What is the difference between PM10 and PM2.5?
Particles come in a wide variety of sizes and have been historically assessed based on size, typically measured by the diameter of the particle in micrometers. PM10 refers to particles that are 10 micrometers in diameter or less. PM2.5, or fine PM, refers to particles that are 2.5 micrometers in diameter or less. (Note: a human hair is about 70 micrometers in diameter and a grain of sand is about 90 micrometers in diameter). Areas of the country are designated nonattainment or attainment separately for the PM10 and PM2.5 standards.
What are the National Ambient Air Quality Standards for PM?
Both PM10 and PM2.5 have two standards related to the average concentration over different time periods:
50 µg/m3 **
To attain this standard, the expected annual arithmetic mean PM10 concentration at each monitor within an area must not exceed 50 µg/m3.
Not to be exceeded more than once per year.
To attain this standard, the 3-year average of the annual arithmetic mean PM2.5 concentrations from single or multiple community-oriented monitors must not exceed 15.0 µg/m3.
To attain this standard, the 3-year average of the 98th percentile of 24-hour concentrations at each population-oriented monitor within an area must not exceed 65 µg/m3.
|** µg/m3 is micrograms per a cubic meter.|
When are air quality plans for PM2.5 due, and when must areas attain the standards?
State implementation plans (SIPs) for PM2.5 are due by April 5, 2008. Areas must attain the standards as expeditiously as practicable, with a maximum attainment date of April 2010. An extension of an additional one to five years may be possible for certain areas based on the severity of the problem and the availability of control measures. Attainment must be demonstrated based on monitoring data for the three years prior to the attainment date. Under interagency consultation requirements, transportation agencies should be involved in the development of the SIP, transportation-related control measures, and the SIP motor vehicle emissions budgets.
How does surface transportation contribute to PM?
Motor vehicles (i.e., cars, trucks, and buses) emit direct PM from their tailpipes, as well as from normal brake and tire wear. In addition, vehicles cause dust from paved and unpaved roads to be re-entrained, or re-suspended, in the atmosphere. Also, highway and transit project construction may cause dust. Finally, precursors in vehicle exhaust may react in the atmosphere to form PM, including nitrogen oxides (NOx), volatile organic compounds (VOCs), and additionally for PM2.5, sulfur oxides (SOx) and ammonia (NH3).