Ambient PT

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					BCH 6112   Environmental Analysis

Analysis of Ambient Air Pollution
1. General considerations
2. Specific measurements and results
a. Suspended air particulate matter
b. Ambient gases
3. Other measurements
Ambient air quality monitoring
 Safeguard public health, and determine overall social
and environmental effects of ambient air pollution (e.g. Air
Quality Index)
 To determine influence of specific emission source on
local air quality (e.g. SO2 emission from power plants)
 To aid the planning of overall pollution control strategies
(e.g. whether or not to adopt ultra-low sulfur diesel)
 Research purposes (e.g. identification of new emission
sources, studying reaction mechanisms of pollutants in
ambient atmosphere)
How to establish a sound ambient air quality monitoring
 proper selection of sampling site(s) / station(s)
 proper establishment of sampling / monitoring schedule
(e.g. sample frequency, sampling duration)
 proper selection of sampling methods and instruments
 proper selection of analytical methods
 proper gathering of auxiliary meteorological data to
facilitate interpretation of data / information
Airborne pollutants in ambient atmosphere:
 Aerosols (dispersion of microscopic solid or liquid
particles in ambient air)
 Particulates (i.e. smoke, suspended particulates, dry
deposition/dustfall, asbestos)
 Gaseous primary pollutants (e.g. NOx, SO2, petroleum
hydrocarbons etc)
 Gaseous secondary pollutants (e.g. O3, NO2, N2O5,
HNO2, HNO3, H2O2, PAN, aldehydes, organic acids, organic

 Monitoring of different types of pollutants needs different
sampling & analytical methods
A typical sampling system for the monitoring of airborne
 Sample intake & transfer
 Sample collection
 Air flow measurement
Isokinetic requirement in sampling intake and transfer

                        Sampling velocity < air stream velocity
                         Underestimation of level of pollutant

                        Sampling velocity > air stream velocity
                         Overestimation of level of pollutant

                        Sampling velocity = air stream velocity
                         Isokinetic sampling
2. Specific measurements of
Monitoring of airborne particulate matter
 Classification:
       Smoke – mainly elemental C; very fine suspended
       particles (< 15 m); related to the staining capability
       of air
       Total suspended particulates (TSP) – suspended
       particles in the ambient air collected by filtration
       Dry deposition / dustfall – refers to suspended
       particles in the ambient air collected by deposition
 Sampling by filtration – the use of Hi-Vol samplers with
various choice of filtering media (cellulose filters, glass fibre
filters, membrane filters and nuclepore filters)
 Determination of suspended particulates – light reflection
method and gravimetric method
High volume (Hi-Vol) air sampler
useful in sampling ambient suspended air
  particulate matter
(see lab notes: choice of filter)
Used for TSP and PM10
3h - 1 d sampling.
Total concentration of TSP/PM10 and
 concentrations of elements/ions after
TSP high volume air sampler
      Continuous measurement of
            PM10 and PM2.5,1055,10122682,00.html

tapered element oscillating microbalance
Some PM10 results: comparison of sampling locations
Comparison of PM2.5 and PM10 trends
Comparison of TSP and PM10 at CityU
Size-selective sampling of air
 particulate matter: Cascade
Cascade impactor: cross-section
                                  Masses of particles
                                  collected on
                                  (greased) filters
                                  determined by
                                  Or perform
                                  quantitative chemical
                                  analysis on filters
                                  Usually, collect
                                  particles between
                                  0.1 and 10 μm
Some cascade
impactor results
at CityU
Monitoring of primary gaseous pollutants in ambient
 Primary gaseous pollutants refer to pollutants that are produced /
emitted directly from their sources – NOx, SO2, NH3, H2S, CH3SH etc.

                                   Concentration (ppb)

     Gas         Source    Source          Urban         Rural     Remote
     SO2        2  106      103          20 - 500       5 - 50      1
     NOx          106        103          20 - 500       5 - 50      3
     H2S           --       > 50           1 - 10        0.1 - 1    0.1
     NH3           --        103           2 - 25        2 - 25      5
note: x μg m-3 = x (24/[M.Wt.]) ppb
 Ambient levels of these pollutants vary greatly depending on
distance from sources. Thus, it is important to choose an analytical
method of appropriate sensitivity for the monitoring
Collection of gaseous pollutants
 Adsorption – a temperature dependent process; more effective at low
temperature; common absorbents include activated charcoal, silica gel,
alumina and GC stationary phases
 Absorption – dissolving gaseous pollutants into suitable solvents;
usually employ an irreversible reaction to ensure complete collection of
pollutant (e.g. collection of ambient H2S by bubbling air into NaOH

                            Various bubblers
                            (impingers) for
                            the collection of
                            pollutants by

Condensation – cooling air sample below boiling point of the target
pollutant in order to condense and collect pollutants
      Gas Sampling Devices:
• Annular denuders: glass or metal tubes,
  opened on each end to allow an air stream
  to pass through
• When air flows through a denuder / coated
  with a porous material
• The gas phase chemically reacts and
  adsorbs into the coated material. The
  coated material then is extracted and
  analyzed to identify the components of the
Views of Denuders

 Annular denuders
                    Honeycomb denuders
Passive sampling of ambient gases
  • By use of small Ogawa passive
    – Consists of 2 filters in each sampler
    – Sampler will be open only when sampling
  • After sampling
    – Filters transferred back to laboratory
    – Filters are put into specific chemicals to
      dissolve/react with the ions from filters
    – Analysis is performed by IC or CE
Ogawa Passive Sampler

• 3 cm in length and 2 cm in

• collection filters are held
  between stainless steel screens

• End-caps have 25 holes to
  allow diffusion
Exploded view of

Impregnate with
suitable reagent,
e.g. citric acid or
boric acid for
sampling NH3(g).

Usually sampling
time is 1 week
 Principle of measurement of
       passive samplers
• Using Fick’s Law of diffusion

• CA = M / mB t

  where CA = ambient concentration
        M = mass uptake
        mB = sampling rate
        t = deployment time
   Passive sampling methodology
             for NH3(g)
• Coating filters in the
  atmospheric bag
• The filter was impregnated
  with 100 µl of 2% (w/v) boric
  acid-2% (v/v) glycerol in
  methanol. The sampler body
  was snapped into a pin-clip
• The completed badge was
  placed into the protective
• The bottle was placed into a
  plastic bag.
Sampling in Tat Chee Avenue
  Results for NH3 passive sampling
 • Roof of CityU                 •Tat Chee Avenue

Kadoorie Farm & Botanic Garden
             Real-time automatic gas
    give automatic data-logging, usually on 1-hour basis. Need calibration by zero
                    gas and standard gas. Usually fixed location.

•   CO analyzer
•   NOx analyzer
•   NH3 analyzer
•   O3 analyzer
•   NOy analyzer
•   H2S analyzer
•   SO2 analyzer
     Carbon monoxide analyzer
      NO/NO2/NOx Analyzer

a) NO + O3  NO2* + O2
b) NO2*  NO2 + h1100 nm
  •   NOx Analyzer uses chemiluminescence to
      detect the target gas
      1. NO is reacted with ozone to generate excited
      2. Excess energy given off by releasing a photon
      3. Resulting photons are detected by a
         photomultiplier tube
          NO/NO2/NOx analyzer,1055,10120715,00.html


    Sample (NO)

                                        Reaction Cell

             O3 Generator

Continuous monitoring of ambient NOx by
 Nitric oxide (NO) reacts with O3 to produce excited NO2 which goes
back to ground state by emission of a photon at about 1100 nm –
        NO + O3                   NO2* + O2
        NO2*                      NO2 + h

          M = N2, O2, H2O, etc

 Chemiluminescent intensity I  [NO][O3] / [M]
[M] is a function of total pressure and at a fixed pressure, [M] is constant.
Thus, if O3 is made to be in large excess  [O3] becomes constant
                                            I  [NO]
Continuous monitoring of ambient NOx by
 Thus, chemiluminescence method is able to measure level of NO. To
measure level of NOx as a whole, NO2 in the ambient air has to be
converted into NO before reaction with O3.
 NO2  NO conversion can be done by:
       Thermal converter – conversion at 600 – 800 ºC
       Chemical converter – use Mo or C as catalyst; temp. 200 – 400 ºC
 Chemiluminescence method is very sensitive (MDL  0.01 ppm) and is
a continuous monitoring method
 NH3, PAN, organic amines, nitrates and nitriles in ambient air may
interfere the measurement as they may decompose to NO in the thermal
  Continuous monitoring of ambient NOx by
                O3 generator                                     O3 generator
 Air dryer         3-way valve       Reaction       Air dryer    Balance valve          chambers

Sample inlet                         PMT        Sample inlet                             PMT

                NO2 converter                                   NO2 converter
  Pump                                             Pump

                                                               Scrubber (O 3 and NO2)
               Scrubber (O3 & NO2)

   Cyclic mode – measure NO and                    Dual mode – two reaction chambers,
   NOx alternatively every 30 s                    one for NO, another for NOx, an
                                                   optical chopper is need to guide the
                                                   PMT to measure these chambers
 NH3 automatic gas analyzer

• NH3 analyzer also uses
  chemiluminescence to detect
  atmospheric ammonia
  – by converting ammonia into NO which is then

    • using two converters: a molybdenum
      converter at 315 oC and a thermal/catalytic
      converter at 800 oC
                             NH3 analyzer

                Thermal             Converter
                                       (315 oC )
                (800 oC)


                                                   Reaction Cell

                                 O3 Generator

Thermal: converts NO2, NH3
into NO
Mo: converts NO2 into NO
               Ozone analyzer

•UV photometer determines ozone concentration by
measuring the attenuation of light due to ozone in the
absorption cell.
•. Absorption wavelength is 254 nm.
•. The concentration of ozone is directly related to the

         WHAT IS NOy?
• NOy is not the sum of NO and NO2
• NOy is the sum of all reactive oxides of
  nitrogen, including NO and NO2
• NOy is equal to NO + NO2 + NO3 +
  2N2O5 + HONO + HNO3 + PAN + organic
  nitrates + etc.
• NOy does not include NH3, N2O, or N2
• Evaluation of photochemical models
• Emission inventory verification
• Understanding of tropospheric chemistry
  reaction pathways
• Acid deposition/transport studies
• Stratospheric ozone depletion
    NOy                 NO
    NO + O3             NO2*
    NO2*                NO2 + light
    Intensity of emitted light is proportional
    to NOy concentration

• Adaptation of traditional chemiluminescence
• Simplified measurement scheme
• Typically very low concentration levels
  – 0.1 to 10 ppb
• Quantitative conversion is difficult
• Highly reactive/surface losses cause
• Chemiluminescence from organic
  interferents at low NOy levels may be
       Continuous H2S analyzer
SO2 gas analyzer,1055,10120716,00.html
Monitoring of secondary pollutants in ambient
 Secondary pollutants refer to pollutants which are produced as a result
of chemical reactions of primary gaseous pollutants within the atmosphere
 They can be classified into three groups:
        Oxidants – oxidizing 2nd pollutants that are able to oxidize
        potassium iodide; e.g. O3, PAN, NO2
        Organic 2nd pollutants – all partially oxidized hydrocarbons, e.g.
        alcohols, aldehydes, organic acids, peroxides and organo-nitro
        Inorganic oxyacids – aerosols of HNO2, HNO3 and H2SO4;
        HNO3 can also be present as vapour in ambient atmosphere
Monitoring of total oxidants in ambient air
 Ozone (O3) is the major component of “total oxidants” in ambient air
 The most widely used reference compound for total oxidants is KI

       O3 + 3KI + H2O          KI3 + 2KOH + O2

 I3- can be measured spectrophotometrically at 352 nm
 Reductants in ambient atmosphere, e.g. SO2 and H2S, cause
negative interference to the determination
Monitoring of airborne asbestos
 Health hazard of airborne asbestos (crocidolite, “blue asbestos”):
           Asbestosis – fibrosis of the lungs

           Mesothelioma – tumor of the pleura or peritoneum

           Lung cancer – as a consequence of lung fibrosis
 Monitoring method – sampling by filtration using membrane filter (0.8
m pore size) and counting of asbestos fibres (length > 5 m) collected
under light microscope.
 The sampling area is divided into random fields. Level of asbestos
pollution is determined by TEM.

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