METO 637

LESSON 18

Schematic of ozone production from VOC

SMOG

• NEEDS• Hydrocarbons and nitrogen oxides• Strong sunlight to start reactions• Warm temperatures to maintain reactions – the higher

the temperature the faster the rate.• Peak ozone will be close to peak temperature –

afternoon

Standards

• Ozone: 1-hour 125 ppbv*, 8-hour 85 ppbv– The 8-hour standard is much more stringent, and

encompasses many areas where transport is the only issue (e.g Shenandoah National Park)

• PM2.5: daily 65 g/m3, yearly 15 g/m3

– Most areas will have trouble only with the annual standard

• Visibility: a 60+ year glide path back to “natural conditions”

*parts per billion by volume

Ozone isopleths (NMHC vs NOx)

Ozone vs NOx for NHMC=0.6 ppmc

Daily Ozone Cycle

Sunrise Sunset

Time of day

Ozone Concentration

Ozone productionfollows a dailycycle with maximumconcentrations typically observedin the late afternoon.

This cycle is a signature of the dynamic processes of atmospheric air pollution

Comparison of ozone data at Fort Meade for August 2 and 8 2002

Overplot of 2 and 8 Aug 2002 and the difference between the two days

Difference 2 Aug minus 8 Aug*1.2

Back Trajectories, Hysplit

AUG 2, 2002 AUG 8, 2002

Comparison of Aug 2 and 8, 2002

• Ozone data for August 8 is typical for local pollution on a clear warm day.

• The NOx and VOC are emitted early in the morning and the ozone amount slowly increases as the temperature increases. The peak production is at about 3-4 in the afternoon when the temperature at the ground is a maximum.

• The back trajectory shows fast upper level winds, which start at a high altitude and then subside to boundary levels at Baltimore.

• Small probability of upper air being polluted.

Comparison of Aug 2 and 8, 2002• On the 2nd of August the back trajectories show that the

air is moving slowly at the boundary layer, and the probability of this air being polluted is high.

• The nocturnal inversion typically breaks down at about 10-11 in the morning.

• Hence the peak in ozone at this time must come from downward transport.

• The overall shape of the ozone data on Aug 2 is a combination of locally produced ozone peaking at about 3 pm and a downward movement of ozone from above at about 10.00 am.

• This ozone above the boundary layer is yesterday’s ozone

• The winds above the boundary layer are usually high. Hence the ozone has been transported some distance

Natural levels of Acidity in Rain• Carbon dioxide dissolves in the rain drop

CO2(g) + H2O(aq) ↔ H2CO3(aq)

• Henry’s Law states that

[H2CO3] = KHP(CO2)

• KH = 3.4E-2 M ATM-1

• In the liquid some of the H2CO3 ionizes

H2CO3(aq) ↔ H+(aq) + HCO3-(aq)

• This reaction has an equilibrium constant of 4.2E-7 M-3

• For the overall reaction

CO2(g) + H2O(aq) ↔ H+(aq) + HCO3(aq)

The equilibrium constant Kc is 1.43E-8 M2.ATM-1

Natural levels of Acidity in Rain

)(

]][[ where

2

3

COP

HCOHKc

• in a liquid [H+] = [HCO3-] hence

[H+]2 = KcP[CO2]

given that CO2 has a mixing ratio of 320 ppm we get

[H+] = 2.14E-6 and a pH of 5.67

Dispersion of acid rain

pH of precipitation over the US

Acid rain

• Acid rain over the Eastern States is the highest – most of the sulfur containing coal occurs in this region

• Shaded areas in the figure show where granite is found.

• If the soil/rocks are carbonates (chalk, limestone) then the acid rain can be neutralized, and does not change the pH of the streams and lakes

• If the soil/rocks are granite then acid rain is not neutralized, and can also leach out the heavy metals. Thus the pH of the lakes and streams can be lowered, and the heavy metal concentration raised.

Sources of pollutants in the USA

Sulfur Dioxide emissions

67%

17%

4%

3%2%

7%

Fuel Combustion- Electric Utility

Fuel Combustion- Industrial

Fuel Combustion- Other

MetalsProcessing

Non-RoadVehicles

All Other

Annual mean haze, United States

IMPROVE network

Flight area

Detection Techniques

• In situ via UV pulsed fluorescence.– Modified commercial instrument– Detection limit ~70 ppt ()– Response time ~3 min

• Satellite via UV (315 to 327 nm ) spectroscopy

(modified DOAS)

Median (25% 75%) Mixing Ratio (ppb)

650

700

750

800

850

900

950

1000

1050

0 1 2 3 4 5 6

SO2 over the Eastern USSummer, 2002 (over 100 profiles)

Pre

ssu

re (

hP

a) Scale height ~1.1 km