High-Resolution Image

The map shows the day of the week when mean aerosol pollution is maximum at various Environmental Protection Agency (EPA) sites around the country. Arrow directions indicate the day of the week (see circular diagram on the right side for key to the meaning of arrow directions), and the length and color of the arrows indicate how strong the weekly changes in concentration are relative to the unpredictable random changes due to the vagaries of weather and aerosol emissions. (The "strength" of the weekly cycle is measured by the significance level p of the cycle, indicating the probability p that the cycle might have been generated by accident from random fluctuations of the aerosol concentration.)

The EPA has established a number of monitoring stations around the country that measure the concentrations of particulate matter in the air on a daily basis as part of their mission to protect human health and the environment. Because human activity varies significantly with the work week, it is not surprising to find that pollution varies somewhat predictably with the day of the week. Since pollution concentrations are reduced when it rains and moved around by winds, however, the weekly pattern cannot always be seen without averaging aerosol measurements over many months. The map was produced by averaging over EPA summertime data for particulate matter smaller than 2.5 µm ("PM2.5") from 1998 to 2005 (though not all sites have data for all years). Note that very few sites show peak concentrations on Saturday or Sunday, and those tend to have low significance levels (large p). This is consistent with most urban dwellers' experience that skies are dirtier looking during the middle of the week than on weekends. (Surface ozone levels, on the other hand, tend to be higher on weekends, apparently due to chemical interactions of pollutants and sunlight.)

Not surprisingly, climate researchers have found that properties of the atmosphere such as temperature and pressure also vary with the day of the week. This is sometimes referred to as the "weekend effect". The weekly "experiments" in changing pollution by humans may allow climate researchers to learn more about how aerosols affect our climate.

(submitted by Thomas Bell)