Wednesday, Sept. 4, 2019

We'll use page 14a, page 15, page 16, page 13c, page 23a, page 23b, page 23c, page 23d, page 24a from the ClassNotes today.
Good (stratospheric) and bad (tropospheric) ozone
Ozone has a kind of Dr. Jekyll and Mr Hyde personality.


The figure above can be found on page 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).

Here's a nice visual depiction of the role that stratospheric ozone plays.










Essentially all of the UV-C light, the most dangerous form, is absorbed by the ozone layer,  About 5% of the UV-B and perhaps 50% of the UV-A makes it to the ground. (Fig. source:
https://www.nasa.gov/larc/celebrate-world-ozone-day)
 The UV Index forecast for Tucson on Jan 10, 2019.  Available at https://www.epa.gov/sunsafety/uv-index-1. 
You can find explanation of the colors on the scale at https://www.epa.gov/sunsafety/uv-index-scale-0
 The UV index forecast for Tucson early in the Fall 2018 semester (Aug. 28).  Sunlight is much more intense in the summer.

Ozone in the troposphere (surface level ozone in the figure above) 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 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.



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 American Lung Association's 2019 State of the Air report mentions that 10 of the 25 cities with the highest tropospheric ozone concentrations are found in California (Los Angeles is at the top of the list).  Texas has 3 cities in the top 25, Colorado has two.  Arizona (Phoenix), Utah, and Nevada each have one city.    Here's a link to the full report (167 pages long).  Here's a shorter report with the lists of most polluted cities.  Here a list of the cities in the United States with the cleanest air.

Tucson exceeded the EPA NAAQS for ozone for the first time in August 2018. Phoenix had already exceeded the NAAQS 39 times that year.  Here's a link to the entire article.


The violation in Tucson, which could impact the availability of federal transportation funds, is partly because the allowed ozone concentration is lower than it used to be.  The 80 parts per billion (ppb) 8 hour average value was lowered to 75 ppb in 2008 and to 70 ppb in 2015.

Photochemical (LA-type) smog

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.

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 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 a 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.

Here's the nearly completed air pollutants list.


Volcanic smog (vog) and laze (lava and haze)

Volcanic smog can form when the sulfur dioxide (SO2) from an erupting volcano reacts with oxygen and moisture.  The product of the reaction can form small drops or particles that  begin to scatter light and lower the visibility. 



Diamond Head is just visible in this picture of volcanic smog in Honolulu.  This picture (Marco Garcia, AP) was published in the Seattle Times on May 6, 2008 (http://old.seattletimes.com/html/nationworld/2004394494_hawaii06.html).  Kiluaea was erupting at that time also.  The volcanic smog can kill crops and cause respiratory problems.  The vog can also spread to nearby  islands. 

An entirely different type of haze cloud is formed when hot molten lava pours into the ocean as shown in the photo below (photo credit: Brocken Inaglory in https://en.wikipedia.org/wiki/Laze_(geology) ). 



In this case the hot lava splits water molecules into hydrogen and oxygen ions.  The hydrogen ions react with chlorine ions from dissociated salt (sodium chlorida NaCl splits into Na+ and Cl- ions) in the ocean water.  The sudden cooling of lave creates small fragments of glass.  You are left with a haze cloud consisting of small droplets of  hydrochloric acid and particles of glass.  The dangerous haze cloud can cause skin and eye irritation, lung damage, even death.




Particulate matter (PM)

The last pollutant that we will cover is Particulate Matter (PM).  PM refers to small solid particles or drops of liquid 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 site.  The following is on page13c in the ClassNotes.


Sizes (in µm) of some common items are sketched above. 

Effects of particulate matter on health


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

 
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

Better than sketches are some actual photographs.  Many of these particles are so small that they are invisible to the naked eye and 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 µm 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 µm 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.
You've probably heard or read about plastic pollution in the ocean.  Much smaller pieces or fibers of plastic are also becoming a serious air pollution concern (see https://www.theguardian.com/environment/2019/aug/14/microplastics-found-at-profuse-levels-in-snow-from-arctic-to-alps-contamination)

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).  Again, PM25 refers to particles with diameters of 2.5 micrometers (µm) or less; particles this small can penetrate deeply into the lungs.  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.




CBS news has ranked the 30 cities in the world with the most polluted air (based on World Health Organization data for 2016) (https://www.cbsnews.com/pictures/the-most-polluted-cities-in-the-world-ranked/ ).   The report is interesting because there is a photograph of each location and more detailed information about the source of the pollution.  Here is some of what was mentioned: sandstorms, vehicle exhaust, aluminum production, deforestation, burning waste, coal burning power plants, oil production, leather tanning, brick factories, chemical factories, burning tires to extract iron, steel mills.  Cities in China, India, Pakistan, Iran, and Saudi Arabia appear multiple times in the list.


This map shows where some of the most polluted places on earth are located (PM25 pollution) and comes from a World Health Organization report "Exposure to ambient air pollution from particulate matter for 2016" (http://www.who.int/airpollution/data/AAP_exposure_Apr2018_final.pdf?ua=1).

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. 
 


We will look at some photographs from Beijing (if that link doesn't work try this one) where 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) and India (November, 2017).

Last summer smoke from forest fires in British Columbia (Canada) was blown southward into Vancouver, Seattle, and Portland (and smaller cities in Washington and Oregon).  The air quality was for a few days as bad as you'll find in the most polluted cities in the world.  (see "Wildfire Smoke Makes Seattle and Portland World's Dirtiest Cities" published online by National Geographic (https://www.nationalgeographic.com/environment/2018/08/news-seattle-portland-dirtier-air-quality-than-parts-of-asia/).

Satellite photograph of smoke from fires burning in British Columbia.
(source: https://www.cbc.ca/news/technology/bc-fires-satellite-1.4789298)





These two photograph of the smoke in Seattle (left) and Portland (right) are from the National Geographic article referenced above.


This photograph is from an updated report published on Aug. 22, 2018 by the Vancouver Sun
(https://vancouversun.com/news/local-news/b-c-wildfires-2018-air-quality-in-vanderhoof-double-hazardous-levels)

Smoke from fires in California will often be seen in Tucson.  Smoke from Canada and the Pacific Northwest does also sometimes move into our area.



Satellite photograph taken early in the Fall 2017 semester (with the new GOES16 satellite) showing smoke from wildfires burning in Washington, Oregon, Idaho and Montana being carried across much of the continental US (Hurricane Harvey is also shown).  Smoke from these fires made it into southern Arizona where, at times, it had a noticeable effect on visibility.
Photograph taken Saturday Aug. 26, 2017 when the air was free of smoke and visibility was pretty good.
 Photograph taken Tuesday on Aug. 29, 2017 when smoke from the fires in the Pacific northwest 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.



Abrupt change to another completely different topic.There is a good chance that we will not get through all of this today.


Mass, weight, density, and pressure.

Pressure is a pretty important concept, that's what we'll start on next.   Differences in atmospheric pressure create winds which can then cause storms.  To better understand pressure we need to first review mass and weight.

Weight is something you can feel.  I'll pass an iron bar around in class (it's sketched below) - lift it and try to guess it's weight.  The fact that it is a 1" by 1" is significant.  More about the bar later.
I used to pass around a couple of small plastic bottles (see below).  One contained some water, the other an equal volume of mercury (here's the source of the nice photo of liquid mercury below at right).  I wanted you to appreciate how much heavier and denser mercury is compared to water.   But the plastic bottles have a way of getting brittle with time and if the mercury were to spill in the classroom the hazardous material people would need to come in and clean it up.  That would probably take a few days, would be very expensive, and I would get into a lot of trouble.  So this semester I'll pass around a smaller, much safer, sample of mercury so that you can at least see what mercury it looks like (it's a recent purchase from a company in London).  I'll keep the plastic bottles of mercury up at the front of the room just in case you want to see how heavy the stuff is.


It isn't so much the liquid mercury that is a hazard, but rather the mercury vapor.  Mercury vapor is used in fluorescent bulbs (including the new energy efficient CFL bulbs) which is why they need to be disposed of properly (you shouldn't just throw them in the dumpster).  That is a topic that will come up again later in the class.  Mercury and bromine are the only two elements that are found naturally in liquid form.  All the other elements are either gases or solids.

I am hoping that you will understand and remember the following statement


atmospheric pressure at any level in the atmosphere
depends on (is determined by)
the weight of the air overhead


We'll first review the concepts of mass, weight, and density but understanding pressure is our main goal.  I've numbered the various sections  to help with organization.  There's a summary at the end of today's notes.

1. weight
This is a good place to start because this is something we are pretty familiar with.  We can feel weight and we routinely measure weight.
 
 


A person's weight also depends on something else.


In outer space away from the pull of the earth's gravity people are weightless.  Weight depends on the person and on the pull of gravity.


 



We measure weight all the time.  What units do we use?  Usually pounds, but sometimes ounces or maybe tons.  Students sometimes mention Newtons, those are metric units of weight (force).

2.mass
Rather than just saying the amount of something it is probably better to use the word mass




It would be possible to have equal volumes of different materials or the same total number of atoms or molecules of two different materials, and still have different masses.

Grams (g) and kilograms (kg) are commonly used units of mass (1 kg is 1000 g).  They're metric units (slugs are the units of mass in the English (American) system of units).

3.  gravitational acceleration
On the surface of the earth, weight is mass times a constant, g,  known as the gravitational acceleration.  The value of g is what tells us about the strength of gravity on the earth; it is determined by the size and mass of the earth.  On another planet the value of g would be different.  If you click here you'll find a little (actually a lot) more information about Newton's Law of Universal Gravitation.  You'll see how the value of g is determined and why it is called the gravitational acceleration.  These aren't details you need to worry about but they're there just in case you're curious.
Here's a question to test your understanding.



The masses are all the same.  On the earth's surface the masses would all be multiplied by the same value of g.  The weights would all be equal.  If all 3 objects had a mass of 1 kg, they'd all have a weight of 2.2 pounds.  That's why we can use kilograms and pounds interchangeably.

The following figure show a situation where two objects with the same mass would have different weights.


On the earth a brick has a mass of about 2.3 kg and weighs 5 pounds.  If you were to travel to the moon the mass of the brick wouldn't change (it's the same brick, the same amount of stuff).  Gravity on the moon is weaker (about 6 times weaker) than on the earth because the moon is smaller and it has less mass, the value of g on the moon is different than on the earth.  The brick would only weigh 0.8 pounds on the moon.  The brick would weigh almost 12 pounds on the surface on Jupiter where gravity is stronger than on the earth.  On the moon, a brick would have the same mass, the same volume, the same density, but a different weight as(than) it would on the earth.



The three objects below were not passed around class (one of them is pretty heavy).  The three objects all have about the same volumes. 





The point of all this was to get you thinking about density.  Here we had three objects of about the same size with very different weights.  Different weights means the objects have different masses (since weight depends on mass).   The three different masses, were squeezed into roughly the same volume producing objects of very different densities. 

4. density






The three objects were a brick (in the back), a piece of lead (on the left) and a piece of wood (redwood) on the right.
The wood is less dense than water (see the table below) and will float when thrown in water.  The brick and the lead are denser than water and would sink in water.

We'll be more concerned with air in this class than wood, brick, or lead.
In the first example below we have two equal volumes of air but the amount (mass) of air squeezed into each volume is different (the dots represent air molecules). 



The amounts of air (the masses) in the second example are the same but the volumes are different.  The left example with air squeezed into a smaller volume has the higher density.
material
 density g/cc
air
 0.001
redwood
 0.45
water
 1.0
iron
 7.9
lead
 11.3
mercury
 13.6
gold
 19.3
platinum
 21.4
iridium
 22.4
osmium
 22.6


g/cc = grams per cubic centimeter
cubic centimeters are units of volume - one cubic centimeter is about the size of a sugar cube
1 cubic centimeter is also 1 milliliter (mL)

I would sure like to get my hands on a brick-size piece of iridium or osmium just to be able to feel how heavy it would be - it's about 2 times denser than lead.

Here's a more subtle concept.  What if you were in outer space with the three wrapped blocks of lead, wood, and brick?  They'd be weightless.
Could you tell them apart then?  They would still have very different densities and masses but we wouldn't be able to feel how heavy they were.

5. inertia






I think the following illustration will help you to understand inertia.






Two stopped cars.  They are the same size except one is made of wood and the other of lead.  Which would be hardest to get moving (a stopped car resists being put into motion).  It would take considerable force to get the lead car going.  Once the cars are moving they resist a change in that motion.  The lead car would be much harder to slow down and stop.

This is the way you could try to distinguish between blocks of lead, wood, and brick in outer space.  Give them each a push.  The wood would begin moving more rapidly than the block of lead even if  both are given the same strength push.

I usually don't mention in class that this concept of inertia comes from Newton's 2nd law of motion

F = m a
force = mass x acceleration

We can rewrite the equation
a = F/m

This shows cause and effect more clearly.  If you exert a force (cause) on an object it will accelerate (effect).  Acceleration can be a change in speed or a change in direction (or both).  Because the mass is in the denominator, the acceleration will be less when mass (inertia) is large.

Not clear we'll have time to cover the following section in class today.
Here's where we're at


From left to right the brick, the iron bar, the piece of wood, and the lead block. They're all standing on end.   The weight of the iron bar is still unknown.

Now we're close to being ready to define (and hopefully understand) pressure.  It's a pretty important concept.  A lot of what happens in the atmosphere is caused by pressure differences.  Pressure differences cause wind.  Large pressure differences (such as you might find in a tornado or a hurricane) can create strong and destructive storms.


The air that surrounds the earth has mass.  Gravity pulls downward on the atmosphere giving it weight.  Galileo conducted a simple experiment to prove that air has weight (in the 1600s).






We could add a very tall 1 inch x 1 inch column of air to the picture.  Other than being a gas, being invisible, and having much lower density, it's really no different from the other objects.

 
6. pressure



Atmospheric pressure at any level in the atmosphere
depends on (is determined by)
the weight of the air overhead 

This is one way, a sort of large scale, atmosphere size scale, way of understanding air pressure.

Pressure depends on, is determined by, the weight of the air overhead.  To determine the pressure you need to divide by the area the weight is resting on.
 
and here we'll apply the definition to a column of air stretching from sea level to the top of the atmosphere (the figure below is on page 23d in the ClassNotes)


Pressure is defined as force divided by area.  Atmospheric pressure is the weight of the air column divided by the area at the bottom of the column (as illustrated above). 

Under normal conditions a 1 inch by 1 inch column of air stretching from sea level to the top of the atmosphere will weigh 14.7 pounds.  Normal atmospheric pressure at sea level is 14.7 pounds per square inch (psi, the units you use when you fill up your car or bike tires with air).

Now back to the iron bar.  The bar actually weighs 14.7 pounds (many people I suspect think it's heavier than that).  When you stand the bar on end, the pressure at the bottom would be 14.7 psi.




The weight of the 52 inch long 1" x 1" steel bar is the same as a 1" x 1" column of air that extends from sea level to the top of the atmosphere 100 or 200 miles (or more) high.  The pressure at the bottom of both would be 14.7 psi.

7. pressure units

Pounds per square inch, psi, are perfectly good pressure units, but they aren't the ones that meteorologists use most of the time.


Typical sea level pressure is 14.7 psi or about 1000 millibars (the units used by meteorologists and the units that we will probably mostly use in this class) or about 30 inches of mercury (refers to the reading on a mercury barometer).    Milli means 1/1000 th.  So 1000 millibars is the same as 1 bar.  You sometimes see typical sea level pressure written as 1 atmosphere.

We covered a lot of material in the 2nd part of today's class.  Here's a summary of the main points.
Summary