# Unit 8: Pollution: module 4.43 Photochemical smog, thermal inversions

## <span style="font-weight: bold;">Module 43: Photochemical smog, thermal inversions, particulates</span>

<span style="font-weight: bold;">Two types:</span>

- Photochemical smog: LA smog, brown smog, caused by surface ozone reactions
- Sulfurous smog: grey smog, VOG (includes bits of volcanic glass)

<div id="bkmrk-here%27s-how-they-look"><span style="font-weight: bold;">Here's how they look:</span></div><div id="bkmrk--2"></div><div class="sandbox" id="bkmrk--4"><div id="bkmrk--5"><div id="bkmrk--6"><div><div class="container"><div class="frontside"><div class="iframe">![](https://bookstack.kamuela.org/uploads/images/gallery/2026-09/CmYBkWzhcZ6P8Wnp-screen-shot-2023-02-26-at-4-56-17-pm-qi4fyx2k.png)</div></div></div></div></div></div></div><span style="font-weight: bold;">Smog and other photochemical reactions:</span>

<span style="font-weight: bold;">Primary and secondary pollutants:</span>

<div class="sandbox" id="bkmrk-unavailable-apple-wi"><div id="bkmrk-unavailable-apple-wi-1"><div id="bkmrk-unavailable-apple-wi-2"><div><div class="container"><div class="frontside"><div class="iframe">**Unavailable Apple Wiki media:** the exported data did not include file e9e5330a-8909-4077-b73a-333c80e052aa.

</div></div></div></div></div></div></div>

- <span style="font-weight: bold;">Secondary pollutants:</span><span style="font-weight: bold;"> need a chemical reaction (often energy from sunlight) to form (see figures above)</span>
- <span style="font-weight: bold;">Example: PhotoChemical Smog: Photo (light) Chemical (reaction) smog</span>
- <span style="font-weight: bold;">See PANs: Peroxyacyl nitrates: formed from VOCs, NOx </span>[https://en.wikipedia.org/wiki/Peroxyacyl\_nitrates](https://en.wikipedia.org/wiki/Peroxyacyl_nitrates)

  
<div class="sandbox" id="bkmrk--15"><div id="bkmrk--16"><div id="bkmrk--17"><div><div class="container"><div class="frontside"><div class="iframe">![](https://bookstack.kamuela.org/uploads/images/gallery/2026-09/eKsQhaVyJ5nKfzfe-photochemical-smog-ndymtv7w.png)</div></div></div></div></div></div></div><div class="sandbox" id="bkmrk--20"><div id="bkmrk--21"><div id="bkmrk--22"><div><div class="container"><div class="frontside"><div class="iframe">![](https://bookstack.kamuela.org/uploads/images/gallery/2026-09/vnf7Bp3cajcRPUoZ-industrial-smog-formation-jhwtcnfq.png)</div></div></div></div></div></div></div><span style="font-weight: bold;">Thermal inversions:</span> L[ondon fog](https://en.wikipedia.org/wiki/Great_Smog_of_London), US, [Donora](https://en.wikipedia.org/wiki/1948_Donora_smog), PA 1948 k.20, sick 7000

- 1952 London 4000-12,000 dead, three nights
- Mexico city 1996 300 dead, 400K sick
- How?
- Normally, the sun heats the surface, warming the air there, which rises. Cooler air aloft then falls to replace this rising air.

<div class="sandbox" id="bkmrk--28"><div id="bkmrk--29"><div id="bkmrk--30"><div><div class="container"><div class="frontside"><div class="iframe">![](https://bookstack.kamuela.org/uploads/images/gallery/2026-09/d7NoSc5AbpbfJUTV-convection-jxpknjui.png)</div></div></div></div></div></div></div><div class="sandbox" id="bkmrk--31"><div id="bkmrk--32"><div id="bkmrk--33"><div><div class="container"><div class="frontside"><div class="iframe">![](https://bookstack.kamuela.org/uploads/images/gallery/2026-09/Aq2hmxbH5IRVxNhL-inversion-fqh9t8xf.png)</div></div></div></div></div></div></div>In an inversion, warmer air aloft traps the air at the surface (no temperature difference, so no convection), causing the pollution to be trapped in the lower levels. Most dangerous in valleys or bowl shaped cities (London, Mexico City, Denver, Donora).

Los Angeles has severe smog in the daytime, when hotter air draws in ocean air ("onshore breeze"), but the cycle reverses at night when the water is warmer than the land, so they have an "offshore breeze". Surfers like the first one.

Another diagram:  
<div class="sandbox" id="bkmrk--35"><div id="bkmrk--36"><div id="bkmrk--37"><div><div class="container"><div class="frontside"><div class="iframe">![](https://bookstack.kamuela.org/uploads/images/gallery/2026-09/p9Q8nofJko5LHYL5-screen-shot-2023-02-26-at-5-02-24-pm-spcc60bd.png)</div></div></div></div></div></div></div>  
<div id="bkmrk-acid-rain-and-acid-d"><span style="font-weight: bold;">Acid Rain and Acid deposition:</span></div><div class="sandbox" id="bkmrk--39"><div id="bkmrk--40"><div id="bkmrk--41"><div><div class="container"><div class="frontside"><div class="iframe">![](https://bookstack.kamuela.org/uploads/images/gallery/2026-09/tbKrduFM8MAlAd9t-acid-rain-raccudi7.png)</div></div></div></div></div></div></div><div class="sandbox" id="bkmrk--43"><div id="bkmrk--44"><div id="bkmrk--45"><div><div class="container"><div class="frontside"><div class="iframe">![](https://bookstack.kamuela.org/uploads/images/gallery/2026-09/BCvKSbiB0JcR9T6E-acid-deposition-g8ttlxvv.png)</div></div></div></div></div></div></div>  

- SO2 + O2, water = H2SO4 (sulfuric acid)
- NO3 + O2, water = HNO3 (nitric acid)
- “Acid snow” yes, acid snow...
- W. VA had rain more acidic than stomach acid in the 1970's

<span style="font-weight: bold;">Important</span>: Know how a power plant air scrubber works (just like the nuclear plant diagram)

  

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