There is a new paper in Science this week on changes to atmospheric visibility. In clear sky conditions (no clouds), this is related mainly to the amount of aerosols (particulate matter) in the air (but is slightly dependent on the amount of water vapour as well, which is corrected for in this study). The authors report that the clear-sky visibility has decreased almost everywhere (particularly in Asia) from 1973 to 2007, with the exception of Europe where visibility has increased (consistent with the ‘brightening trend’ reported recently). Trends in North American stations seem relatively flat.
There is another story that didn’t get as much press when it came out late last year but that is highly relevant to this issue – whether any of the efforts that the Chinese authorities to reduce air pollution ahead of the Olympics last year had any impact. To the extent that they did, they might point the way to reducing aerosols and other pollutants across Asia, but it might also reveal how hard it is to do so.
The press release and abstract for the Science paper link their results to the ‘global dimming’ trends we have reported on in the past, but it’s worth perhaps pointing out that previous studies (and the term ‘global dimming” itself) have referred to all-sky conditions. So that includes changes in clouds – which are obviously a big factor in how much sunlight gets to the surface. Looking at the clear sky conditions (i.e. only when there are no clouds) can help attribute changes to aerosols or atmospheric dynamics say, but since aerosols affect clouds (the ‘indirect effect’) as well as circulation too, it is only a partial estimate of the true impact of aerosols.
But getting back to the Olympics…. Monitoring of pollutants near the surface has improved enormously in recent years with the various satellite instruments now in orbit (MOPITT, GOME, OMI and TES for instance (sounds like a comedy revue team, no?)). These instruments detect specific frequencies where pollutants are known to absorb and so can give a birds eye view of where the pollutants are and how they are changing. Among other things, the satellites can detect ozone, NOx, SO2, the total amount of aerosols and carbon monoxide. Each of these have different atmospheric lifetimes and so can be used either to detect point sources (from pollutants that only last a short time) or long range transports of pollution (from the longer lived pollutants). NO2 (a big component of NOx – which lumps together NO and NO2
all of the reactive nitrogen oxides), is very short-lived and so tells you a lot about local sources. Carbon monoxide has a longer lifetime (a couple of months) and so can show the long-range impacts. Many of these pollutants have related industrial sources (car exhausts, coal burning, industrial production etc) and so can be used as proxies for many other pollutants (such as specific aerosols) which can’t (yet) be directly measured.
What do the results show? The team at GSFC have released preliminary images from the NO2 analysis showing the before and during the pollution controls. In both images, Beijing shows up as a huge hotspot of pollution, but relatively, the levels during the Olympics were significantly smaller:
August 2008 levels were therefore about 50% less than a similar period the year before. Meanwhile values at other hotspots in China were steady or got even worse. So there was a significant effect, but the scale of the task was indeed Olympian.