What is Air Pollution Dispersion Modeling ?
A model provides a fundamental link between emissions and air quality changes by simulating transport, dispersion, transformation, and deposition.
Why do we model air pollution?
- 1. Emission Assessments
- 2. To discriminate against sources
- 3. To evaluate alternative control strategies
- 4. To compliment ambient monitoring
- 5. To evaluate accidental releases
- Degree of stability must be known if we are to estimate the ability of the atmosphere to be able to disperse pollutants from anthropogenic sources.
- Stable atmospheres do not allow much vertical mixing. As a result, pollutants near the earth’s surface tend to stay there
- Mixing is dependant upon: Mechanical turbulence due to shearing action of wind and; Temperature gradient
- Comparing actual environmental temperature gradient (lapse rate) to adiabatic lapse rate can help determine possibility of thermal mixing
- Stable – does not exhibit much vertical mixing or motion
- Unstable – mechanical structure is enhanced by thermal structure
- Neutral – thermal structure neither enhances nor resists mechanical turbulence
Lapse Rate is:
- Rate of decrease in temperature as one ascends through the atmosphere
- oK/ Km rise (0oK = -273oC)
Dry adiabatic lapse rate
Rate of temperature decrease of parcel of air as it rises
Environmental lapse rate
Temperature gradient of ambient air as changes with altitude.
What is zero drift?
- Drift dictates the frequency of calibration.
- Zero drift is the change in response to zero pollutant concentration, over 12 and 24 hours of continuous unadjusted operation
What is span drift?
the percent change in a response to a pollutant concentration over a 24-hour period of continuous unadjusted operation
Occurring without the addition or loss of heat.
Unstable (B-C) conditions:
Atmospheric lapse rate cooling faster then adiabatic lapse rate in plume.
Stability letters ABC?
Stability letter D?
Stability letters E,F?
- From lab/clinical studies, assumes risk at every dose, no safe risk.
- = Risk/Dose (mg/kg/d)-1
Risk-specific Dose (RsD):
- for contaminant known to cause cancer
- = Risk/Slope Factor (mg/kg/day)
- should be < 1/100,000 for carcinogens
- Tolerable Daily Intake (Rfd - reference dose)
- for non-cancer effects; non-carcinogens
- = NOAEL/(UF1 x UF2 x ... x MF)
Uncertainty Factors for TDI:
- Heterogeneous Population = x10
- Animals to Humans = x10
- Chronic NOAEL from subchronic data = x10
- NOAEL rather than LOAEL = x10
- MF = x10 (general uncertainty)
Estimated Daily Intake through exposure pathways: inhaled, ingested, etc.
Estimated Dose (Air):
- ED = Ca x IRA x AFinh/BW
- Ca - concentration of contaminant (mg/m3)
- IRA - inhalation rate (m3/h)
- AFinh - inh absorption factor = 1.0
- BW - body weight
- EDI < RsD
- minimal risk of cancer from exposure to that contaminant
- EDI < TDI
- exposure to contaminant likely does not pose signif risk to human health
Slope-factor vs. TDI
Cancer risk per bite vs. Threshold number of bites resulting in toxic effect.
Hazard Quotient (HQ):
- Non-carcinogens (air-borne contaminant)
- HQ = Air [ ] (ug/m3) x Fraction of time exposed/Tolerable air [ ] ug/m3
- HQ = ED/TDI (ED - calculated without D's and LE)
- HQ < 1, acceptable risk
Incremental Lifetime Cancer Risk (ILCR):
- Carcinogens (air-borne) (ug/m3)-1
- ILCR = Air [ ] ug/m3 x Fraction of time exposed x Cancer Unit Risk (ug/m3)
- ILCR < 1/105 , acceptable risk.
Limits of Risk assessment:
- Lack of studies to back up
- Lack of long term effects evidence
- Difficult to assess risk posed by trace amounts in tissues
- With small doses, dose-response difficult to quantify
- Individual differences
- Lifestyle differences
- Conventional approaches inadequate to measure delayed effects
- Effects only seen in synergism
Continuous Emission Monitors (CEMs):
have built in calibration gases to correct for zero drift and span drift daily i.e. continuous calibration.
Parameters monitored at station:
SO2, TRS, NOx, ppm (TSP, PM10, PM2.5, & dustfall), PAHs, PCBs, VOCs, fluoridation rate, meteorological (wind speed/direction, temp, solar radiation)
Sampling System Design:
- Temperature stability of shelter
- Location of sampling probe(s)
- Manifold or sample inlet line system
- Length of probe
- Probe material
- Determine frequency of routine site visits
- Provide training
- Plan approp. level of surveillance
- Plan equipment operations and data checking
- Calibration checks (daily, manual, multi-point)
- Traceability, unique identifiers
Summa Cannister, fills after 24 hours.
TSP and Metals monitoring
- high vol sampler, quartz filter
- Q = 40-60 ft3/min
PAH and PCB monitoring:
- high volume sampler, PUF/XAD module
- Q = 7.9 ft3/min
- high vol sampler, quartz filter, selective inlet
- big round top
- Q = 40 ft3/min
- low vol sampler, PTFE filter, size selective filter
- Q = 16.7 L/min
Sydney Tar Ponds Agency - Ambient Air Monitoring Program
As Fg accelerates particle downward, speed increases and FD:
Drag Force increases.
Net force: Fg - FD:
decreases with acceleration (eventually reaching 0)
Fg is constant, 9.81 m2/sec, FD:
increases with speed.
When net force = 0, then FD =
- If the particles are falling in the viscous fluid by their own weight
- due to gravity, then a terminal velocity, also known as the settling velocity, is
- reached when this frictional force combined with the buoyant force exactly balance the gravitational force.
- The result is settling velocity (or terminal velocity) = ut.
Optimal Particle Ranges:
- Settling Chamber: 40-10,000 um
- Cyclone: <10-20 um
- Wet scrubber: 0.1 - 30 um
- Fabric filter: 0.01 - 20 um
- ESP: 0.001 - 10 um
Electrostatic Precipitators work by:
giving particles an electrostatic charge then puts them in an electrostatic field that drives them to a collecting wall.
Two types of filters are:
- Surface filters (coffee filter - form a cake) &
- Depth filters (HEPA - brownian diffusion)
Brownian diffusion - 2 important effects:
- Rate of collisions are not balanced
- Significant force in the imbalanced direction
Scrubbers collect particles:
- in dirty gas stream with liquid drops (eg. ventruri scrubber)
- Particles collide with droplets, separated in cyclone
4 ways of reducing pollutants:
Manual used on Sydney Tar Ponds Project AQ monitoring:
Operations Manual for Air Quality Monitoring in Ontario