(BRAVO) Study: Final Report. - Desert Research Institute

Final Report — September 2004
Table 9-22. CMAQ-MADRID performance for 24-hr concentrations at the K-Bar site over the duration
of the BRAVO study. 6
Statistic units SO4 2– NO3 – NH4 + EC OM Other PM2.5
Observed mean µg/m 3 2.52 0.20 0.82 0.12 1.27 1.56 6.49
Simulated mean µg/m 3 2.63 0.04 0.68 0.06 0.44 0.80 4.66
Coeff. of determination (r 2 ) 0.53 0.03 0.50 0.00 0.10 0.02 0.23
Mean bias µg/m 3 0.11 -0.16 -0.13 -0.06 -0.82 -0.76 -1.83
Mean error µg/m 3 1.08 0.20 0.34 0.09 0.84 1.47 3.13
Mean Normalized Bias % 19 26 6 8 -60 -45 -13
Mean Normalized Error % 54 194 51 89 6 160 51
Fractional Bias % -1 -154 -13 -13 -116 163 -31
Fractional Error % 46 169 48 19 121 282 87
RMS error µg/m 3 1.51 0.24 0.45 0.14 1.10 2.37 4.11
particle mass were measured at the K-Bar site. The “Other” component represents the
difference between the measured PM 2.5 mass concentration and the sum of the major
components; this “other” component is likely to contain dust, crustal material, metal oxides,
and sea-salt and may also represent water remaining in the sample at the time of weighing.
In addition, since some components of PM 2.5 may volatilize during sample handling and
measurement, the “other” component could also reflect negative contributions. Finally,
“other” bears the burden of absorbing all of the measurement uncertainties of the other
components.
The stacked bar charts in Figure 9-28 illustrate the composition information in Table
9-22. Over the analysis period, the mean PM 2.5 mass is underestimated by a factor of 1.4,
equivalent to an underestimate of 1.83 µg/m 3 . On the other hand, the average simulated
absolute concentrations of sulfate and ammonium agree quite well with the measured
concentrations. However, due to the underestimation of all non-sulfate components, the
model estimates a higher contribution of fine sulfate to PM 2.5 mass. The simulated
proportional contributions of the various other components of PM 2.5 are also quite different
from the measured proportions, except for ammonium (NH 4 + ) and elemental carbon.
The difference between model estimates and observations of total fine particle mass
cannot be explained solely by discrepancies in model predictions for sulfate. CMAQ-
MADRID predicted a much tighter relation between fine sulfate and PM 2.5 mass (r 2 = 0.91)
6 Statistics in Table 9-22 were computed without the lower cutoff value of 0.1 µg/m 3 used in previous
tables in this section, and thus these statistics test model performance at lower concentration levels
than those that apply for Table 9-17. Also, only those days with valid measurements for all
components, and thus with the ability to derive a value for “Other”, were used.
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