This is an interesting paper at the intersection of chemistry, botany, and environmental science, and I think it’s probably going to get a fair amount of attention in the press. Unfortunately, it’s also likely that some of the headlines around it are going to mangle the details, so let’s try to straighten those out in advance.
The authors are looking at ground-level ozone concentrations in large cities worldwide, and I think we can stipulate up front that such elevated ozone is detrimental to human health. This is different from the stratospheric ozone layer, of course - urban ozone is right down there on the streets where people are breathing it, not miles up at the edge of space. Both of these have photochemistry as their source, but that’s where the similarity ends. The stratospheric ozone layer results from hard UV radiation from the sun hitting oxygen molecules in the upper atmosphere and splitting them into two oxygen atoms. This atomic oxygen goes on to combine with regular O2 molecules to produce ozone (O3), which is a strong absorber of that ultraviolet radiation. On absorption, it splits back to a molecule of oxygen and a single atomic oxygen, which can then recombine, and this ozone/oxygen cycle is continuous at such altitudes. The great majority of UV-C(200-280 nm) and UV-B (280-315 nm( ultraviolet light from the sun is absorbed by this ozone layer (UV-B intensity is millions of times higher at the top of the atmosphere than it is at the Earth’s surface).
Meanwhile, ground level ozone is produced through a whole set of photochemical reactions between hydrocarbons and nitrogen oxides, both of which are important classes of air pollution already. Automobile and industrial emissions are of course a real concern here, because these contain both of those chemical classes simultaneously, and this is of course a big factor behind existing emissions controls. This new paper, though, calls attention to another part of the problem that gets occasional attention: volatile organic compounds (VOCs) emitted by plants - in this case, urban trees.
The main VOC emitted by trees is isoprene (2-methylbutadiene, the smallest unit in the terpene pathway, which in its polymerized form is natural rubber), and it’s responsible for the summertime haze in forested areas which existed before there were any automobile tailpipes or industrial smokestacks at all. Its emission is in fact a heat-stress response in plants and it is involved in resistance to reactive oxygen species - emissions fall to low levels in darkness and/or cooler temperatures. The authors studied ozone levels and VOC emissions in Beijing, and found that trees (especially willow and poplar species) contributed about 10% of the VOC total with human activities responsible for the other 90%. So far, that’s about what you would have expected.
But on further study, it appears that the tree-derived VOCs account for at least half of the ozone-forming chemistry in the air, because of the reactivity of isoprene and related compounds with hydroxyl radicals. So the tree emissions are hitting far more heavily than that 10/90 split would make you believe, and that’s the surprise take-away from this work.
Here are the details, though, that might get overlooked if someone writes a “Trees Responsible for Smog” headline. For one thing, it’s not all trees. Some species emit far more isoprene than others (the biggest culprits are the willows and poplars as mentioned, along with oaks and eucalyptus). That means that cities in Australia (where eucalypts are everywhere) have even more of a problem with this ozone source than do most cities in China. So secondly, these results don’t mean that Trees Are Bad, because they’re actually very beneficial in urban environments, not least for shading. It just means that we need to pay more attention to which kinds of trees we’re using, and to ease up on the isoprene emitters. Third, the main reason that tree emissions are this big a contributor now is because of clean-air emissions laws that have reduced the human contributions over the decades - if you’d looked at this in (say) 1950 the tree contributions would not have looked so impressive. Fourth, this most certainly does not mean that we can ease off on regulating our own emissions. As mentioned, nitrogen oxides are a key ingredient in ozone formation, and if we can continue to bring those down (and they’re almost completely our fault), ground-level ozone will improve no matter how many weeping willows are around. We planted the high-isoprene varieties ourselves for the most part, and if we didn’t keep dumping volatile nitrogen oxides into the system it wouldn’t be much of a problem anyway. It’s up to us to fix it.
