Philippe Le Sager

The effect of stratospheric sulfur from Mount Pinatubo on tropospheric oxidizing capacity and methane

The eruption of Mount Pinatubo in 1991 injected a large amount of SO2 into the stratosphere, which formed sulfate aerosols. Increased scattering and absorption of UV radiation by the enhanced stratospheric SO2 and aerosols decreased the amount of UV radiation reaching the troposphere, causing changes in tropospheric photochemistry. These changes affected the oxidizing capacity of the atmosphere and the removal rate of CH4 in the years following the eruption. We use the three-dimensional chemistry transport model TM5 coupled to the aerosol microphysics module M7 to simulate the evolution of SO2 and sulfate aerosols from the Pinatubo eruption. Their effect on tropospheric photolysis frequencies and concentrations of OH and CH4 is quantified for the first time. We find that UV attenuation by stratospheric sulfur decreased the photolysis frequencies of both ozone and NO2 by about 2% globally, decreasing global OH concentrations by a similar amount in the first 2 years after the eruption. SO2 absorption mainly affects OH primary production by ozone photolysis, while aerosol scattering also alters OH recycling. The effect of stratospheric sulfur on global OH and CH4 is dominated by the effect of aerosol extinction, while SO2 absorption contributes by 12.5% to the overall effect in the first year after the eruption. The reduction in OH concentrations causes an increase in the CH4 growth rate of 4 and 2 ppb/yr in the first and second years after the eruption, respectively, contributing 11 Tg to the 27 Tg observed CH4 burden change in late 1991 and early 1992.

Impact of the Ulysses velocity on the diagnosis of the electron density by the Unified Radio and Plasma Wave sounder in the outskirts of the Io torus

The resonance spectra collected on February 8, 1992, in the outskirts of the Io plasma torus by the Unified Radio and Plasma wave (URAP) relaxation sounder on board the Ulysses spacecraft present significant differences from the active spectra gathered by earthbound spacecraft in similar plasma conditions. The most striking anomaly is the lack of resonances at the harmonics of the electron gyrofrequency, one of the most common signatures found in usual sounder spectra. These differences are interpreted, when computing the dispersion relation of the Bernstein modes triggered by the sounder, by taking into account the large frequency shift induced on the resonances by the high speed of the Ulysses spacecraft in the plasma. The frequency of the shifted resonances depends on the ratio Vsat/p/Vth of the spacecraft speed relative to the plasma and the thermal speed of the electrons. The observed resonances are all found to be in excellent agreement with the computed frequencies obtained in this way. Some predicted resonances are not observed on the Ulysses spectra, but it is suggested that they have too long a wavelength to be easily detected by the URAP antenna. As a by-product of the electron density diagnosis allowed by this analysis a rough estimate of the electron temperature can be deduced.

Study of CO surface pollution in Europe based on observations and nested-grid applications of GEOS-CHEM global chemical transport model.

Carbon monoxide (CO) is studied over Europe for 2001 using measurements from 31 rural-background stations and the nested-grid application of the global CTM GEOS-CHEM. The model reveals lowest (highest) biases in warm (cold) periods, tracking observations in most cases more closely than the global model. The role of CO production and destruction processes and the atmospheric conditions are investigated. A rotated Principal Component Analysis is applied to all stations, based on daily CO modelled concentrations in 2001, yielding three principal components (PCs) with stations of common characteristics. CO concentrations are studied for these groups in relation to the circulation patterns prevailing over Europe in 2001, at mean sea level and 850 hPa. The nested-grid model improves results in comparison to those calculated by the global model by up to ∼22% for first principal component subregion, where emissions are high and elevation is low. Improvement reaches∼17 and∼7%, respectively, for second and third principal component subregions, where emissions are lower and altitudes are higher. Better performance is achieved for patterns associated with westerly flow, whereas poor skills are revealed during stagnant conditions. During pollution events, the nesting model’s ability in capturing CO surface concentrations improves by up to ∼40% in comparison to the global simulation.

The impact of precipitation evaporation on the atmospheric aerosol distribution in EC-Earth v3.2.0

The representation of aerosol–cloud interaction in global climate models (GCMs) remains a large source of uncertainty in climate projections. Due to its complexity, precipitation evaporation is either ignored or taken into account in a simplified manner in GCMs. This research explores various ways to treat aerosol resuspension and determines the possible impact of precipitation evaporation and subsequent aerosol resuspension on global aerosol burdens and distribution. The representation of aerosol wet deposition by large-scale precipitation in the EC-Earth model has been improved by utilising additional precipitation-related 3D fields from the dynamical core, the Integrated Forecasting System (IFS) general circulation model, in the chemistry and aerosol module Tracer Model, version 5 (TM5). A simple approach of scaling aerosol release with evaporated precipitation fraction leads to an increase in the global aerosol burden (+7.8 to +15 % for different aerosol species). However, when taking into account the different sizes and evaporation rate of raindrops following Gong et al. (2006), the release of aerosols is strongly reduced, and the total aerosol burden decreases by −3.0 to −8.5 %. Moreover, inclusion of cloud processing based on observations by Mitra et al. (1992) transforms scavenged small aerosol to coarse particles, which enhances removal by sedimentation and hence leads to a−10 to −11 % lower aerosol burden. Finally, when these two effects are combined, the global aerosol burden decreases by −11 to −19 %. Compared to the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite observations, aerosol optical depth (AOD) is generally underestimated in most parts of the world in all configurations of the TM5 model and although the representation is now physically more realistic, global AOD shows no large improvements in spatial patterns. Similarly, the agreement of the vertical profile with CloudAerosol Lidar with Orthogonal Polarization (CALIOP) satellite measurements does not improve significantly. We show, however, that aerosol resuspension has a considerable impact on the modelled aerosol distribution and needs to be taken into account.