Measurement of gas phase pollutants by automatic analyzers

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Measurement of gas phase pollutants by automatic
analyzers
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Nitrogen Oxides (NOx)

• Most nitrogen oxides found in ambient air are
  emitted from automobiles and incinerators.
• Nitrogen oxides consist of nitrogen monoxide and
  nitrogen dioxide.

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1. Chemiluminescence method

• NOO3 NO2O2

NO2hv (chemiluminescence)
Emitted light hv is directly proportional to
concentration of NO
If the sample air is passed through a converter
to convert nitrogen dioxide into nitrogen
monoxide, the concentration of nitrogen oxides
(nitrogen monoxide and nitrogen dioxide) can be
obtained by measuring the intensity of the
chemiluminescence.
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Flow diagram of NOx analyzer
Moisture quenches (reduce) the chemiluminescence
resulting from the reaction between nitrogen
monoxide and ozone, causing negative interference
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Effect of water (humidity)

• If there is a great difference between the water
  content of the calibration gas and that of the
  measured air, the measurement is particularly
  affected. For this reason, the monitoring system
  is equipped with a semipermeable membrane filter,
  humidity controller, or the like that is used to
  maintain a constant water content of the sample
  air.

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2. Absorptiometry using Saltzman reagent

• Nitrogen dioxide in air reacts with the Saltzman
  reagent in an absorption solution (an aqueous
  solution of N-1 naphthyldiamine dihydrochloride,
  sulfanilic acid, and acetic acid) producing azo
  pigment.
• The concentration of nitrogen dioxide can be
  determined by measuring the absorbance of this
  pigment in a range near 545 nm
• Since nitrogen monoxide does not react with the
  Saltzman reagent, it is oxidized by passing it
  through an oxidation solution of sulfuric acid
  potassium permanganate, and its concentration
  determined similarly.

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Flow diagram for the impinger method
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Sulfur dioxide(SO2) 1. Ultraviolet fluorescence
Method

• This method is based on the principle that SO2
  molecules absorb ultraviolet(UV) light and become
  excited at one wavelength,
• SO2hv1 SO2
• SO2 SO2 hv2 (fluorescence)
• The intensity of fluorescence is directly
  proportional to SO2 concentration

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Flow diagram of SO2 analyzer
The hydrocarbon scrubber shall remove
hydrocarbons contained in ambient air, which are
excited with W light and consequently emit
fluorescence. The SO2 molecules pass through the
hydrocarbon scrubber unaffected. -Fluorescence
Chamber The fluorescence chamber shall have a
structure to efficiently emit the fluorescence of
SO2 . -UV Light Source The exciting light
source shall generate W light energy by an
electric discharge and so on. -Photoelectric
Detector The photoelectric detector shall be
located adjacent to the fluorescence chamber via
an optical filter which selectively passes the
fluorescence to an electrical signal of required
level.
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2. Conductimetric Method

• This is the method to measure continuously the
  concentration of SO2 in ambient sample air from
  change in the conductivity of absorbent (hydrogen
  peroxide solution acidified by sulfuric acid)
  which appears when the ambient sample air passes
  through the absorbent by air bubbling.

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3. Coulometry

• This is the method to measure continuously the
  concentration of sulfur dioxide in ambient sample
  air detecting with the electrodes the change of
  bromine concentration decreased by the reaction
  of SO2 in ambient sample air and bromine which is
  dissociated in the electrolyte of potassium
  bromide by electrolysis.

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4. Flame Photometry

• This is the method to continuously measure sulfur
  compounds in ambient sample air as the
  concentration of SO2 by measuring the light
  intensity with a photomultiplier tube utilizing
  the light emission phenomenon which appears in
  the near ultraviolet region when the sulfur
  compounds are thermally decomposed in a hydrogen
  flame.

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Carbon monoxide (CO) Infrared absorption method

• When infrared rays are radiated to gases of
  different kinds of molecules, each of those gases
  absorbs the infrared rays of a specific
  wavelength range in accordance with its
  concentration. Based on this principle, the
  absorption of infrared rays of 4.7 micron by
  carbon monoxide is measured to obtain the
  concentration of this substance.

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Flow diagram of Infrared absorption method
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• Function of CO gas filter
• To produce a reference beam
• As the gas correlation filter rotates, two
  different kinds of light alternately enter the
  sample gas cell. One is the light which has
  passed through the CO-filled cell of that filter
  and therefore contains no infrared rays of the
  wavelength band absorbed by CO, and the other is
  the light which has passed through the other cell
  filled with N2 or a similar inert gas. The
  concentration of CO is calculated from the
  difference between these two kinds of light.

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Ozone
A reference air is produced by eliminating ozone
in the converter and send to cell A and air
sample without ozone elimination is directed to
Cell B and their UV absorbances were measured.
Concentration is calculated using Beers Lambert
Law
K absorption coefficient 134cm-1 atm-1 I
Transmitted light quantity I0 Incident light
quantity L Cell length (cm) T Cell outlet
temperature P Cell outlet pressure (corrected
to 760mm Hg)
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Methane and Nonmethane hydrocarbon (NMHC)
measurements

• There are hundreds of hydrocarbon exist in the
  atmosphere.
• CH4 is particularly important being a greenhouse
  gas.
• Terminology of the concentration is a little bit
  confusing.
• e.g. 1ppmV C2H6 2 ppm C C2H6

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Selective combustion method
In NMHC cutter, NMHCs are selectively oxidized
and only methane was sended to FID
detector. Difference between THC and CH4 gives
the NMHC concentrations.
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Chromatographic Method
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How do they look like?