How do air pollution and acid rain affect the quality of vines, grapes and wine, and ultimately our health?

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Specific pollutants or classes of pollutants are known to have damaging effects on the environment, such as ozone (triatomic oxygen) depletion and global warming, as well as our health effects when pollutants exceed ‘acceptable’ levels. Vineyards are usually located in areas with less pollution, so this is not a major concern. That doesn’t mean pollution should be ignored. However, research on the effects of pollution on vineyards is limited. This is a very complex field of study. Let’s look specifically at the chemical composition of the pollutants and their impact on viticulture.

The Earth’s atmosphere contains a variety of gases, primarily diatomic nitrogen and oxygen as well as carbon dioxide and water vapour. Every moment, pollutants from vehicle exhaust, landfills, and industrial processes are released into the atmosphere and interact with their constituents. The main pollutants are sulfur oxides, carbon dioxide, nitrogen oxides, and chlorofluorocarbons (CFCs), which, when they reach the atmosphere, react with sunlight and atmospheric (diatomic) oxygen to produce harmful substances.

Sulfur dioxide is known to have damaging effects on plants in high concentrations. It is the result of oxidation of elemental sulfur, for example during coal combustion. It also reacts with atmospheric oxygen to form sulfur trioxide, which then reacts with water vapor in acid rain to form sulfuric acid. The second component of acid rain is carbonic acid, formed by a similar reaction from emitted carbon dioxide. The most damaging components are hydrofluoric acid, formed from hydrogen fluoride, released from smelters and phosphate fertilizer production, and water vapor in the atmosphere. When acid rain reaches the soil in a vineyard, it lowers the pH and throws the soil chemistry out of balance, making it difficult to grow quality grapes. Some of these acids are particularly corrosive and toxic, which is not conducive to the growth of vines.

Nitrogen oxides are present in vehicle exhaust as a result of the high temperature combustion of (diatomic) nitrogen. In the atmosphere, nitrogen oxides react with molecular oxygen to form nitrogen dioxide, the reddish-brown gas that causes smog. Nitrogen dioxide is then photochemically converted back to nitrogen oxides together with oxygen atoms. The oxygen radicals then react with molecular oxygen to form ozone in the lower atmosphere. Ozone is highly irritating and destructive to vines and crops.

Vehicle exhaust also releases hydrocarbons that react with nitrogen oxides to form peroxyacetyl nitrate, a compound belonging to the peroxyacyl nitrate or PAN class, which is a powerful compound found in photochemical smog. Toxic irritant, causes ozone buildup. PAN is highly destructive to the physiology of the vine, resulting in reduced yields. Before the phase-out of leaded (tetraethyl lead) gasoline, wines produced from vineyards near heavy traffic roads showed higher levels of lead, a potent neurotoxin that causes lead poisoning. If lead enters the bloodstream, it interferes and disables the enzyme delta-aminolevulinic acid dehydratase (ALAD), which is responsible for making hemoglobin. Hemoglobin is an iron-containing protein pigment found in the red blood cells of vertebrates and is primarily responsible for transporting oxygen from the lungs to body tissues. Lead poisoning can cause irreversible nerve damage as well as abdominal pain, gastrointestinal problems, headaches, anemia, reproductive problems, and many other effects.

Chlorofluorocarbons (CFCs) are haloalkanes—that is, they contain alkanes, such as methane or ethane, and halogens, such as chlorine or fluorine—that are known to have harmful effects associated with ozone depletion. The halogens in the CFC react with ozone to form halogen oxides and molecular oxygen.

As for pesticides, it has been clearly demonstrated that, without knowledge of the environmental impact, the residues in wine are negligible. This, of course, assumes that the pesticide was properly applied within the recommended treatment period prior to harvest. And the various processes of winemaking, namely crushing, pressing, fermentation, clarification, filtration and aging, make pesticide residues disappear.

What about the smoke from the devastating wildfires that ravaged Northern California’s wine country in the summer of 2008? Some red wines from hard-hit regions like Mendocino County have distinctly smoky, burnt aromas and charred wood, ash flavors. The compounds responsible for the smoke flavor are guaiacol and 4-methylguaiacol, volatile phenolics that are absorbed by the skins of grapes—especially thin-skinned varieties such as Pinot Noir—and then absorbed by grapes during maceration. Extracted and intensified by fermentation. White wines are mostly spared because the juice has not macerated the skins. There has been a lot of research into smoked wines in Australia, and in 2003, smoke from wildfires had a huge impact on vineyards.

Using reverse osmosis and nanofiltration techniques, Australians have been able to reduce the culprit compound to undetectable levels. VA Filtration (VAF), a company that specializes in removing volatile acidity (VA), Brett (yeast infection) and TCA (cork wine) among other services, claim they can now remove up to 99% of targeted organoleptic characteristics by treating affected wines with Food grade resin developed in Germany. Interesting, if not puzzling, VAF’s website The statement “Removed offensive compounds still does not [known]”

But astute readers will also know that guaiacol and 4-methylguaiacol are compounds found in roasted wines aged in oak barrels and are therefore very popular. What a polarization!

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