(Invited Speaker) Role of the free troposphere on surface air quality

New research on health effects from ozone has shown impacts, including premature mortality, at lower concentrations than previously identified. As a result air quality standards for ozone are becoming increasingly stringent. Ozone is unique among the regulated pollutants in that background concentrations are approximately 50% of the air quality standard. Also ozone is the only regulated pollutant where the background has a positive gradient with height. These two facts make understanding the sources and variability of ozone in the free troposphere especially important for surface air quality.

Starting in 1997, we have made observations from surface, aircraft and mountain top observatories in the Pacific Northwest. These observations have shown that many sources contribute to enhanced concentrations of ozone and other pollutants in the free troposphere over North America. This includes Asian industrial emissions, biomass burning and strat-trop exchange. Starting in 2004 we began observations at the summit of Mt. Bachelor (2.7 km) in central Oregon. The data from this station have provided us with an improved understanding of the sources of ozone, particulate matter and mercury. By using multiple tracers (e.g. CO, O3, PM1, PM2.5, Hg, etc) we can often confirm the type of source being sampled using both chemistry and meteorology.

In implementing new and stricter ozone standards, one of the key questions is: How much of the ozone is under local control and how much is contributed by background air? While this question has been looked at using various models, we have started to look at this using a variety of observational based approaches. This includes examining the relationship between free trop and surface concentrations on various timescales. While free tropospheric data are relatively sparse we have made use of ozonesonde data, mountain top data and satellite observations to identify a significant contribution from the free troposphere on surface ozone, especially at elevated sites in the western US.