[1] We report continuous whole canopy isoprene emission fluxes from a northern hardwood forest in Michigan for the 1999-2002 growing seasons. The eddy covariance fluxes of isoprene, CO 2 , latent heat, and sensible heat are presented along with an analysis of the seasonal and year-to-year variations. Measurements were made in collaboration with the AmeriFlux site located at the University of Michigan Biological Station (UMBS) and the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET). In general, isoprene emissions increased throughout the day with increasing temperature and light levels, peaked at midafternoon, and declined to zero by night. There were significant variations from one 30-min period to the next, and from one day to the next. Average midday isoprene fluxes were 2.8, 3.2, and 2.9 mg C m À2 h À1 for 2000 through 2002, respectively. Insufficient data were available to include 1999. Last frost and full leaf out were significantly later in 2002 compared to the other years; however, total accumulated isoprene emissions for each year varied by less than 10%. Fully developed isoprene emissions occurred between 400 and 500 heating degree days, roughly half those required at other sites. Using long-term net ecosystem exchange measurements from the UMBS$Flux group, isoprene emissions represent between 1.7 to 3.1% of the net carbon uptake at this site. Observations for 2000-2002 were compared with the BEIS3 emission model. Estimates agree well with observations during the midsummer period, but BEIS3 overestimates observations during the spring onset of emissions and the fall decline in emissions. This work provides a unique long-term data set useful for verifying canopy scale models and to help us better understand the dynamics of biosphere-atmosphere exchange of isoprene.

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An atmospheric tracer dispersion study known as Joint Urban 2003 was conducted in Oklahoma City, Oklahoma, during July of 2003. As part of this field program, vertical concentration profiles were measured at approximately 1 km from the downtown ground-level tracer gas release locations. These profiles showed that the urban landscape was very effective in mixing the plume vertically. In general, the lowest concentration measured along the profile was within 50% of the highest concentration in any given 5-min measurement period. The general slope of the concentration profiles was bounded by a Gaussian distribution with Briggs's urban equations (stability classes D and E/F) for vertical dispersion. However, measured concentration maxima occurred at levels above the surface, which would not be predicted by Gaussian formulations. Variations in tracer concentration observed in the time series between different release periods were related to changes in wind direction as opposed to changes in turbulence. This was demonstrated using data from mobile analyzers that captured the width of the plume by traveling east to west along nearby streets. These mobile-van-analyzer data were also used to compute plume widths. Plume widths increased for wind directions at larger angles to the street grid, and a simple model comprising adjusted open-country dispersion coefficients and a street channeling component, were used to describe the measured widths. This dispersion dataset is a valuable asset not only for developing advanced tools for emergency-response situations in the event of a toxic release but also for refining air-quality models.