Distribution Of HNO3 Research Articles

On the consistency of HNO3 and NO2 in the Aleutian High region from the Nimbus 7 LIMS Version 6 data set.

This study uses photochemical calculations along kinematic trajectories in conjunction with Limb Infrared Monitor of the Stratosphere (LIMS) observations to examine the changes in HNO3 and NO2 near 30 hPa in the region of the Aleutian High (AH) during the minor warming event of January 1979. An earlier analysis of Version 5 (V5) LIMS data indicated increases in HNO3 without a corresponding decrease in NO2 in that region and a quasi-wave 2 signature in the zonal distribution of HNO3, unlike the wave 1 signal in ozone and other tracers. Version 6 (V6) LIMS also shows an increase of HNO3 in that region, but NO2 is smaller than from V5. The focus here is to convey that V6 HNO3 and NO2 are of good quality, as shown by a re-examination of their mutual changes in the AH region. Photochemical model calculations initialized with LIMS V6 data show increases of about 2 ppbv in HNO3 over 10 days along trajectories terminating in the AH region on 28 January. Those increases are mainly a result of the nighttime heterogeneous conversion of N2O5 on background stratospheric sulfuric acid aerosols. Changes in the composition of the air parcels depend on the extent of exposure to sunlight and, hence, on the dynamically controlled history of the trajectories. Trajectories that begin in low latitudes and traverse to across the North Pole in a short time lead to the low HNO3 in the region separating the anticyclone from the polar vortex, both of which contain higher HNO3. These findings help to explain the observed seasonal evolution and areal extent of both species. V6 HNO3 and NO2 are suitable, within their errors, for the validation of stratospheric chemistry-climate models.

Nitric Acid and Water Extraction by T2EHDGA in n-Dodecane

ABSTRACTLiquid–liquid distribution behavior of nitric acid (HNO3) and water by a diglycolamide (DGA) ligand, N,N,N’,N’-tetra-2-ethylhexyldiglycolamide (T2EHDGA), into n-dodecane diluent was investigated. Spectroscopic Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) characterization of the organic extraction solutions indicate the T2EHDGA carbonyl coordinates with HNO3 and progressively aggregates at high acid conditions. Water extraction increases in the presence of HNO3. The experimentally observed distribution of HNO3 was modeled using the computer program SXLSQI. The results indicated that the formation of two organic-phase species—HNO3·T2EHDGA and (HNO3)2·T2EHDGA—satisfactorily describes the acid distribution behavior. Temperature-dependent solvent extraction studies allowed for the determination of thermodynamic extraction constants and ΔH and ΔS parameters for the corresponding extractive processes.

Influence of pressure (temperature) on the nitric acid distribution between the liquid and vapor in the course of evaporation of nitric acid radioactive waste

Analysis of the published dependences of the HNO3 distribution coefficient between the liquid and vapor in the system with water shows that these dependences are power functions with the exponent depending on the temperature (pressure) and emerge from the azeotropic point. The exponent is inversely proportional to the temperature, and its logarithm is directly proportional to the logarithm of the pressure. The pattern is preserved when a salting-out agent is present in the system and is described similarly when taking into account the hydration and hydrolysis of the cation and the degree of dissociation of sulfuric acid, as shown previously for atmospheric pressure. The dependences obtained are interpreted within the framework of the law of mass action (without activity coefficients) as dilution of the azeotropic compound HNO3(H2O)2 with water with the formation of an associated ionic liquid.

Effect of salting-out agents on nitric acid distribution between liquid and vapor in the course of evaporation of nitric acid radioactive waste

The database on the influence of concentrations of salting-out agents with different cation valence (Li+, Na+, K+, Cs+, NH4+, Mg2+, Ca2+, Cu2+, Zn2+, Al3+, Fe3+, La3+, Pu4+, UO22+ nitrates) and of H2SO4 on the HNO3 distribution between liquid and vapor at atmospheric pressure was verified and expanded by performing equilibrium evaporation experiments in an evaporator without reflux with circulation of the bottoms. This equilibrium can be described by the equation \(\log {\kern 1pt} {\alpha _H} = - 2.35 + 2\log \;{X_{\Sigma N{O_3}}}\) (where \({X_{\Sigma N{O_3}}}\) is the total concentration of nitrate ions in aqueous solution), taking into account partial binding of water due to hydration of the salting-out agent and apparent hydrolysis of polyvalent cations, and also incomplete dissociation of sulfuric acid. The suggested model is valid at \({X_{\Sigma N{O_3}}}\) from 3 to 16 M (azeotrope); at \({X_{\Sigma N{O_3}}}{\text{ < }}\;3\), Raoult’s law is observed (αH = const), which is confirmed by a number of examples. In this model, the additivity of the properties of the salting- out agents, taking into account the concentration of “free” water, is assumed, which allows calculation of the composition of the bottoms and distillate when evaporating waste of any composition from radiochemical plants.

Distribution of HDBP in two-phase systems consisting of aqueous nitric acid, organic diluent (with or without TBP), and HDBP or its zirconium salt

The distribution of HDBP between 0–5 M aqueous HNO3 and various solvents or solutions of acidic zirconium salt of HDBP (HDBP ZS, HDBP : Zr ratio 6–18) in the same solvents [decane, xylene, CCl4, m-nitrobenzotrifluoride (F-3), 30 vol % TBP in decane and xylene] was studied. The HDBP concentration in aqueous solutions decreases with an increase in the HNO3 concentration. At HDBP concentrations in the organic phase below 0.1 M, its distribution follows the Nernst equation; in the range 0.1–0.4 M, its concentration in the aqueous phase at a constant acidity is approximately constant; and with a further increase in the HDBP concentration in the organic phase, its concentration in the aqueous phase starts to grow again. When the organic solvent is TBP in diluent, the HDBP concentration in more concentrated HNO3 solutions decreases by almost an order of magnitude compared to the systems without TBP, whereas with water and dilute HNO3 solutions the difference from the TBP-free systems is insignificant. The presence of Zr somewhat increases the HDBP distribution ratio. The extraction of HNO3 with a solution of HDBP in xylene and CCl4 apparently involves the HDBP dimer. A mathematical description of the HDBP and HNO3 distribution between aqueous nitric acid solutions and xylene or CCl4, based on equations of chemical reactions between the components, was suggested.

Extraction of U(VI), Th(IV), and rare-earth elements from nitric acid solutions with phosphoryl-and carbonyl-containing podands

The distribution of HNO3 and microamounts of nitrates of U(VI), Th(IV), La, Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y between aqueous solutions of HNO3 and solutions of N,N′-dimethyl-N,N′-diphenyl-3-oxapentanediamide (I) and 1,3-bis(diphenylphosphinoyl)-2-oxapropane (II) in organic diluents was studied. The stoichiometry of the extractable complexes was determined, and the effect exerted by the extractant structure and by the organic diluent on the efficiency of recovery of rare-earth elements (REE) into the organic phase was examined. In nitric acid solutions, carbonyl-containing podand I surpasses in the extraction ability phosphoryl-containing podand II. The possibility of using a macroporous polymeric sorbent impregnated with compound I for recovering REE from nitric acid solutions was demonstrated.

Denitrification in the Arctic mid‐winter 2004/2005 observed by airborne submillimeter radiometry

We present measurements of unusually low mixing ratios of HNO3 in the exceptionally cold Arctic vortex of late‐January and early‐February 2005. The measurements were obtained by the airborne submillimeter radiometer ASUR during the polar aura validation experiment (PAVE). The distribution of HNO3 inside the vortex reaches minima below 4 ppbv around 22 km altitude and maxima above 13 ppbv around 16 km altitude, with a considerable spatial variability. We estimate a vortex averaged denitrification of 3.1 ± 0.8 ppbv around 20 km altitude, and slight renitrification below ∼15.5 km altitude. The observed HNO3 deficit is largest (∼6 ppbv) near the center of the vortex, where the air masses had experienced temperatures below the NAT formation threshold for 80–100% of the previous 20 days according to back trajectories. This suggests that the main denitrification mechanism is based on sedimenting nitric acid trihydrate particles.

Three‐dimensional structure and evolution of stratospheric HNO3 based on UARS Microwave Limb Sounder measurements

The Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) measured the global distribution of stratospheric HNO3 through more than seven complete annual cycles in both hemispheres. Here we present an overview of the seasonal, interhemispheric, and interannual variations in the distribution of HNO3 throughout the lower and middle stratosphere from 420 to 960 K potential temperature based on the UARS MLS version 6 HNO3 measurements. The version 6 MLS data have much better precision and a larger vertical range than previous MLS HNO3 data sets and have also been corrected to account for an oversight in the retrieval algorithms that led earlier versions to overestimate HNO3 abundances by as much as 35% at some levels in the stratosphere. HNO3 exhibits little vertical, seasonal, or interannual variability in the tropics. For the first ∼1.5 years of the mission, however, a persistent enhancement is seen at low and middle latitudes that we attribute to perturbations in reactive nitrogen chemistry under conditions of high aerosol loading from the eruption of Mount Pinatubo. The signature of Pinatubo‐induced HNO3 enhancement is considerably weaker at 420 and 465 K than it is at higher altitudes, and it is also considerably weaker at northern middle and high latitudes than it is in the Southern Hemisphere. HNO3 abundances increase toward the pole in both hemispheres at all levels and in all seasons, with the exception of the severely denitrified region inside the Antarctic vortex. A pronounced seasonal cycle is present at middle and high latitudes up to at least 960 K (∼34 km), with a winter maximum and a summer minimum. Large interannual variability in the timing, magnitude, and duration of enhanced wintertime HNO3 abundances is seen in both hemispheres. Even in the coldest Arctic winters, HNO3 depletion is modest and limited in both horizontal and vertical extent. In contrast, virtually complete removal of gas‐phase HNO3 occurs at the highest southern latitudes by July in every year throughout the lower stratosphere. Indications of denitrification are present up to at least 740 K, well above the highest altitude at which dehydration is observed, providing further evidence that denitrification can proceed in the absence of dehydration.

A new Chemical Lagrangian Model of the Stratosphere (CLaMS) 2. Formulation of chemistry scheme and initialization

The first simulations of stratospheric chemistry using the Chemical Lagrangian Model of the Stratosphere (CLaMS) are reported. A comprehensive chemical assimulation procedure is described that combines satellite, airborne, and balloon‐borne tracer observations with results from a two‐dimensional photochemical model simulation. This procedure uses tracer‐tracer and tracer‐potential vorticity mapping techniques. It correctly reproduces all basic features of the observed tracer distribution. This methodology is used to generate the initial composition fields that will be used for subsequent chemical simulations. Results from a 6‐day simulation starting on 20 February 1997 show that the simulated HNO3 distribution displays the correct morphology, although the extremes of the observed HNO3 distribution are underestimated. The simulated ClO distribution exhibits a similar morphology to the observed Microwave Limb Sounder ClO distribution. Because of unseasonally low temperatures in the arctic lower stratosphere during spring 1997, high levels of chlorine activation are maintained in the simulation, resulting in up to 1.8 ppmv of chemical ozone loss over a 5‐week period. Furthermore, simulations show strong spatially inhomogeneous chemical ozone depletion within the polar vortex and show that greatest ozone loss is confined to the vortex core. These results are confirmed by several Halogen Occultation Experiment and ozone sonde profiles, although the minimum ozone concentrations are overestimated. These studies demonstrate that CLaMS is capable of simulating vortex isolation, an essential feature of the polar vortex.

Tropospheric reactive odd nitrogen over the South Pacific in austral springtime

The distribution of reactive nitrogen species over the South Pacific during austral springtime appears to be dominated by biomass burning emissions and possibly lightning and stratospheric inputs. The absence of robust correlations of reactive nitrogen species with source‐specific tracers (e.g., C2H2 [combustion], CH3Cl [biomass burning], C2Cl4 [industrial], 210Pb [continental], and 7Be [stratospheric]) suggests significant aging and processing of the sampled air parcels due to losses by surface deposition, OH attack, and dilution processes. Classification of the air parcels based on CO enhancements indicates that the greatest influence was found in plumes at 3–8 km altitude in the distributions of HNO3 and peroxyacetyl nitrate (PAN). Here mixing ratios of these species reached 600 parts per trillion by volume (pptv), values surprisingly large for a location several thousand kilometers removed from the nearest continental areas. The mixing ratio of total reactive nitrogen (the NOy sum), operationally defined in this paper as measured (NO + HNO3 + PAN + CH3ONO2 + C2H5ONO2) + modeled (NO2), had a median value of 285 pptv within these plumes compared with 120 pptv in nonplume air parcels. Particle NO−3 was not included in this analysis of the NOy sum due to its 10‐ to 15‐min sampling time resolution, but, in general, it was &lt;10% of the NOy sum. Comparison of the two air parcel classifications for NOy and alkyl nitrate distributions showed no perceivable plume influence, but recycling of reactive nitrogen may have masked this direct effect. In the marine boundary layer, the NOy sum averaged 50 pptv in both air parcel classifications, being somewhat isolated from the polluted conditions above it by the trade wind inversion. In this region, however, alkyl nitrates appear to have an important marine source where they comprise 20–80% of the NOy sum in equatorial and high‐latitude regions over the South Pacific.

Is there change over time in the season-of-birth effect for schizophrenia?: Data from the Southern hemisphere

Data from the draft COSPAR reference model for stratospheric nitric acid (HNO3) are analysed. Eight months of LIMS HNO3 measurements allow the analysis of dynamics of regimes associated with the annual HNO3 maximum followed by the HNO3 decrease in the Northern Hemisphere and the annual HNO3 minimum followed by the HNO3 increase in the Southern Hemisphere. The HNO3 minimum is noted earlier (in November) in the Southern Hemisphere subtropical upper stratosphere, from where the regime of minimum HNO3 values propagates to the southern high-latitude middle stratosphere, and then (in Austral summer) the equatorward propagation of the regime is observed, with a persistent downward component. The regime of the HNO3 annual maximum in the Northern Hemisphere propagates from the Arctic lower stratosphere (in autumn) and from the tropical middle stratosphere (in late summer), so that in the mid-latitude middle stratosphere the downward propagation of the regime is observed. Evolution of areas with HNO3 increase and decrease by 1 ppbv against the January HNO3 distribution quantifies intensity of the HNO3 decrease in winter-spring in the Northern Hemisphere and the HNO3 increase in Austral summer-autumn in the Southern Hemisphere.

Six years of UARS Microwave Limb Sounder HNO3 observations: Seasonal, interhemispheric, and interannual variations in the lower stratosphere

We present an overview of the seasonal, interhemispheric, and interannual variations in the distribution of HNO3 in the lower stratosphere based on measurements of gas‐phase HNO3 made by the UARS Microwave Limb Sounder (MLS) through six complete annual cycles in both hemispheres. Outside of the winter polar regions, zonal‐mean HNO3 mixing ratios on the 465‐K potential temperature surface are comparable in the two hemispheres in all latitude bands and in all years examined. Except at high latitudes, interannual variability is minimal, and there is no significant hemispheric asymmetry in the overall HNO3 distribution or its seasonal cycle. Although the Antarctic experiences widespread severe denitrification, the MLS data indicate that the denitrification is not complete; that is, not all polar stratospheric cloud (PSC) particles sediment out of the lower stratosphere. Replenishment of HNO3 at 465 K during the mid‐ to late‐winter period (when temperatures, though still low, are generally rising) is most likely achieved through a combination of PSC evaporation and continuing weak diabatic descent. Despite large interhemispheric and interannual differences in the extent and duration of PSC activity and denitrification, HNO3 recovers to similar values at the end of every winter in both the Arctic and the Antarctic. Zonal‐mean HNO3 values for the two hemispheres are virtually indistinguishable for the latitudes equatorward of 65°, even during the winter months. Thus the effects of severe denitrification are confined in both space and time to the regions poleward of 65°S during the winter and early spring.

Influence of biomass combustion emissions on the distribution of acidic trace gases over the southern Pacific basin during austral springtime

This paper describes the large‐scale distributions of HNO3, HCOOH, and CH3COOH over the central and South Pacific basins during the Pacific Exploratory Mission‐Tropics (PEM‐Tropics) in austral springtime. Because of the remoteness of this region from continental areas, low part per trillion by volume (pptv) mixing ratios of acidic gases were anticipated to be pervasive over the South Pacific basin. However, at altitudes of 2–12 km over the South Pacific, air parcels were encountered frequently with significantly enhanced mixing ratios (up to 1200 pptv) of acidic gases. Most of these air parcels were centered in the 3–7 km altitude range and occurred within the 15°−65°S latitudinal band. The acidic gases exhibited an overall general correlation with CH3Cl, PAN, and O3, suggestive of photochemical and biomass burning sources. There was no correlation or trend of acidic gases with common industrial tracer compounds (e.g., C2Cl4 or CH3CCl3). The combustion emissions sampled over the South Pacific basin were relatively aged exhibiting C2H2/CO ratios in the range of 0.2–2.2 pptv/ppbv. The relationships between acidic gases and this ratio were similar to what was observed in aged air parcels (i.e., &gt;3–5 days since they were over a continental area) over the western North Pacific during the Pacific Exploratory Mission‐West Phases A and B (PEM‐West A and B). In the South Pacific marine boundary layer a median C2H2/CO ratio of 0.6 suggested that this region was generally not influenced by direct inputs of biomass combustion emissions. Here we observed the lowest mixing ratios of acidic gases, with median values of 14 pptv for HNO3, 19 pptv for HCOOH, and 18 pptv for CH3COOH. These values were coincident with low mixing ratios of NOx(&lt;10 pptv), CO (≈50 parts per billion by volume (ppbv)), O3 (&lt; 20 ppbv), and long‐lived hydrocarbons (e.g., C2H6 &lt;300 pptv). Overall, the PEM‐Tropics data suggest an important influence of aged biomass combustion emissions on the distributions of acidic gases over the South Pacific basin in austral springtime.

Large‐scale distributions of tropospheric nitric, formic, and acetic acids over the western Pacific basin during wintertime

We report here measurements of the acidic gases nitric (HNO3), formic (HCOOH), and acetic (CH3COOH) over the western Pacific basin during the February‐March 1994 Pacific Exploratory Mission‐West (PEM‐West B). These data were obtained aboard the NASA DC‐8 research aircraft as it flew missions in the altitude range of 0.3–12.5 km over equatorial regions near Guam and then further westward encompassing the entire Pacific Rim arc. Aged marine air over the equatorial Pacific generally exhibited mixing ratios of acidic gases &lt;100 parts per trillion by volume (pptv). Near the Asian continent, discrete plumes encountered below 6 km altitude contained up to 8 parts per billion by volume (ppbv) HNO3 and 10 ppbv HCOOH and CH3COOH. Overall there was a general correlation between mixing ratios of acidic gases with those of CO, C2H2, and C2Cl4, indicative of emissions from combustion and industrial sources. The latitudinal distributions of HNO3 and CO showed that the largest mixing ratios were centered around 15°N, while HCOOH, CH3COOH, and C2Cl4 peaked at 25°N. The mixing ratios of HCOOH and CH3COOH were highly correlated (r2 = 0.87) below 6 km altitude, with a slope (0.89) characteristic of the nongrowing season at midlatitudes in the northern hemisphere. Above 6 km altitude, HCOOH and CH3COOH were marginally correlated (r2 = 0.50), and plumes well defined by CO, C2H2, and C2Cl4 were depleted in acidic gases, most likely due to scavenging during vertical transport of air masses through convective cloud systems over the Asian continent. In stratospheric air masses, HNO3 mixing ratios were several parts per billion by volume (ppbv), yielding relationships with O3 and N2O consistent with those previously reported for NOy.

Reactive nitrogen and ozone over the western Pacific: Distribution, partitioning, and sources

Measurements of important reactive nitrogen species (NO, NO2, HNO3, PAN, PPN, NO3−, NOy), C1 to C6 hydrocarbons, O3, chemical tracers (C2Cl4, CO), and meteorological parameters were made in the troposphere (0 to 12 km) over the western Pacific (0°–50°N) during the Pacific Exploratory Mission‐West A campaign (September–October 1991). Under clean conditions, mixing ratios of NO, NO2, NOy, and O3 increased with altitude and showed a distinct latitudinal gradient. PAN showed a midtropospheric maximum, while nitric acid mixing ratios were generally highest near the surface. Measured NOy concentrations were significantly greater than the sum of individually measured nitrogen species (mainly NOx, PAN, and HNO3), suggesting that a large fraction of reactive nitrogen present in the atmosphere is made up of hitherto unknown species. This shortfall was larger in the tropics (≈65%) compared to midlatitudes (≈40%) and was minimal in air masses with high HNO3 mixing ratios (&gt;100 ppt). A global three‐dimensional photochemical model has been used to compare observations with predictions and to assess the significance of major sources. It is possible that the tropical lightning source is much greater than commonly assumed, and both lightning source and its distribution remain a major area of uncertainty in the budgets of NOy and NOx. A large disagreement between measurement and theory exists in the atmospheric distribution of HNO3. It appears that surface‐based anthropogenic emissions provide nearly 65% of the global atmospheric NOy reservoir. Relatively constant NOx/NOy ratios imply that NOy and NOx are in chemical equilibrium and the NOy reservoir may be an important in situ source of atmospheric NOx. Data are interpreted to suggest that only about 20% of the upper tropospheric (7–12 km) NOx is directly attributable to its surface NOx source, and free tropospheric sources are dominant. In situ release of NOx from the NOy reservoir, lightning, direct transport of surface NOx, aircraft emissions, and small stratospheric input collectively maintain the NOx balance in the atmosphere. It is shown that atmospheric ratios of reactive nitrogen and sulfur species, along with trajectory analysis, can be used to pinpoint the source of Asian continental outflow. Compared to rural atmospheres over North America, air masses over the Pacific are highly efficient in net O3 production. Sources of tropospheric NOx cannot yet be accurately defined due to shortcomings in measurements and theory.

A comparison of Nimbus 7 limb infrared monitor of the stratosphere and radiosonde temperatures in the lower stratosphere poleward of 60N

Discrepancies have been noted in the literature between distributions of HNO3 and NO2 from Nimbus 7 limb infrared monitor of the stratosphere (LIMS) and from gas‐phase model calculations, particularly in polar winter. The inclusion of heterogeneous chemistry processes into models gives improved agreement for HNO3 but not for NO2. For this reason we considered whether there were temperature bias errors for LIMS that would have an impact on its species retrievals in polar regions. The 7 months of Nimbus 7 LIMS mapped temperatures are compared with daily, lower stratospheric radiosonde (RAOB) observations at 1200 UTC from a set of 22 stations poleward of 60N. The 7‐month average LIMS minus RAOB differences for the set are within ± 0.3 K at 30, 50, and 70 hPa; LIMS minus RAOB equals −1.2 K ± 2.2 K at 100 hPa. Springtime comparisons at 30 hPa were improved after recommended solar corrections were applied to the subset of daylight RAOB temperatures. Time series comparisons were conducted at 10 hPa for four stations in the Aleutian sector. After a correction for radiative cooling was applied to the RAOB data, the LIMS minus RAOB differences at 10 hPa were within ± 2 K, except in winter when LIMS was still significantly warmer by several degrees. Much of that bias is attributed to spatial temperature variations for LIMS/RAOB comparisons in regions of temperature extremes and to the finite vertical resolution of LIMS. All of this means that there is almost no significant error in the LIMS temperatures that could impact the accuracy of LIMS constituent retrievals throughout much of the polar lower stratosphere. The LIMS cold bias at 100 hPa is most pronounced for the 22 station set in November and December (about −1.9 K) and is statistically significant for many of the stations. LIMS tends to smooth through the vertical structure associated with elevated warm layers near 100 hPa. This negative temperature bias leads to retrieved overestimates of LIMS species concentrations below about the 70‐hPa level.

Nitric Acid Trihydrate Particle Formation in the Polar Stratosphere and Its Effect on Nitric Acid Transport to the Troposphere.

Growth and sedimentation of type-I polar stratospheric clouds (type-I PSCs) particles including nitric acid are examined using a numerical model. The PSCs particles which grow in the upper particle layer have enough large size (-10μm) to descend rapidly to the tropopause within several days. In the lower part of PSCs layer, evaporation of HNO3 from particulate matter occurs and acts as source of HNO3 vapor. From view point of stratospheric HNO3 distribution, the particle sedimentation effectively functions as vertical transport mechanism of HNO3 vapor from the upper PSCs layer to the lower layer and/or to the troposphere.

Nitric acid forecast experiments

Three-dimensional forecasts of the time evolution of the distribution of HNO3 in the stratosphere during two time periods (17-24 February 1979, 13-23 March 1979) are reported. The February time period was during a major wave number 2 stratospheric warming, and dynamics dominate the HNO3 evolution. Dynamics and photochemistry are both important in the March experiment.The zonal mean results of the model calculations are in general qualitative agreement with the LIMS HNO3 observations. However, the calculated 3-dimensional fields show significant differences from the observations. The analysis of the differences between the model results and the data gives insight into the problems of the modeling technique and to the consistency of the dynamical and constituent data.

Accuracy and precision of the nitric acid concentrations determined by the Limb Infrared Monitor of the Stratosphere Experiment on NIMBUS 7

The LIMS is a six channel limb scanning IR radiometer flying on the NIMBUS 7 spacecraft. It measured radiances from October 24, 1978, to May 28, 1979, from which vertical profiles of temperature, ozone, water vapor, nitrogen dioxide, and nitric acid were determined. Nitric acid (HNO3) plays an important role in stratospheric chemistry, both as reservoir and as sink for the nitrogen compounds and because of its relationship to the hydroxyl radical. This paper describes the validation of the HNO3 results, beginning with an outline of the measurements and data reduction leading to the retrievals. The error sources due to instrumental effects and data reduction are described, and their effects on the results are calculated. The predicted random errors are shown to be somewhat larger than the observed values of ∼0.10 ppbv. The LIMS results are within 20% of a set of 15 correlative balloon‐borne measurements, obtained with several techniques over a range of season, latitude, and pressure between 100 and 20 mbar, the region of largest HNO3 mixing ratio. This is about the accuracy of the correlative measurements. The percent differences are larger at altitudes above 10 mbar. LIMS results agree with earlier measurements at all but the highest levels. Total overburdens above 200 mbar calculated from LIMS agree with those measured spectroscopically from aircraft. LIMS and other HNO3 measurements show similar agreement with model predictions. The consistency suggests that the major LIMS error sources are reasonably well understood. The HNO3/NO2 ratio also leads to reasonable OH concentrations. The global distribution of HNO3 in November shows strong latitudinal variation at the altitude of the maximum, with a large build up over the northern hemisphere pole at ∼40 mbar, but lower values over the southern pole. This reverses by early northern spring. Some characteristics and limits on the data are noted, but the LIMS HNO3 determinations are usually of comparable accuracy to other determinations. These results provide a valuable addition to our knowledge of the distributions of trace gases in the stratosphere. By themselves, but especially in conjunction with the LIMS measurements of temperature, ozone, water vapor, and nitrogen dioxide, they form the basis for a wide range of atmospheric studies.

Measurements of NO2 and HNO3 during a stratospheric warming at 54°N in February 1979

The altitude distribution of HNO3 was measured during a stratospheric warming on a balloon flight from Cold Lake, Alberta, on 11 February, 1979. The HNO3 profile measured appears to be layered and have a greater thickness than that of a typical summer profile. Ground based observations of the zenith sky were made from 31 January to 12 February and yield the total abundance of stratospheric NO2. Stratospheric NO2 column amounts deduced from these ground based measurements and another set taken during February, 1977 exhibit a diurnal variation under dynamical conditions of both stable vortex flow (1977) and stratospheric warming (1979). Using these measurements, the NO measurements of Knight et al. (1982), and model calculations we find that above 20 km there does not appear to be a pronounced seasonal change in total odd nitrogen. However, below 24 km the conversion of NOw = NO + NO2 + 2 × N2O5 + HO2NO2 to HNO3 occurs more rapidly than calculated using current homogeneous chemistry.