Particulate Matter - PM10-2.5 Data Analysis

The size of particles is directly linked to their potential for causing health problems. EPA is concerned about particles that are 10 micrometers in diameter or smaller because those are the particles that generally pass through the throat and nose and enter the lungs. Once inhaled, these particles can affect the heart and lungs and cause serious health effects. EPA groups particle pollution into two categories:

1. "Inhalable coarse particles," such as those found near roadways and dusty industries, are larger than 2.5 micrometers and smaller than 10 micrometers in diameter.
2. "Fine particles," such as those found in smoke and haze, are 2.5 micrometers in diameter and smaller. These particles can be directly emitted from sources such as forest fires, or they can form when gases emitted from power plants, industries and automobiles react in the air.

This analysis is concerned only with total particulate concentrations. No aspect of fine particle or coarse particle speciation is addressed.

Distributions of Coarse Particle Data Concentrations

PM data with which to calculate PM_10-2.5 have been available from six to ten sites during each year since 2009, including a duplicate set of monitors at one of the sites. Four of these sites (Garinger, Montclaire, Durham and Raleigh) provided direct measurement of PM_10-2.5 data through the use of a paired monitor federal method. The other sites were covered by collocated but unpaired PM_2.5 and PM₁₀ monitors, so that PM_10-2.5 is indirectly measured and computed. PM10 concentrations are conventionally reported with respect to air volumes that are adjusted to a standardized temperature and atmospheric pressure, whereas PM_2.5 concentrations are reported at the average ambient temperature and pressure during the sample collection. Therefore, we first readjust PM₁₀ to ambient conditions and then compute PM_10-2.5 = PM₁₀ - PM_2.5.

The federal method is designed so that PM₁₀ concentrations are virtually never smaller than the corresponding PM_2.5 concentrations, but for the unpaired collocated monitors there is no such limitation. This means that negative PM-coarse concentrations can occasionally be reported.

It is a standard practice with particulate data to weight calendar quarters equally when calculating the annual mean concentration. However, all the datasets have been approximately balanced over four consecutive calendar quarters, so I have used unweighted arithmetic means here. All the percentiles reported here have been computed as interpolated order statistics, rather than following the standard EPA definition that coincides exactly with an appropriate order statistic.

To see a panel of boxplots and a summary table describing PM_10-2.5 statistics in North Carolina, select a year2012201120102009