C. T. McElroy

Evidence for Large Upward Trends of Ultraviolet-B Radiation Linked to Ozone Depletion

Spectral measurements of ultraviolet-B radiation made at Toronto since 1989 indicate that the intensity of light at wavelengths near 300 nanometers has increased by 35 percent per year in winter and 7 percent per year in summer. The wavelength dependence of these trends indicates that the increase is caused by the downward trend in total ozone that was measured at Toronto during the same period. The trend at wavelengths between 320 and 325 nanometers is essentially zero.

Observed OH and HO2 in the upper troposphere suggest a major source from convective injection of peroxides

ER-2 aircraft observations of OH and HO_2 concentrations in the upper troposphere during the NASA/STRAT campaign are interpreted using a photochemical model constrained by local observations of O_3, H_2O, NO, CO, hydrocarbons, albedo and overhead ozone column. We find that the reaction Q(^(1)D) + H_2O is minor compared to acetone photolysis as a primary source of HO_x (= OH + peroxy radicals) in the upper troposphere. Calculations using a diel steady state model agree with observed HO_x concentrations in the lower stratosphere and, for some flights, in the upper troposphere. However, for other flights in the upper troposphere, the steady state model underestimates observations by a factor of 2 or more. These model underestimates are found to be related to a recent (< 1 week) convective origin of the air. By conducting time-dependent model calculations along air trajectories determined for the STRAT flights, we show that convective injection of CH_3OOH and H_2O_2 from the boundary layer to the upper troposphere could resolve the discrepancy. These injections of HO_x reservoirs cause large HO_x increases in the tropical upper troposphere for over a week downwind of the convective activity. We propose that this mechanism provides a major source of HO_x in the upper troposphere. Simultaneous measurements of peroxides, formaldehyde and acetone along with OH and HO_2 are needed to test our hypothesis.

Evidence for bromine monoxide in the free troposphere during the Arctic polar sunrise

During the Arctic polar springtime, dramatic ozone losses occur not only in the stratosphere but also in the underlying troposphere. These tropospheric ozone loss events have been observed over large areas, in the planetary boundary layer (PBL) throughout the Arctic. They are associated with enhanced concentrations of halogen species and are probably caused by catalytic reactions involving bromine monoxide (BrO) and perhaps also chlorine monoxide (ClO). The origin of the BrO, the principle species driving the ozone destruction, is thought to be the autocatalytic release of bromine from sea salt accumulated on the Arctic snow pack, followed by photolytic and heterogeneous reactions which produce and recycle the oxide. Satellite observations have shown the horizontal and temporal extent of large BrO enhancements in the Arctic troposphere, but the vertical distribution of the BrO has remained uncertain. Here we report BrO observations obtained from a high-altitude aircraft that suggest the presence of significant amounts of BrO not only in the PBL but also in the free troposphere above it. We believe that the BrO is transported from the PBL into the free troposphere through convection over large Arctic ice leads (openings in the pack ice). The convective transport also lifts ice crystals and water droplets well above the PBL, thus providing surfaces for heterogeneous reactions that can recycle BrO from less-reactive forms and thereby maintain its ability to affect the chemistry of the free troposphere.

Altitude distributions of stratospheric constituents from ground‐based measurements at twilight

A technique for extracting height profiles from ground-based column measurements at twilight is introduced. Its sensitivities to chemical processes, initialization, and air mass factors are investigated. The method is applied to observations made at Lauder, New Zealand, in 1987. The technique provides information on the vertical structure of atmospheric absorbers such as ozone or NO2 from the surface to about 50 km and is particularly valuable for identifying the influence of pollution on such measurements. When tropospheric pollution is low, it yields profiles in reasonable agreement with model predictions and with satellite measurements.

A comparison of observations and model simulations of NOx/NOy in the lower stratosphere

Extensive airborne measurements of the reactive nitrogen reservoir (NO_(y)) and its component nitric oxide (NO) have been made in the lower stratosphere. Box model simulations that are constrained by observations of radical and long-lived species and which include heterogeneous chemistry systematically underpredict the NO_x (= NO + NO_2) to NO_y ratio. The model agreement is substantially improved if newly measured rate coefficients for the OH + NO_2 and OH + HNO_3 reactions are used. When included in 2-D models, the new rate coefficients significantly increase the calculated ozone loss due to NO_x and modestly change the calculated ozone abundances in the lower stratosphere. Ozone changes associated with the emissions of a fleet of supersonic aircraft are also altered.

Partitioning of the Reactive Nitrogen Reservoir in the lower stratosphere of the southern hemisphere: Observations and modeling

Measurements of nitric oxide (NO), nitrogen dioxide (NO2), and total reactive nitrogen (NOy = NO + NO2 + NO3 + HNO3 + ClONO2 + 2N2O5 + …) were made during austral fall, winter, and spring 1994 as part of the NASA Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft mission. Comparisons between measured NO2 values and those calculated using a steady state (SS) approximation are presented for flights at mid and high latitudes. The SS results agree with the measurements to within 8%, suggesting that the kinetic rate coefficients and calculated NO2 photolysis rate used in the SS approximation are reasonably accurate for conditions in the lower stratosphere. However, NO2 values observed in the Concorde exhaust plume were significantly less than SS values. Calculated NO2 photolysis rates showed good agreement with values inferred from solar flux measurements, indicating a strong self-consistency in our understanding of UV radiation transmission in the lower stratosphere. Model comparisons using a full diurnal, photochemical steady state model also show good agreement with the NO and NO2 measurements, suggesting that the reactions affecting the partitioning of the NOy reservoir are well understood in the lower stratosphere.

Long‐term ozone decline over the Canadian Arctic to early 1997 from ground‐based and balloon observations

Column ozone measurements in the Canadian High Arctic (north of 70°N) started in 1957 and there have been regular ozone sonde flights there since 1966. Column ozone values over the High Arctic were as much as 45% below normal for some days in March 1997. During March 1996 arctic ozone values were also very low. In both years, temperatures in the arctic stratosphere were extremely cold and, especially in 1997, the vortex wind pattern was unusual for the arctic and quite similar to the antarctic spring vortex. Similar cold stratospheric temperatures were present in 1967, but ozone deviations were much smaller. Despite the very low values in the High Arctic during March 1997, the average column ozone from three Canadian sites in the 50–60°N latitude range was only 3.5% below normal.

Observational evidence for the role of denitrification in Arctic stratospheric ozone loss

Severe and extensive denitrification, chlorine activation, and photochemical ozone loss were observed throughout the lower stratosphere in the 1999–2000 Arctic vortex. A large number of air parcels sampled between late February and mid-March, 2000, were photochemically intercomparable for chemical O3 loss rates. In these air parcels, the temporal evolution of the correlations of O3 with the NOy remaining after denitrification provides strong evidence for the role of NOy in moderating O3 destruction. In 71%-denitrified air parcels, a chemical O3 destruction rate of 63 ppbv/day was calculated, while in 43%-denitrified air parcels the destruction rate was only 43 ppbv/day. These observational results show that representative denitrification models will be required for accurate prediction of future Arctic O3 changes.

Comparison of modeled and observed values of NO2 and JNO2 during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission

Stratospheric measurements of NO, NO_(2), O_(3), ClO, and HO_(2) were made during spring, early summer, and late summer in the Arctic region during 1997 as part of the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) field campaign. In the sunlit atmosphere, NO_(2) and NO are in steady state through NO2 photolysis and reactions involving O_(3), ClO, BrO, and HO_(2). By combining observations of O_(3), ClO, and HO_(2), observed and modeled values of the NO_(2) photolysis rate coefficient (JNO_(2)), and model estimates of BrO, several comparisons are made between steady state and measured values of both NO_(2) and JNO_(2). An apparent seasonal dependence in discrepancies between calculated and measured values was found; however, a source for this dependence could not be identified. Overall, the mean linear fits in the various comparisons show agreement within 19%, well within the combined uncertainties (±50 to 70%). These results suggest that photochemistry controlling the NO_(2)/NO abundance ratio is well represented throughout much of the sunlit lower stratosphere. A reduction in the uncertainty of laboratory determinations of the rate coefficient of NO + O_(3) → NO_(2) + O_(2) would aid future analyses of these or similar atmospheric observations.

Derivation of UV‐A irradiance from pyranometer measurements

Two statistical models have been developed from 6 years of simultaneous measurements of global radiation by pyranometers, UV-A by a Brewer spectrophotometer and from total ozone, dew point temperature and snow cover data at Toronto. The models estimate instantaneous UV-A irradiance at 324 nm from pyranometer data with an uncertainty as low as 3.5% (1σ) for summer sunny conditions and between 6–10% for cloudy conditions. These uncertainties are reasonably small when compared with the uncertainty of UV-A and global solar irradiance measurements (2–3%). The uncertainty is reduced when daily and longer-term averages are considered. The major source of error in the models is likely linked to rare occurrences of absorbing aerosols in the atmosphere. The models were also tested on a 6-year, independent record from Edmonton. The uncertainties at Edmonton are 30–45% larger than at Toronto for the instantaneous data, approximately 20% larger for daily integrated values.