Lecture 3: Particles Size Distribution

Question 1

The properties of Particle Size Distribution

Answer 1

• Most aerosols in the ambient air are polydisperse
• the distribution of particles with respect to size - PSD

Characterizing PSD: statistical means (follows log normal distribution)

• PSD classification;
• • Number distribution
• • Mass distribution

Question 2

Number Distribution

Answer 2

Mode: the most frequent size
Median: the diameter for which 1/2 the total number of particles are smaller and 1/2 are larger
Frequency function: fraction of the total number of particles per unit of size interval

Question 3

Mass Distribution

Answer 3

• the fraction of the total mass of the particles contributed by particles in any size range
• Mass Median Diameter (MMD): the diameter for which half the mass is contributed by particles larger than the MMD and half by particles smaller than the MMD.

Question 4

Regulation of PM_course (PM2.5-PM10)

Answer 4

• to avoid double counting, EPA proposed a new standard PM_course to differentiate between the “course” and “fine” fraction of inhalable particles
• PMcourse regulation: goal is to prevent adverse health effects due to the penetration of inhalable particles to the thoracic region of the human respiratory system

Question 5

2005 PMcourse standard

Answer 5

Primary 24-hr avg concentration of 85 micrograms/m^3

• later reduced to 70 to be conservative
• applied to property lines or as an ambient conc. limit.

Question 6

Problems with current PMcourse standard???

Answer 6

• by regulating PMc based on the difference in the NAAQS, we assume that ALL PM has the same PSD
• Agri. operations emit PM with wide range of PSD
• Urban PM is much smaller than the PM emitted by agri. operations.

Question 7

Problems with current PMcourse standard???

Answer 7

There is no scientific basis for regulating PMc based on the difference in the existing PM standards

• a PMc standard of 75-85 is;
• -unattainable by many agri. operations as well ass urban environments in non attainment for PM10
• • effectively reduces the PM10 and PM2.5 standards to unattainable levels — impact of PSD