On June 28, 1989, a severe thunderstorm over North Dakota developed into a squall line and then into a mesoscale convective complex (MCC) with overshooting tops as high as ∼14 km and a cirrus anvil that covered more than 300,000 km2. In this paper we describe the trace gas concentrations prior to, in, and around the storm; paper 2 presents numerical simulations. Observations of O3 and θeq unaffected by upstream convection for at least 3 days prior to the flights placed the undisturbed tropopause between 10.7 and 11 km. The anvil outflow, sampled at altitudes of 10.8 to 12.2 km, extended well into what used to be the stratosphere. Air inside the anvil was characterized by notably low concentrations of O3 and high CO relative to the out‐of‐cloud environment. Elevated concentrations of NO and NOy, due to lightning and upward transport, were observed in the anvil. A tongue of air with tropospheric characteristics lay above stratospheric air, showing that extensive stratosphere‐troposphere exchange had occurred. The effects of this mechanism on atmospheric budgets of trace species depend on the fate of the air that enters the anvil and on the frequency of MCCs. Assuming that the symmetry was cylindrical and that the material transported during the observations at the east edge of the anvil was representative of the entire cirrus anvil cloud, we estimate a minimum flux of 2 × 1010 g of O3 into the troposphere and a maximum flux of 3–7 × 1013 g of H2O into the stratosphere. This is a greater flux of water than the stratospheric water budget can support, and thus most of this water must return to the troposphere; the ice crystals were of sufficient size to have substantial settling velocity. If, however, even a small fraction of the mass of such anvils remains in the stratosphere, then convective transport of reactive tropospheric trace species such as NOy, CO, and NMHC may dominate the chemistry of the lower stratosphere in this midlatitude region. More detailed estimates of the fluxes, taking into account the rear anvil as well, are presented in the companion paper.

Mixing across the tropopause due to intense convective events may significantly influence the atmospheric chemical balance. Stratosphere‐troposphere exchange acts as an important natural source of O3 in the troposphere, and a source of H2O, HCs, CFCs, HCFCs, and reactive nitrogen in the stratosphere. The redistribution of atmospheric trace gases produces secondary radiative, dynamical and climate effects, influencing lower stratospheric temperatures and the tropopause height. During the 1989 North Dakota Thunderstorm Project, a severe storm which evolved into a mesoscale convective complex (MCC) on June 28–29 showed the unusual feature of an anvil formed well within the stratosphere and produced strong vertical mixing of atmospheric trace gases including H2O, CO, O3 and NOy as discussed by Poulida et al. [this issue] in Part 1 of this paper. In this paper the two‐dimensional NASA Goddard Cumulus Ensemble (GCE) model was employed to simulate this convective storm using observed initial and boundary conditions. The sensitivity to the domain size, initial and boundary conditions, stability, and time resolution are evaluated. Synoptic‐scale moisture convergence, simulated by moist boundary inflow, influences significantly the storm intensity, spatial structure, and trace gas transport, and produces a storm that reintensifies after the initial decay, mimicking the observed behavior of the MCC. The deformation of the tropopause documented with aircraft observations was qualitatively reproduced along with transport of stratospheric ozone downward into the troposphere, and the transport of trace species from the boundary layer upward into the stratosphere. If the chemistry and dynamics of this storm are typical of the roughly 100 MCCs occurring annually over midlatitudes, then this mechanism plays an important role in CO, NOy, and O3 budgets and could be the dominant source of H2O in the lower stratosphere and upper troposphere over midlatitudes.

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Carbon monoxide (CO) and ozone (03) play a central role in the oxidizing capacity of the atmosphere. Standard meteorological parameters and concentrations of these trace gases at Big Meadows, Shenandoah National Park, Virginia, were monitored almost continuously from October 1988 to October 1989. The National Park Service has been measuring 03 at this and two other sites in the park since 1983. Seasonal, monthly, and diurnal variations of hourly averages are examined. In the winter, dry deposition dominates; ozone values are relatively low with CO and 03 negatively correlated. In the summer, photochemistry dominates; ozone values are relatively high, and CO and 03 are positively correlated. Ozone shows a yearly mean mixing ratio of 33 (c•= 12) ppbv and did not exceed the ambient air quality standard during this year. CO mixing ratios averaged 204 (c•= 51) ppbv with no discernible diurnal or seasonal variation. Histograms of hourly means of 03 and CO appear lognormal, but the chi-square tests for goodness of fit reject the hypotheses. Several lines of evidence suggest that the data are little affected by local sources and are reasonably representative of the regional air quality. The summer of 1989 was cooler than normal, and the average ozone concentration was lower than the 7-year mean, although an analysis of the full record illustrates no statistically significant trend. INTR OD U CTIONThe downward transport of ozone (03) from the stratosphere was thought to be the main source of tropospheric ozone [Junge, 1962], but the appearance of urban, Los Angeles type smog stimulated research on photochemical ozone production in the troposphere, where 03 is formed as a by-product of the photoxidation of hydrocarbons and is destroyed by dry deposition.The current interest in tropospheric ozone has two roots. One is the global importance of ozone in generating hydroxyl radicals (OH) by the photoproduction of O(1D) and its reaction with water vapor (reactions (R1) and (R2)). The concentration of OH in turn influences the concentrations of many trace species, such as CH4, CO, SO2, CH3CC13 (reactions (R3) and (R4), for example). O3+ hv --> O2+ O(1D) O(1D) + H20 --> 2OH OH + CO (+ 02) --> CO2 + HO2 The increase of tropospheric ozone over North America, Europe, and Japan [Logan, 1985] results in the greater oxidation of hydrocarbons. The second root is the oxidation strength of 03, which can be dangerous to human health [Folinsbee et al., 1988]. It is deleterious to vegetation and is thought to be responsible for most of the crop damage caused by air pollution in the United States [Heck et al., 1982, 1983, 1984; Douchelie et al., 1982; Reich and Aroundson, 1985] and to contribute to the observed decline of forests in Europe and the eastern U.S. [Skarby and Sellden, 1984]. Ozone plays a key role in biogeochemical cycles, air quality, and global change. As a pollutant and greenhouse gas, 03 should be brought under effective control, but the variability of its natural background level in the boundary layer makes difficult the def...

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