Characterization of an Aerodyne Aerosol Mass Spectrometer (AMS ...

768 N. TAKEGAWA ET AL.
TABLE 3
Summary of intercomparison results
Fit parameters a
Species Slope Intercept, µg m −3 r 2
Nitrate 1.12 ± 0.03
Phase 1 (May 10–12, 2003)
0.33 ± 0.09 0.99
Sulfate 1.24 ± 0.11 −0.34 ± 0.32 0.85
Chloride 1.16 ± 0.05 0.10 ± 0.03 0.96
Ammonium 1.26 ± 0.06 −0.38 ± 0.15
Phase 2 (July 23–30, 2003)
0.96
Nitrate 0.81 ± 0.03 0.46 ± 0.18 0.95
Sulfate 0.90 ± 0.06 0.44 ± 0.18 0.88
Chloride 0.91 ± 0.05 0.07 ± 0.02 0.92
Ammonium 0.95 ± 0.06 0.25 ± 0.17 0.89
Organics 1.79 ± 0.29 0.98 ± 0.84
Phase 3 (October 1–15, 2003)
0.67
Organics 1.61 ± 0.09 0.86 ± 0.52 0.83
a Linear regression fit parameters of AMS versus PILS-IC or OC correlation. Errors are 95% confidence intervals.
(dva < 200 nm) was dominated by aliphatic or aromatic hydrocarbons
that are the major constituents of combustion-generated
primary organic aerosols and that the accumulation mode (dva >
200 nm) was dominated by oxygenated organic compounds that
are the major constituents of secondary organic aerosols. The
size distributions suggest that the former is externally mixed
with inorganic species and the latter is internally mixed with
inorganic species (e.g., Allan et al. 2003b; Zhang et al. 2005a).
Therefore, the AMS particle collection efficiency of 0.5 is a
reasonable assumption for the accumulation-mode organics. In
this analysis we assume a constant collection efficiency of 0.5
for the small-mode organics as well as the accumulation-mode
organics.
Figure 7 shows scatter plots of OMAMS versus OC mass
concentrations in July and October 2003. In general, the mass
concentrations of OMAMS were systematically higher than those
of OC for almost the entire dataset, which is expected since organics
contain several elements other than carbon (H, O, N, etc.).
The linear regression slope was found to be 1.8 for July 2003 and
1.6 for October 2003. Turpin and Lim (2001) summarized the ratio
of OM to OC for various cases. The ratio of molecular weight
to carbon weight is 1.2 for n-alkanes (C23–C34) and 1.0–1.1 for
polycyclic aromatic hydrocarbons (PAHs), while it reaches as
high as 1.7–3.8 for aliphatic dicarboxylic acids (C2–C9). They
suggested that the OM/OC ratio could be approximated as
1.6 ± 0.2 for urban aerosols and 2.1 ± 0.2 for aged (non-urban)
FIG. 7. Scatter plots of AMS organics (OMAMS) and OC mass concentrations in July and October 2003. The data are 1-h averages. The shaded line represents
the linear regression line. The dashed lines indicate 1:1 and 2:1 correspondence lines.