E. R. Keim

Mixing of polar vortex air into middle latitudes as revealed by tracer‐tracer scatterplots

The occurrence of mixing of polar vortex air with midlatitude air is investigated by examining the scatterplots of insitu measurements of long-lived tracers from the NASA ER-2 aircraft during the Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE, April, May 1993; northern hemisphere) and the Airborne Southern Hemisphere Ozone Experiment / Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA, March-October 1994; southern hemisphere) campaigns. The tracer-tracer scatterplots from SPADE form correlation curves which differ from those measured during previous aircraft campaigns (Airborne Antarctic Ozone Experiment (AAOE), Airborne Arctic Stratospheric Experiments I (AASE I) and II (AASE II)). It is argued that these anomalous linear correlation curves are mixing lines resulting from the recent mixing of polar vortex air into the middle latitude environment. Further support for this mixing scenario is provided by contour advection calculations and calculations with a simple one-dimensional strain-diffusion model. The scatterplots from the midwinter deployments of ASHOE/MAESA are consistent with those from previous midwinter measurements (i.e., no mixing lines), but the spring CO 2 :N 2 O scatterplots form altitude-dependent mixing lines which indicate that air from the vortex edge region (but not from the inner vortex) is mixing with midlatitude air during this period. These results suggest that at altitudes above about 16 km the mixing of polar vortex air into middle latitudes varies with season: in northern and southern midwinter this mixing rarely occurs, in southern spring mixing of vortex-edge air occurs, and after the vortex breakup mixing of inner vortex air occurs.

Tracer-tracer relationships and lower stratospheric dynamics: CO2 and N2O correlations during SPADE

Simultaneous measurements of CO2 and N2O from the NASA ER-2 aircraft during SPADE deployments in November 1992, April/May 1993, and October 1993 provide new information on transport rates in the lower stratosphere. The tropospheric seasonal cycle in CO2, superimposed on the long-term trend, is observed to propagate into the stratosphere. The compact correlations observed between CO2 and N2O indicate that meridional transport is sufficiently rapid to create a uniform set of relationships over the northern hemisphere up to at least 21 km even though CO2 changes significantly on a time scale of 8 to 12 weeks. The observed seasonal dependence of the correlations indicates that vertical transport above 20 km is slower in northern summer than in winter and slow throughout the year between 19 km and the tropopause. The inferred amplitude of the seasonal CO2 oscillation in the stratosphere, viewed relative to N2O, places constraints on the mean latitude for air entering the stratosphere.

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.

Subsidence, mixing, and denitrification of Arctic polar vortex air measured during POLARIS

We determine the degree of denitrification that occurred during the 1996-1997 Arctic winter using a technique that is based on balloon and aircraft borne measurements of NO y , N 2 O, and CH 4 . The NO 3 /N 2 O relation can undergo significant change due to isentropic mixing of subsided vortex air masses with extravortex air due to the high nonlinearity of the relation. These transport related reductions in NO y can be difficult to distinguish from the effects of denitrification caused by sedimentation of condensed HNO 3 . In this study, high-altitude balloon measurements are used to define the properties of air masses that later descend in the polar vortex to altitudes sampled by the ER-2 aircraft (i.e., ∼20 km) and mix isentropically with extravortex air. Observed correlations of CH 4 and N 2 O are used to quantify the degree of subsidence and mixing for individual air masses. On the basis of these results the expected mixing ratio of NO y resulting from subsidence and mixing, defined here as NO y ** , is calculated and compared with the measured mixing ratio of NO y . Values of NO y and NO y ** agree well during most parts of the flights. A slight deficit of NO y versus NO y ** is found only for a limited region during the ER-2 flight on April 26, 1997. This deficit is interpreted as indication for weak denitrification (∼2-3 ppbv) in that air mass. The small degree of denitrification is consistent with the general synoptic-scale temperature history of the sampled air masses, which did not encounter temperatures below the frostpoint and had relatively brief encounters with temperatures below the nitric acid trihydrate equilibrium temperature. Much larger degrees of denitrification would have been inferred if mixing effects had been ignored, which is the traditional approach to diagnose denitrification. Our analysis emphasizes the importance of using other correlations of conserved species to be able to accurately interpret changes in the NO y /N 2 O relation with respect to denitrification.

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.

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.

The Coupling of ClONO2, ClO, and NO2 in the Lower Stratosphere From in Situ Observations Using the NASA ER-2 Aircraft

The first in situ measurements of ClONO 2 in the lower stratosphere, acquired using the NASA ER-2 aircraft during the Polar Ozone Loss in the Arctic Region in Summer (POLARIS) mission, are combined with simultaneous measurements of ClO, NO 2 , temperature, pressure, and the calculated photolysis rate coefficient (J ClONO2 ) to examine the balance between production and loss of ClONO 2 . The observations demonstrate that the ClONO 2 photochemical steady state approximation, [ClONO 2 ] PSS = k × [ClO] × [NO 2 ] / J ClONO2 , is in good agreement with the direct measurement, [ClONO 2 ] MEAS . For the bulk of the data (80%), where T > 220 K and latitudes > 45°N, [ClONO 2 ] PSS = 1.15±0.36 (1σ) × [ClONO 2 ] MEAS , while for T 300 nm. These measurements confirm the mechanism by which active nitrogen (NO x = NO + NO 2 ) controls the abundance of active chlorine (Cl x = ClO + Cl) in the stratosphere.

Comparison of modeled and observed values of NO2 and JNO2 during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission

Stratospheric measurements of NO, NO_(2), O_(3), ClO, and HO_(2) were made during spring, early summer, and late summer in the Arctic region during 1997 as part of the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) field campaign. In the sunlit atmosphere, NO_(2) and NO are in steady state through NO2 photolysis and reactions involving O_(3), ClO, BrO, and HO_(2). By combining observations of O_(3), ClO, and HO_(2), observed and modeled values of the NO_(2) photolysis rate coefficient (JNO_(2)), and model estimates of BrO, several comparisons are made between steady state and measured values of both NO_(2) and JNO_(2). An apparent seasonal dependence in discrepancies between calculated and measured values was found; however, a source for this dependence could not be identified. Overall, the mean linear fits in the various comparisons show agreement within 19%, well within the combined uncertainties (±50 to 70%). These results suggest that photochemistry controlling the NO_(2)/NO abundance ratio is well represented throughout much of the sunlit lower stratosphere. A reduction in the uncertainty of laboratory determinations of the rate coefficient of NO + O_(3) → NO_(2) + O_(2) would aid future analyses of these or similar atmospheric observations.

Influence of Antarctic denitrification on two‐dimensional model NO y /N2O correlations in the lower stratosphere

The mechanisms responsible for latitudinal and seasonal variations in the stratospheric NOy/N2O correlation, represented by the effective NOy yield from N2O loss, or FNOy, are explored using the Garcia-Solomon two-dimensional model. The model is run with and without Antarctic denitrification. Model results are compared to in situ NOy/N2O measurements taken onboard the NASA ER-2 high-altitude aircraft in the lower stratosphere during the 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft campaign, and to global-scale measurements taken onboard the Upper Atmosphere Research Satellite (UARS) from 1992 to 1993. The southern hemisphere midlatitude seasonal cycle observed by the ER-2 and the latitudinal gradients observed by UARS are consistent with the results of the denitrified model, although some aspects of the model results are sensitive to prescribed and/or calculated horizontal diffusion coefficients. The consistency with observations supports the models prediction of a seasonal cycle in which FNOy, increases at southern midlatitudes during winter due to descent of FNOy-enriched air from above and decreases in spring due to mixing with FNOy-depleted air from the denitrified polar vortex. Antarctic denitrification appears to affect midlatitudes mainly by a one-time dilution of the polar vortex following the final warming rather than by flow-through vortex processing during the winter. Because of the high concentrations of NO3, at polar latitudes before denitrification a large fraction of total stratospheric NOy can be removed by a one-time dilution of the denitrified polar vortex. The nondenitrified model results generally do not agree well with observations, suggesting that denitrification strongly influences latitudinal and seasonal variations in FNOy in the southern hemisphere.

Constraints on N2O sinks inferred from observed tracer correlations in the lower stratosphere

Recent isotopic studies have suggested that the trace gas N 2 O has a missing stratospheric sink of potentially major significance. While these studies have raised interesting questions, the constraints on N 2 O photochemistry imposed by correlations between N 2 O, total reactive nitrogen, and other tracers measured in situ in the lower stratosphere also should be considered. Measured tracer correlations, when compared to the results of models using standard photochemistry, provide evidence in support of conventional N 2 O sinks. Stratospheric tracer correlations, however, cannot be used to preclude a new atmospheric source of N 2 O in the troposphere or to argue against an undiscovered stratospheric sink that contributes less than ∼20% of the total sink.