Evaluating the Mixing of Organic Aerosol Components Using High-Resolution Aerosol Mass Spectrometry

Small particles in the atmosphere can scatter or absorb radiation, as well as influence cloud formation and lifetime, and therefore affect climate (IPCC, 2007). The aerosol particles also affect human health by, among others, damaging the respiratory and cardiovascular systems (Dockery et al., 1993; Davidson et al., 2005; Pope and Dockery, 2006). Even though organic material represents about half of the fine-particle mass on average (Kanakidou et al., 2005; Zhang et al., 2007), the formation and properties of organic aerosol are not well understood.

Traditionally, the organic particulate matter is classified as primary or secondary organic aerosol (POA or SOA). In this classification, POA refers to compounds that are emitted as particles. SOA is formed when volatile organic compounds (VOCs) undergo one or more chemical transformations in the gas phase, forming less volatile compounds that then partition to the particle phase. SOA is further separated into anthropogenic SOA (ASOA) and biogenic SOA (BSOA). The formation of SOA is modeled as absorptive partitioning of semivolatile compounds between the gas and particle phases (Pankow, 1994; Odum et al., 1996; Donahue et al., 2006). According to these models, the addition of (anthropogenic) organic aerosol will enhance the formation of biogenic organic aerosol, if anthropogenic and biogenic components mix. Practically all air-quality models employ a pseudo-ideal-mixing assumption, by which organic-aerosol components from different sources mix ideally. Recent studies measuring SOA yields in the presence of POA have challenged this assumption (Song et al., 2007).

Experimentally, it is very difficult to distinguish between different types of organic aerosol such as ASOA versus BSOA. Organic aerosol looks increasingly similar as it is processed in the atmosphere, and even different types of fresh organic aerosol formed in the laboratory have very similar organic mass spectra when analyzed with a unit-resolution aerosol mass spectrometer (AMS). This is partly because the aerosol in the AMS is vaporized at 600˚C. The high temperature is necessary for vaporization, but it also enhances fragmentation of the organic species after electron-impact ionization, making the species lose most of their chemical distinction. In our mixing experiments, we want to measure how much aerosol is formed from each source, and we therefore need to be able to distinguish the aerosol from different sources. Our approach is to use isotopically labeled compounds and a High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-AMS) from Aerodyne, Inc. (DeCarlo et al., 2006). The HR-AMS is able to distinguish between mass spectral peaks at the same m/z. We use this capability to distinguish ¹³C-containing and ¹²C-containing mass fragments.

We performed experiments in Carnegie Mellon's environmental chamber to form ASOA from isotopically labeled toluene (¹³C-toluene), and non-labeled BSOA from a-pinene or limonene, or non-labeled POA from diesel exhaust. These experiments thus allow us to investigate the mixing of ASOA/BSOA and ASOA/POA by comparing SOA yields in the presence of organic precursor seed to the yields in the presence of inorganic, non-reactive seed (Hildebrandt et al., 2009).

We use, compare and discuss three approaches to extract the labeled and non-labeled organic mass spectra. The first method identifies all of the peaks corresponding to organic fragments containing ¹³C, and those containing only ¹²C. The sum of these fragments should correspond to the ¹³C-toluene aerosol versus the non-labeled organic aerosol, respectively. The second method uses positive matrix factorization (PMF) on the mass spectra, an increasingly popular method used to extract the mass spectra of different types of organic aerosol (Ulbrich et al., 2008; Zhang et al., 2005; Zhang et al., 2007). By applying PMF to the unit mass resolution (UMR) data and the high resolution (HR) data, we can infer how much additional information the HR data provide and whether these experiments could also be conducted with a UMR instrument. Finally, the third method separates the organic mass spectra by using the known organic mass spectra of the different types of organic aerosol (e.g. the ¹³C-toluene spectrum and the a-pinene spectrum) and reconstructing the observed, mixed organic mass spectrum as a linear combination of the two known mass spectra.

Separation of the organic mass spectra is possible and the results from the three methods agree reasonably well. We can thus calculate the mass yield of one type of aerosol in the presence of the other and compare these yields to the yields in the presence of inorganic seed. This allows us to infer the extent of mixing of the different types of organic aerosol. Understanding the mixing behavior is necessary to understand the formation of organic aerosol, and how organic aerosol concentrations will respond to emission reductions.

This work was supported by the National Science Foundation and the Environmental Protection Agency.
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