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 Running simple simulation models

Why are we interested in the Smog Chemistry???

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 the lungs for air 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 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

Serial dilutions from stock solution 5 mg/Liter

Serial dilutions from stock solution 5 3 mg/Liter

Serial dilutions from stock solution 5 3 2 mg/Liter

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

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

Making a plot I2 adsorbances are plotted vs. concentration

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 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- a LC

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. 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.

from simple kinetics we can show that:. d[org]/dt = -krate [org] [OH] from simple kinetics we can show that: d[org]/dt = -krate [org] [OH] If [OH.] is constant ln [org]t = ln [org]t=o -krate[OH.]x time1/2 Let’s say we want to know the time it takes for the organic to go to 1/2 its original [conc].

ln [org]t = ln [org]t=o -krate[OH.]x time1/2 rearranging ln {[org]t / [org]t=o }= -krate[OH.]x t1/2 The time that it takes for the conc to go to half means [org]t will be 1/2 of its starting conc. [org]t=o . This means [org]t / [org]t=o = 1/2 and ln (1/2) = -0.693= -krate[OH.]x t1/2

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 or 69.7 days

What this means is that if we emit CO from a car, 69 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 Why is the reaction of OH. with ethylene so much faster than with methane? H H 1. H-C-H....OH . -> H-C. + .H OH . H H 2. H2C=CH2 attack by OH.is at the double bond, which is rich in electrons

What happens in urban air?? In urban air, we have the same reactions as we discussed before NO2 + uv light -> NO + O. O. + O2 +M --> O3 + M O3 + NO -> NO2 + O2 This is a do nothing cycle

What is the key reaction that generates ozone at the surface of the earth? What is the main reaction that generates it in the stratosphere? How would you control O3 formation?

In the urban setting there are a lot of ground base combustion sources Exhaust hydrocarbons NO & NO2 CO

If organics are present they can photolyze or generate radicals H2C=O + hn -> .HC=O + H. H. + O2 -> .HO2 if we go back to the cycle NO2 + uv light -> NO + O. O. + O2 +M --> O3 + M O3 + NO -> NO2 + O2 .HO2 can quickly oxidize NO to NO2 NO + .HO2 -> NO2 + OH.

OH. + can now attack hydrocarbons which makes formaldehyde and other radical products for ethylene CH2=CH2 + OH. -> OHCH2CH2. OHCH2CH2. + O2 -> OHCH2CH2O2. OHCH2CH2O2. + NO ->NO2 + OHCH2CH2O. OHCH2CH2O. + O2 -> H2C=O + .CH2OH O2 + .CH2OH -> H2C=O + .HO2

These reactions produce a host of radicals which “fuel” the smog reaction process First OH radicals attack the electron rich double bond of an alkene Oxygen then add on the hydroxy radical forming a peroxy-hydroxy radical the peroxy-hydroxy radical radical can oxidize NO to NO2 ,just like HO2 can

Further reaction takes place resulting in carbonyls and HO2 which now undergo further reaction; the process then proceeds…

There is similar chemistry for alkanes OH. + H 3-C-CH3 --> products and for aromatics OH. + aromatics --> products

Aromatic Reactions * toluene ring cleavage benzaldehyde o-cresol CH 3 OH 2 . O=CH NO +O H * O + HO + H + toluene o-cresol benzaldehyde rearrangement methylglyoxal butenedial oxygen bridge radical ? ring cleavage Aromatic Reactions

Nitrogen Storage (warm vs. cool) OH H C-C=O + H2O. H C-C=O 3 . 3 H PAN warm cool

Nitrogen Loss (HNO3 formation) NO2 + O3  NO3.+ O2 NO3.+ NO2  N2O5 N2O5 + H2O  2HNO3 (surface) NO2 + OH.  HNO3 (gas phase)

Nitrogen Loss (alkylnitrates) butane O 2 -C-C-C-C- 2-butylnitrate NO -C-C-C-C- -C-C-C-C- + H. 2 2-butanal

How can we easily estimate O3 if we know NO and NO2? The rate of of formation of O3 is governed by the reaction: NO2 + uv light -> NO + O. and its rate const k1 because: O. + O2 +M --> O3 + M is very fast so the rate of formation O3 is: rateform = +k 1 [NO2]

The rate of removal of O3 is governed by the reaction: O3 + NO -> NO2 + O2 and its rate const k3 so the rate of removal of O3 is: rateremov = -k 3 [NO] [O3] the overall ratetot =rateform +rateremov

ratetot = -k 3 [NO] [O3] +k 1 [NO2] if ratetot at steady state = 0, then k 1 [NO2]= k 1 [NO2][O3] and [O3] = k 1 [NO2] / {k 1 [NO2] } This means if we know NO, NO2, k1 and k3 we can estimate O3

Calculate the steady state O3 from the following:. NO2 = 0. 28 ppm Calculate the steady state O3 from the following: NO2 = 0.28 ppm NO = 0.05 ppm k1 = 0.4 min-1 k3 = 26 ppm-1min-1

What is the key reaction that generates ozone at the surface of the earth? What is the main reaction that generates it in the stratosphere? How would you control O3 formation?

Can we use computers to predict the amount of ozone formed if we know what is going into the atmosphere? yes but we need to create experimental systems to see of our models are working correctly.

In 1972 we built the first large outdoor smog chamber, which had an interior volume of 300 m3. We wanted to predict oxidant formation in in the atmosphere. The idea was to add different hydrocarbon mixtures and NO + NO2, to the chambers early in the morning.

Samples would be taken through out the day Samples would be taken through out the day. We would then compare our data to the predictions from chemical mechanisms. If we could get a chemical mechanism to work for many different conditions, we would then test it under real out door- urban conditions.

The Chamber had two sides Or Darkness NO &NO2 300 m3 chamber Teflon Film walls propylene Formaldehyde

Example experiment with the following chamber concentrations: NO = 0.47 NO2 = 0.11 ppm Propylene = 0.99 ppmV temp = 15 to 21oC

Solar Radiation Profile

Example Mechanism dNO2/dt = -k1[NO2]; DNO2=-k1 [NO2] Dt NO2+ hn -> NO + O. k1 keyed to sunlight O. + O2 --> O3 k2 O3 +NO2 --> NO + O3 k3 H2C=O + hn --> .HC=O + H. k4 keyed to sunlight H. +O2 --> HO2. k5 HO2. + NO --> NO2+OH. k6 (fast) OH.+ C=C ---> H2C=O + HO2 + H2COO. k7 dNO2/dt = -k1[NO2]; DNO2=-k1 [NO2] Dt

Photochemical System

Photochemical System

let’s see how this kinetics model works 1st we will look at a mechanism 2nd we will look at the model inputs 3rd we will run the model with reduced hydrocarbons (formaldehyde) to see the effect of reducing HC run the model with reduced NOx

We need to get mixing height into the model height in kilometers 0.0 0.1 0.2 0.3 0.4 1.1 1.5 Balloon temperature Temp in oC 20 25 30 35 Dry adiabatic lines

How do we get light into the mechanism?? A molecule photolyzes or breaks apart when it absorbs photons that have energy that is greater than the bond strength Let’s look at the energy in a mole of photons which have a wavelength 288 nm The energy E, in a mole of light at 288 nm is E= 6.02x1023x hc/l c= 3x108m/s; h=6.63x10-34Js, l=288x10-9m E= 416kJ/mole If all this light was absorbed it would break C-H bond

Light and rate constants The question is, is all the light absorbed?? Actually not, but this brings up the concept of quantum yields, f, and light absorption s f= # molecules reacted/# photons absorbed The light flux is the # of photons of light cm-2 sec-1 I have called this I but often it is called J or F at a given l The rate constant for photolyis can be written as kratel= Jl x f l x absorption coefl

Light and rate constants kratel= Jl x f l x absorption coefl the absortion coef. s has units of cm2/molecule and comes from Beer’s law I=Io e-sl[C] kratel= Jl1 x f l1 x s l1 This is at one wavelength l1; what do we do when we have two wavelengths l1 and l2? krateltotal = Jl1 x f l1 x s l1 + Jl2 x f l2x s l2

Light and rate constants so across all wavelengths krateltotal = S Jl x f l x s l What this says is that if we know the light flux or “intensity” at each wavelength, Jl , the absorption coef., s l at each wavelength and the quantum yield s l , we can calculate krateltotal for the real atmosphere

Light and rate constants Lets calculate kratel for NO2 at the wave length of 400-405 nm and a zenith angle of 20 degrees J400-405nm= photons cm-2 sec-1 = 1.69x1015 f400nm = quantum yield = ~0.65 s400-405nm = ~6x10-19 cm2 molecule-1 kratel= Jl x f l x s l= 0.00067sec-1

Light and rate constants so in the reaction NO2 + light at 400-405nm -> NO + O. kratel= Jl x f l x s l= 0.00067sec-1 dNO2/dt = krate l [NO2]

Solar flux striking the earth

Light and rate constants The angle of the sun is called the zenith angle. When the sun is directly over head the zenith angle is zero degrees when it has just gone down it is 90o

Light and rate constants (zenith angle) Sun q

Light and rate constants When the sun is at lower angles in the sky, less energy strikes the surface of the earth; sun up and sun down the zenith angle is zero Algorithms have been written to relate zenith angle to light flux at each wavelength

Light and rate constants The zenith angle for given latitude and time of year can known, as well as the time that the sun comes up and how high in the sky it will go at noon in the winter time it will not go as high in the sky as in summer.

Light and rate constants from these tables if we know f and s for a compound we can calculate the photolysis rate constants for any compound over the course of the day as the zenith angle changes NO2, H2C=O, O3, acetaldehyde