E. L. Woodbridge

Removal of Stratospheric O3 by Radicals: In Situ Measurements of OH, HO2, NO, NO2, ClO, and BrO

Simultaneous in situ measurements of the concentrations of OH, HO2, ClO, BrO, NO, and NO2 demonstrate the predominance of odd-hydrogen and halogen free-radical catalysis in determining the rate of removal of ozone in the lower stratosphere during May 1993. A single catalytic cycle, in which the rate-limiting step is the reaction of HO2 with ozone, accounted for nearly one-half of the total O3 removal in this region of the atmosphere. Halogen-radical chemistry was responsible for approximately one-third of the photochemical removal of O3; reactions involving BrO account for one-half of this loss. Catalytic destruction by NO2, which for two decades was considered to be the predominant loss process, accounted for less than 20 percent of the O3 removal. The measurements demonstrate quantitatively the coupling that exists between the radical families. The concentrations of HO2 and ClO are inversely correlated with those of NO and NO2. The direct determination of the relative importance of the catalytic loss processes, combined with a demonstration of the reactions linking the hydrogen, halogen, and nitrogen radical concentrations, shows that in the air sampled the rate of O3 removal was inversely correlated with total NOx, loading.

In situ observations in aircraft exhaust plumes in the lower stratosphere at midlatitudes

Instrumentation on the NASA ER-2 high-altitude aircraft has been used to observe engine exhaust from the same aircraft while operating in the lower stratosphere. Encounters with the exhaust plume occurred approximately 10 min after emission with spatial scales near 2 km and durations of up to 10 s. Measurements include total reactive nitrogen, NO(y), the component species NO and NO2, CO2, H2O, CO, N2O, condensation nuclei, and meteorological parameters. The integrated amounts of CO2 and H2O during the encounters are consistent with the stoichiometry of fuel combustion (1:1 molar). Emission indices (EI) for NO(x) (= NO + NO2), CO, and N2O are calculated using simultaneous measurements of CO2. EI values for NO(x) near 4 g/(kg fuel) are in good agreement with values scaled from limited ground-based tests of the ER-2 engine. Non-NO(x) species comprise less than about 20% of emitted reactive nitrogen, consistent with model evaluations. In addition to demonstrating the feasibility of aircraft plume detection, these results increase confidence in the projection of emissions from current and proposed supersonic aircraft fleets and hence in the assessment of potential long-term changes in the atmosphere.

The distribution of hydrogen, nitrogen, and chlorine radicals in the lower stratosphere: Implications for changes in O3 due to emission of NOy from supersonic aircraft

In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained in the lower stratosphere during SPADE are compared to results from a photochemical model that assimilates measurements of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N_2O_5 agree well with measured concentrations of NO and ClO, but concentrations of HO_2 and OH are underestimated by 10 to 25%, concentrations of NO_2 are overestimated by 10 to 30%, and concentrations of HCl are overestimated by a factor of 2. Discrepancies for [OH] and [HO_2] are reduced if we allow for higher yields of O(^1D) from O_3 photolysis and for heterogeneous production of HNO_2. The data suggest more efficient catalytic removal of O_3 by hydrogen and halogen radicals relative to nitrogen oxide radicals than predicted by models using recommended rates and cross sections. Increases in [O_3] in the lower stratosphere may be larger in response to inputs of NO_y from supersonic aircraft than estimated by current assessment models.

The diurnal variation of hydrogen, nitrogen, and chlorine radicals: Implications for the heterogeneous production of HNO2

In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained through sunrise and sunset in the lower stratosphere during SPADE are compared to results from a photochemical model constrained by observed concentrations of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N205 on sulfate aerosols agree with measured concentrations of NO, NO2, and C10 throughout the day, but fail to account for high concentrations of OH and HO2 observed near sunrise and sunset. The morning burst of (OH) and (HO2) coincides with the rise of (NO) from photolysis of NO 2, suggesting a new source of HOx that photolyzes in the near UV (350 to 400 nm) spectral region. A model that allows for the heterogeneous production of HNO2 results in an excellent simulation of the diurnal variations of (OH) and (HO2).

The seasonal evolution of reactive chlorine in the Northern Hemisphere stratosphere

In situ measurements of chlorine monoxide (ClO) at mid- and high northern latitudes are reported for the period October 1991 to February 1992. As early as mid-December and throughout the winter, significant enhancements of this ozone-destroying radical were observed within the polar vortex shortly after temperatures dropped below 195 k. Decreases in ClO observed in February were consistent with the rapid formation of chlorine nitrate (ClONO2) by recombination of ClO with nitrogen dioxide (NO2) released photochemically from nitric acid (HNO3). Outside the vortex, ClO abundances were higher than in previous years as a result of NOx suppression by heterogeneous reactions on sulfate aerosols enhanced by the eruption of Mount Pinatubo.

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.

Measurements of the NO y ‐N2O correlation in the lower stratosphere: Latitudinal and seasonal changes and model comparisons

The tracer species nitrous oxide, N2O, and the reactive nitrogen reservoir, NOy, were measured in situ using instrumentation carried aboard the NASA ER-2 high altitude aircraft as part of the NASA Airborne Southern Hemisphere Ozone Expedition/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) and Stratospheric Tracers of Atmospheric Transport (STRAT) missions. Measurements were made throughout the latitude range of 70°S to 60°N over the time period of March to October 1994 and October 1995 to January 1996, which includes the period when the Antarctic polar vortex is most intense. The correlation plots of NOy with N2O reveal compact, near-linear curves throughout data obtained in the lower stratosphere (50 mbar to 200 mbar). The average slope of the correlation, ΔNOy/ΔN2O, in the southern hemisphere (SH) exhibited a much larger seasonal variation during this time period than was observed in the northern hemisphere (NH). Between March and October in the potential temperature range of 400 K to 525 K, the correlation slope in the SH midlatitudes increased by 28%. A smaller but still positive increase in the correlation slope was observed for higher-latitude data obtained within or near the edge of the SH polar vortex. At NH midlatitudes the correlation slope did not significantly change between March and October, while between October and January the slope increased by +7%. The larger SH midlatitude increase is consistent with ongoing descent throughout the winter and spring and also suggests that denitrification, the irreversible loss of HNO3 through sedimentation of cloud particles, is not a significant term (<10–15%) in the budget of NOy at SH midlatitudes during the wintertime. A secular increase in the correlation slope is ruled out by comparison with SH data obtained during the 1987 Airborne Antarctic Ozone Expedition (AAOE) aircraft campaign. These results suggest that a seasonal cycle exists in the correlation slope for both hemispheres, with the SH correlation slope returning to the April value during the SH spring and summer. Changes in stratospheric circulation also probably play a role in both the SH and the NH correlation slope seasonal cycles. Comparisons with two-dimensional model results suggest that the slope decreases when the denitrified Antarctic vortex is diluted into midlatitudes upon vortex breakup in the spring and that through the descent of stratospheric air, the slope recovers during the following fall/winter period.

New observations of the NOy/N2O correlation in the lower stratosphere

During the Airborne Arctic Stratospheric Expedition II (AASE II), September 1991 through March 1992, in situ measurements of reactive nitrogen (NO[sub y]) and N[sub 2]O were made in the Northern Hemisphere lower stratosphere. The authors present an analysis of this new data and compare it with results from similar data taken during AASE in the winter of 1989. In the Northern Hemisphere there is a consistent linear correlation of N[sub 2]O and NO[sub y] which shows no interannual variation. Cases of departure from a linear correlation are examined and classified as being due to denitrification (NO[sub y] loss) or sampling air from a region where the photochemical lifetime of NO[sub y] is decreased. The latter case was observed for the first time in the winter of 1992. 19 refs., 4 figs., 1 tab.

Evaluating the role of NAT, NAD, and liquid H2SO4/H2O/HNO3 solutions in Antarctic polar stratospheric cloud aerosol: Observations and implications

Airborne measurements of total reactive nitrogen (NOy) and polar stratospheric cloud (PSC) aerosol particles were made in the Antarctic (68°S) as part of the NASA Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAES A) campaign in late July 1994. As found in both polar regions during previous studies, substantial PSC aerosol volume containing NOy was observed at temperatures above the frost point, confirming the presence of particles other than water ice. The composition of the aerosol particles is evaluated using equilibrium expressions for nitric acid trihydrate (NAT), nitric acid dihydrate (NAD), and the supercooled ternary solution (STS) composed of nitric acid (HNO3), sulfuric acid (H2SO4), and water (H2O). The equilibrium abundance of condensed HNO3 is calculated for each phase and compared to estimates made using observations of aerosol volume and NOy. The best agreement is found for STS composition, using criteria related to the onset and abundance of aerosol volume along the flight track. Throughout the PSC region, a comparison of the number of particles between 0.4 and 4.0 μm diameter with the number of available nuclei indicates that a significant fraction of the background aerosol number participates in PSC growth. Modeled STS size distributions at temperatures below 191 K compare favorably with measured size distributions of PSC aerosol. Calculations of the heterogeneous loss of chlorine nitrate (ClONO2) show that the reactivity of the observed PSC surface area is 30 to 300% greater with STS than with NAT composition for temperatures less than 195 K. The total volume of STS PSCs is shown to be more sensitive than NAT to increases in H2O, HNO3, and H2SO4 from supersonic aircraft fleet emissions. Using the current observations and perturbations predicted by the current aircraft assessments, an increase of 50 to 260% in STS aerosol volume is expected at the lowest observed temperatures (190 to 192 K), along with an extension of significant PSC activity to regions ∼0.7 K higher in temperature. These results improve our understanding of PSC aerosol formation in polar regions while strengthening the requirement to include STS aerosols in studies of polar ozone loss and the effects of aircraft emissions.

New photolysis system for NO2 measurements in the lower stratosphere

A new system for NO2 detection has been developed for use on the NASA ER-2 aircraft. The system converts NO2 to NO using UV photolysis with the NO product subsequently detected with an on-board chemiluminescence detector. The new system is compact, light weight, has high time resolution (- 1 s), and is significantly more efficient then some previous designs. Details of the system design and airborne performance are discussed.