Showing papers by "Paul O. Wennberg published in 1997"
TL;DR: In this paper, the photodissociation quantum yields for acetone in the 290-320 nm wavelength region for pressures and temperatures characteristic of the upper troposphere were compared with those obtained during the NASA and NOAA sponsored Stratospheric Tracers of Atmospheric Transport (STRAT) field campaign.
Abstract: This paper summarizes measured photodissociation quantum yields for acetone in the 290-320 nm wavelength region for pressures and temperatures characteristic of the upper troposphere. Calculations combine this laboratory data with trace gas concentrations obtained during the NASA and NOAA sponsored Stratospheric Tracers of Atmospheric Transport (STRAT) field campaign, in which measurements of OH, HO_(2), odd-nitrogen, and other compounds were collected over Hawaii, and west of California during fall and winter of 1995/1996. OH and HO_(2) concentrations within 2 to 5 km layers just below the tropopause are ∼50% larger than expected from O_(3), CH_(4), and H_(2)O chemistry alone. Although not measured during STRAT, acetone is inferred from CO measurements and acetone-CO correlations from a previous field study. These inferred acetone levels are a significant source of odd-hydrogen radicals that can explain a large part of the discrepancy in the upper troposphere. For lower altitudes, the inferred acetone makes a negligible contribution to HO_(x) (HO+HO_(2)), but influences NO_(y) partitioning. A major fraction of HO_(x) production by acetone is through CH_(2)O formation, and the HO_(x) discrepancy can also be explained by CH_(2)O levels in the 20 to 50 pptv range, regardless of the source.
201 citations
TL;DR: In this paper, a photochemical model constrained by local observations of O_3, H_2O, NO, CO, hydrocarbons, albedo and overhead ozone column is used to estimate HO x in the upper troposphere during the NASA/STRAT campaign.
Abstract: 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
172 citations
TL;DR: In this article, high-resolution, high air mass solar spectra taken at the National Solar Observatory, Kitt Peak, Arizona, in March 1995 were used to demonstrate that atmospheric iodine chemistry is not responsible for the reductions observed in lower stratospheric ozone during the last several decades.
Abstract: Absorption attributable to atmospheric IO is observed in high-resolution, high air mass solar spectra taken at the National Solar Observatory, Kitt Peak, Arizona, in March 1995. These observations, together with cross sections measured in the laboratory for the IO {A^(2)Π3/2←X^(2)Π_(3/2) (2,0)} rotationally resolved electronic transition, are consistent with a total stratospheric iodine mixing ratio of 0.2 (+0.3 −0.2) parts per trillion by volume. This result, combined with recent laboratory measurements of the rate of the reactions of IO with other halogen species, suggests that iodine chemistry is not responsible for the reductions observed in lower stratospheric ozone during the last several decades.
52 citations
TL;DR: The ER-2 made two descents through upper tropospheric biomass burning plumes during ASHOE/MAESA as mentioned in this paper, and HO_x concentrations were consistently underestimated by a model constrained by observed NO_x levels inside the plumes.
Abstract: The ER-2 made two descents through upper tropospheric biomass burning plumes during ASHOE/MAESA. HO_x (= OH + HO_2) concentrations are largely self-limited outside the plumes, but become progressively more limited by reactions with NO_x (= NO + NO_2) at the higher NO_x concentrations inside the plumes. Sources of HO_x in addition to H_(2)O and CH_4 oxidation are required to balance the known HOx sinks both in the plumes and in the background upper troposphere. HO_x concentrations were consistently underestimated by a model constrained by observed NO_x concentrations. The size of the model underestimate is reduced when acetone photolysis is included. Models which do not include the additional HO_x sources required to balance the HO_x budget are likely to underestimate ozone production rates.
47 citations
TL;DR: In this article, the authors used a low pressure mercury lamp to produce low concentrations (10.5 - 10.9 cm^(-3)) of water vapor in air with a low-pressure mercury lamp.
Abstract: Calibration of laser induced fluorescence (LIF) instruments
that measure OH is challenging because it is difficult to
reliably introduce a known amount of this reactive radical into a measurement apparatus. In a recent paper, Hofzumahaus et al., [1996] describe a novel and seemingly simple technique to accomplish this goal: they dissociate trace quantities of water vapor in air with a low pressure mercury (Hg) lamp to produce low concentrations (10^5 - 10^9 cm^(-3)) of OH (R1).
38 citations
TL;DR: In this article, a photochemical trajectory model is used to constrain the role of different heterogeneous reactions and PSC types, which includes heterogeneous sulfate and pyrite chemistry.
Abstract: Simultaneous in situ measurements of HCl and ClO have been made for the first time in the southern hemisphere, allowing a systematic study of the processes governing chlorine activation between 15 and 20 km in the 1994 Antarctic winter. Data for several other gases (O_3, NO, NO_y, OH, HO_2, N_(2)O, CH_4, CO, H_(2)O, CFCs), particulates, and meteorological parameters were collected from the ER-2 aircraft out of New Zealand as part of the 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements of Atmospheric Effects of Stratospheric Aircraft (ASHOE/MAESA) campaign. Observations from the ER-2 in the fall (April–May), prior to polar night, show that chlorine activation begins with 60–75% of inorganic chlorine as HCl. By midwinter (July–August), near-total removal of HCl is observed. The wintertime loss of HCl in air recently exposed to extreme temperatures is found to be correlated with high levels of reactive chlorine (ClO and its dimer, Cl_(2)O_2) in the linear fashion expected from the stoichiometry of the heterogeneous reaction of hydrochloric acid with chlorine nitrate on polar stratospheric clouds (PSCs): HCl + ClONO_2 → Cl_2 + HNO_3. To constrain the role of different heterogeneous reactions and PSC types, we have used a photochemical trajectory model which includes heterogeneous sulfate and PSC chemistry. Model calculations of the evolution of reactive gases are compared with the in situ observations. In addition, simultaneous measurements of OH and HO_2 are used as a diagnostic for the occurrence of the heterogeneous reaction HOCl + HCl → Cl_2 + H_(2)O, which contributes to suppressed levels of HO_x inside the vortex. It is shown that the amount of chlorine activation is not strongly dependent on the composition of PSCs. However, HO_x levels exhibit different signatures depending on the type of heterogeneous surfaces that affected chlorine activation. Furthermore, this analysis implies that in the edge region of the Antarctic vortex, the observed near-total removal of HCl can result from latitudinal excursions of air parcels in and out of sunlight during the winter, which photochemically resupply HOCl and ClONO_2 as oxidation partners for HCl.
27 citations
TL;DR: In this article, in situ measurements of sulfuric acid aerosols in aircraft exhaust have emerged as a critical issue in determining the impact of supersonic aircraft on stratospheric ozone, and they imply that reactions with OH are responsible for oxidizing only a small fraction of SO2 (2%), and thus cannot explain the large number of particles observed in the exhaust wake of the Concorde.
Abstract: The generation of sulfuric acid aerosols in aircraft exhaust has emerged as a critical issue in determining the impact of supersonic aircraft on stratospheric ozone. It has long been held that the first step in the mechanism of aerosol formation is the oxidation of SO2 emitted from the engine by OH in the exhaust plume. We report in situ measurements of OH and HO2 in the exhaust plumes of a supersonic (Air France Concorde) and a subsonic (NASA ER-2) aircraft in the lower stratosphere. These measurements imply that reactions with OH are responsible for oxidizing only a small fraction of SO2 (2%), and thus cannot explain the large number of particles observed in the exhaust wake of the Concorde.
19 citations