It used to be my job to teach the environmental health survey course for public health students and air pollution was a topic I spent a lot of time on because it interested me and intersected some of my research work. One of the things I taught my students was that some air pollutants were very local — carbon monoxide (CO) being a good example; levels of CO on one side of the street could vary significantly from those on the other side by virtue of traffic patterns or street canyon effects — while others were considered regional pollutants. Ozone (O3) was my example of choice. It isn’t emitted by sources by formed as a secondary pollutant via chemical reactions in the atmosphere from chemical precursors like volatile organics and nitrogen oxides (juiced with sunlight) which were emitted by sources (primary pollutants). Thus ozone was an area-wide pollutant with not as much spatial variation, although there was a paradoxical suburban-rural high ozone effect caused by additional reactions of high precursors in the city “eating up” some of the ozone they had produced, leaving a relatively lower ozone level. It turns out my former teaching didn’t go far enough in its areal reach. A new paper in Nature suggests that a significant proportion of background ozone in the lower levels of the atmosphere may be a result of long range transport from East Asia, especially China:
?In springtime, pollution from across the hemisphere, not nearby sources, contributes to the ozone increases above western North America,? said lead author Owen R. Cooper, Ph.D., of the NOAA-funded Cooperative Institute for Research in Environmental Sciences at the University of Colorado at Boulder. ?When air is transported from a broad region of south and east Asia, the trend is largest.?
Cooper and colleagues from NOAA?s Earth System Research Laboratory in Boulder, Colo., and eight other research institutes used historical data of global atmospheric wind records and sophisticated computer modeling to match each ozone measurement with air-flow patterns for several days before it was recorded. This approach essentially let the scientists track ozone-producing emissions back to a broad region of origin.
When the dominant airflow came from south and east Asia, the scientists saw the largest increases in ozone measurements. When airflow patterns were not directly from Asia, ozone still increased but at a lower rate, indicating the possibility that emissions from other places could be contributing to the ozone increases above North America. The study used springtime ozone measurements because previous studies have shown that air transport from Asia to North America is strongest in spring, making it easier to discern possible effects of distant pollution on the North American ozone trends. (NOAA Press release; I also read the paper, but the press release is more comprehensible and clearer; Cooper et al., Increasing springtime ozone mixing ratios in the free
troposphere over western North America, Nature Vol 463| 21 January 2010| doi:10.1038/nature08708)
Just so there’s no confusion, ozone in the lower layer of the atmosphere, where we live, is bad. It’s not only a greenhouse gas, but it damages the lungs and vegetation and crops. Ozone is the primary constituent of photochemical oxidant pollution, otherwise known as “smog.” Ozone in the next layer of the atmosphere (the stratosphere) is good for human health because it absorbs higher frequency ultraviolet light that might otherwise damage human health. This is why we are worried about the “ozone hole” in the stratosphere. So ozone is “good” up there, “bad” down here, where we are.
This study is still preliminary and doesn’t quantify the contribution from across the Pacific. But it does show once again how tightly interconnected we are. Ozone precursors and hence ozone production are decreasing in the US and western Europe, but a rapidly industrializing East and South Asia could potentially cancel out the benefits. The rest of the world is now returning the favor we did them when we were industrializing.
What goes around, comes around. And around. And around.