Air Monitoring

Basic Calculations for Air Monitoring

• Calculating Concentration: Measurements for particulate matter, gases, and vapors are often expressed as the mass of contaminant per cubic meter of air, μg/m³ or mg/m³. Consider the following example. An IH sample for cobalt dust at 2 liters per minute (lpm) for 7 hours and after laboratory analysis finds that 0.11 mg of cobalt has been collected. If we wanted to calculate the average concentration of cobalt dust in the air during the 7 hours, we would use the following equation:
  • Step 1:
    $2\mathrm{lpm}x7\mathrm{hr}x\frac{60\min }{1\mathrm{hr}}$
  • Step 2:
    $\left(\frac{0.11\mathrm{mg}\text{Cobalt}}{840\mathrm{liters}\text{of air}}\right)x\left(\frac{1000\mathrm{liters}}{1{\mathrm{m}}^3}\right)=0.13\mathrm{mg}/{\mathrm{m}}^3$
• Converting mg/m³ to ppm: When measuring vapors or gases, measurements can be expressed as mg/m³ or parts per million (ppm) or parts per billion (ppb). Below is the formula to convert mg/m³ to ppm. MW stands for molecular weight of the agent. This formula can be used when measurements are taken at 25° C and 1 atmosphere.
  $\mathrm{ppm}=\frac{(\mathrm{mg}/{\mathrm{m}}^{3}x24.45)}{\mathrm{MW}}$
• Time-Weighted Average: Time-weighted average is generally used in occupational health as a measure of workers daily exposure to a hazardous substance and is averaged to an 8-hour workday, taking into account the average levels of the agent and the time spent in the area. A time-weighted average is equal to the sum of the portion of each time period (as a decimal, such as 0.25 hour) multiplied by the levels of the substance or agent during the time period divided by the hours in the workday (usually 8 hours). Below is the calculation where C1 is the concentration during T1 and Ttotal is total time.
  $\mathrm{TWA}=\frac{(C_1{T}_1+C_2{T}_2+C_3{T}_3\dots C_{\mathrm{n}}{T}_{\mathrm{n}})}{{\mathrm{T}}_{\text{total}}}$

Active Sampling of PM

There are several methods available for actively sampling particulate matter. Size-selective sampling is the collection of one particle size fraction, such as the collection of PM₁₀ or PM2.5. If the objective is to obtain data regarding a particular PM size, cyclones and single stage impactors can be used to remove large particles from the sample followed by a filter for aerosol collection. Size-segregated sampling refers to the physical separation of airborne particles into several size fractions, which can be achieved with the use of cascade impactors (right) and virtual impactors.

Shown in the image below is a cascade impactor with its schematic illustrating how it collects particles of different sizes on separate plates.

Active Sampling of Gases/Vapors

A sampling pump is required to actively sample gases and vapors. One method involves the use of sorbent tubes, which contain collection materials such as activated charcoal, silica gel, tenex, etc.

Another method involves the use of annular denuders shown below in which the agent of interest is adsorbed onto the wall coated with suitable collection material. The particles and undesired gases will pass through while the target gases will removed adsorbed onto the wall. While this method still requires a pump, it also relies on diffusion to collect the gas or vapors.

Passive Sampling of Gases/Vapors

Passive sampling does not require a pump. Passive samplers have been developed for many gaseous pollutants, such as NO₂, O₃, and VOCs. The sampling rate is controlled by a physical process, such as diffusion through a static air layer or permeation through a membrane. It's important to note that factors such as temperature, high humidity, and face velocity can affect the sampling process.

Shown below is a vapor diffusion badge that can be used to passively sample vapors. It's relatively easy to use and there are many types (contaminant-specific) available.