Emission rate and chemical state estimation by 4-dimensional variational inversion

[摘要] This study aims to assess the potential and limits ofan advanced inversion method to estimate pollutantprecursor sources mainlyfrom observations. Ozone, sulphur dioxide,and partly nitrogen oxides observations are taken to infer sourcestrength estimates.As methodology, the four-dimensional variational data assimilationtechniquehas been generalised and employed to include emission rateoptimisation, in addition to chemical state estimates as usual objective of data assimilation.To this end, theoptimisation space of thevariational assimilation system has been complemented by emission ratecorrection factors of 19 emitted species at each emitting gridpoint, involving the University of Cologne mesoscale EURAD model.For validation, predictive skills were assessedfor an August 1997 ozone episode,comparing forecast performances ofpure initial value optimisation, pure emission rateoptimisation, and joint emission rate/initial value optimisation.

Validation procedures rest on both measurements withheld from dataassimilation and prediction skill evaluation offorecasts after the inversion procedures.Results show that excellentimprovements can be claimed for sulphur dioxide forecasts, afteremission rate optimisation. Significant improvements can be claimedfor ozone forecastsafter initial value and joint emission rate/initial valueoptimisation of precursor constituents. The additional benefitsapplying jointemission rate/initial value optimisation are moderate, and veryuseful intypical cases, where upwind emission rate optimisation isessential. In consequence of the coarse horizontal model grid resolution of 54 km, applied in this study,comparisons indicate that the inversionimprovements can rest onassimilating ozone observations only, as the inclusion of NO_xobservations does not provide additional forecast skill.Emission estimates were found to be largely independent from initialguesses from emission inventories, demonstrating the potential of the4D-var method to infer emission rate improvements. The study alsopoints to the need for improved horizontal model resolution to moreefficient use of NO_x observations.

[学科分类] 大气科学