Introduction to Air Quality Monitoring Systems

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Introduction to Air Quality Monitoring Systems
Met 163 5 May 2011

• Topics to be covered
• Basic urban air quality
• Measurements of Ozone
• Measuring CO
• Measuring NOx
• Measuring Black Carbon
• Issues with monitoring systems

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What is in Urban Air?
O3, NO, NO2, CO, CO2, SO2, HCHO
And other things like particulates and Black
Carbon!
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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.
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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.
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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.
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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
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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.
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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.

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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

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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 lt 0.35
µm and between 0.45-0.75 µm.
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Monitor Labs 9810B Ozone Analyzer
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Description

• UltraViolet Photometer
• Model 9810b
• Released in 1996 by Monitor Labs, Inc.
• Now owned by Teledyne Monitor Labs
  http//www.teledyne-ml.com/

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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.

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PneumaticsFlow Rate 0.5 slpm
Flow control
External Pump
Measurement cell absorption
UV lamp source
Measurement Valve
Ozone Scrubber
Inlet
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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).

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Technology

• Beer/Lambert Law gives ozone concentration

O3 O3 concentration, ppm a absorption
coefficient of O3 at 245 nm308 atm-1 cm-1 at 0C
and 760 torr (760 torr101kPa) l optical path
length, cm T sample temperature, K P sample
pressure, torr L correction factor for O3
losses
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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
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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.
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Technology

• Beer/Lambert Law gives ozone concentration

O3 O3 concentration, ppm a absorption
coefficient of O3 at 245 nm308 atm-1 cm-1 at 0C
and 760 torr (760 torr101kPa) l optical path
length, cm T sample temperature, K P sample
pressure, torr L correction factor for O3
losses
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Measuring NOx
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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

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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.

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Parts diagram

• FRONT

MOLYCON Catalytic converter
DELAY COIL
PERMAPURE DRYER
OZONE GENERATOR
OPTICAL BENCH
REACTION CELL
FAN
FAN
VALVE MANIFOLD
NOx PRE-PROCESSOR BOARD
POWER SUPPLY
VOLTAGEREGULATOR BOARD
MICROPROCESSOR BOARD
FRONT
REAR
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Air flow diagram
AIR INTAKE
OZONE GENERATOR
DRYER ASSY
VALVE MANIFOLD
REACTION CELL ASSY
NO2
NO2
hn
MOLYCON ASSY
OPTICAL BENCH ASSY
NO2 ? NO CONVERTER
SAMPLE PORT
PARTICULATE FILTER
DELAY COIL
EXTERNAL PUMP
EXHAUST PORT
EXHAUST
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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.)
• Establish a reliable and stable calibrating
  source.
• Calibrate the analyzer against the calibrating
  source.
• Zero/span calibration

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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)
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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, (aefa????) 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.
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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 )
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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 ? may be written as ATN(?) s
(1/?) BC
where BC is the mass of black carbon, and s
(1/?) 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.
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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.
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