Size-dependent molecular-level characterization of secondary organic aerosol from various oxidants using nanospray desorption electrospray (nano-DESI) high-resolution mass spectrometry
EMSL Project ID
48347
Abstract
Secondary organic aerosol (SOA) formation from oxidation of biogenic volatile organic compounds (BVOCs) contributes a large fraction of global aerosol loading, affecting regional and global climate. In polluted forests at night, the oxidation reactions that initiate aerosol formation may be from a combination of O3 and NO3 oxidants, with potentially drastically different aerosol yields depending on the dominant oxidant. Recently, our lab has begun a series of experiments exploring SOA production as a function of oxidant gradient from pure O3 to pure NO3, by adding increasing NO2 concentrations to dark chamber ozonolysis experiments of alpha-pinene, beta-pinene, delta3-carene, and limonene. We observe systematic changes in the total number of particles, the total aerosol mass, and the shape of the aerosol growth curve with increasing [NO2]. In particular, when NO2 is added to the chamber, in some cases we observe a bifurcation of the SOA growth curve into two size modes. Preliminary ESI-MS analysis of filter-collected aerosol suggests a changing composition corresponding to these oxidant differences. With the support of this proposal, we will explore the hypothesis that the NO3 and O3 oxidation products form separate particle populations of SOA that grow at different rates, suggesting their immiscibility in the condensed phase. Specifically, we will conduct a new series of chamber experiments, collecting size-dependent samples of SOA from various oxidant regimes using a mirco-orifice uniform deposit impactor (MOUDI). During these chamber experiments, the growth rates of aerosol formation will be monitoring using a scanning electrical mobility sizer (SEMS), and when bifurcation occurs, the MOUDI sampler will be employed to separately collect the multiple size modes. These impacted particles will then be analyzed for chemical composition at high mass accuracy using using state-of-the-art high-resolution mass spectrometry (HR-MS) combined with the efficient ionization and high sensitivity offered by nano-desorption electrospray ionization (nano-DESI). The objective of this project is the molecular-level characterization of the separate size modes of SOA, in order to identify the molecular structure differences that underpin the differing growth rates, and to confirm or disprove the hypothesis that these structural differences arise from different oxidants. This research conducted during this project will provide an important step forward towards better mechanistic understanding for differences in SOA formation from the oxidation of BVOC, and interpretation of the mixing and physical state of organic aerosol. Additionally, this project will have a very valuable influence on the education of Reed College undergraduates, providing them the opportunity to learn about state-of-the-art instrumentation and mass spectrometric analysis.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2014-10-01
End Date
2016-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members