AQS Assessment & Monitoring Methods 3 — Questions and Answers
Question 1: A Pasquill-Gifford stability class of 'A' indicates which atmospheric condition, and how does it affect pollutant dispersion?
- Strongly unstable; promotes rapid vertical mixing and faster dilution (Correct answer)
- Strongly stable; suppresses vertical mixing and increases ground-level concentrations
- Neutral; causes moderate dispersion proportional to wind speed
- Highly stable; causes fumigation events near the surface
Correct answer: Strongly unstable; promotes rapid vertical mixing and faster dilution
Class A is strongly unstable (typically sunny, low wind), which drives vigorous turbulent mixing that rapidly dilutes emissions.
Question 2: What distinguishes a field blank from a trip blank in an air quality sampling program?
- A field blank is opened at the sampling site; a trip blank travels with samples but is never opened (Correct answer)
- A field blank is used only for gaseous samples; a trip blank is for particulate samples
- A field blank stays in the laboratory; a trip blank goes to the field but is not exposed to air
- A field blank measures instrument zero; a trip blank measures reagent purity
Correct answer: A field blank is opened at the sampling site; a trip blank travels with samples but is never opened
Field blanks are opened and handled at the site to detect site contamination, while trip blanks travel sealed to detect contamination during transport.
Question 3: Which optical remote sensing technique uses differential absorption of two laser wavelengths to measure pollutant column concentrations along an open path?
- Differential Optical Absorption Spectroscopy (DOAS)
- Differential Absorption Lidar (DIAL) (Correct answer)
- Fourier Transform Infrared (FTIR) spectroscopy
- Cavity Ring-Down Spectroscopy (CRDS)
Correct answer: Differential Absorption Lidar (DIAL)
DIAL uses two wavelengths—one absorbed by the target pollutant and one not—measured via lidar backscatter to determine spatially resolved concentrations.
Question 4: When conducting a fence-line monitoring program under EPA's Petroleum Refinery Sector Rule, which pollutant triggers action level calculations using a two-week rolling average?
- Benzene (Correct answer)
- PM2.5
- Hydrogen sulfide
- Toluene
Correct answer: Benzene
40 CFR Part 63 Subpart CC (MACT CC) requires refineries to monitor fence-line benzene concentrations with a two-week rolling average action level of 9 µg/m³.
Question 5: In stack testing, isokinetic sampling requires that the gas velocity at the probe nozzle tip match the stack gas velocity. What error results from sampling at velocities HIGHER than isokinetic?
- Negative bias — particles with inertia bypass the nozzle, under-sampling coarse particles (Correct answer)
- Positive bias — additional particles are drawn in, over-sampling coarse particles
- No error — only fine particles are affected by isokinetic deviation
- Positive bias — additional gas volume inflates the concentration
Correct answer: Negative bias — particles with inertia bypass the nozzle, under-sampling coarse particles
Super-isokinetic sampling causes coarse particles to miss the nozzle due to their inertia, resulting in under-collection and a negative bias.
Question 6: A UV photometric ozone analyzer operates on which measurement principle?
- Ozone absorbs UV light at 254 nm; absorbance is proportional to concentration via Beer-Lambert law (Correct answer)
- Ozone emits chemiluminescence when reacting with ethylene; light intensity equals concentration
- Ozone is reduced electrochemically; current flow is proportional to concentration
- Ozone quenches fluorescence of a dye; signal reduction indicates concentration
Correct answer: Ozone absorbs UV light at 254 nm; absorbance is proportional to concentration via Beer-Lambert law
UV photometric ozone analyzers measure the attenuation of 253.7 nm UV light through a sample cell, applying Beer-Lambert law to calculate O3 concentration.
Question 7: During receptor modeling using Positive Matrix Factorization (PMF), what does each 'factor' represent?
- A source profile that contributes a specific chemical fingerprint to ambient air (Correct answer)
- A meteorological variable linked to pollutant transport
- A laboratory blank correction applied to each measured species
- A seasonal adjustment factor for temperature normalization
Correct answer: A source profile that contributes a specific chemical fingerprint to ambient air
In PMF, each resolved factor represents a distinct emission source or source type characterized by its chemical species profile.
AQS Assessment & Monitoring Methods 3
A Pasquill-Gifford stability class of 'A' indicates which atmospheric condition, and how does it affect pollutant dispersion?