Why are there negative values in the pollution series?
In short, the pollutant data have been detrended—more details can be found in the document PollutantProcess.pdf. Before the pollution data are averaged across monitors they have a very smooth trend subtracted off. That is why variables with the “tmean” suffix have negative values (the same is true for variables with the “mean” suffix). The median of the trends is stored in a variable with suffix “mtrend”. Adding a variable ending in “tmean” with its corresponding “mtrend” variable should get you something resembling the original averaged values. There is a basic flowchart describing the processing of the pollutant data. Adding the “tmean” and “mtrend” variables adds the average detrended series with the median of the long term trends from each monitor. It is not an exact reconstruction of any particular series. Variables ending with the “mean” suffix have been processed similarly, but instead of trimmed mean, a standard arithmetic mean is used to combine data across monitors.
