CHEM. 250

Dixon First Homework Problems

Ch. 2: 12, 14, 15, 17, 18, 19

12. Describe the difference between an aerosol and a particulate.

An aerosol is a collection of suspended particles and the surrounding gases. A particulate (which is more commonly referred to as particulate matter) refers just to the suspended solid or liquid matter.

[Note: this differs significantly from the book's answer, which I think is wrong. I believe the Wikipedia definition is more accurate – see: http://en.wikipedia.org/wiki/Aerosol and http://en.wikipedia.org/wiki/Particulate.]

14. Give four examples of particulate matter that arises from the following:

a) natural sources. Sulfates from oxidation of sulfur dioxide from volcanoes, sulfates from oxidation of biogenic reduced sulfur species (primarily dimethylsulfide from phytoplankton), smoke from forest fires, organic compounds from oxidation of terpenes and related volatile organic compounds emitted from plants, sea-salt particles from sea-spray, dust from winds over arid regions.

b) anthropogenic sources. Fly ash from burning coal, soot from diesel smoke, sulfates from burning coal or other sulfur containing fossil fuels, smoke from human set wildfires or fireplaces, carbonaceous aerosol from oxidation of anthropogenic volatile organic hydrocarbons.

15. Describe Raleigh scattering by particulates.

Raleigh scattering occurs when aerosol particles are considerably smaller than the wavelength of light. Raleigh scattering is most efficient for smaller wavelengths of light (e.g. blue light).

17. Describe the TSP test.

TSP, which stands for total suspended particulates, is measured in the TSP test by pulling air through a filter at a controlled flow rate for a defined period of time. The weight gained by the filter is measured and the TSP is calculated as the ratio of the mass gained over the volume of air pulled through the filter (usually in μg m^-3). [Note: it often is difficult to collect the larges particles with unit efficiency – so TSP often has some bias associated with how efficient large particles are actually collected]

18. Define the following:

PM_2.5: This is the particulate matter under 2.5 μm in diameter.

PM₁₀: This is the particulate matter under 10 μm in diameter.

19. Explain how the surface areas of particulates and aerosols are a factor in causing them to be health hazards. Compounds within aerosol particles with larger surface areas can be released into the body more quickly since there is no need to migrate from the interior of an aerosol particle to be released. Additionally, fine aerosol particles are smaller, have a greater surface area, and travel more readily to the lungs because they are not impacted out as air traveling to the lung makes bends in the airway.

Ch. 6: 44a,b, 45, 48

44. List the following:

a) Industrial sources of particulate matter

fly ash, sulfates from combustion of sulfur containing fuels, soot from inefficient combustion

b) mobile sources of particulate matter

soot from diesel exhaust, dust from vehicle use on unpaved roads, carbonaceous aerosol from oxidation of volatile organic hydrocarbons in fuel exhaust.

45. A high-volume TSP station has been constructed near the toll booth of the Delaware Bay Bridge. During a busy summer day, the collection pump pulls a 1-L/min sample into the air sampler. An empty collection filter weighed 75.34 g before it was inserted into the air sampler. At the end of a 24-hour sampling period, the filter weighed 75.68 g. What was the TSP concentration of the air that was sampled.

TSP = mass/volume = (75.68 – 75.34 g)/[(1L/min)(24 hrs)(60 min/hr)] = 0.000236g/L

TSP = 236 mg m^-3. (This is an extremely high concentration).

48. What is XRF used to measure?

a) What is XRF measuring in PM?

It is measuring concentrations of major elements (except lightest elements).

b) Does XRF measure the total metallic composition of the PM?

Nearly all metals are heavy enough to be measured by XRF. There can be some problems in quantifying metals because of matrix effects combined with changes in aerosol composition from the surface to the interior of large particles.

c) Why is XRF used rather than inductively coupled plasma-optical emission spectroscopy?

XRF measures concentrations of non-metals like Cl and S (these require special ICP spectrometers). Additionally, little sample treatment is needed for XRF.

[Note regarding text answers: XRF does only measure the surface of materials analyzed, but fine particles are small enough that essentially the whole particle is analyzed. For large coarse particles, XRF composition may differ from concentrations found using other techniques]