We're interested in the changes made to our atmosphere as a result of comet or asteroid impacts. Even moderate ones, such as the famous 1908 Tunguska (Russia, Siberian region) airburst can have a measurable effect on atmospheric chemistry. We have simulated the event and produced results that agree with ice core data. We then consider a million-times larger event which may have happened over the North American ice sheet near the end of the last ice age. The results are interesting--at this point I believe I will just link a PowerPoint presentation, which should be self-explanatory. Or pdf in case you prefer that. For a more formal presentation, our paper about this has been published on Geology. An account in Science News explains the main points.