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Lecture ThreeLecture ThreeAtmospheric Dispersion

and Air Quality Monitoring

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LearningObjectivesfor Today Educational ObjectivesEducational Objectives

What is Atmospheric Dispersion

Factors affecting atmospheric dispersion and transport – Wind and atmospheric stability.

Air quality monitoring

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The atmosphere as a sink The atmosphere as a sink • Volcanoes and pollutants from other natural

sources• Pollutants from human activities

– From the discovery of the utility of fire to the present

• Imperfect sink• Limited ability to disperse and remove

pollutants– Affected by atmospheric motion phenomena

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The atmosphere as a sink The atmosphere as a sink • Medium for chemical reactions that may

remove pollutants• Buildup of long lived pollutants

– Methane– Nitrous oxide– Carbon dioxide– Halocarbons

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Atmospheric Dispersion and Atmospheric Dispersion and TransportTransport

• Dispersion – mixing of pollutants with air

• Transport - air motions carry pollutants from one region of the atmosphere to another

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Atmospheric DispersionAtmospheric Dispersion

• Air pollutants are dispersed in the atmosphere in both horizontal and vertical directions.

• Winds and the atmospheric stability are important factors on the degree of dispersion.

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Dispersion and transportDispersion and transport• Pollutants released into planetary boundary

layer(PBL)– Subject to fluxes of heat and water vapor– One to 2 km thick

• Air above the PBL is the free troposphere– Is relatively stable

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Dispersion and transportDispersion and transport• Atmospheric/environmental conditions that

affect pollutant concentrations– Horizontal wind– Turbulence– Topography– Atmospheric stability– Inversions

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Atmospheric DispersionAtmospheric Dispersion• If emission is constant from a source, the

concentration is an inverse function of the wind speed. (Thus doubling wind speed will halve the pollutant concentration.)

• Wind direction is also important as it affects the concentration of pollutant at downwind location.

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WindWind• Characterized by speed and direction• Affected by:

– Horizontal pressure gradients– Horizontal temperature gradients– Friction related to surface roughness

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Atmospheric Dispersion and Atmospheric Dispersion and TransportTransport

• Horizontal wind speed is affected by friction which is proportional to surface roughness

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Atmospheric stabilityAtmospheric stability• Vertical temperature gradients

– Described as lapse rates– Normally temperature decreases with height

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Super-adiabatic lapse ratesSuper-adiabatic lapse rates• Temperature decreases are greater than –10

o C/ kilometer (-1˚C/100 meters)• Occur on sunny days• Characterized by intense vertical mixing• Excellent dispersion conditions• The atmosphere is unstable

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Neutral lapse ratesNeutral lapse rates

• Temperature decreases are similar to the adiabatic lapse rate (-1˚C/100 meters)

• Results from:– Cloudy conditions– Elevated wind speeds– Day/night transitions

• Describes good dispersion conditions

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Isothermal lapse ratesIsothermal lapse rates

• Characterized by no temperature change with height

• Atmosphere is somewhat stable• Dispersion conditions are moderate

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Inversion lapse ratesInversion lapse rates

• Characterized by increasing temperature with height

• May occur at night• Associated with high-pressure systems• Dispersion conditions are poor• The atmosphere is stable

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Effects of topographyEffects of topography• Intensify nocturnal inversions in river

valleys• Reduces dispersion along coastal areas on

calm days– Results from lake/sea/land breezes

• Restricts horizontal air flows• May affect microscale air flow around point

sources

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Topography related windTopography related wind

Adapted from The Atmosphere, Eighth edition, by F. Lutgens and Tarbuck, Prentice Hall

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Mountain-valley windMountain-valley wind

Adapted from The Atmosphere, Eighth edition, by F. Lutgens and Tarbuck, Prentice Hall

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Dispersion from point sourcesDispersion from point sources• Pollutants emitted in plume form• Plume history depends on:

– Physical/chemical nature of pollutants– Meteorological factors– Location of source relative to potential

obstructions– Topography

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Plume rise and transportPlume rise and transport• Affected by:

– Physical stack height (h)– Plume rise (Δh)– The effective stack height (H = h + Δh)

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Dispersion of a point sourceDispersion of a point source

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Plume rise and transportPlume rise and transport• Effective stack height

– Dispersion enhanced with increasing height– Can be increased by building taller stacks– Higher wind speed can decrease effective stack

height– Can be increased by higher stack gas temperatures

and exit velocity

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Long range transportLong range transport• Large-scale wind can transport pollution to a long

distance (hundreds/thousands of miles).

– Transport of Asian dusts to North America in April 2001

– Westerly winds transport Asia pollution across the Pacific Ocean

– Transport of air pollution from the Pearl River Delta to Hong Kong

25Results from the RAMS model for March 27th and 29th 1997. The pink “cloud” shows the region with the tracer concentration (SO2) greater than 500 pptv.

Air-quality monitoring• to determine compliance status relative to air quality

standards/objectives

• to determine baseline levels before control programs

• to determine progress toward compliance after control programs

• Others uses: epidemiological studies, long-term trend, atmospheric research etc.

Aspects of air-quality monitoring• Sampling: bring air sample to collection medium

or a sensor• consideration: enough materials collected or

enough sensitivity for a sensor

• Sampling train may include a vacuum pump, flow regulator, and a collecting device or a sensing unit.

• Sampling methods: static, grab, intermittent, and continuous

Sampling principles

• Absorption (dissolve sample air into a liquid)– e.g., SO2

• Adsorption (collect sample on a solid surface)– e.g., air toxic

• Condensation (freeze out sample)– e.g., hydrocarbon

Analysis methods• Wet chemistry• Ultraviolet absorption

– absorption of UV, O3

• Infrared – absorption of infrared light, CO, CO2

• Chemiluminescence – emission of light a as result of chemical reactions, e.g., NO

• Electrochemistry – electric current from chemical reactions, e.g., ozonesonde sensor

• Chromatography – mainly used for separation and detection of organic compounds.

The instrument is mainly used in laboratory.

O3 analysis method (UV absorption)

I=Io exp(-a[O3])

CO analysis method (Infrared)

SO2 analysis method (UV fluorescence)

SO2 + hv (214 nm)->SO2*

SO2* -> SO2 + hv (340 nm)

NOx analysis method (Chemiluminescence)

NO + O3 -> NO2*

NO2* -> NO2 +hr

Particulate Matter (PM)

-collect particles on a filter mediumHi-Vol sampler

Paper tape samplerSize selective sampler

Calibration and Quality Assurance

• Calibration- measured signal are compared against standard (known) values.– Standards:

• permeation tube• gas bottles• UV ozone generator

– Zero air: pollutant free air

• Continuous gas analyzer- calibration involves the use of gas mixture and dynamic calibration system.

The Hong Kong Polytechnic University Research

Guangzhou

Macau

The location of the Tai O Site in the Pearl River delta

Hong Kong

Tai O

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M300

43S

42S

Inlet

Manifold

Catalyst Box

Roof Top

Inlet

VOC

IMPROVE

NephInlet

PSAP

Inlet

Measurement

O3

CO

SO2

NONOy

Bscat

Bap

PM2.5

VOC

Canister

The Tai O Supersite for Studying Ozone Pollution

Sampling inlet, NOy inlet, wind, temp, and RH sensors

Ambient nephelometer

Real-time monitors

PSAP Aerosol absorption

VOC sampling pump

Solar radiation-global

Solar radiation-TUV

Transporting equipment/supplies to the field site

Accuracy and Precision

• Accuracy: relative closeness of a measured value to the true value

• Precision: a measure of a method’s reproducibility

Quality Assurance/Quality Control

• A comprehensive QA/QC program for a network of continuous analyzers might involve:– Daily automatic calibration checks of the analyzers– Site checks every one or two months– Regular analyzer servicing by trained instrument engineers– Regular intercalibrations (either taking several analyzers to the same

calibrator, or taking one calibrator around all the analyzers in succession)– Periodic checking of the calibrator against primary standards– Detailed manual and automatic scrutiny of the data to eliminate false

values caused by instrument malfunction or transmission faults. It is much better to have holes in the data set than to have spurious values

– Comprehensive review of period data sets– Telemetry of the data to the processing centre, so that faults can be

spotted as soon as they occur

Hong Kong Air Quality Objectives (Unit: µg/m3)

Air Pollutant 1-hour [1] 8-hour [2] 24-hour [2] 3-month [2] 1-year

SO2 800 -- 350 -- 80

NO2 300 -- 150 -- 80

CO 30000 10000 -- -- --

O3 240 -- -- -- --

TSP -- -- 260 -- 80

RSP -- -- 180 -- 55

Lead -- -- -- 1.5 --

Note: Not to be exceeded more than three times per year

Not to be exceeded more than once per year

Characteristics of air quality monitoring stations in HK

Central/Western

Urban: residential area

Sham Shui Po Urban: densely populated residential areas with commercial developments

Eastern

Urban: densely populated residential area

Kwun Tong Urban: densely populated residential areas with mixed

commercial/industrial developmentsKwai ChungTsuen WanTai Po

New town: residential areaSha TinTung ChungYuen Long New town: residential areas with fairly rapid

developmentTap Mun RuralCauseway Bay

Urban roadside with heavy trafficCentralMong Kok

Air Pollution Index• API is the conversion of the ambient RSP, SO2, CO, O3 and NO2

concentrations to a scale of 0 to 500.

• An index of 100 corresponds to the short-term HKAQO (1-24 hours)

• API is calculated by first computing the subindices of scale extending from 0 to 500 for each air pollutant measured at each air quality monitoring station based on the 1-hour, 8-hour and/or 24-hour average concentrations.

• API is subsequently calculated for each air quality monitoring station by taking the maximum of the subindices among all the parameters measured at that station to indicate the overall pollution level

API Conversion TableAPI

subindex level

Relationship with

HKAQO

Corresponding Concentrations (μg/m3)

RSP 24-hr

SO2 24-hr

SO2 1-hr

NO2 24-hr

NO2 1-hr

CO 8-hr

CO 1-hr

O3 1-hr

0 - 0 0 0 0 0 0 0 0

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50% Annual HKAQO/

25% HKAQO

28 40 200 40 75 2500 7500 60

50

Annual HKAQO/

50% HKAQO

55 80 400 80 150 5000 15000 120

100 HKAQO 180 350 800 150 300 10000 30000 240

200 - 350 800 1600 280 1130 17000 60000 400

300 - 420 1600 2400 565 2260 34000 90000 800

400 - 500 2100 3200 750 3000 46000 120000 100

500 - 600 2620 4000 940 3750 57000 150000 1200

API versus O3 concentration

0

100

200

300

400

500

600

0 200 400 600 800 1000 1200 1400

Ozone Conc. (ug/m3)

API

Legislation for Management of Air Quality in Hong Kong

• The Air Pollution Control Ordinance– Smoke regulation limit continuous dark smoke to three minutes– Fuel restriction limit sulfur content of liquid fuel to 0.5%– Prior approval needed before work on furnace, oven and chimney– Ban of open burning of construction waste, tyes and cables for

metal salvage– Tightened asbestos control– Prior notification of certain construction works– Installation of vapor recovery to reduce benzene at gas stations– Motor vehicle fuel limit sulfur in diesel less than 0.2% and further

down to 0.05%– Ban the sale of leaded petrol after April 1999– Restrict use of ozone-depleting substances