Environmental Engineering Reference
In-Depth Information
segments from a “cold start” from 3 June 2008 at 00 UTC to 19 June at 00 UTC.
Seventy-two 48-h forecasts were then made 12 h apart from 19 June at 12 UTC to
24 July at 00 UTC, alternating between 12 and 00 UTC starts.
Figure 1 shows the mean Day-1 20-UTC PM 2.5 concentration fields for the
period 20 June to 23 July predicted by CHRONOS and GEM-MACH15. The
overall PM 2.5 spatial patterns are similar for the two models, but GEM-MACH15-
predicted PM 2.5 magnitudes are generally higher, especially over eastern North
America, and PM 2.5 transport over the Atlantic Ocean is greater.
Fig. 1. Mean Day-1 20-UTC PM 2.5 concentration fields (μg m −3 ) for the 20 June-23 July 2008
period from averaging 12 (left) CHRONOS and (right) GEM-MACH15 00-UTC runs
Table 1 presents some objective scores for both CHRONOS and GEM-MACH15
against measurements for a similar period. Canadian and U.S. O 3 and PM 2.5
ambient measurements were obtained in near-real time from Canadian stations and
the AIRNow meta-network (see http://airnow.gov). For both O 3 and PM 2.5 ,
CHRONOS predictions are on average biased low for this period whereas GEM-
MACH15 predictions are biased high. Overall, the GEM-MACH15 scores for
hourly O 3 are slightly better than those for CHRONOS. For hourly PM 2.5 the
GEM-MACH15 mean bias and correlation coefficient (r) scores are better than the
CHRONOS scores but the unbiased RMSE score is worse. Except for the O 3
correlation coefficient, all of these differences are statistically significant based on
a bootstrapping calculation.
Table 1. Objective scores for 72 CHRONOS and GEM-MACH15 runs for all hours of period
from 19 June (12 UTC) to 26 July 2008 (00 UTC)
Statistics
Hourly O 3 (ppbv)
Hourly PM 2.5 (μg m 3 )
CHRONOS
GEM-MACH15
CHRONOS
GEM-MACH15
Mean bias
2.5
0.7
−4.3
−2.1
Unbiased RMSE
15.4
15.3
12.8
13.5
R
0.68
0.68
0.30
0.40
 
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