Introduction to Photochemical Smog Chemistry

Introduction to Photochemical Smog Chemistry • Basic Reactions that form O3 • Distinguish between O3 formation in the troposphere and stratosphere • How hydrocarbons and aldehydes participate in the formation of smog ozone • Formation of free radicals • Nitrogen loss mechanisms • Secondary aerosol formation • Running simple simulation models

Why are we interested in the Smog Chemistry??? Let’s start with the formation of ozone Then continue on to fine particles that form in a smog atmosphere

Ozone • ozone is a form of oxygen; it has three atoms of oxygen per molecule • It is formed in the lower troposphere (the atmosphere we live up to 6 km) from the photolysis of NO2 • NO2 + light --> NO + O. • O. + O2 -----> O3 (ozone) • its concentration near the earth’s surface ranges from 0.01 to 0.5 ppm

Ozone • background ranges from 0.02 to 0.06 ppm • What is a ppm?? • A ppm in the gas phase is one molecule per 106 molecules air or • 1x10-6 m3 O3 per 1 m3 air or • 1x10-6 atmospheres per 1 atmosphere of air • A ppm in water is 1x10-3grams /L water

Ozone • let’s convert 1 ppm ozone to grams/m3 • start with: 1x10-6 m3 per 1 m3 air • we need to convert the volume 1x10-6 m3 of O3 to grams • let’s 1st convert gas volume to moles and from the molecular weight convert to grams • at 25oC or 298K one mole of a gas= 24.45liters or 24.45x10-3 m3

Ozone • we have 1x10-6 m3 of ozone in one ppm • so: 1x10-6 m3 --------------------- = #moles O3 24.45x10-3 m3/mol • O3 has a MW of 48 g/mole • so # g O3 in 1ppm = #moles Ox 48g/mole per m3 • = 4.1x10-5 g/m3

Ozone Health Effects • Ozone causes dryness in the throat, irritates the eyes, and can predispose the lungs to bacterial infection. • It has been shown to reduce the volume or the capacity of air that enters the lungs • School athletes perform worse under high ambient O3 concentrations, and asthmatics have difficulty breathing • The current US standard has been just reduced from 0.12 ppm for one hour to 0.08 ppm for one hour

Lung function before exposure to O.32 ppm O3

Lung function after exposure to O.32 ppm O3

Athletic performance

How do we measure Ozone • 40 years ago chemists borrowed techniques that were developed for water sampling and applied them to air sampling • for oxidants, of which O3 is the highest portion, a technique called “neutral buffered KI was used. • a neutral buffered solution of potassium iodide was placed in a bubbler

How do we measure Ozone • a neutral buffered solution of potassium iodide is placed in a bubbler • KI + O3 --> I2 • measure I2

How do we measure Ozone • a neutral buffered solution of potassium iodide is placed in a bubbler • KI + O3 --> I2 • measure I2 KI solution

How do we measure Ozone • A top is added to the bubbler so that air can enter the KI solution • KI + O3 --> I2 • measure I2 KI solution

How do we measure Ozone • a pump is attached to the bubbler pump KI solution

How do we measure Ozone • Air goes in through the top of the bubbler and oxidants are trapped in the KI liquid and form I2 Air goes in KI solution + I2

How do we measure Ozone • The absorbance of the I2 in the KI solution is then measured with a spectrophotometer KI solution + I2

How do we measure Ozone • The absorbance of the I2 in the KI solution is then measured with a spectrophotometer Spectrophotometer KI solution + I2

A calibration curve • A standard curve is constructed from known serial dilutions of I2 in KI solution • to do this I2 is weighed out on a 4 place balance and diluted with KI solution to a known volume

A calibration curve • A standard curve is constructed from known serial dilutions of I2 in KI solution • to do this I2 is weighed out on a 4 place balance and diluted with KI solution to a known volume I2

Serial dilutions from stock solution I2

Serial dilutions from stock solution I2 5 mg/Liter

Serial dilutions from stock solution I2 5 3 mg/Liter

Serial dilutions from stock solution I2 5 3 2 mg/Liter

Serial dilutions from stock solution I2 5 3 2 1 mg/Liter

Spectrophotometer absorbance absorbances are measured for each of the serially diluted standards

Making a plot I2 adsorbances are plotted vs. concentration

Standard Curve I2 absorbances are plotted vs. concentration absorbance 1 2 3 4 5 concentration (mg/liter)

How do we measure Ozone • The absorbance of the I2 in the KI solution is then measured with a spectrophotometer Spectrophotometer KI solution + I2

We then compare our sample absorbance to the standard curve I2 absorbances are plotted vs. concentration absorbance air sample 1 2 3 4 5 concentration (mg/liter)

Problems • anything that will oxidize KI to I2 will give a false positive response • NO2, PAN, CH3-(C=O)-OO-NO2, give positive responses • SO2 gives a negative response

Instrumental techniques of measuring Ozone • Chemilumenescene became popular in the early 1970s • For ozone, it is reacted with ethylene • ethylene forms a high energy state of formaldehyde, [H2C=O]* • [H2C=O]*--> light + H2C=O • A photomultiplyer tube measures the light • The amount of light is proportional O3

Chemilumenescence measurement of Ozone PM tube

Chemilumenescence measurement of Ozone pump PM tube

Chemilumenescence measurement of Ozone pump PM tube waste ethylene ethylene

Chemilumenescence measurement of Ozone pump sample air with O3 O3 PM tube PM tube waste ethylene ethylene

Chemilumenescence measurement of Ozone PM tube picks up light from {H2CH=O}* pump sample air with O3 O3 PM tube {H2C=O}* waste ethylene ethylene

Chemilumenescence measurement of Ozone pump sample air with O3 O3 PM tube {H2C=O}* waste ethylene ethylene catalytic converter CO2 + H2O

Using UV photometry to measure Ozone • This is the most modern technique for measuring ozone • sample air with O3 enters a long cell and a 254 nm UV beam is directed down the cell. • at the end of the cell is a UV photometer which is looking at 254 nm light • we know that: light Intensityout= light intensityin e- aLC

Photochemical Reactions • Oxygen (O2) by itself does not react very fast in the atmosphere. • Oxygen can be converted photochemically to small amounts of ozone (O3). O3 is a very reactive gas and can initiate other processes. • In the stratosphere O3 is good, because it filters uv light. At the earth's surface, because it is so reactive, it is harmful to living things

In the stratosphere O3 mainly forms from the photolysis of molecular oxygen (O2) • O2 + uv light -> O. • O. + O2 +M --> O3 + M • In the troposphere nitrogen dioxide from combustion sources photolyzes • NO2 + uv or visible light -> NO + O. • O. + O2 +M --> O3 (M removes excess energy and stabilizes the reaction)

O3 can also react with nitric oxide (NO) • O3 + NO -> NO2 + O2 • both oxygen and O3 photolyzes to give O.O2 + hn-> O. +O. (stratosphere) O3 + hn -> O. + O2 • O. can react with H2O to form OH. radicalsO. + H2O -> 2OH.

O. can react with H2O to form OH. Radicals O. + H2O -> 2OH. • OH. (hydroxyl radicals) react very quickly with organics and help “clean” the atmosphere; for example: • OH. + H2C=CH2products ;very very fast • If we know the average OH. radical concentration, we can calculate the half-life or life time of many organics[org]in the atmosphere.

if we use CO as an example, it has a known rate constant for reaction with OH. • CO + OH. -> CO2 krate= 230 ppm-1 min-1 • If the average OH. conc. is 3 x10-8 ppm • for t1/2 we have: ln(1/2) = -krate[OH.] x t1/2 • -0.693= -230 ppm-1 min-1 x 3 x10-8ppm x t1/2 • t1/2 = 100456 min or69.7days

What this means is that if we emit CO from a car, 69.7 days later its conc. will be 1/2 of the starting amount. In another 69.7 days it will be reduced by 1/2 again. • For the same average OH. conc. that we used above, what would be the t1/2 in years for methane and ethylene, if their rate constants with OH. radicals are 12.4 and 3840 ppm-1 min-1 respectively? CH4 H2C=CH2

Why is the reaction of OH. with ethylene so much faster than with methane? H H1. H-C-H....OH . -> H-C. + .HOH . H H 2. H2C=CH2attack by OH.is at the double bond, which is rich in electrons

Introduction to Photochemical Smog Chemistry