Thu., Sep. 1 notes

Good (stratospheric) and bad (tropospheric) ozone. Easy & harder ways of making ozone.

We'll first turn our attention to ozone. Ozone has a kind of Dr. Jekyll and Mr Hyde personality.

The figure above can be found on p. 14a in the photocopied ClassNotes. The ozone layer (ozone in the stratosphere) is beneficial, it absorbs dangerous high energy ultraviolet light (which would otherwise reach the ground and cause skin cancer, cataracts, and actually there are some forms of UV light that would quite simply kill us).

Ozone in the troposphere is bad, it is toxic and a pollutant. Tropospheric ozone is also a key component of photochemical smog (also known as Los Angeles-type smog)

We'll be making some photochemical smog in a class demonstration. To do this we'll first need some ozone; we'll make use of the simple stratospheric recipe (shown above) for making what we need instead of the more complex tropospheric process (the 4-step process in the figure below). You'll find more details a little further down in the notes.

At the top of this figure (p. 15 in the packet of ClassNotes) you see that a more complex series of reactions is responsible for the production of tropospheric ozone. The production of tropospheric ozone begins with nitric oxide (NO). NO is produced when nitrogen and oxygen in air are heated (in an automobile engine for example) and react.

The NO can then react with oxygen in the air to make nitrogen dioxide, the poisonous brown-colored gas that I used to make in class.

Sunlight can dissociate (split) the nitrogen dioxide molecule producing atomic oxygen (O) and NO. O and O₂ react in a 4th step to make ozone (O₃) just like happens in the stratosphere. Because ozone does not come directly from an automobile tailpipe or factory chimney, but only shows up after a series of reactions in the air, it is a secondary pollutant. Nitric oxide (NO) would be the primary pollutant in this example.

NO is produced early in the day (during the morning rush hour). The concentration of NO₂ peaks somewhat later. Because sunlight is needed in step #3 and because sunlight is usually most intense at noon, the highest ozone concentrations are usually found in the afternoon. Ozone concentrations are also usually higher in the summer when the sunlight is more intense than at other times of year.

Once ozone is formed, the ozone can react with a hydrocarbon of some kind to make a product gas. The ozone, hydrocarbon, and product gas are all invisible, but the product gas sometimes condenses to make a visible smog cloud or haze. The cloud is composed of very small droplets or solid particles. They're too small to be seen but they are able to scatter light - that's why you can see the cloud. This is similar to invisible water vapor condensing to form a visible cloud composed of water droplets or ice crystals.

Photochemical smog demonstration
Here's a pictorial summary of the photochemical smog demonstration.

We started by putting a small "mercury vapor" lamp inside a flash. The bulb produces a lot of ultraviolet light (the bulb produced a dim bluish light that we could see, but the UV light is invisible so we had no way of really telling how bright it was). The UV light and oxygen in the air produced a lot of ozone (you could easily have smelled it if you had taken the cover off the flask).

After a few minutes we turned off the lamp and put a few pieces of lemon peel into the flash. Part of the smell of lemon is limonene, a hydrocarbon. The limonene gas reacted with the ozone to produce a product gas of some kind. The product gas condensed, producing a visible smog cloud (the cloud was white, not brown as shown above). We shined the laser beam through the smog cloud to reinforce the idea that we are seeing the cloud because the drops or particles scatter light.

Here's a video that I found of a slightly different version of the demonstration (you really don't miss much if you don't come to class). Instead of using UV light to produce the ozone the demonstration uses an electrical discharge (the discharge travels from the copper coil inside the flask to the aluminum foil wrapped around the outside of the flask). The overall effect is the same. The discharge splits an oxygen molecule O₂ into two oxygen atoms.

O₂ + spark ---> O + O

One of the oxygen atoms reacts with an oxygen molecule to form O₃

O + O₂ ---> O₃

The smog cloud produced in the video is a little thicker than the one produced in class. I suspect that is because they first filled the flask with pure oxygen, 100% oxygen, before making the ozone. I used air in the room which is 20% oxygen. More oxygen in the flask means more ozone and a thicker cloud of Los Angeles type smog.

Back to our summary list

Particulate matter (PM)

The last pollutant that we will cover is Particulate Matter (PM). This is small solid particles or drops of liquid, not gases, that remain suspended in the air.

Carbon monoxide (CO), O₃ , and Particulate Matter are the three main pollutants of concern in Tucson. PM is a year round problem in Tucson.

PM pollution is often split into two groups: PM10 and PM2.5. These refer to particles with diameters less than 10 micrometers and 2.5 micrometers, respectively. A micrometer (µm) is one millionth of a meter (10^-6 m). You'll find examples of metric distances ranging from kilometers to nanometers at this interesting site.

Sizes (in µm) of some common items are sketched above. Better than sketches are some actual photographs. The particles are so small they need to be examined using a microscope.

Photographs of micrometer and 10s of micrometer size objects

Electron microscope photograph of human red blood cells..
Individual cells in this example are a little over 5 um in diameter.
This is not something you'd find in the atmosphere.
(image source: Dartmouth College Electron Microscope Facility)

This is something that is commonly found in the air. This is a photograph of a mixture of different types of pollen.
The largest pollen grain comes from morning glory (I think) and is about 100 um in diameter
(image source: Dartmouth College Electron Microscope Facility)

Scanning electron microscope photograph of volcanic ash
(USGS image by A.M. Sarna-Wojcick from this source)

Airborne particulate matter collected on the surface of a tree leaf (source). These particles are pretty small with diameters of 1 to 2 µm.
According to the source, trees capture appreciable amounts of particulate matter and remove it from the air in urban areas.

Sources of particulate matter
Particulate matter can be produced naturally (wind blown dust, clouds above volcanic eruptions, smoke from lightning-caused forest and brush fires). Many human activities also produce particulates (automobile exhaust for example). Gases sometimes react in the atmosphere to make small drops or particles (this is what happened in the photochemical smog demonstration). Just the smallest, weakest gust of wind is enough to keep these small particles suspended in the atmosphere.

A recent study estimates that more than 3.2 million people die each year across the globe because of exposure to unhealthy levels of PM25 (click here to see a summary and some discussion of the study and here to see the study itself). The study also attempted to determine the sources of the PM25 pollution. The figure below summarizes their findings.

Information like this is important because you need to know what is adding particulate matter to the air if you want to try and reduce emissions.

Note the PM10 annual National Ambient Air Quality Standard (NAAQS) value of 50 micrograms/cubic meter (µg/m³) at the bottom of p. 13c in the photocopied ClassNotes.

The following list (p. 13d in the ClassNotes) shows that there are several cities (in bold font) around the world where PM concentrations are 2 or 3 times higher than the NAAQS value.

Effects of PM on health
One of the main concerns with particulate pollution is that the small particles might be a health hazard ( a health advisory is sometimes issued during windy and dusty conditions in Tucson).

Particles with dimensions of 10 µm and less can be inhaled into the lungs (larger particles get caught in the nasal passages). These inhaled particles may be poisonous, might cause cancer, damage lung tissue, or aggravate existing respiratory diseases. The smallest particles can pass through the lungs and get into the blood stream (just as oxygen does) and damage other organs in the body.

The figure below identifies some of the parts of the human lung mentioned above. The key point is that the passageways get smaller and smaller the deeper you move into the lungs. The smallest particles are the most dangerous because they can penetrate furthest into the lungs.

Crossectional view of the human lungs
from: http://en.wikipedia.org/wiki/Lung

1 - trachea
2 - mainstem bronchus
3 - lobar bronchus

4 - segmental bronchi

5 - bronchiole
6 - alveolar duct
7 - alveolus

from http://en.wikipedia.org/wiki/Image:Illu_quiz_lung05.jpg

The 2008 Summer Olympics were held in Beijing and there was some concern that the polluted air would affect the athletes performance. Chinese authorities restricted transportation and industrial activities before and during the games in an attempt to reduce pollutant concentrations. Rainy weather during the games may have done the greatest amount of good.

Clouds and precipitation are the best way of cleaning pollutants from the air. We'll learn later in the semester that cloud droplets form on small particles in the air called condensation nuclei. The cloud droplets then form raindrops and fall to the ground carrying the particles with them.

The second main concern with particulates is the effect they may have on visibility (esthetics below should actually be spelled aesthetics - i.e. qualities that might make something appear beautiful or not).

Here's a view of the Catalina mountains taken from the Gould Simpson Building on the south side of campus.

Some rainy weather had occurred just a day to two earlier, cleaned the air, and the visibility was very good. Clouds and rain have done a really good job of cleaning the air.

Windy weather a few days later stirred up a lot of dust that was carried into town.

This picture was taken the day after the windy weather. There is still a lot of fine dust particles in the air and the visibility is pretty bad.

We looked at some photographs from Beijing (January, 2013) very particulate pollution can be quite severe. Here are some pictures from Harbin, China (October, 2013). That's about as bad as visibility can get, visibility in some cases is just a few 10s of feet. The problem is limited to China, here's a picture from Paris (March, 2014).