R. Vautard

A comparison of simulated and observed ozone mixing ratios for the summer of 1998 in Western Europe

Abstract This study describes and evaluates the newly developed European scale Eulerian chemistry transport model CHIMERE-continental. The model is designed for seasonal simulations and real time forecasts without the use of super-computers. For the purpose of model evaluation simulated ozone mixing ratios for the period between 1 May 1998 and 30 September 1998 are compared to observational data from 115 European surface sites. In order to facilitate the interpretation of future forecasts a statistic is established to estimate the reliability of a simulated pollution level. Besides this, the comparison is done by means of time series, scatter plots, a spectral analysis and the calculation of RMS-errors and biases of the model results corresponding to each observation site. It turns out that the mean RMS-error of the simulated daily maximum ozone mixing ratio for the sites considered a priori as well suited for a model comparison is about 10 ppb . For the same period but a reduced number of sites observed concentrations of NO2 and ethene are compared to simulated values. Difficulties encountered with the representativeness of observations when trying to evaluate a mesoscale air pollution model are discussed.

Summertime European heat and drought waves induced by wintertime Mediterranean rainfall deficit

[1]xa0The risk of extreme heat waves in Europe like the unprecedented one of summer 2003 is likely to increase in the future, calling for increased understanding of these phenomena. From an analysis of meteorological records over 58 years, we show that hot summers are preceded by winter rainfall deficits over Southern Europe. Subsequent drought and heat spreads northward throughout Europe in early summer, due to atmospheric transport of anomalously warm and dry air from Southern Europe in southerly wind episodes. This is shown by the observations and supported by mesoscale meteorological sensitivity simulations for Summer 1994. Moreover previous winter and early spring rainfall frequency in the Mediterranean regions is correlated with summer temperature in central continental Europe. These results emphasize the critical role of the water reservoir in the soil of continental Mediterranean areas for the maintenance of European climate.

Validation of a hybrid forecasting system for the ozone concentrations over the Paris area

Abstract A simplified hybrid statistical-deterministic chemistry-transport model, is used in real time for the prediction of ozone in the area of Paris during Summer 1999. We present here a statistical validation of this experiment. We distinguish the forecasts in the urban area from forecasts in the pollution plume downwind of the city. The validation of model forecasts, up to 3 days ahead, is performed against ground based observations within and up to 50xa0km outside of Paris. In the urban area, ozone levels are fairly well forecast, with correlation coefficients between forecast and observations ranging between 0.7 and 0.8 and root mean square errors in the range 15–20xa0μgxa0m−3 at short lead times. While the bias of urban forecast is very low, the largest peaks are somehow underestimated. The ozone plume amplitude is generally well reproduced, even at long lead times (root mean square errors of about 20–30xa0μgxa0m−3), while the direction of the plume is only captured at short lead times (about 70% of the time). The model has difficulties in forecasting the direction of the plume under stagnant weather conditions. We estimate the model ability to forecast concentrations above 180xa0μgxa0m−3, which are of practical relevance to air quality managers. It is found that about 60% of these events are well forecast, even at long lead times, while the exact monitoring station where the exceedance is observed can only be forecast at short lead times. Finally, we found that about half of the forecast error is due to the error in the estimation of the boundary conditions, which are forecast by a simple linear regression model here.

Intercomparison of SCIAMACHY nitrogen dioxide observations, in situ measurements and air quality modeling results over Western Europe

[1] The Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) satellite spectrometer provides detailed information on the nitrogen dioxide (NO 2) content in the planetary boundary layer. NO 2 tropospheric column retrievals of SCIAMACHY and its predecessor Global Ozone Monitoring Experiment are characterized by errors of the order of 40%. We present here a new SCIAMACHY tropospheric retrieval data set for the year 2003. The cloud free satellite observations are compared to surface measurements and simulations over western Europe performed with the regional air-quality model CHIMERE. The model has a resolution of 50 km similar to the satellite observations. For these comparisons, averaging kernels are applied to the collocated model profiles to remove the dependency of the comparison on a priori NO 2 profile information used in the retrieval. The consistency of both SCIAMACHY and CHIMERE outputs over sites where surface measurements are available allows us to be confident in evaluation of the model over large areas not covered by surface observations. CHIMERE underestimates surface NO 2 concentrations for urban and suburban stations which we mainly attribute to the low representativeness of point observations. No such bias is found for rural locations. The yearly average SCIAMACHY and CHIMERE spatial NO 2 distributions show a high degree of quantitative agreement over rural and urban sites: a bias of 5% (relative to the retrievals) and a correlation coefficient of 0.87 (n = 2003). On a seasonal basis, biases are smaller than 20% and correlation coefficients are larger than 0.75. Spatial correlations between both the model and satellite columns and the European Monitoring and Evaluation Program (EMEP) emission inventory are high in summer (r = 0.74, n = 1779) and low in winter (r = 0.48, n = 1078), related to seasonal changes in lifetime and transport. On the other hand, CHIMERE and SCIAMACHY columns are mutually consistent in summer (r = 0.82) and in winter (r = 0.79). This shows that CHIMERE simulates the transport and chemical processes with a reasonable accuracy. The NO 2 columns show a high daily variability. The daily NO 2 pollution plumes observed by SCIAMACHY are often well described by CHIMERE both in extent and in location. This result demonstrates the capabilities of a satellite instrument such as SCIAMACHY to monitor the NO 2 concentrations over large areas on a daily basis. It provides evidence that present and future satellite missions, in combination with CTM and surface data, will contribute to improve quantitative air quality analyses at a continental scale. Citation: Blond, N., K. F. Boersma, H. J. Eskes, R. J. van der A, M. Van Roozendael, I. De Smedt, G. Bergametti, and R. Vautard (2007), Intercomparison of SCIAMACHY nitrogen dioxide observations, in situ measurements and air quality modeling results over Western Europe,

Paris emission inventory diagnostics from ESQUIF airborne measurements and a chemistry transport model

[1]xa0During the Atmospheric Pollution Over the Paris Area (ESQUIF) experiment a series of airborne measurements were collected in the vicinity of the city of Paris during smog episodes. They are used in combination with an air quality photochemical model in order to diagnose uncertainties in the current emission inventory. Diagnostics are made by comparing simulated with observed concentrations for nitrogen oxides, carbon monoxide, and primary hydrocarbons, taking into account the chemistry and transport processes of these compounds. An emphasis is put on the uncertainty of the results, taking into account the finiteness of the measurement samples, possible errors in the model transport, and chemistry and measurement errors. We examine, in particular, possible sources of bias in the model. For instance, we show that boundary layer depth is underestimated by at most 30% on average. However, sensitivity experiments showed that these model biases, taken individually, cannot alter the qualitative aspects of our results. Only a conspiracy of these biases could possibly shift all our diagnostics toward significantly different results. There is reasonable consistency between simulated and measured concentrations. NOy simulations agree with measured concentrations to within 35%; CO concentrations agree to within a factor of 2. There are significant underestimations and overestimations in some individual primary hydrocarbons. However, the total mass and reactivity of the measured hydrocarbon mixture, which accounts for only about half of the total emitted mass, agree with modeled values to within an estimated uncertainty of 40%. The analysis of results provides clues for improving emission inventories. It is found, for instance, that temperature dependence, which is not considered here, can be a key factor and that hydrocarbon emissions from solvent use may suffer from inaccurate totals or speciation. Another source of uncertainties may be the temporal or spatial distributions of solvent activities.

Sensitivity of photochemical pollution using the adjoint of a simplified chemistry-transport model

The nature of two summertime photooxidant pollution episodes over Paris is investigated by means of adjoint modeling. The first episode (July 12, 1994) is characteristic of a local production episode, while the second (August 12, 1997) is mostly due to advection into the urban area of high concentrations of ozone. We calculate the sensitivities to emissions of individual primary species and to reaction rate coefficients using the adjoint of a simplified multibox model representing the urban and suburban areas of the city of Paris. Sensitivity to emissions demonstrates that the first episode is essentially sensitive to volatile organic compounds (VOC)/NOx emissions, while the second is sensitive to NOx emissions. These results also point out the two important emission activity categories: traffic and solvent use, the second one being more sensitive than the first one. Sensitivity to reaction rates indicates that oxidant production is only sensitive to a few reactions. The high sensitivity of photochemical pollutants peaks to particular chemical reactions points out the necessity to refine the knowledge of their reaction rates.

Future global tropospheric ozone changes and impact on European air quality

[1]xa0A global chemistry climate model is used in conjunction with a regional chemistry-transport model dedicated to air quality studies to investigate the impact of anthropogenic emission changes, under several scenarios, on western European summertime surface O3 levels in 2030. The implementation of presently decided emission control legislation in the individual countries worldwide leads to a geographically heterogeneous impact on summertime surface O3 levels over Europe. A decrease of the averaged O3 mixing ratio reaching −3 ppbv is predicted in southern areas whereas an increase reaching up 4 ppbv is calculated in northwestern Europe. The benefit of European emission control measures is found to be significantly counterbalanced by increasing global O3 levels and subsequent long range transport since both are of the same magnitude (up to 4 ppbv) but opposite in sign. However, the net effect of both global and European emission changes is a significant decrease of O3 extreme episodes during summertime.

O3‐NOx‐VOC sensitivity and NOx‐VOC indicators in Paris: Results from models and Atmospheric Pollution Over the Paris Area (ESQUIF) measurements

[1]xa0A three-dimensional photochemical model has been used to interpret aircraft measurements from the Atmospheric Pollution Over the Paris Area campaign near Paris, with special attention to measurements that are related to predicted O3-NOx-volatile organic compound (VOC) sensitivity. The model (CHIMERE) includes a representation of ozone formation over Europe and a more detailed spatial representation of the region around Paris. A series of model scenarios were developed with varying wind speeds and emission rates. Comparisons are shown with measured O3, total reactive nitrogen (NOy), summed VOCs, and isoprene. Results show that model NOx-VOC sensitivity predictions are correlated with the ratio O3/NOy but not with O3/peroxyacetyl nitrate. Measured O3 and NOy on high-ozone days tends to agree with model values when models predict NOx-sensitive or transitional chemistry but not when models predict VOC-sensitive chemistry. Model values for O3/NOy and the O3-NOy slope are lower than measured values, suggesting the possibility of missing, unmeasured VOCs in the Paris plume. Standard performance tests for ozone models, such as normalized bias, show good agreement between models and measurements, even in cases when significant differences appear in the O3-NOy correlation. Model predictions shift slightly toward NOx-sensitive chemistry when model wind speeds are increased. Isoprene represents 20% of total VOC reactivity–weighted carbon in the center of the Paris plume and 50% in the surrounding rural area during high-ozone events.

Are decadal anthropogenic emission reductions in Europe consistent with surface ozone observations

The consistency between pollutant emission reductions in Europe during the 1990 - 2002 period and ozone observations is quantitatively verified by 13-year long simulations over the whole period using the regional chemistry-transport model and the EMEP emission inventory. A statistically significant decadal tendency of 0.65 ppb/year is found in the difference between simulated and observed summer 90th percentiles of ozone daily maxima when model emissions are kept constant from year to year. By contrast the use of yearly dependent emissions does not yield a statistically significant percentile difference tendency. The regional structure of the 90th percentile differences shows that emissions may have decreased with a higher rate than assumed in the U. K. and at a lower rate in central Europe. The observed 10th percentiles are also compatible with the assumed emission reductions in Europe during 1990 - 2002, but are of lesser agreement with simulations using a uniform trend in the baseline ozone.

Three‐dimensional ozone analyses and their use for short‐term ozone forecasts

[1]xa0A statistical interpolation method is evaluated for routine production of ozone three-dimensional fields over western Europe. These fields are used for initializing short-term ozone forecasts issued from a chemistry-transport model. We mainly address two questions: (1) To what extent can the use of surface ozone observation data improve the description of ozone fields relative to raw simulations? (2) Does the use of ozone analysis improve short-term forecasts of the tropospheres chemical composition? The method consists of combining ozone simulations with surface ozone measurements. The resulting analyses are compared with independent observations in a statistical way over a long period of time (four consecutive summers). The improvement of the root-mean-square (RMS) error of the analyses relative to the raw simulations is ∼30%. The short-term (1–2 days in advance) ozone forecasts are improved on average (by ∼1 ppb of RMS error) if ozone analyses are used for initialization. The improvement is almost lost after a lead time of 36 hours. However, in cases where a model error propagates throughout the model domain, the improvement can be much larger (∼10 ppb). We analyze one such case.