Ammonium Nitrate Chemistry

Old Men Yell at Trees

An atrocious piece of journalism has appeared in the editorial pages of the Wall Street Journal, blaming air pollution on trees, people, dirt, and pretty much everything but cars and the burning of fossil fuels.

On the campaign trail in 1980 Ronald Reagan was famously reported to quip that "trees cause more pollution than automobiles do".  He was widely mocked - and rightly so - for this naive interpretation of atmospheric science.  Now another old white man, Paul Gigot, is shaking his fist at the sky in vain, and dragging his like-minded and trouble plagued editorial board along with him.

In their editorial, the WSJ inform us that volatile organic compounds (VOCs) react with nitrogen oxides (NOx) to form ozone and smog.  So far so good.  They then go on to cite the latest scientific literature showing that in many cities personal care and household cleaning products - plus industrial solvents and commercial chemicals but they curiously don't get much attention here - provide a larger total VOC burden than automobiles do.  Finally, they remind us that trees also emit VOCs.  Ultimately and unsurprisingly this is all used to argue in support of the Trump Administration's proposal to relax automobile efficiency regulations.

It is true that trees provide a large source of VOCs to the atmosphere and that they are, to quote the WSJ piece, "to blame for the blue haze in the Appalachian Mountains" not to mention in other beautiful and pristine parts of the world.  But it is also true that we would all prefer to breathe in this mountain air than city smog, and that is where the science gets interesting.  Although we have a pretty good picture of the chemistry taking place in polluted urban air, it has emerged in recent years that we don't have a great understanding of how the world's relatively untouched forests cleanse themselves of the chemicals that they naturally emit.  In cities, the presence of NOx from burning fossil fuels and farming accelerates the destruction of VOCs emitted from all sources, yet it does this at the expense of making toxic ozone and particulate matter.  What we have unexpectedly found over the last decade or so is that forests are also capable of efficiently cleansing their air without the interference of NOx and ozone, by naturally recycling the reactive free radicals responsible for breaking down VOCs.  In other words, the new science of smog has actually revealed that man made air pollution is having even more of an impact upon the natural environment than we had previously thought.

So, what should we take away from the latest research revealing that cities have high VOC loads from sources other than cars?  For one, we should celebrate that fuel efficiency standards have been cutting the contribution of cars to air pollution, particularly in developed nations.  But there is further progress to be made, and this is not the time to take our foot off the pedal.  We also need to recognise that all human activities impact upon our environment, be mindful of what we are releasing into the air that we breathe, and minimise these emissions where we can.  What we shouldn't be doing is twisting scientific findings to support the profits of commercial interests at the expense of the public's health.