Introduction to Air Quality Monitoring Systems
Met 163
5 May 2011
Topics to be covered:
1. Basic urban air quality
2. Measurements of Ozone
3. Measuring CO
4. Measuring NOx
5. Measuring Black Carbon
6. Issues with monitoring systems
What is in Urban Air?
O3, NO, NO2, CO, CO2, SO2, HCHO
And other things like particulates and Black Carbon!
Primary Pollutants
Carbon Monoxide (CO): is a tastless, colorless and
odorless gas.
Sources of CO include: fossil-fuel combustion
(Incomplete combustion in autos, trucks and
airplanes, some industrial process), biomass
burning, and photolysis reactions.
Concentrations of CO in urban air are typically 2-10
ppmv. On freeways and in traffic tunnels values
rise to more than 100 ppmv.
Primary Pollutants
Sulfur Dioxide (SO2): is a colorless gas that
exhibits a taste at levels of 0.3 ppmv and a strong
odor at levels greater than 0.5 ppmv.
Sources of SO2 include: coal-fired power plants,
automobile tailpipes, and volcanoes.
Background concentrations in the troposphere
range from 10 pptv to 1 ppbv. In polluted air, 1-30
ppbv.
Primary Pollutants
Nitric Oxide (NO): is a colorless gas and a free
radical. Precursor to tropospheric ozone.
Sources include: fossil-fuel combustion/autos and
planes, and biomass burning.
Nitrogen Dioxide (NO2): is a brown gas with
a strong odor. Absorbs short wavelengths.
Intermediary between NO and O3 production.
Primary Pollutants
Background concentrations range from 20 to 50
pptv. In urban areas, concentrations are 0.1-0.25
ppmv. In midmorning NO2 is more prevalent than
during midday because sunlight breaks down most
NO2 past midmorning.
NOx = NO + NO2
Secondary Pollutants
Secondary pollutants form chemically in the air. Ozone is a
secondary pollutant in the troposphere.
Ozone (O3): is a relatively colorless gas. Ozone exhibits an
odor at concentrations of 0.02 ppmv or greater. Ozone has a
sweet smell (Copy machine).
Ozone is not emitted. Background concentrations in
troposphere are 20 to 40 ppbv. In urban air, concentrations
range from 0.01 ppmv at night to 0.5 ppm during afternoons
in the most polluted cities. Typical mixing ratios are 80-100
ppb in urban air.
In 2008, EPA lowered the 8 hr primary standard from 84
ppbv to 75 ppbv.
Chemistry of Photochemical Smog
Photochemical smog involves reactions among NOx and
reactive organic gases (ROGs) in the presence of sunlight.
We call ROGs volatile organic compounds (VOCs) which
are emitted from cars and processing industry.
On a typical day, ozone forms following emission of NO and
ROGs. ROGs are broken down chemically into peroxy
radicals, RO2.
NO + RO2
NO2 + RO
NO + O3
NO2 + O2
NO2 + hν
NO + O
O + O2
O3
EPA Standards
National Ambient Air Quality Standards
(NAAQS)
• 75 ppb O3 for an 8-hour averaging period
• 120 ppb O3 for a 1-hour averaging period
Gas Absorption: absorption occurs when radiative
energy enters a substance and is converted to internal
energy, increasing the temperature of the substance.
Absorption removes energy from an incident beam,
reducing the amount of radiation received past the
point of absorption.
Gases selectively absorb radiation in different portions
of the electromagnetic spectrum.
Ozone absorbs UV radiation at wavelengths < 0.35 μm
and between 0.45-0.75 μm.
Monitor Labs 9810B
Ozone Analyzer
Description
•
•
•
•
UltraViolet Photometer
Model: 9810b
Released in 1996 by Monitor Labs, Inc.
Now owned by Teledyne Monitor Labs
http://www.teledyne-ml.com/
Technology
• Nondispersive UV photometer alternately switches a
ozone scrubber in and out of measuring stream.
• Scrubber uses manganese dioxide (MnO2) to destroy
only the ozone and pass other common absorbers
(SO2, etc).
• Mercury vapor lamp as the light source with 254 nm
at the center of absorption band.
• Transmitted light density computes ratio of ozone
scrubbed air (ozone free air) to non-scrubbed air
(ozone air).
• Has a microprocessor to monitor and adjust
temperature and pressure.
Pneumatics
Flow Rate = 0.5 slpm
Flow control
External
Pump
Measurement cell absorption
UV lamp source
Ozone
Scrubber
Inlet
Sample
Measurement
Valve
Technology
• Measurement Cell:
– Mercury vapor lamp as source
– vacuum photodiode as detector
• A photodiode is a type of photodetector capable of
converting light into either current or voltage, depending
upon the mode of operation.
– Glass tube used as absorption cell
• Reference cycle: ozone is scrubbed out of
sample and light intensity is determined (Io)
• Sample cycle: light intensity is determined for
ambient air with ozone (I).
Technology
• Beer/Lambert Law gives ozone concentration:
(O3 )out
  1 I  T  760 106 

  ln 


 al I o  273 P  L 
O3= O3 concentration, ppm
a = absorption coefficient of O3 at 245 nm=308 atm-1 cm-1 at 0˚C and 760
torr (760 torr=101kPa)
l = optical path length, cm
T = sample temperature, ˚K
P = sample pressure, torr
L = correction factor for O3 losses
Measure CO
Gas Filter Correlation method
Most analyzers use a nondispersive infrared photometer
Photometer is an instrument for measuring light intensity
Generate a IR radiation that is absorbed by CO.
5 m pathlength.
The gas filter correlation wheel facilitates rejection of
interferents and the narrow band-pass filter ensures only
CO sensitive IR wavelengths are only measured.
US EPA Automated Reference Method RFCA-0992-088
Measure CO
Infrared energy absorbed by a sample to that absorbed by
a reference gas according to the Beer-Lambert law.
This is accomplished with a Gas Filter wheel which
alternately allows a high energy light source to pass
through a CO filled chamber and a chamber with no CO
present (a chamber that scrubs out CO).
The light path then travels through the sample cell, which
has a folded path of 5 meters.
Technology
• Beer/Lambert Law gives ozone concentration:
(O3 )out
  1 I  T  760 106 

  ln 


 al I o  273 P  L 
O3= O3 concentration, ppm
a = absorption coefficient of O3 at 245 nm=308 atm-1 cm-1 at 0˚C and 760
torr (760 torr=101kPa)
l = optical path length, cm
T = sample temperature, ˚K
P = sample pressure, torr
L = correction factor for O3 losses
Measuring NOx
Theory of Operation
• Gas-phase Chemiluminescence detection
to analyze nitric oxide (NO), total oxides of
nitrogen (NOX), and nitrogen dioxide (NO2)
• Instrument has a pneumatic system, an
NO2-to-NO converter, a reaction cell,
detector (PMT), and processing
electronics
Theory of Operation
• The luminescence from an activated
molecular NO2 species produced by the
reaction between NO and O3 :
NO + O3 → NO2 + O2
• As the activated species NO2 reverts to a
lower energy state, it emits broad-band
radiation from 500-3000 nm.
• NO concentration is proportional to the
intensity of the chemiluminescent
→proportional to the current output.
Parts diagram
FRONT
PERMAPURE DRYER
DELAY
COIL
MOLYCON
Catalytic converter
REACTION
CELL
OZONE
GENERATOR
OPTICAL
BENCH
FAN
FAN
VALVE MANIFOLD
NOx PRE-PROCESSOR BOARD
VOLTAGEREGULATOR BOARD
MICROPROCESSOR BOARD
FRONT
POWER
SUPPLY
REAR
Air flow diagram
AIR
INTAKE
DRYER
ASSY
OZONE
GENERATOR
VALVE
MANIFOLD
REACTION CELL ASSY
NO2
NO2
hn
MOLYCON ASSY
PARTICULATE
FILTER
SAMPLE
PORT
DELAY
COIL
EXTERNAL
PUMP
EXHAUST
PORT
EXHAUST
NO2 → NO
CONVERTER
OPTICAL BENCH ASSY
Calibration process
• The analyzer must be calibrated initially and
periodically to determine the reliability and
accuracy of all air quality data.
• Single/multi point calibration: calibrated against
know sources of NO and NO2 (NIST standards etc.)
1. Establish a reliable and stable calibrating
source.
2. Calibrate the analyzer against the calibrating
source.
• Zero/span calibration
Black Carbon
Black Carbon is a primary aerosol component of Diesel Particulate Matter,
a known toxin and regulated pollutant by several regulatory agencies,
including the California Air Resources Board (CARB).
Diesel Particulate Matter is known to cause adverse health effects in
people who are exposed, including premature hospitalization, asthma
attacks, bronchitis, other respiratory and cardiovascular symptoms, and
premature death.
Black Carbon is the second leading cause of Global Warming.
Black Carbon is emitted as a primary pollutant to the atmosphere through
a variety of incomplete combustion of sources and fuels; BC concentration
cannot be modeled or predicted, it must be measured.
Black Carbon is NOT adequately characterized through PM-2.5 mass only
measurements, chemical speciation is necessary. (Magee Scientific)
Black Carbon
Aerosol black carbon (“BC”) is a particulate pollutant species
emitted from the combustion of any carbonaceous fuel.
The name “Aethalometer” is derived from the classical Greek
verb ‘aethaloun’, (αεφαλουν) meaning ‘to blacken with soot’. It
is this optically-absorbing material that the ‘standard’
Aethalometer measures.
The only known source of aerosol black carbon in the
atmosphere is the combustion of carbonaceous fuels. There
are no known significant biological, geological or
meteorological sources.
The Aethalometer: The Optical Attenuation Method
The optical method that we use is a measurement of the attenuation of a
beam of light transmitted through the sample when collected on a fibrous
filter.
When calculated as shown, this quantity is linearly proportional to the amount
of BC in the filter deposit.
Define I0 as the intensity of light transmitted through the original filter, or
through a blank portion of the filter.
Define I as the intensity of light transmitted through the portion of the
filter on which the aerosol deposit is collected.
The ‘Optical Attenuation’ ATN is defined as
ATN = 100 * ln (I0 / I )
The Aethalometer: The Optical Attenuation Method
This measurement is affected by the wavelength of the light with which it is
made, provided that the particle size is somewhat smaller than the
wavelength.
The absorption of light by a broad band absorber such as graphitic carbon
is inversely proportional to the wavelength of the light used.
Thus, for a given mass of black carbon [BC], the optical attenuation at a
fixed wavelength λ may be written as
ATN(λ) = σ (1/λ) * [BC]
where [BC] is the mass of black carbon, and σ (1/λ) is the optical absorption
cross-section (‘sigma’) that is wavelength dependent, and which is referred
to as the ‘Specific Attenuation’.
The Aethalometer model AE-16 uses a solid-state source operating in
the near-infrared at a wavelength of 880 nm.
Important Definitions
Range- the measurand interval over which a sensor is designed to respond.
Span- the algebraic difference between the upper and lower range values.
Resolution- the smallest change in the primary input that produces a
detectable change in the output.
Hysteresis- is present when the sensor output for a given input depends
upon whether the input was increasing or decreasing.
Imprecision or uncertainty- is a measure of the noise or scatter in the
measurement.
Bias- is the systematic error. The purpose of static calibration is to remove the
bias and to numerically define the imprecision.