Ralph Lehmann

Prolonged stratospheric ozone loss in the 1995–96 Arctic winter

It is well established that extensive depletion of ozone, initiated by heterogenous reactions on polar stratospheric clouds (PSCs) can occur in both the Arctic and Antarctic lower stratosphere. Moreover, it has been shown that ozone loss rates in the Arctic region in recent years reached values comparable to those over the Antarctic,. But until now the accumulated ozone losses over the Arctic have been the smaller, mainly because the period of Arctic ozone loss has not—unlike over the Antarctic—persisted well into springtime. Here we report the occurrence—during the unusually cold 1995–96 Arctic winter—of the highest recorded chemical ozone loss over the Arctic region. Two new kinds of behaviour were observed. First, ozone loss at some altitudes was observed long after the last exposure to PSCs. This continued loss appears to be due to a removal of the nitrogen species that slow down chemical ozone depletion. Second, in another altitude range ozone loss rates decreased while PSCs were still present, apparently because of an early transformation of the ozone-destroying chlorine species into less active chlorinenitrate. The balance between these two counteracting mechanisms is probably a fine one, determined by small differences in wintertime stratospheric temperatures. If the apparent cooling trend in the Arctic stratosphere is real, more dramatic ozone losses may occur in the future.

Regional climate model of the Arctic atmosphere

A regional climate model of the whole Arctic using the dynamical package of the High- Resolution Limited Area Model (HIRLAM) and the physical parameterizations of the Hamburg General Circulation Model (ECHAM3) has been applied to simulate the climate of the Arctic north of 65 oN at a 50-km horizontal resolution. The model has been forced by the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses at the lateral boundaries and with climatological or actual observed sea surface temperatures and sea ice cover at the lower boundary. The results of simulating the Arctic climate of the troposphere and lower stratosphere for January 1991 and July 1990 have been described. In both months the model rather closely reproduces the observed monthly mean circulation. While the general spatial patterns of surface air temperature, mean sea level pressure, and geopotential are consistent with the ECMWF analyses, the model shows biases when the results are examined in detail. The largest biases appear during winter in the planetary boundary layer and at the surface. The underestimated vertical heat and humidity transport in the model indicates the necessity of improvements in the parameterizations of vertical transfer due to boundary layer processes. The tropospheric differences between model simulations and analyses decrease with increasing height. The temperature bias in the planetary boundary layer can be reduced by increasing the model sea ice thickness. The use of actual observed sea surface temperatures and sea ice cover leads only to small improvements of the model bias in comparison with climatological sea surface temperatures and sea ice cover. The validation of model computed geopotential, radiative fluxes, surface sensible and latent heat fluxes and clouds against selected station data shows deviations between model simulations and observations due to shortcomings of the model. This first validation indicates that improvements in the physical parameterization packages of radiation and in the description of sea ice thickness and sea ice fraction are necessary to reduce the model bias.

Chemical reaction pathways affecting stratospheric and mesospheric ozone

Catalytic cycles and other chemical pathways affecting ozone are normally estimated empirically in atmospheric models. In this work we have automatically quantified such processes by applying a newly developed analysis package called the Pathway Analysis Program (PAP). It used modeled chemical rates and concentrations as input. These were supplied by the Module Efficiently Calculating the Chemistry of the Atmosphere MECCA box model, itself initialized by the Free University of Berlin Climate Middle Atmosphere Model with Chemistry. We analyzed equatorial, midlatitude and high-latitude locations over 24-hour periods during spring in both hemispheres. We present results for locations in the lower stratosphere, upper stratosphere and midmesosphere. Oxygen photolysis dominated (>99%) in situ ozone production in the equatorial lower stratosphere, in the upper stratosphere and in the mesosphere. In the lower stratosphere midlatitudes the ozone smog cycle (already established in the troposphere) rivaled oxygen photolysis as an in situ ozone source in both hemispheres. However, absolute ozone production rates in midlatitudes were rather slow compared with at the equator, typically 1650 ppt ozone/day. In the equatorial lower stratosphere, five catalytic sinks were important (each contributing at least 5% to chemical ozone loss): a HOx cycle, a HOBr cycle and its HOCl analog, a water-HOx cycle and a mixed chlorine-bromine cycle. Important in midlatitudes were the HOx cycle, a NOx cycle, the HOBr cycle and the mixed chlorine-bromine cycle. In lower-stratosphere high latitudes, the chlorine dimer cycle and the mixed chlorine-bromine cycle dominated in both hemispheres. A variant on the latter, involving BrCl formation, also featured. In the upper stratosphere high latitudes (where strong negative ozone trends are observed), a nitrogen cycle, a chlorine cycle, and a mixed chlorine-nitrogen cycle were found. In the mesosphere, three closely related HOx cycles dominated ozone loss.

Nitric acid enhancements in the mesosphere during the January 2005 and December 2006 solar proton events

We investigate enhancements of mesospheric nitric acid (HNO(3)) in the Northern Hemisphere polar night regions during the January 2005 and December 2006 solar proton events (SPEs). The enhancements are caused by ionization due to proton precipitation, followed by ionic reactions that convert NO and NO(2) to HNO(3). We utilize mesospheric observations of HNO(3) from the Microwave Limb Sounder (MLS/Aura). Although in general MLS HNO(3) data above 50 km (1.5 hPa) are outside the standard recommended altitude range, we show that in these special conditions, when SPEs produce order-of-magnitude enhancements in HNO(3), it is possible to monitor altitudes up to 70 km (0.0464 hPa) reliably. MLS observations show HNO(3) enhancements of about 4 ppbv and 2 ppbv around 60 km in January 2005 and December 2006, respectively. The highest mixing ratios are observed inside the polar vortex north of 75 degrees N latitude, right after the main peak of SPE forcing. These measurements are compared with results from the one-dimensional Sodankyla Ion and Neutral Chemistry (SIC) model. The model has been recently revised in terms of rate coefficients of ionic reactions, so that at 50-80 km it produces about 40% less HNO(3) during SPEs compared to the earlier version. This is a significant improvement that results in better agreement with the MLS observations. By a few days after the SPEs, HNO(3) is heavily influenced by horizontal transport and mixing, leading to its redistribution and decrease of the SPE-enhanced mixing ratios in the polar regions.

The effectiveness of N2O in depleting stratospheric ozone

Recently, it was shown that of the ozone-depleting substances currently emitted, N2O emissions (the primary source of stratospheric NOx) dominate, and are likely to do so throughout the 21st century. To investigate the links between N2O and NOx concentrations, and the effects of NOx on ozone in a changing climate, the evolution of stratospheric ozone from 1960 to 2100 was simulated using the NIWA-SOCOL chemistry-climate model. The yield of NOx from N2O is reduced due to stratospheric cooling and a strengthening of the Brewer-Dobson circulation. After accounting for the reduced NOx yield, additional weakening of the primary NOx cycle is attributed to reduced availability of atomic oxygen, due to a) stratospheric cooling decreasing the atomic oxygen/ozone ratio, and b) enhanced rates of chlorine-catalyzed ozone loss cycles around 2000 and enhanced rates of HOx-induced ozone depletion. Our results suggest that the effects of N2O on ozone depend on both the radiative and chemical environment of the upper stratosphere, specifically CO2-induced cooling of the stratosphere and elevated CH4 emissions which enhance HOx-induced ozone loss and remove the availability of atomic oxygen to participate in NOx ozone loss cycles.

Comparison of Arctic and Antarctic trace gas column abundances from ground-based Fourier transform infrared spectrometry

Column abundances of several atmospheric trace gases have been derived from solar absorption spectra measured from McMurdo, Antarctica (77.9°S, 166.7°E), in September and October 1986 and from solar and lunar absorption spectra recorded in Ny Alesund, Spitsbergen (78.9°N, 11.9°E), during winter and spring 1992–1995. The same analysis software, including the molecular spectroscopic parameters and initial volume mixing ratio profile shapes, was employed for both data sets to minimize the possibility of introducing systematic biases. The results clearly show that denitrification in the Antarctic lower stratosphere results in much smaller column abundances of HNO3 than in the Arctic. The springtime recovery of HCl in the Antarctic showed a stronger increase than in the Arctic. The ClONO2 peak occurred about 1 month later in the Antarctic and was found to be less pronounced than in the Arctic. After accounting for the 30% increase in total chlorine between 1986 and 1993, the minimum values for HCl + ClONO2 are similar in the Arctic and the Antarctic, indicating that both polar regions show nearly the same activation of chlorine during the polar night. However, in the Arctic the low values of HCl + ClONO2 start to recover in February, whereas in the Antarctic the lack of NO2, caused by the denitrification, delays the increase of HCl + ClONO2 by about 1 month. A simple one-dimensional model was able to reproduce the behavior of HCl and ClONO2, simply by assuming a one month later date for the last Antarctic polar stratospheric clouds together with greater latitude excursions of the Arctic air parcel trajectories. The model runs imply that in the Antarctic the reconversion of ClONO2 to HCl occurs about 1 month later than in the Arctic. Furthermore, the results imply that any differences in the O3 depletion are caused mainly by differences in the stratospheric temperatures and dynamics and only to a small extent by the increased chlorine loading. The total column abundances of the short-lived tropospheric trace gases C2H6, C2H2, CO, and CH2O are found to be up to 10 times higher in the Arctic compared with the Antarctic, reflecting the hemispheric imbalance in production.

Potential biosignatures in super-Earth atmospheres II. Photochemical responses.

Spectral characterization of super-Earth atmospheres for planets orbiting in the habitable zone of M dwarf stars is a key focus in exoplanet science. A central challenge is to understand and predict the expected spectral signals of atmospheric biosignatures (species associated with life). Our work applies a global-mean radiative-convective-photochemical column model assuming a planet with an Earth-like biomass and planetary development. We investigated planets with gravities of 1g and 3g and a surface pressure of 1 bar around central stars with spectral classes from M0 to M7. The spectral signals of the calculated planetary scenarios have been presented by in an earlier work by Rauer and colleagues. The main motivation of the present work is to perform a deeper analysis of the chemical processes in the planetary atmospheres. We apply a diagnostic tool, the Pathway Analysis Program, to shed light on the photochemical pathways that form and destroy biosignature species. Ozone is a potential biosignature for complex life. An important result of our analysis is a shift in the ozone photochemistry from mainly Chapman production (which dominates in Earths stratosphere) to smog-dominated ozone production for planets in the habitable zone of cooler (M5-M7)-class dwarf stars. This result is associated with a lower energy flux in the UVB wavelength range from the central star, hence slower planetary atmospheric photolysis of molecular oxygen, which slows the Chapman ozone production. This is important for future atmospheric characterization missions because it provides an indication of different chemical environments that can lead to very different responses of ozone, for example, cosmic rays. Nitrous oxide, a biosignature for simple bacterial life, is favored for low stratospheric UV conditions, that is, on planets orbiting cooler stars. Transport of this species from its surface source to the stratosphere where it is destroyed can also be a key process. Comparing 1g with 3g scenarios, our analysis suggests it is important to include the effects of interactive chemistry.

Arctic and Antarctic ozone layer observations: chemical and dynamical aspects of variability and long-term changes in the polar stratosphere

The altitude dependent variability of ozone in the polar stratosphere is regularly observed by balloon-borne ozonesonde observations at Neumayer Station (70°S) in the Antarctic and at Koldewey Station (79°N)in the Arctic. The reasons for observed seasonal and interannual variability and long-term changes are discussed. Differences between the hemispheres are identified and discussed in light of differing dynamical and chemical conditions. Since the mid- 1980s, rapid chemical ozone loss has been recorded in the lower Antarctic stratosphere during the spring season. Using coordinated ozone soundings in some Arctic winters, similar chemical ozone loss rates have been detected related to periods of low temperatures. The currently observed cooling trend of the stratosphere, potentially caused by the increase of anthropogenic greenhouse gases, may further strengthen chemical ozone removal in the Arctic. However, the role of internal climate oscillations in observed temperature trends is still uncertain. First results of a 10000 year integration of a low order climate model indicate significant internal climate variability. on decadal time scales, that may alter the effect of increasing levels of greenhouse gases in the polar stratosphere.

Enhancement of odd nitrogen modifies mesospheric ozone chemistry during polar winter

Energetic particle precipitation (EPP) enhances odd nitrogen (NOx) in the polar upper atmosphere. Model studies have reported a solar cycle response in mesospheric ozone (O3) caused by EPP-related NOx enhancements which are included by applying a vertical NOx flux at around 80 km. However, it is not clear how O3 can be affected when the main chemical catalyst of odd oxygen (Ox = O + O(1D) + O3) loss in the mesosphere is odd hydrogen (HOx). Here we use a 1-D atmospheric model and show how enhanced NOx affects mesospheric chemistry and changes HOx partitioning, which subsequently leads to increase in Ox loss through standard HOx-driven catalytic cycles. Another, smaller increase of Ox loss results from HOx storage in HNO3 during night and its release by daytime photodissociation. Our results suggest that EPP, through NOx enhancements, could have a longer-term effect on mesospheric HOx and Ox in polar winter.

The moon as light source for atmospheric trace gas observations: measurement technique and analysis method

Abstract The moon as light source has been used to measure trace gas concentrations in the infrared spectral region. The paper describes the measurement technique and the characteristics of the analysis. Below a wavenumber of 1500 cm −1 the atmospheric emission needs to be considered in the analysis. We have derived a simple correction factor which allows to use available solar absorption retrieval codes and correct the retrieved results for the effect of the atmospheric emission later on. Furthermore, the approach presented allows us to estimate the influence of the atmospheric emission on the results. Below 1500 cm −1 and in the polar regions the uncertainties of the simple correction method are within the uncertainties caused by the signal-to-noise ratio. At midlatitudes or in the tropics the atmospheric emission contribution increases to more than 50%. Spectra recorded above 2600 cm −1 show a negligible emission contribution, even in the tropics, and can be treated without considering the atmospheric emission.