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.
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.
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.
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 the bulb was). The UV light and oxygen in the air produced a lot of ozone (you could have easily 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 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.
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)
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.
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.
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.
 |
 1 - trachea
from http://en.wikipedia.org/wiki/Image:Illu_quiz_lung05.jpg2 - mainstem bronchus 3 - lobar bronchus 4 -
segmental bronchi
5 - bronchiole6 - alveolar duct 7 - alveolus |
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).
_____
Satellite photograph taken Thursday
evening (with the new GOES16 satellite) showing smoke
from wildfires burning in Washington, Oregon, Idaho
and Montana being carried across much of the
continental US. Smoke from these
fires has made it into southern Arizona and has, at
times, had a noticeable effect on visibility.
 |
 |
| Photograph taken last
Saturday (Aug. 26) when the air was free of
smoke and visibility was pretty good. |
Photograph taken Tuesday
this week (Aug. 29) when smoke from the fires
in the NW was present. There has been a
noticeable drop in visibility.The camera was
tilted down slightly in this picture but the
field of view is the same as the other
photograph. |