Eun-Su Yang

Evidence for slowdown in stratospheric ozone loss: First stage of ozone recovery

[1] Global ozone trends derived from the Stratospheric Aerosol and Gas Experiment I and II (SAGE I/II) combined with the more recent Halogen Occultation Experiment (HALOE) observations provide evidence of a slowdown in stratospheric ozone losses since 1997. This evidence is quantified by the cumulative sum of residual differences from the predicted linear trend. The cumulative residuals indicate that the rate of ozone loss at 35– 45 km altitudes globally has diminished. These changes in loss rates are consistent with the slowdown of total stratospheric chlorine increases characterized by HALOE HCl measurements. These changes in the ozone loss rates in the upper stratosphere are significant and constitute the first stage of a recovery of the ozone layer. INDEX TERMS: 0340 Atmospheric Composition and Structure: Middle atmosphere—composition and chemistry; 0341 Atmospheric Composition and Structure: Middle atmosphere—constituent transport and chemistry (3334); 1610 Global Change: Atmosphere (0315, 0325); KEYWORDS: stratospheric ozone trends, CFCs, Montreal Protocol Citation: Newchurch, M. J., E.-S. Yang, D. M. Cunnold, G. C. Reinsel, J. M. Zawodny, and J. M. Russell III, Evidence for slowdown in stratospheric ozone loss: First stage of ozone recovery, J. Geophys. Res., 108(D16), 4507, doi:10.1029/2003JD003471, 2003.

Springtime transitions of NO2, CO, and O3 over North America : Model evaluation and analysis

Surface observations from AIRNow and Southeastern Aerosol Research and Characterization Study networks, aircraft observations from the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft program, ozonesondes, and remote sensing measurements from Global Ozone Mapping Experiment, Total Ozone Mapping Spectrometer (TOMS), and Stratospheric Aerosol and Gas Experiment (SAGE) II for February-May 2000 over North America are used to characterize the springtime transitions of O3 and its precursors. These measurements provide a comprehensive data set to evaluate the performance of the 3-D Regional Chemical Transport Model (REAM). The model is then applied to analyze the key factors affecting the springtime transitions of trace gas concentrations and export. The global GEOS-CHEM model is used to provide chemical initial and boundary conditions. Generally, the model results are in good agreement with the observations in the troposphere except for a low bias of upper tropospheric O3; the bias decreases toward the summer and lower latitudes. The rate of observed surface O3 increase in spring is simulated well by REAM. It is overestimated by GEOS-CHEM over the eastern United States. A key factor driving the model difference is daytime mixing depth. A shallow boundary layer in REAM leads to more efficient removal of radicals and hence slower activation of photochemistry in spring, when the primary radical source is relatively small. Comparison of top-down estimates of fossil fuel NOx emissions between REAM and GEOS-CHEM shows model dependence. The associated uncertainty is up to 20% on a monthly basis. Averaging over a season reduces this uncertainty. While tropospheric column NO2 decreases over the continent, it increases over the western North Atlantic due to lightning NOx production. Consequently, the REAM model simulates significant increases of tropospheric O3 over the region as indicated by column data derived from TOMS-SAGE II. Lightning impact is also evident in model-simulated NOx exports.

Upper-Stratospheric Ozone Trends 1979-1998

Extensive analyses of ozone observations between 1978 and 1998 measured by Dobson Umkehr, Stratospheric Aerosol and Gas Experiment (SAGE) I and II, and Solar Backscattered Ultraviolet (SBUV) and (SBUV)/2 indicate continued significant ozone decline throughout the extratropical upper stratosphere from 30–45 km altitude. The maximum annual linear decline of −0.8±0.2 % yr−1 (2σ) occurs at 40 km and is well described in terms of a linear decline modulated by the 11-year solar variation. The minimum decline of −0.1±0.1% yr−1 (2σ) occurs at 25 km in midlatitudes, with remarkable symmetry between the Northern and Southern Hemispheres at 40 km altitude. Midlatitude upper-stratospheric zonal trends exhibit significant seasonal variation (±30% in the Northern Hemisphere, ±40% in the Southern Hemisphere) with the most negative trends of −1.2% yr−1 occurring in the winter. Significant seasonal trends of −0.7 to −0.9% yr−1 occur at 40 km in the tropics between April and September. Subjecting the statistical models used to calculate the ozone trends to intercomparison tests on a variety of common data sets yields results that indicate the standard deviation between trends estimated by 10 different statistical models is less than 0.1% yr−1 in the annual-mean trend for SAGE data and less than 0.2% yr−1 in the most demanding conditions (seasons with irregular, sparse data) [World Meteorological Organization (WMO), 1998]. These consistent trend results between statistical models together with extensive consistency between the independent measurement-system trend observations by Dobson Umkehr, SAGE I and II, and SBUV and SBUV/2 provide a high degree of confidence in the accuracy of the declining ozone amounts reported here. Additional details of ozone trend results from 1978 to 1996 (2 years shorter than reported here) along with lower-stratospheric and tropospheric ozone trends, extensive intercomparisons to assess relative instrument drifts, and retrieval algorithm details are given by WMO [1998].

Comment on “Enhanced upper stratospheric ozone: Sign of recovery or solar cycle effect?” by W. Steinbrecht et al.

[01] Steinbrecht et al. [2004] (hereinafter referred to as S4) have discussed the trend in upper stratospheric ozone at 35 -45-km altitude determined from their lidar measurements at Hohenpeissenberg (47.8degN, 11.0degE) from 1987 to 2003. They question the conclusion of Newchurch et al. [2003] (hereinafter referred to as N3) that after approximately 1997 the downward trend of upper stratospheric ozone at 35-45-km altitude has diminished significantly. They argue instead that recent ozone changes are associated with the recent solar maximum (i.e., the solar cycle effect on ozone). In this comment we question their procedure for identifying the solar cycle effect. Moreover, we argue that the solar cycle effect was appropriately accounted for in the N3 analysis, and we buttress our argument by demonstrating that the more extensive data set used by N3 shows that the trend in upper stratospheric ozone has diminished significantly since 1997 and that this is evidence of the first stage of ozone recovery.