale of hours in Mexico City. Additionally, the AMS is now routinely used in chamber studies of the chemistry of SOA formation, allowing for the chemical characterization of laboratory-generated organic aerosol [Bahreini et al., 2005; Alfarra et al., 2006]. Recently we have developed a time-of-flight mass spectrometer version of the AMS (ToF-AMS), leading to substantial increases in the sensitivity, resolution, and data acquisition rate of the instrument . This development, which included significant DoE support, has led to the deployment of over 20 ToF-AMS systems worldwide, including two in DoE laboratories, and deployment on the DoE G-1 aircraft.The goal of this project is to enhance the molecular information that can be derived from aerosol mass spectra. The standard mode of AMS operation involves ionization by electron impact (EI), which is a proven, quantitative technique, but involves the extensive fragmentation of individual ions, making the identification and characterization of individual organics in the aerosol rather difficult. During this DOE SBIR Phase II project, we have developed several soft ionization techniques, i.e., ionization schemes which involve less fragmentation of the ions, for use with the ToF-AMS. This SBIR program has successfully accomplished the tasks in the Phase II proposal, including:1. Vacuum ultraviolet single photon ionization was demonstrated in the laboratory and deployed in field campaigns [Northway et al., 2007]. This demonstration has led to collaboration with a German group [Mühlberger et al., 2005b] to continue VUV lamp development undertaken by the University of Colorado during the SBIR. 2. Dissociative electron attachment is also now a routine approach with AMS systems configured for bipolar, negative ion detection. This technique is particularly powerful for detection of acidic and other highly oxygenated SOA chemical functionality. 3. Low energy electron ionization is now a "softer" ionization approach routinely available to AMS users. 4. Lithium ion (Li + ) attachment has been shown to be sensitive to more alkyl-like chemical functionality in SOA. Results from Mexico City are particularly exciting in observing changes in SOA molecular composition under different phot...