M. Ramonet

Contribution of anthropogenic and natural sources to atmospheric methane variability

Methane is an important greenhouse gas, and its atmospheric concentration has nearly tripled since pre-industrial times. The growth rate of atmospheric methane is determined by the balance between surface emissions and photochemical destruction by the hydroxyl radical, the major atmospheric oxidant. Remarkably, this growth rate has decreased markedly since the early 1990s, and the level of methane has remained relatively constant since 1999, leading to a downward revision of its projected influence on global temperatures. Large fluctuations in the growth rate of atmospheric methane are also observed from one year to the next, but their causes remain uncertain. Here we quantify the processes that controlled variations in methane emissions between 1984 and 2003 using an inversion model of atmospheric transport and chemistry. Our results indicate that wetland emissions dominated the inter-annual variability of methane sources, whereas fire emissions played a smaller role, except during the 1997–1998 El Niño event. These top-down estimates of changes in wetland and fire emissions are in good agreement with independent estimates based on remote sensing information and biogeochemical models. On longer timescales, our results show that the decrease in atmospheric methane growth during the 1990s was caused by a decline in anthropogenic emissions. Since 1999, however, they indicate that anthropogenic emissions of methane have risen again. The effect of this increase on the growth rate of atmospheric methane has been masked by a coincident decrease in wetland emissions, but atmospheric methane levels may increase in the near future if wetland emissions return to their mean 1990s levels.

Satellite climatology of African dust transport in the Mediterranean atmosphere

A daily analysis of African dust concentrations in the Mediterranean atmosphere has been made between June 1983 and December 1994 using the International Satellite Cloud Climatology Project (ISCCP-B2) archive of Meteosat visible (VIS) channel images. The ISCCP-B2 archive of Meteosat infrared (IR) images has also been used to determine the frequencies of dust mobilization over the continent, north of 30°N. Despite a large daily variability, climatological results show a clear seasonal cycle with a maximum during the dry season: dust transport begins over the eastern basin in spring and spreads over the western basin in summer. These patterns are shown to be related to both cyclogenesis over North Africa and rainfall over the Mediterranean Sea. Indeed, the frequency of dust mobilization over the continent and of dust outbreaks over the sea are strongly related to the climatology of depressions affecting North Africa. Precipitations appear to be an important factor explaining both the seasonal east-west shift in transport location and the south-north gradients of dust concentrations over the Mediterranean.

CO2 surface fluxes at grid point scale estimated from a global 21 year reanalysis of atmospheric measurements

This paper documents a global Bayesian variational inversion of CO2 surface fluxes during the period 1988-2008. Weekly fluxes are estimated on a 3.75 degrees x 2.5 degrees (longitude-latitude) grid throughout the 21 years. The assimilated observations include 128 station records from three large data sets of surface CO2 mixing ratio measurements. A Monte Carlo approach rigorously quantifies the theoretical uncertainty of the inverted fluxes at various space and time scales, which is particularly important for proper interpretation of the inverted fluxes. Fluxes are evaluated indirectly against two independent CO2 vertical profile data sets constructed from aircraft measurements in the boundary layer and in the free troposphere. The skill of the inversion is evaluated by the improvement brought over a simple benchmark flux estimation based on the observed atmospheric growth rate. Our error analysis indicates that the carbon budget from the inversion should be more accurate than the a priori carbon budget by 20% to 60% for terrestrial fluxes aggregated at the scale of subcontinental regions in the Northern Hemisphere and over a year, but the inversion cannot clearly distinguish between the regional carbon budgets within a continent. On the basis of the independent observations, the inversion is seen to improve the fluxes compared to the benchmark: the atmospheric simulation of CO2 with the Bayesian inversion method is better by about 1 ppm than the benchmark in the free troposphere, despite possible systematic transport errors. The inversion achieves this improvement by changing the regional fluxes over land at the seasonal and at the interannual time scales. (Less)

Inverse modeling of annual atmospheric CO2 sources and sinks. 2. Sensitivity study

Atmospheric transport models can be used to infer surface fluxes of atmospheric CO 2 from observed concentrations using inverse methods. One of the main problem of these methods is the question of their sensivity to all the parameters involved in the calculation. In this paper we study precisely the influence of the main parameters on the net CO 2 fluxes inferred by an annual Bayesian three-dimensional (3-D) inversion of atmospheric CO 2 monthly concentrations. This inversion is described as the control inversion (S 0 ) of Bousquet et al. [this issue]. Successively, at regional to global spatial scales we analyze the numerical stability of the solution to initial fluxes and errors, the influence of a priori flux scenario, the sensitivity to the atmospheric transport model used, the influence of δ 13 C measurements, and the influence of the atmospheric network. We find that the atmospheric transport model introduces a large uncertainty to the inferred budget, which overcomes our control run uncertainties. The effects of vertical transport on CO 2 concentrations appear to be a critical point that has to be investigated further. Spatial patterns of fluxes also have significant influence on a regional basis. We notice that accounting for the Baltic Sea station (BAL) deeply modifies the Europe versus Asia partition of the land uptake at mid and high latitudes of the Northern Hemisphere. We also analyze the weak influence of using δ 13 C measurements as additional constraints. In the tropics we find that the low level of constraints imposed by the atmospheric network limits the analysis of fluxes to zonal means. Finally, we calculate overall estimates of CO 2 net sources and sinks at continental scale, accounting for all sensivity tests. Concerning the controversial partition of CO 2 sink at mid and high latitudes of the Northern Hemisphere, we find (on average, for the 1985-1995 period) an overall partition of the sink of 0.7±0.7 Gt C yr -1 for North America, 0.2±0.3 Gt C yr -1 for the North Pacific Ocean, 0.5±0.8 Gt C yr -1 for Europe, 0.7±0.3 Gt C yr -1 for the North Atlantic Ocean, and 1.2±0.8 Gt C yr -1 for north Asia. This overall partition tends to place an important land uptake over north Asia. However, uncertainties remain large when we account for all the sensitivity tests.

Global CO2 fluxes inferred from surface air-sample measurements and from TCCON retrievals of the CO2 total column

We present the first estimate of the global distribution of CO_2 surface fluxes from 14 stations of the Total Carbon Column Observing Network (TCCON). The evaluation of this inversion is based on 1) comparison with the fluxes from a classical inversion of surface air-sample-measurements, and 2) comparison of CO_2 mixing ratios calculated from the inverted fluxes with independent aircraft measurements made during the two years analyzed here, 2009 and 2010. The former test shows similar seasonal cycles in the northern hemisphere and consistent regional carbon budgets between inversions from the two datasets, even though the TCCON inversion appears to be less precise than the classical inversion. The latter test confirms that the TCCON inversion has improved the quality (i.e., reduced the uncertainty) of the surface fluxes compared to the assumed or prior fluxes. The consistency between the surface-air-sample-based and the TCCON-based inversions despite remaining flaws in transport models opens the possibility of increased accuracy and robustness of flux inversions based on the combination of both data sources and confirms the usefulness of space-borne monitoring of the CO_2 column.

Radon-222 measurements during the Tropoz II campaign and comparison with a global atmospheric transport model

The aim of the 222Rn measurements during the airborne campaign TROPOZ II, was first to help in the interpretation of the photochemical studies, and secondly to furnish a data set of 222Rn in the troposphere, for validation of atmospheric transport models. In this paper we present the 222Rn measurements, and their simulation with a 3-D atmospheric transport model based on observed winds. The 222Rn was measured using the active daughters deposit technique with isokinetic aerosol sampling. We have obtained 44 measurements distributed between 65° North and 55° South, from 1 to 11 km height. In 25% of cases, we found relatively high concentrations (> 300 mBq·scm) of 222Rn in the high troposphere (>8 km). The results of 3D simulations and the calculations of back-trajectories allow us to find the origins of the high 222Rn concentrations. The transport model reproduced most of the observed synoptic variations, but it overestimates the concentrations which implies a vertical transport of excessive velocity.

The YAK-AEROSIB transcontinental aircraft campaigns: new insights on the transport of CO2, CO and O3 across Siberia

Two airborne campaigns were carried out to measure the tropospheric concentrations and variability of CO2, CO and O3 over Siberia. In order to quantify the influence of remote and regional natural and anthropogenic sources, we analysed a total of 52 vertical profiles of these species collected in April and September 2006, every ∼200 km and up to 7 km altitude. CO2 and CO concentrations were high in April 2006 (respectively 385–390 ppm CO2 and 160–200 ppb CO) compared to background values. CO concentrations up to 220 ppb were recorded above 3.5 km over eastern Siberia, with enhancements in 500–1000 m thick layers. The presence of CO enriched air masses resulted from a quick frontal uplift of a polluted air mass exposed to northern China anthropogenic emissions and to fire emissions in northern Mongolia. A dominant Asian origin for CO above 4 km (71.0%) contrasted with a dominant European origin below this altitude (70.9%) was deduced both from a transport model analysis, and from the contrasted ΔCO/ΔCO2 ratio vertical distribution. In September 2006, a significant O3 depletion (∼ –30 ppb) was repeatedly observed in the boundary layer, as diagnosed from virtual potential temperature profiles and CO2 gradients, compared to the free troposphere aloft, suggestive of a strong O3 deposition over Siberian forests.

Investigation of the atmospheric boundary layer depth variability and its impact on the 222Rn concentration at a rural site in France

Continuous monitoring of the atmospheric boundary layer (ABL) depth (zi) is important for investigations of trace gases with near-surface sources. The aim of this study is to examine the temporal variability of zi on both diurnal and seasonal time scales over a full year (2011) and relate these changes to the atmospheric 222Rn concentrations (CRn) measured near the top of a 200 m tower at a rural site (Trainou) in France. Continuous zi estimates were made using a combination of lidar and hourly four-height carbon dioxide (CO2) profile measurements. Over the diurnal cycle, the 180 m CRn reached a maximum in the late morning as the growing ABL passed through the inlet height (180 m) transporting upward high CRn air from the nocturnal boundary layer. During late afternoon, a minimum in the CRn occurred mainly due to ABL-mixing. We argue that ABL dilution occurs in two stages: first, during the rapid morning growth into the residual layer, and second, during afternoon with the free atmosphere when zi has reached its quasi-stationary height (around 750 m in winter or 1700 m in summer). An anticorrelation (R2 of −0.49) was found while performing a linear regression analysis between the daily zi growth rates and the corresponding changes in the CRn illustrating the ABL-dilution effect. We also analyzed the numerical proportions of the time within a season when zi remained lower than the inlet height and found a clear seasonal variability for the nighttime measurements with higher number of cases with shallow zi (<200 m) in winter (67.3%) than in summer (33.9%) and spring (54.5%). Thus, this pilot study helps delineate the impact of zi on CRn at the site mainly for different regimes of ABL, in particular, during the times when the zi is above the measurement height. It is suggested that when the zi is well below the inlet height, measurements are most possibly indicative of the residual layer 222Rn, an important issue that should be considered in the mass budget approach.

Origins of 210Po in the atmosphere at lamto, ivory coast: Biomass burning and saharan dusts

Abstract Measurements of 210 Po and 210 Pb activities in surface air at Lamto (Ivory Coast) during 18 months are presented and discussed in relation to the regional sources of 210 Po. High seasonal variation is observed with maximum in winter, reaching an activity of 210 Po as high as I Bq per 10 3 m 3 . Our estimation shows that the high 210 Po activity in winter is not closely related to the local biomass burning emissions but rather due to the particular atmospheric circulation patterns and geographical location of this region: in winter, surface winds having continental origin bring soil dusts, enriched in 210 Po and 210 Pb, from Sahara desert. However, biomass burning and the growth from 222Rn within the atmosphere appear to be the major sources of unsupported 210 Po in the other seasons.

A recent build‐up of atmospheric CO2 over Europe. Part 1: observed signals and possible explanations

We analysed interannual and decadal changes in the atmospheric CO2 concentration gradient (ΔCO2) between Europe and the Atlantic Ocean over the period 1995–2007. Fourteen measurement stations are used, with Mace-Head being used to define background conditions. The variability of ΔCO2 reflects fossil fuel emissions and natural sinks activity over Europe, as well as atmospheric transport variability. The mean ΔCO2 increased by 1–2 ppm at Eastern European stations (∼30% growth), between 1990–1995 and 2000–2005. This built up of CO2 over the continent is predominantly a winter signal. If the observed increase of ΔCO2 is explained by changes in ecosystem fluxes, a loss of about 0.46 Pg C per year would be required during 2000–2005. Even if severe droughts have impacted Western Europe in 2003 and 2005, a sustained CO2 loss of that magnitude is unlikely to be true.We sought alternative explanations for the observed CO2 build-up into transport changes and into regional redistribution of fossil fuel CO2 emissions. Boundary layer heights becoming shallower can only explain 32% of the variance of the signal. Regional changes of emissions may explain up to 27% of the build-up. More insights are given in the Aulagnier et al. companion paper.