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Constraining Acceptance Criteria

The analysis above uses all of the overpass data supplied, for which the maximum separations sometimes exceed 300 km. In some regions a 300 km radius represents a wide range in climate regimes, so the analysis was also repeated with more stringent acceptance limits (Table 3).

Measurement Site

Subset of Overpass data

No of days

Correlation Coeff (R2)

Regression Gradient

Toronto, Canada All
100km
50km
10km
50 km, z>45
50 km, r<20
50 km, z<45
50 km, z<45,r<20
50 km, z>45,r<20

1125
1049
1015
91
529
434
486
287
147

0.974
0.975
0.976
0.974
0.963
0.989
0.977
0.989
0.989

1.166
1.167
1.164
1.200
1.165
1.187
1.164
1.187
1.181

Thessaloniki, Greece All
100 km
50 km
10 km
50 km, z>45
50 km, r<20
50 km, z<45
50 km, z<45,r<20
50 km, z>45,r<20

738
686
662
67
319
408
343
248
160

0.984
0.985
0.985
0.984
0.980
0.990
0.986
0.990
0.992

1.248
1.246
1.249
1.277
1.259
1.263
1.248
1.261
1.289

Garmisch-Partenkirchen, Germany All
100 km
50 km
10 km
50 km, z>45
50 km, r<20
50 km, z<45
50 km, z<45,r<20
50 km, z>45,r<20

1013
967
937
106
511
231
426
139
92

0.936
0.937
0.938
0.952
0.956
0.985
0.985
0.985
0.985

1.376
1.376
1.377
1.342
1.259
1.278
1.396
1.282
1.251

Lauder, New Zealand All
100 km
50 km
10 km
50 km, z>45
50 km, r<20
50 km, z<45
50 km, z<45,r<20
50 km, z>45,r<20

1209
1150
1119
112
550
524
569
273
251

0.962
0.967
0.968
0.978
0.969
0.989
0.968
0.989
0.990

0.977
0.984
0.985
0.972
1.006
0.993
0.984
0.993
0.996

Table 3. Effect on statistics of changing the co-incidence criteria for TOMS E-P data. The criterion z>45 corresponds to data selected at sza > 45, corresponding to winter data, whereas, z<45 corresponds to summer data. The criterion r<20 corresponds to selecting only data with reflectivity less than 20% to avoid complications of overcast conditions, or snow-covered terrain. There were no significant changes in satellite footprint pressure when using different selection criteria.

As the selection criteria are tightened, the number of points included decreases and the correlation coefficients increase as expected. While at most sites the relationship between satellite-derived UV and measured UV is relatively e these selection criteria. The largest differences occur at Garmisch-Partenkirchen, where the gradients are smaller during the winter months (sza > 45), or for cloud and snow free conditions (reflectivities < 20). It appears that at this site the high surrounding terrain contributes significantly to the deviations. However, at the other sites, errors arising from these selection criteria are not important.

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