I found a case where there are deposition contours where there are no corresponding air concentration contours. Why?

Although it appears you have deposition where there is no air concentration, this cannot be the case. If you look at the minimum air concentration for the plots, you will see that it is much smaller than the minimum contour shown. Normally the concentration plotting program automatically scales the contour interval and these smaller contours would have been shown. However, due to the popularity of the concentration animations, we set the default plot to fix the contours, so that the same color would represent the same concentration with each frame. Therefore on subsequent frames the air concentration plume appears to dissapear while deposition continues to occur. You can change this default to have the contours change from plot to plot.

Roland Draxler