13.2 Tropospheric photochemistry and air pollution

Tropospheric Photochemistry and Air Pollution

Tropospheric ozone forms through complex reactions involving sunlight, nitrogen oxides, and volatile organic compounds. Unlike stratospheric ozone (which shields us from UV), tropospheric ozone is a harmful pollutant that damages lungs, crops, and materials. Understanding how it forms is the key to figuring out how to reduce it.

Primary Reactions in Ozone Formation

Ozone production in the troposphere starts with the photostationary state, a cycle involving $NO$, $NO_2$, and $O_3$ that, on its own, produces no net ozone. VOCs then disrupt this cycle and tip the balance toward ozone accumulation.

Step 1: Photolysis of nitrogen dioxide. UV-A and violet light (wavelengths < 420 nm) break apart $NO_2$:

$NO_2 + h\nu \rightarrow NO + O$

Step 2: Ozone formation. The oxygen atom reacts almost immediately with $O_2$. A third body $M$ (typically $N_2$ or $O_2$) absorbs excess energy and stabilizes the product:

$O + O_2 + M \rightarrow O_3 + M$

Step 3: Ozone destruction by NO. $NO$ reacts with the ozone just formed, regenerating $NO_2$:

$NO + O_3 \rightarrow NO_2 + O_2$

If only these three reactions operated, ozone would be created and destroyed at roughly equal rates, so no net ozone would build up. This is the photostationary state.

How VOCs break the cycle. VOCs react with hydroxyl radicals ($OH$) in the atmosphere to produce peroxy radicals ($RO_2$ and $HO_2$). These radicals convert $NO$ to $NO_2$ without consuming ozone:

$RO_2 + NO \rightarrow RO + NO_2$

That extra $NO_2$ feeds back into Step 1, generating more ozone while the ozone-destroying Step 3 is bypassed. This is why VOCs are so important: they shift the cycle toward net ozone production.

Formaldehyde as a radical source. Photolysis of $HCHO$ is one important pathway for generating $HO_2$ radicals:

$HCHO + h\nu \rightarrow H + HCO$

$H + O_2 \rightarrow HO_2$

$HCO + O_2 \rightarrow HO_2 + CO$

Each molecule of formaldehyde can produce two $HO_2$ radicals, making it a significant contributor to the radical pool that drives ozone formation.

Role of NOx and VOCs

NOx ($NO$ + $NO_2$) comes primarily from combustion: vehicle engines, power plants, and industrial boilers. It acts as the catalyst in the ozone-forming cycle described above.

VOCs come from both anthropogenic sources (vehicle exhaust, industrial solvents, fuel evaporation) and biogenic sources (isoprene and terpenes emitted by trees). VOCs react with $OH$ radicals to form the peroxy radicals that amplify ozone production.

The NOx-to-VOC ratio determines which chemical regime controls ozone levels in a given area:

• VOC-limited (NOx-saturated) regime: Common in dense urban centers. There's plenty of $NOx$ but not enough VOCs to generate radicals. Reducing VOC emissions is more effective at lowering ozone here. Counterintuitively, reducing $NOx$ alone can actually increase ozone in this regime because excess $NO$ that was scavenging ozone (Step 3) is removed.
• NOx-limited regime: Common in suburban and rural areas downwind of cities. VOCs are abundant (often from biogenic sources), but $NOx$ is the bottleneck. Reducing $NOx$ emissions is the most effective strategy here.

Getting this distinction right matters enormously for policy. Applying the wrong control strategy can be ineffective or even counterproductive.

Peroxyacetyl nitrate (PAN) forms as a secondary pollutant from the reaction of VOC-derived radicals with $NO_2$. PAN is thermally unstable, so it decomposes at higher temperatures, releasing $NO_2$ far downwind of the original emission source. This makes PAN an important reservoir species that transports reactive nitrogen over long distances and can cause ozone formation in remote areas.

Effects of Tropospheric Pollution

Human health. Ground-level ozone is a powerful oxidant that irritates airways. Short-term exposure causes coughing, wheezing, and chest tightness, and it aggravates asthma. Long-term exposure is linked to increased susceptibility to respiratory infections and premature mortality. The WHO estimates that outdoor air pollution contributes to millions of premature deaths annually.

Vegetation. Ozone enters leaves through stomata and damages cells, reducing photosynthesis. Visible symptoms include stippling and chlorosis on leaf surfaces. Crop yield losses are significant: studies estimate global yield reductions of 7–12% for wheat and 6–16% for soybeans due to ozone exposure. Over time, chronic ozone stress alters ecosystem composition by favoring ozone-tolerant species over sensitive ones.

Materials. Ozone accelerates degradation of many materials. Limestone and marble on buildings erode faster. Copper and bronze surfaces corrode (the green patina on copper statues is partly driven by atmospheric oxidants). Rubber loses elasticity and cracks, and textiles and dyes fade more rapidly.

Strategies for Ozone Reduction

Emissions control targets the precursors directly:

• Tightening vehicle emission standards and mandating catalytic converters to reduce both $NOx$ and VOC tailpipe emissions
• Adopting low-$NOx$ burner technology in power plants
• Regulating industrial VOC emissions through vapor recovery systems and solvent substitution

Urban planning reduces emissions at the source:

• Expanding public transit systems (e.g., bus rapid transit) to cut vehicle miles traveled
• Establishing low-emission zones that restrict high-polluting vehicles in city centers
• Increasing urban green spaces, though with care since some tree species (e.g., oaks, poplars) emit significant biogenic VOCs

Alternative energy addresses the root cause by transitioning electricity generation to renewables (solar, wind) and promoting electric vehicles, which eliminate tailpipe $NOx$ and VOC emissions entirely.

Air quality monitoring networks provide real-time ozone and precursor data. These enable health advisories on high-ozone days and supply the data needed to evaluate whether control strategies are working.

International cooperation is necessary because ozone precursors travel across borders. The Convention on Long-Range Transboundary Air Pollution (CLRTAP) in Europe is one example of a framework for coordinated emission reductions.

Public education helps people understand air quality indices and take protective action on high-pollution days. It also encourages behaviors that reduce emissions, such as carpooling, reducing idling, and choosing low-VOC consumer products.