Engineering topics

Kawa, S. R.

Publications and source records attributed to Kawa, S. R..

At least 55 records · Page 3

NASA NTRS1998

Contrast Between 1992 and 1997 High Latitude Spring Haloe Observations of Lower Stratospheric HCl

HCl measurements from HALOE in the northern hemisphere during mid-May 1997 revealed vortex fragments in which the chlorine reservoir partitioning was strongly pushed toward HCl (approx. 90% HCl, approx. 10% ClONO2), similar to partitioning previously observed in the Antarctic vortex region. In contrast, observations of ClONO2 and HCl in the northern polar spring, 1992, and in other years, show these species established the balance typical for gas phase photochemical reactions in this region (approx. 60% HCl, approx. 40% ClONO2). Annually, chlorine reservoirs in the winter lower stratosphere polar vortex are converted to chlorine radicals via heterogeneous reactions on particle surfaces at very cold temperatures (less than about 200 K). As temperatures warm in spring, the heterogeneous processes become insignificant compared with gas phase reactions, and the chlorine reservoirs are reformed. Measurements through the northern winter/spring in 1992 show rapid formation of ClONO2, followed by steady loss of ClONO2 and increasing HCl. Although ClONO2 measurements are not available for 1997, the HCl increase in 1997 is observed to be much more rapid and the eventual HCl mixing ratio is about 50% greater than that of 1992. The observations are examined through comparison with the Goddard three-dimensional chemistry and transport model. This model utilizes winds and temperatures from the Goddard Earth Observing System Data Assimilation System and a complete integration scheme for stratospheric photochemistry. Analysis of the evolution of HCl and ClONO2 shows that the observed difference in the overall rate of HCl formation is explained by the sensitivity of the gas-phase chemistry to the ozone mixing ratio and the temperature. The results show that the model accurately simulates HCl and ClONO2 evolution during these two winters. Model validity is further supported by comparisons with O3 and reactive nitrogen species NO and NO2. This analysis provides a sensitive test of the lower stratospheric chlorine photochemistry, particularly because the analysis considers constituent evolution at a time when the HCl and ClONO2 are far from a photochemical stationary state.

Douglass, A. R.↗

NASA NTRS1998

Model Simulations of Ozone in the Summer Lower Stratosphere

The Goddard 3D chemistry and transport model (CTM) uses winds and temperatures from the Goddard Earth Observing System Data Assimilation System (GEOS DAS); thus CTM simulations can be compared directly with observations from satellite, balloon and aircraft. In general, aspects of these comparisons show remarkable agreement between observation and model. One significant difference is that the model ozone is high biased below the ozone peak. The bias is apparently largest at high latitudes during the summer months. At the same time, comparisons with HALOE observations show that at mid to high latitudes, the ozone mixing ratio peak appears persistently at a lower altitude than observed by HALOE; the peak mixing ratio is also overestimated by the model. Both transport and photochemistry are possible contributors to the biased ozone in the lower stratosphere - excessive downward motion would increase lower stratospheric ozone, as would a too large vertical gradient in ozone. On the other hand, comparisons of model N2O and NOy with observations suggest transport deficiencies in the opposite sense, i.e., model N2O can be high relative to observations (particularly during winter), suggesting the need for stronger downward transport. Sensitivity studies have been carried out using parameterizations for ozone production and loss, NOy production and loss, and N2O loss. The goal of these studies is to clarify how problems in the photochemical scheme at and above the ozone peak influence the lower stratospheric ozone.

Douglass, Anne R.↗

NASA NTRS1998

The Influences of Airmass Histories on Radical Species During POLARIS

The POLARIS mission focused on understanding the processes associated with the decrease of polar stratospheric ozone from spring to fall at high latitudes. This decrease is linked primarily to in situ photochemical destruction by reactive nitrogen species, NO and NO2, which also control other catalytic loss cycles. Steady state models have been used to test photochemistry and radical behavior but are not always adequate in simulating radical species observations. In some cases, air mass history can be important and trajectory models give an improved simulation of the radical species. Trajectory chemistry models, however, still consistently underestimate NO and NO2 abundances compared to measurements along the ER-2 flight track. The Goddard chemistry on trajectory model has been used to test updated rate constants for NO2 + OH, NO2 + O and OH + HNO3, key reactions that affect NO and NO2 abundances. We present comparisons between the modified Goddard chemistry on trajectory model, the JPL steady state model and observations from selected flights.

Pierson, James M.↗

NASA NTRS1993

Interpretation of NO(x)/NO(y) observations from AASE-2 using a model of chemistry along trajectories

In situ measurements of NO and NO(y) are used to derive the ratio NO(x)/NO(y) along the flight track of the NASA ER-2 aircraft. Data are presented for two flights at midlatitudes in October 1991 during the Airborne Arctic Stratospheric Expedition-2 (AASE-2). Aerosol particle surface area was concurrently measured. The observations are compared with a photochemical model integrated along back trajectories from the aircraft flight track. Comparison of observations with the model run along trajectories and at a fixed position clearly and quantitatively demonstrates the importance of an air parcel's dynamic history in interpretation of local chemical observations. Comparison of the data with model runs under different assumptions regarding heterogeneous chemistry further reinforces the case for occurrence of the reaction of N2O5 + H2O on sulfate aerosol surfaces in the atmosphere. Finally, comparisons for which relative changes in the model and the data are not consistent caution that our ability to resolve all the observations is not yet complete.

Kawa, S. R.↗

NASA NTRS1993

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

During the Airborne Arctic Stratospheric Expedition 2 (AASE 2), September 1991 through March 1992, in situ measurements of reactive nitrogen (NO(y) and N20 were made in the Northern Hemisphere lower stratosphere. We 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 consistent linear correlation of N2O and NO(y) which shows no interannual variation. Cases of departure from a linear correlation are examined and classified as being due to denitrification (NO(y) loss) or sampling air from a region where the photochemical lifetime of NO(y) is decreased. The latter case was observed for the first time in the winter of 1992.

Loewenstein, M.↗

NASA NTRS1993

New Observations of the NO(y)/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(y)) and N2O were made in the Northern Hemisphere lower stratosphere. We 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 N2O and NO(y) which shows no interannual variation. Cases of departure from a linear correlation are examined and classified as being due to denitrification (NO(y) loss) or sampling air from a region where the photochemical lifetime of NO(y) is decreased. The latter case was observed for the first time in the winter of 1992.

Lowenstein, M.↗

NASA NTRS1993

In situ measurements constraining the role of sulphate aerosols in mid-latitude ozone depletion

In situ measurements of stratospheric sulphate aerosol, reactive nitrogen and chlorine concentrations at middle latitudes confirm the importance of aerosol surface reactions that convert active nitrogen to a less active, reservoir form. This makes mid-latitude stratospheric ozone less vulnerable to active nitrogen and more vulnerable to chlorine species. The effect of aerosol reactions on active nitrogen depends on gas phase reaction rates, so that increases in aerosol concentration following volcanic eruptions will have only a limited effect on ozone depletion at these latitudes.

Fahey, D. W.↗

NASA NTRS1993

Reactive nitrogen and its correlation with ozone in the lower stratosphere and upper troposphere

High resolution in situ measurements of reactive nitrogen (NO(y)) and O3 were made in the upper troposphere and lower stratosphere at a variety of latitudes and seasons. In the lower stratosphere, NO(y) and O3 are very highly positively correlated at all times and spatial scales sampled. The ratio NO(y)/NO3 is much less variable than either species measured separately. The ratio has a much weaker gradient with altitude than the mixing ratios of O3 or NO(y). The ratio is smaller and decreases more rapidly with altitude in the tropics than at midlatitudes. In the upper troposphere NO(y) and O3 are only weakly correlated. Their ratio in the tropical upper troposphere is about 0.005-0.025 and the ratio in the midlatitude upper troposphere is about 0.004-0.010. The NO(y) in the upper troposhere is probably partly due to lightning.

Murphy, D. M.↗

NASA NTRS1992

Polar stratospheric cloud processed air and potential vorticity in the Northern Hemisphere lower stratosphere at mid-latitudes during winter

The present study compares small-scale (less than 100 km) features in ER-2 measurements of ClO, O3, H2O, N2O, and NO(y) outside the lower stratospheric Arctic vortex of 1988-1989 with features on potential vorticity maps from ECMWF. The potential vorticity maps are obtained from T106 analyses and forecasts. Some of the plots were truncated to lower resolution (T63 or T42) which smooths out the finer-scale structure. Comparison of these lower resolution plots shows how much detail is lost by excessive smoothing. It is also evident that the forecast plots lose fine-scale structure due to dissipation in the model resulting mainly from horizontal diffusion. It is concluded that blobs of air on the maps at latitudes between the vortex edge and 25 deg N having potential vorticities characteristic of the vortex did indeed originate from the vortex, but that the real atmosphere is more sharply differentiated than the meteorological analyses, implying that the potential vorticity maps underestimate the amount of peeled-off material.

Tuck, A. F.↗

NASA NTRS1992

Photochemical partitioning of the reactive nitrogen and chlorine reservoirs in the high-latitude stratosphere

The correlated set of measurements in the lower stratosphere from polar missions of the NASA ER-2 is used to derive partitioning of the major components of the reactive nitrogen and inorganic chlorine reservoirs. The results provide a consistent method for comparing distributions, and hence the controlling processes, between different areas of the near-polar regions. Clear evidence of the effects of heterogeneous processes in the atmosphere is found. Values for NO2, ClONO2, N2O5, and Cl2O2 are derived in a simplified steady-state model based on in situ NO, ClO, O3, temperature, and pressure measurements, laboratory-measured reaction rates; and modeled photodissociation rates. Values for the reservoir totals are independently derived from measurements of N2O, organic chlorine, and total reactive nitrogen. The relative abundances of the measured and derived species within the reservoirs are calculated, and the longer-lived species HCl and HNO3 are estimated as the residuals of their respective reservoirs.

Kawa, S. R.↗

NASA NTRS1992

The Arctic polar stratospheric cloud aerosol - Aircraft measurements of reactive nitrogen, total water, and particles

In situ aircraft measurements in the lower stratosphere are used to investigate the reactive nitrogen, NO(y), total water, and particle components of the polar stratospheric cloud (PSC) aerosol in the Arctic. The results are compared to findings from the Antarctic derived using similar measurements and interpretive techniques. The Arctic data show that particle volume well above background values is present at temperatures above the frostpoint, confirming the result from the Antarctic that the observed PSCs are not water ice particles. NO(y) measurements inside a PSC are enhanced above ambient values consistent with anisokinetic sampling of particles containing NO(y). In the Arctic data over long segments of several flights, calculations show saturation with respect to nitric acid trihydrate without significant PSC particle growth above background.

Kawa, S. R.↗

NASA NTRS1992

Particle size distributions in Arctic polar stratospheric clouds, growth and freezing of sulfuric acid droplets, and implications for cloud formation

The paper uses particle size and volume measurements obtained with the forward scattering spectrometer probe model 300 during January and February 1989 in the Airborne Arctic Stratospheric Experiment to investigate processes important in the formation and growth of polar stratospheric cloud (PSC) particles. It is suggested on the basis of comparisons of the observations with expected sulfuric acid droplet deliquescence that in the Arctic a major fraction of the sulfuric acid droplets remain liquid until temperatures at least as low as 193 K. It is proposed that homogeneous freezing of the sulfuric acid droplets might occur near 190 K and might play a role in the formation of PSCs.

Dye, James E.↗

NASA NTRS1991

The potential for ozone depletion in the Arctic polar stratosphere

The nature of the Arctic polar stratosphere is observed to be similar in many respects to that of the Antarctic polar stratosphere, where an ozone hole has been identified. Most of the available chlorine (CHl and ClONO2) was converted by reactions on polar stratospheric clouds to reactive ClO and Cl2O2 thoroughout the Arctic polar vortex before midwinter. Reactive nitrogen was converted to HNO3, and some, with spatial inhomogeneity, fell out of the stratosphere. These chemical changes ensured characteristic ozone losses of 10 to 15 percent at altitudes inside the polar vortex where polar stratospheric clouds had occurred. These local losses can translate into 5 to 8 percent losses in the vertical column abundance of ozone. As the amount of stratospheric chlorine inevitably increases by 50 percent over the next two decades, ozone losses recognizable as an ozone hole may well appear.

Brune, W. H.↗

NASA NTRS1990

Interpretation of aircraft measurements of NO, ClO, and O3 in the lower stratosphere

Results are presented from an October 17, 1988, flight of the NASA ER-2 stratospheric research aircraft. The flight sampled a nearly constant air mass at 20 km altitude, near 39 deg N latitude, from before sunrise until near noon. The instrumentation on board simultaneously measured NO, ClO, O3, temperature, and pressure. The measurements are combined with modeled photodissociation coefficients and known reaction kinetics to infer abundances of other important species, and the results are compared to previous estimates as a test for consistency in the understanding of the photochemical processes governing the species distributions.

Kawa, S. R.↗

NASA NTRS1990

A diagnostic for denitrification in the winter polar stratospheres

The pairwise correlation of NO(y) and N2O data from the Southern and Northern Hemispheres is presented. Both data sets show a linear correlation region, defined as a reference state, and regions of denitrification where the correlation breaks down. Using two-dimensional photochemical model simulations of the atmosphere, a similar linear correlation is found between NO(y) and N2O, thereby establishing a theoretical framework for the reference state. This general approach, which can be extended to other pairs of molecules, should prove to be powerful in further comparisons of aircraft data with numerical models.

Fahey, D. W.↗

NASA NTRS1990

Observations of denitrification and dehydration in the winter polar stratospheres

It is argued that denitrification of the Arctic stratosphere can be explained by the selective growth and sedimentation of aerosol particles rich in nitric acid. Because reactive nitrogen species moderate the destruction of ozone by chlorine-catalyzed reactions by sequestering chlorine in reservoir species such as ClONO2, the possibility of the removal of reactive nitrogen without dehydration should be allowed for in attempts to model ozone depletion in the Arctic. Indeed, denitrification along with elevated concentrations of reactive chlorine observed in 1989 indicate that the Arctic was chemically primed for ozone destruction without an extended period of temperatures below the frost point, as is characteristic of the Antarctic.

Fahey, D. W.↗

NASA NTRS1990

Measurements of total reactive nitrogen during the Airborne Arctic Stratospheric Expedition

Composite distributions of measured total reactive nitrogen NO(y), from the NASA ER-2 during the Airborne Arctic Stratospheric Expedition are presented. The observed features of these distributions are discussed in terms of the controlling dynamical, chemical and microphysical processes. In the latitudinal profile from 58 deg N to within about 4 deg poleward of the polar vortex boundary, NO(y) conforms closely to predictions of NO(y) based on N2O measurements. Poleward of 5 deg of latitude within the boundary, the average NO(y) decreases sharply and is significantly lower than that predicted from N2O. This feature is consistent with loss of NO(y) through sedimentation of particles containing NO(y) in polar stratospheric clouds.

Kawa, S. R.↗

NASA NTRS1990

Nitric oxide measurements in the Arctic winter stratosphere

Measurements of NO from five flights of the NASA ER-2 aircraft during the Airborne Arctic Stratospheric Expedition are presented. The NO values and vertical gradient near 60 deg N latitude are similar to previous measurements near 50 deg N in winter (Ridley et al., 1984, 1987). The NO latitudinal gradient is distinctly negative outside of the polar vortex, approaching zero at the boundary of the vortex, and remaining below the 20 pptv detection limit inside the vortex. Steady state NO2 and NO(x) (NO + NO2) are calculated from measured NO, O3, and ClO, and modeled photodissociation rates. NO(x) outside the vortex shows a negative dependence on latitude and solar zenith angle. Low NO(x) and NO(x)/NO(y), inside and near the vortex boundary may be indications of heterogeneous removal of ClONO2 and N2O5.

Fahey, D. W.↗