Tropospheric Ozone Across North America in Summer 2004 From Ozonesonde Observations During IONS

During July - August 2004 an intensive series of ozone vertical profile measurements was carried out at locations across the United States and southeastern Canada. The ozonesonde observations were carried out under the umbrella of the INTEX/ICARTT/ITCT-2K4/NEAQS Ozone Network Study (IONS), part of an international campaign focused on the transport of atmospheric constituents over the United States, their transformation, and eventual export over the North Atlantic. The multinational effort included several aircraft and ground-based measurement platforms as well as the ozonesonde observations. Eleven sites made ozonesonde launches with nine of the locations making intensified observations. Three of the sites made daily soundings over an ~40-day period. A prominent feature of Summer 2004 was the absence of high surface ozone episodes in the northeastern U.S. The summer was characterized by the passage of weak cold frontal systems and the lack of stagnant high pressure. The middle and upper troposphere often saw significant stratosphere/troposphere exchange with relatively high ozone amounts. This feature showed up in the ozone profiles from Michigan to the NOAA Ship Ron Brown operating in the Gulf of Maine. In the southeastern and western U.S. ozone in the troposphere was nearer average conditions. A Lagrangian tracer model (FLEXPART) was used to identify various processes influencing the tropospheric ozone distribution. Signatures of stratospheric ozone transport into the upper and middle stratosphere were seen at most locations. Ozone production from precursor emissions was seen most prominently in the southeastern U.S. Comparison with longer measurement records at several of the sites showed that with the exception of the lower altitudes in the northeast U.S., Summer 2004 exhibited characteristics that were not atypical over the mid latitudes of North America. The summertime ozone distribution across mid latitude North America had highest low altitude ozone concentrations in the southeastern U.S., higher upper tropospheric ozone in the northeast, and lowest ozone amounts in both the upper and lower troposphere in the western U.S.