High-Resolution Image

Local concentrations of particulate matter smaller than 10 microns, referred to as "PM₁₀", are measured each day at many Environmental Protection Agency (EPA) sites around the country. The image shows for each site whether PM₁₀ concentration exhibits a regular variation with the day of the week, that is, whether PM₁₀ tends to be higher or lower on some days of the week than others. Most of the EPA sites are classified according to their setting as either "rural", "suburban", or "urban and center city". The upper map shows the measured weekly cycle for sites classified as "rural". The lower map shows the same thing for sites classified either as "suburban" or "urban and center city".

The maps show, for each site, the day of the week when maximum average particulate concentrations tend to occur, using the directions of the arrows to indicate the day of the week: see the clock-style guide to the directions at the right of the maps. (The foot of each arrow is placed at the site location.) The maps show that particulate concentrations tend to be maximum between Tuesday and Thursday at the vast majority of sites.

The lengths of the arrows show the amplitude of the average weekly fluctuations as a fraction of the weekly mean. The key to the meaning of the lengths of the arrows is shown to the right of the color-bar. A fraction f = 0.1 indicates that the average daily PM₁₀ concentration may rise about 10% above the mean on the day it peaks. The colors indicate the likelihood that the weekly fluctuations at that site are truly correlated with the day of the week. A significance level p = 0.1 indicates that there is a 10% chance that a weekly cycle as big as what is seen could be just an accident owing to lack of sufficient data. Thus, blue arrows indicate some uncertainty about the regularity of the weekly cycle, while red arrows indicate fairly strong, regular changes with the day of the week.

As one would expect, the weekly changes tend to be stronger in the urban areas than in the rural areas. (Summertime data from 1998 to 2005 were used, although not all site records include all years.)

More information can be found in the paper by Bell et al. (2007).

(submitted by Thomas Bell)