(644c) 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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