Lec. 29: Human Effects: Air Pollution & Heat Islands (Ch 14)

• Atmospheric pollutants

• Atmospheric controls on air pollution

• Urban heat islands http://www3.gov.ab.ca/env/air/index.html

Science & air pollutionScience & air pollution

Science & air pollutionScience & air pollution

Science & air pollutionScience & air pollution

In air: by “resolved” wind

“Turbulent diffusion” (same as “turbulent dispersion”) is name given to mixing due to the unresolved scales of motion, ie. the turbulent component of the wind

3. Responses/consequences • deposition to soil, plant, ocean, animal…

Particulates soot, pollen, etc. and (eg.) asbestos... strongly affect visibility, so most noticeable. Classified by size: eg. PM10 (diameter < 10 m) remains suspended and can enter lungs… PM2.5 (“fine particles”)

Sources: natural fires, eruptions, breaking ocean waves, wind-entrained dust** and pollen. Secondary particulates formed by coagulation of gases.

Small terminal velocities (slow “gravitational settling”) imply long residence time – precipitation the most effective removal process (aerosols are the CCN at heart of precipitates; and “scavenging” by falling droplets removes other particulates

**soil erosion – dust dislodged by wind stress on surface in combination with bombardment by “saltating” particles in the “saltation layer” above which is the suspension layer• soil condition (moisture, friability) plays a role• leaving stubble strips to shelter the soil – means to reduce wind

stress on the soil (wind “pulls on” the straws instead)

Human effects – shelter – Human effects – shelter – controlling the environment controlling the environment to mitigate soil erosion to mitigate soil erosion

Human effects – shelter researchHuman effects – shelter research

-10 -5 0 5 10 15

x/h

0

0.2

0.4

0.6

0.8

1

1.2

S/S

0

Wind fromwesteast

observations

Human effects – shelter researchHuman effects – shelter research

-4 0 4 8 12 16 20

x/H

0.4

0.6

0.8

1

S/S

0

Influence of unstable stratification

L = - 5 m

neutral

Lines - computations

Influence of windbreak is “felt” as a mean momentum sink...Influence of windbreak is “felt” as a mean momentum sink...

U-mtm :U-mtm :

uu22::

H)δ(x,0)s(z, u |U| 4k '2r

x

uwua

zz

uwua

x

kuc

z

Uwu

x

Uu

z

uwauW

zx

uuauU

x

tt

tt

2'2'

2'112'2'

2'2'2'

2'2'

''''

3

2

3

2''22

H)δ(x,0)s(z,|U|Uk r

x

PwuUW

zUU

x ou

1''2

Windbreak sink kr U2 … localised by delta-function at x=0, step function at z=H

horiz.U-mtm.fluxhoriz.U-mtm.flux

horiz.fluxhoriz.flux shear pro.shear pro. redistr.redistr.

and as a turbulent kinetic energy sink… eg. in a 2nd-order closure:and as a turbulent kinetic energy sink… eg. in a 2nd-order closure:

Basis of computed winds…Basis of computed winds…

heuristic

vert.U-mtm.fluxvert.U-mtm.flux

(This is just to give you a glimpse of what the govering equations look like)

“emissions of the railway lines are dominated by ‘iron’ particles, which contribute 67% to the railway related PM10. The ‘iron’ class particles were appointed to the wear of tracks, as well as wheels and breaks of the trains. In addition, ‘aluminium’ and ‘calcium’ particles assigned to abrasion of the gravel bed and re-suspension of mineral dust contribute to the PM10 load: 23% for the ‘aluminium’ and 10% for the ‘calcium’ particles.” (Lorenzo et al., Atmospheric Environment Vol. 40, 2006)

iron

Zurich; 700 trains/day on line studied

Carbon Monoxide (CO) from incomplete combustion of carbon fuels; colourless, odourless, poisonous

Sources: fires, eruptions… but taken up by soil micro-organisms so background levels low. Key anthopogenetic source: vehicles

Sulphur oxide (SOx ) gases… many natural sources but background levels low. SO2 the key primary sulphur gas, from burning sulphur-bearing fossil fuels. Colourless but smells, can cause respiratory problems. SO3 typically a secondary gas; in moist air forms sulfuric acid acid rain

Fig. 14-3

Volatile Organic Compounds (VOC’s or VOx) hydrocarbons, eg. methane (natural). Some carcinogenic. Involved in photo-chemistry of smog

Nitrogen Oxides (NOx) Nitric oxide (NO) non-toxic, colourless, odourless… formed naturally in soil, and this is balanced by natural sinks. Anthropogenic NO associated with combustion, produces secondary NO2 (nitrogen dioxide) …

NO2 toxic & corrosive… gives polluted air its yellow/red/brown colour. In moist air reacts to form nitric acid acid rain

Ozone (O3) secondary pollutant formed when sunlight acts on other pollutants. Smells, irritates. Main ingredient of photochemical smog.

Fig. 14-5

Anthropogenic pollutant sources in the U.S.Anthropogenic pollutant sources in the U.S.

Fig. 14-1

(other than vehicles – eg. energy generation from coal, oil, gas…)

Anthropogenic pollutant sources in the U.S.Anthropogenic pollutant sources in the U.S.

Fig. 14-1

Atmospheric controls on air pollutionAtmospheric controls on air pollution

• wind can control or modulate source strength (eg. dust; surface flux of methane off swine lagoons;…

• wind is the transport and mixing agent

• temperature affects reaction rates; stratification influences the vertical wind, ie. mixing

• mixed layer is the short-term sink for pollutants

Fig. 14-6

• plume dilution • assumes source const.• concentration 1/u

From “Meteorology & Atomic Energy”

• elevation of plume means locally no pollution

• potential for severe local pollution

• high levels intermittently

““Box model” of urban air pollutionBox model” of urban air pollution

Contours of mean summer maximum (afternoon) mixing depths h (unit: 100 m)

• from Portelli, 1977

• Edmonton, winter, < 300 m

Urban heat islandUrban heat island

Fig. 14-9a

9 pm LST July 4, 1972(temperature near ground)

Fig. 14-9a

Maximal late evening and night, and during winter. Causes:

• reduced albedo (multiple reflections)• high heat capacity of ashphalt, concrete implies downwelling longwave• precip rapidly drained, dry surfaces low QE results in high QH

• anthropogenic heat