Radiation and chemistry in the stratosphere: Sensitivity to O_2 absorption cross sections in the Herzberg continuum

We propose that a significant overestimate of the molecular oxygen absorption cross sections in the important spectral window from 200-220 nm is in large part responsible for the discrepancy between observed and modeled vertical profiles of some halocarbons (CFCl_3 in particular), as well as for the long-standing problem of simultaneously fitting N_2O, CH_4, CF_2Cl_2 and CFCl_3 profiles with a single eddy diffusion model. Recent measurements of the direct solar flux in the stratosphere by J.R. Herman and co-workers seem to support this idea. Replacing our current O_2 cross sections in the 200-220 nm range by values in better agreement with the results of the above group leads to a reduction in N_2O, CF_2Cl_2 and CFCl_3 concentrations (by factors of 0.70, 0.62 and 0.19, respectively, at 30 km), while CH_4, H_2 and CO profiles are essentially unchanged. Moreover, the predicted concentration of HNO_3 above 30 km is reduced by ∼50%, yielding better agreement with observations. The reduction in O_2 cross sections produces a 10-20% decrease in ozone above about 35 km, but a fairly large increase (∼30%) near the peak around 20-25 km. The changes in other stratospheric species are also briefly discussed

Lower variability of radionuclide activities in upland dairy products compared to soils and vegetation: Implication for environmental survey

Contamination of the environment by radionuclides is usually estimated using soil and grass sampling. However, radionuclides are often not homogeneously distributed in soils. In the alpine Mercantour region (Western Alps, France) a large heterogeneity in Chernobyl 137Cs deposition has been previously observed. Here we report additional 137Cs results together with new 90Sr and Pu data for soil, grass, milk, and cheese samples. The results show that radioisotopes from nuclear weapons tests fallout are more homogeneously distributed than Chernobyl 137Cs. Further, we observe that the 137Cs and 90Sr contents are less variable in milk samples than in grass or soil samples. This can be attributed to the homogenization effect of cow vagrancy during grazing. Hence milk seems to be a more robust sample than soil or grass to evaluate the extent of contamination on a regional scale. We explore this idea by comparing own unpublished 90Sr results and 90Sr results from the literature to establish the relationship between altitude of grazing and contamination of soil and milk for Western Europe. There is a significant positive correlation between soil contamination and altitude and an even closer correlation between milk 90Sr activity (A) and altitude (h): A = A0 + ek·h where A0 is the expected activity of milk sampled at sea level (A0 = 0.064 ± 0.014 Bq g-1 Ca) and h is the altitude of grazing, k being a constant (k = 0.95 × 10-3 ± 0.11 × 10-3 m-1 Bq g-1 Ca). The fact that there is less scattering in the relationship for the 90Srmilk-altitude than for 90Srsoil-altitude suggests, again, that milk is a well-suited sample for environmental survey. The relationship between the altitude of grazing and the 90Sr content of milk and cheese can also be used to assess the authenticity of dairy products. © 2006 Elsevier Ltd. All rights reserved

A critical analysis of ClO and O_3 in the mid-latitude stratosphere

In the upper stratosphere, an altitude range in which ozone should be in photochemical steady-state, calculated ozone abundances that are derived from a one-dimensional photochemical model with updated chemistry are up to 60% smaller than mean observed values. On the other hand, the model results for the key free radicals (HO_x, NO_x, and ClO_x species) in the catalytic destruction of ozone are shown to be in reasonable agreement with available measurements. The general validity of the model simulation of ClO_x chemistry is confirmed through a detailed intercomparison between the computed ClO diurnal variation and recently published ground-based microwave observations. Since many field measurements are performed near sunrise or sunset, the uncertainties in the model results arising from the details of the radiation field calculations at large zenith angles are discussed. Although the calculated ozone discrepancy could be the result of a number of errors in adopted photochemical parameters, a sensitivity analysis shows that no reasonable change in any one or two parameters can resolve this problem. The limited available observations regarding the ratio of atomic oxygen to ozone suggest a possible discrepancy in that quantity, which could be responsible for a large part of the ozone problem

The UARS microwave limb sounder version 5 data set: Theory, characterization, and validation

Nitric acid (HNO3) is a major player in processes controlling the springtime depletion of polar ozone. It is the main constituent of the Polar Stratospheric Clouds (PSCs) and a primary reservoir for reactive nitrogen. Potential variations in the stratospheric circulation and temperature may alter the extent and duration of PSCs activity, influencing the future ozone levels significantly. Monitoring HNO3 and its long-term variability, especially in polar region, is then crucial for better understanding issues related to ozone decline and expected recovery. In this study we present an intercomparison between ground based HNO3 measurements, carried out by means of the Ground-Based Millimeter-wave Spectrometer (GBMS), and two satellite data sets produced by the two NASA/JPL Microwave Limb Sounder (MLS) experiments. In particular, we compare UARS MLS measurements (1991-1999) with those carried out by the GBMS at South Pole, Antarctica (90°S), Fall of 1993 and 1995. A similar intercomparison is made between Aura MLS HNO3 observations (2004 - to date) and GBMS measurements obtained during the period February 2004 - March 2007, at the mid-latitudes/high altitudes station of Testa Grigia (45.9° N, 7.7° E, elev. 3500 m), and during polar winters 2008/09 and 2009/2010 at Thule Air Base (76.5°N 68.8°W), Greenland. We assess systematic differences between GBMS and both UARS and Aura HNO3 data sets at seven potential temperature levels (θ) spanning the range 465 – 960 K. The UARS data set advected to the South Pole shows a low bias, within 20% for all θ levels but the 960 K, with respect to GBMS measurements. A very good agreement, within 5%, is obtained between Aura and GBMS observations at Testa Grigia, while larger differences, possibly due to latitude dependent effects, are observed over Thule. These differences are under further investigations but a preliminary comparison over Thule among MLS v3, GBMS, and ACE-FTS measurements suggests that GBMS measurements carried out during winter 2009 might not be reliable. These comparisons have been performed in the framework of the NASA JPL GOZCARDS project, which is aimed at developing a long-term, global data record of the relevant stratospheric constituents in the context of ozone decline. GBMS has been selected in GOZCARDS since its HNO3 dataset, although sampling different latitudes in different years, is the only one spanning a sufficiently long time interval for cross-calibrating HNO3 measurements by the UARS and Aura MLS experiments

Ground-based stratospheric O3 and HNO3 measurements at Thule, Greenland: An intercomparison with Aura MLS observations

In response to the need for improving our understanding of the evolution and the interannual variability of the winter Arctic stratosphere, in January 2009 a Ground-Based Millimeter-wave Spectrometer (GBMS) was installed at the Network for the Detection of Atmospheric Composition Change (NDACC) site in Thule (76.5° N, 68.8° W), Greenland. In this work, stratospheric GBMS O3 and HNO3 vertical profiles obtained from Thule during the winters 2010 (HNO3 only), 2011 and 2012 are characterized and intercompared with co-located measurements of the Aura Microwave Limb Sounder (MLS) experiment. Using a recently developed algorithm based on Optimal Estimation, we find that the GBMS O3 retrievals show good sensitivity (> 80%) to atmospheric variations between ~ 17 and ~ 50 km, where their 1σ uncertainty is estimated to be the larger of ~ 11% or 0.2 ppmv. Similarly, HNO3 profiles can be considered for scientific use between ~ 17 and ~ 45 km altitude, with a 1σ uncertainty that amounts to the larger of 15% or 0.2 ppbv. Comparisons with Aura MLS version 3.3 observations show that, on average, GBMS O3 mixing ratios are biased negatively with respect to MLS throughout the stratosphere, with differences ranging between ~ 0.3 ppmv (8%) and 0.9 ppmv (18%) in the 17–50 km vertical range. GBMS HNO3 values display instead a positive bias with respect to MLS up to 26 km, reaching a maximum of ~ 1 ppbv (10%) near the mixing ratio profile peak. O3 and HNO3 values from the two datasets prove to be well correlated at all altitudes, although their correlations worsen at the lower end of the altitude ranges considered. Column contents of GBMS and MLS O3 (from 20 km upwards) and HNO3 (from 17 km upwards) correlate very well and indicate that GBMS measurements can provide valuable estimates of column interannual and seasonal variations for these compounds

Record-breaking ozone loss in the Arctic winter 2010/2011: comparison with 1996/1997

We present a detailed discussion of the chemical and dynamical processes in the Arctic winters 1996/1997 and 2010/2011 with high resolution chemical transport model (CTM) simulations and space-based observations. In the Arctic winter 2010/2011, the lower stratospheric minimum temperatures were below 195 K for a record period of time, from December to mid-April, and a strong and stable vortex was present during that period. Simulations with the Mimosa-Chim CTM show that the chemical ozone loss started in early January and progressed slowly to 1 ppmv (parts per million by volume) by late February. The loss intensified by early March and reached a record maximum of ~2.4 ppmv in the late March–early April period over a broad altitude range of 450–550 K. This coincides with elevated ozone loss rates of 2–4 ppbv sh^(−1) (parts per billion by volume/sunlit hour) and a contribution of about 30–55% and 30–35% from the ClO-ClO and ClO-BrO cycles, respectively, in late February and March. In addition, a contribution of 30–50% from the HO_x cycle is also estimated in April. We also estimate a loss of about 0.7–1.2 ppmv contributed (75%) by the NO_x cycle at 550–700 K. The ozone loss estimated in the partial column range of 350–550 K exhibits a record value of ~148 DU (Dobson Unit). This is the largest ozone loss ever estimated in the Arctic and is consistent with the remarkable chlorine activation and strong denitrification (40–50%) during the winter, as the modeled ClO shows ~1.8 ppbv in early January and ~1 ppbv in March at 450–550 K. These model results are in excellent agreement with those found from the Aura Microwave Limb Sounder observations. Our analyses also show that the ozone loss in 2010/2011 is close to that found in some Antarctic winters, for the first time in the observed history. Though the winter 1996/1997 was also very cold in March–April, the temperatures were higher in December–February, and, therefore, chlorine activation was moderate and ozone loss was average with about 1.2 ppmv at 475–550 K or 42 DU at 350–550 K, as diagnosed from the model simulations and measurements

Intercomparison between Aura MLS and ground-based millimeter-wave observations of stratospheric O3 and HNO3 from Thule (76.5° N, 68.7° W)

The Ground-Based Millimeter-wave Spectrometer (GBMS) measures rotational emission spectra of middle atmospheric trace gases, with a spectral window of 600 MHz tunable between approximately 230 and 280 GHz and a resolution of up to 65 kHz. It was designed and built at the State University of New York at Stony Brook in the early 90’s and since then has been regularly upgraded and operated at a variety of sites in both hemispheres, at polar and mid-latitudes. In view of a growing need for long-term data sets of stratospheric constituents, in January 2009 we resolved to establish a long-term GBMS observation site at the Arctic station of Thule Air Base (76.5°N, 68.8°W), Greenland, in order to track the long- and short-term interactions between the changing climate and the seasonal processes tied to the ozone depletion phenomenon. Since then three winter campaigns were carried out from Thule during the period January-March 2009, 2010 and 2011. Observations of O3, HNO3, CO and N2O were performed, mostly on a daily basis, except during periods characterized by poor weather conditions. In this study we compare GBMS stratospheric O3 and HNO3 measurements obtained during these three winter periods at Thule with colocated satellite observations from the Aura Microwave Limb Sounder (MLS) experiment. The Version 3.3 Aura MLS O3 and HNO3 data sets have a resolution of about 2.5 km and 3-4 km, respectively, in the stratosphere. The MLS precisions range from 0.1 to 0.6 ppmv for O3 and about 0.6-0.7 ppbv for HNO3 throughout the stratosphere. Based on preliminary comparisons with correlative data sets and on results obtained for v2.2, systematic uncertainties are estimated to lead to HNO3 measurements biases that vary between ±0.5 and ±2 ppbv and multiplicative errors of ±5 –15% throughout most of the stratosphere. Similarly, a systematic uncertainty of the order of 5-10% has been assessed for O3 data. As for the GBMS, the O3 pure rotational transition line at 276.923 GHz is observed with a ~1.5-hour integration, while the weaker HNO3 spectrum, represented by a cluster of superimposed emission lines centered at 269.1 GHz, needs about 4 hours of integration. Taking advantage of the dependence of the line broadening on atmospheric pressure, inversion techniques allow the retrieval of vertical profiles from approximately 15 to 50 km. In the past, GBMS O3 and HNO3 spectra were deconvolved using a Chahine-Twomey (C-T) and an iterative constrained Matrix Inversion (MI) technique, respectively. More recently, the GBMS retrieval algorithm has been updated to an Optimal Estimation Method (OEM) in order to conform to the standard of the NDACC microwave group, and to easily provide retrievals with a set of averaging kernels that grants more straightforward comparisons with other data sets. The nominal vertical resolution of the retrieved profiles (defined as the FWHM of averaging kernels) is ~8 km for O3 and ~ 12 km for HNO3, although the inversion technique locates the maximum of the mixing ratio profile of both species with a much better accuracy (i.e., ~ ±1 km). The 1σ uncertainty of O3 and HNO3 mixing ratio vertical profiles depends on altitude and is estimated at ~15% or 0.3 ppbv, whichever is larger. Each GBMS profile is compared to the closest MLS profile, with coincidence criteria of ±10° longitude, ±2.5° latitude and ±12 h. In order to avoid of severely compromising the comparison between GBMS and Aura MLS observations due to the much higher resolution of the satellite-derived data sets, we ‘convolved’ the MLS profiles using the GBMS averaging kernels before directly comparing the two data sets. For both species a fairly good agreement between MLS and GBMS profiles is observed, with the GBMS showing, however, a ~10-15% low bias at the mixing ratio peak