Ozone Photochemistry Part I.

Over the US Thanksgiving holiday, well, actually in the news hole right before it, on Wednesday afternoon, US EPA announced new rules for ambient ozone, dropping the current limits from 75 ppb to 65-70 ppm with fairly long timelines to meet those limits. What the EPA means by such a standard is that
the

an area would meet the primary standard if the fourth highest maximum daily 8-hour ozone concentration per year averaged over three years  is equal to or less than the standard

 Neela Banerjee and Tony Barboza in the LA Times describe the new rules and the reaction to it, principally from the usual suspects who have never been right when they declared the end of the world as we know it due to some environmental improvement.  They have a pretty good Mom Rabett level explanation of how the ozone is formed

Ozone is created when unstable gases are released during combustion, whether at power plants, factories or in vehicle engines. The pollutants react with sunlight to create ozone, which can trigger asthma attacks, worsen heart and lung disease and lead to premature deaths.

Because so many sources emit those ozone components, the effect of an ozone standard is far-reaching, which has made politicians leery of regulating it. The Bush administration rejected EPA science advisors' recommendation six years ago for a tougher limit. The Obama administration vowed to implement a tighter standard, but the president shelved it and let the Bush-era limit remain at the start of his reelection bid.

The operation of the rule would take a while

Once finalized, the ozone standard would not go into effect for years. States are given three years to collect air quality data before their status is determined. They then have years to devise a plan to cut pollution and force industry and communities to comply.

The worst-polluted regions in the U.S., including Los Angeles, would have until 2037 to meet a new standard.

Ozone is not good for anyone's health and smog produced as a by product is also not good for living things.

Eli would like to take a few minutes of your time to discuss the chemical mechanism driving ground level ozone formation and its relationship to smog.

At root, three things are necessary, volatile organic compounds (VOCs) from combustion and natural sources (think fossil fuel combustion and pine forests), reactive nitrogen oxide emissions (think fossil fuel combustion) and sunlight (think sunlight).  Oh yeah, you also need some humidity and some oxygen.

But let Eli start with NO2, using the MPI Mainz UV-Viz spectral data base

 NO2 is one of the few molecules found in the atmosphere that absorb strongly in the visible and near UV region.  The thermochemical limit for dissociation into NO and O atoms is roughly 400 nm for ground vibrational level NO2 but because low lying vibrational and rotational levels are thermally excited in the atmosphere, the effective boundary for photodissociation is about 425 nm.  Since the solar spectrum at ground level extends down to about 300 nm,

during the day NO2  will be efficiently dissociated.

 NO2 + hν ∠ 426nm  → NO + O(³P)   [1]

O(³P) is the ground (lowest) electronic state of the oxygen atom.  The next one up as it were, is O(¹D) and Eli will explain why that is important in a moment.  The average time that an NO2 molecule lasts at the surface is of the order of an hour less during the summer when sunlight is more intense, more during the winter and so forth.  

The O(³P) produced in Reaction 1, then combines with oxygen to form ozone

O(³P) + O2 + M  →  O3 + M     [2]

When the oxygen atom and molecule come together they form a reaction intermediate which has enough energy to immediately fall apart.  The role of the third reactant, M, is to carry that energy away leaving a ground state ozone molecule, O3.  Without that no ozone would be formed

In the troposphere NO2 photolysis is the dominant source of ozone.  The ozone absorption spectrum is practically the mirror image of the NO2 absorption spectrum

For the troposphere, the important part of this spectrum is the region to higher wavelengths because the ozone layer in the stratosphere blocks everything  below ~290 nm and that is being generous, but there is some that leaks through above that limt as can be seen in this figure from Slaper, et al, showing the solar spectrum near 300 nm in Bilthoven, Netherlands with two different solar zenith angles.

Time of day and time of year play important roles in how much light is available below ~320 nm and it is only the light between the upper limit of the ozone absorption spectrum and the stratospheric ozone cut off that can be absorbed by tropospheric ozone created in Reaction 2

Oxygen atoms created by photolysis of ozone come in two flavors, ground state oxygen atoms O(³P), and excited state oxygen atoms O(¹D).  In addition to the issue of the decreasing ozone absorption coefficient at wavelengths above 290 nm, the quantum yield of O(¹D) decreases with increasing wavelength as discussed in Matsumi, et al.

Why is this important.  Well, it turns out that O(¹D) from ozone photolysis is the principle source of HO radicals in the troposphere, and HO radicals, sometimes called the atmosphere's vacuum cleaner, are what degrades the volatile organic molecules coming out of fossil fuel combustion and those killer trees.

O^*(¹D) + H2O → HO + HO   [4]

Electronically excited O(¹D) is mostly quenched to O(³P) by collisions with other O2 and N2 molecules which are much more common than water vapor.  However, O(³P) produced will simply cycle back to ozone via reaction [2] keeping the ozone concentration roughly constant.

The rest of the story (stay tuned) is the interplay between NO2, the VOCs, and HO that control ozone concentration and the generation of smog.