Air Quality
Core concepts
Air-quality questions reward fluency with the criteria pollutants (PM10, PM2.5, ozone, NO2, SO2, CO, lead), NAAQS averaging times, and the permitting framework: attainment status drives PSD or nonattainment NSR, with BACT, LAER, and offsets. NAAQS are ambient health-based limits, not stack emission limits. Understand Title V operating permits, MACT/NESHAP for hazardous air pollutants, and New Source Performance Standards. For calculations, know emission factors (AP-42-style), control-device efficiency, combustion stoichiometry, excess air, and concentration-to-mass conversions. Always track actual versus standard conditions, wet versus dry gas, and O2 correction; these are common traps. Distinguish uncontrolled from controlled emissions and stack gas from ambient air. Dispersion modeling concepts-stack height, buoyancy, downwash, stability class, and terrain-often appear qualitatively.
Worked example
A stack emits 100 ppmv NOx (as NO2, MW 46) at 10,000 dscfm. At 68°F and 1 atm, 1 lb-mol occupies about 385 scf. Mass rate = C×10^-6 × Q×60/385 × MW = 100×10^-6 × 10,000×60/385 × 46 ≈ 7.2 lb/hr. The trap is using the wrong molar volume or forgetting the 60 min/hr factor. For O2-corrected limits, correct to the permit's reference O2 before comparing.
Rule of thumb
Control efficiency required = (uncontrolled − limit)/uncontrolled. If uncontrolled is 100 lb/hr and the limit is 10 lb/hr, needed efficiency is 90%, not 10%.