M. H. Proffitt

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.

A barrier to vertical mixing at 14 km in the tropics: Evidence from ozonesondes and aircraft measurements

We use ozonesondes launched from Samoa (14°S) during the Pacific Exploratory Mission (PEM) Tropics A to show that O3 mixing ratios usually start increasing toward stratospheric values near 14 km. This is well below the tropical tropopause (as defined either in terms of lapse rate or cold point), which usually occurs between 16 and 17 km. We argue that the main reason for this discrepancy in height between the chemopause and tropopause is that there is very little convective detrainment of ozone-depleted marine boundary layer air above 14 km. We conjecture that the top of the Hadley circulation occurs at roughly 14 km, that convective penetration above this altitude is rare, and that air that is injected above this height subsequently participates in a slow vertical ascent into the stratosphere. The observed dependence of ozone on potential temperature in the transitional zone between the 14-km chemopause and the tropical tropopause is consistent with what would be expected from this hypothesis given calculated clear-sky heating rates and typical in situ ozone production rates in this region. An observed anticorrelation between ozone and equivalent potential temperature below 14 km is consistent with what would be expected from an overturning Hadley circulation, with some transport of high O3/low θe air from midlatitudes. We also argue that the positive correlations between O3 and N2O in the transitional zone obtained during the 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft) (ASHOE/MAESA) campaign support the notion that air in this region does have trace elements of stratospheric air (as conjectured previously), so that some of the ozone in the transitional zone does originate from the stratosphere rather than being entirely produced in situ.

Fast‐response dual‐beam UV‐absorption ozone photometer suitable for use on stratospheric balloons

Various types of ozone detectors are currently in use, each with different advantages and compromises in response time, portability, sensitivity, accuracy, need for repeated calibration, and expense. We describe here a new dual‐beam UV‐absorption instrument for balloon‐borne measurements of atmospheric ozone. It has two identical absorption chambers, each alternating between reference mode (ozone free) and sample mode by means of a four‐port valve and ozone scrubber. The ratio of the absorption signals, along with the known lengths and ozone absorption cross section, yield the ozone concentration. The dual‐beam feature cancels the effects of lamp intensity fluctuations, while the mode alternation compensates for mechanical changes and also provides continuous measurements. The absorption measurement requires no calibration and, hence, is independent of gas flow rate. The response time is 1 s and, for this measurement duration, the minimum ozone concentration detectable by this instrument (one standard dev...

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.

Stratospheric horizontal wavenumber spectra of winds, potential temperature, and atmospheric tracers observed by high‐altitude aircraft

Abstract : Horizontal wavenumber power spectra of vertical and horizontal wind velocities, potential temperatures, and ozone and N(2)O mixing ratios, as measured in the mid-stratosphere during 73 ER-2 flights (altitude approx. 20km) are presented. The velocity and potential temperature spectra in the 100 to 1-km wavelength range deviate significantly from the uniform -5/3 power law expected for the inverse energy-cascade regime of two-dimensional turbulence and also for inertial-range, three-dimensional turbulence. Instead, steeper spectra approximately consistent with a -3 power law are observed at horizontal scales smaller than 3 km for all velocity components as well as potential temperature. Shallower spectra are observed at scales longer than 6 km. For horizontal velocity and potential temperature the spectral indices at longer scales are between -1.5 and -2.0. For vertical velocity the spectrum at longer scales become flat. It is argued that the observed velocity and potential temperature spectra are consistent with gravity waves. At smaller scales, the shapes are also superficially consistent with a Lumley-Shur-Weinstock buoyant subrange of turbulence and/or nonlinear gravity waves. Contemporaneous spectra of ozone and N(sub 2)O mixing ratio in the 100 to 1-km wavelength range do conform to an approximately uniform -5/3 power law. It is argued that this may reflect interactions between gravity wave air-parcel displacements and laminar or filamentary structures in the trace gas mixing ratio field produced by enstropy-cascading two-dimensional turbulence.

Chemical loss of ozone in the arctic polar vortex in the winter of 1991-1992.

In situ measurements of chlorine monoxide, bromine monoxide, and ozone are extrapolated globally, with the use of meteorological tracers, to infer the loss rates for ozone in the Arctic lower stratosphere during the Airborne Arctic Stratospheric Expedition II (AASE II) in the winter of 1991-1992. The analysis indicates removal of 15 to 20 percent of ambient ozone because of elevated concentrations of chlorine monoxide and bromine monoxide. Observations during AASE II define rates of removal of chlorine monoxide attributable to reaction with nitrogen dioxide (produced by photolysis of nitric acid) and to production of hydrochloric acid. Ozone loss ceased in March as concentrations of chlorine monoxide declined. Ozone losses could approach 50 percent if regeneration of nitrogen dioxide were inhibited by irreversible removal of nitrogen oxides (denitrification), as presently observed in the Antarctic, or without denitrification if inorganic chlorine concentrations were to double.

Comparison of MkIV balloon and ER‐2 aircraft measurements of atmospheric trace gases

On May 8, 1997, vertical profiles of over 30 different gases were measured remotely in solar occultation by the Jet Propulsion Laboratory MkIV Interferometer during a balloon flight launched from Fairbanks, Alaska. These gases included H 2 O, N 2 O, CH 4 , CO, NO x , NO y , HCI, ClNO 3 , CCl 2 F 2 , CCl 3 F, CCl 4 , CHClF 2 , CClF 2 CCl 2 F, SF 6 , CH 3 Cl, and C 2 H 6 , all of which were also measured in situ by instruments on board the NASA ER-2 aircraft, which was making flights from Fairbanks during this same early May time period as part of the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) experiment. A comparison of the gas volume mixing ratios in the upper troposphere and lower stratosphere reveals agreement better than 5% for most gases. The three significant exceptions to this are SF 6 and CCl 4 for which the remote measurements exceed the in situ observations by 15-20% at all altitudes, and H 2 O for which the remote measurements are up to 30% smaller than the in situ observations near the hygropause.

Ozone loss inside the northern polar vortex during the 1991 - 1992 winter

Measurements made in the outer ring of the northern polar vortex from October 1991 through March 1992 reveal an altitude-dependent change in ozone, with a decrease at the bottom of the vortex and a substantial increase at the highest altitudes accessible to measurement. The increase is the result of ozone-rich air entering the vortex, and the decrease reflects ozone loss accumulated after the descent of the air through high concentrations of reactive chlorine. The depleted air that is released out of the bottom of the vortex is sufficient to significantly reduce column ozone at mid-latitudes.

Water vapor and cloud water measurements over Darwin during the STEP 1987 tropical mission

Measurements of stratospheric and upper tropospheric cloud water plus water vapor (total water) and water vapor were made with two Lyman α hygrometers as part of the STEP tropical experiment. The in situ measurements were made in the Darwin, Australia, area in January and February of 1987 on an ER-2 aircraft. Average stratospheric water vapor at a potential temperature of 375 K (the average value of θ at the tropopause) was 2.4 parts per million by volume (ppmv). This water mixing ratio is below the 3.0 to 4.0 ppmv necessary to be consistent with the observed upper stratospheric dryness. Saturation with respect to ice and the potential for dehydration was observed up to θ = 402 K.

Biomass burning and deep convection in southeastern Asia: Results from ASHOE/MAESA

There was extensive biomass burning in Indonesia, northern Australia, and New Guinea during September and October 1994. This paper discusses two accidental encounters of biomass plumes from the 1994 Airborne Southern Hemisphere Ozone Experiment and Measurements for Assessing the Effects of Stratospheric Aircraft campaign (ASHOE/MAESA). During the October 23 descent into Fiji, and an ascent from Fiji on October 24, the NASA ER-2 passed through layers highly enhanced in NO, NO y , CO, and O 3 . These layers occurred near an altitude of 15 km. Back trajectories and satellite images indicate that the layers probably originated as outflow from a convective disturbance near New Guinea. The measurements indicate that deep convection can inject emissions from southeast Asian biomass burning to near tropical tropopause altitudes. Deep convection magnifies the impact of biomass burning on tropospheric chemistry because of the much longer residence times and chemical lifetimes of species in the upper tropical troposphere. Transport of the products of southeast Asian biomass burning into the upper tropical troposphere, followed by southward high-level outflow and advection by the subtropical jet, may play a significant role in dispersing these emissions on a global scale. Anthropogenic emissions from countries in southeast Asia are likely to increase in the future as these countries become more highly industrialized. This transport mechanism may play a role in increasing the impact of these types of emissions as well.