Analysis of two atmospheric dispersion schemes from CO₂ surface concentrations at a rural site

Abstract

Two atmospheric dispersion schemes, the transport index and the Bultynck-Malet scheme, have been analysed and combined by means of a three-year database obtained at a rural site comprising CO 2 surface concentrations and meteorological variables (wind and temperature profiles) from a RASS sodar. The most noticeable feature of the transport index is its ability to split the influence of local effects and transport, whereas the Bultynck-Malet scheme is based on seven stability classes and requires temperature gradient and wind data not too close to the surface. The transport index was obtained in the layer from 40 to 100 m. Medians of the transport index were below 600 m, particularly at night, revealing that local effects prevailed during this period. CO 2 concentrations were parametrized as a function of distance, and a boundary of the transport index was calculated to establish two regions, the inner region affected by local effects and the outer, affected by transport. Dependence of this boundary on height was also analysed. The original Bultynck-Malet scheme was adapted to the database by modifying the limit of high winds. As a result, only a few observations were transferred to more stable classes. A successful relationship between CO 2 concentrations and stability classes was obtained and a detailed analysis revealed the noticeable influence of temperature gradient on the two most stable classes. Finally, the relationship between both dispersion schemes enabled us to propose an extremely simplified scheme retaining the two most stable Bultynck-Malet classes and grouping the remaining observations into a third class with lower concentrations. Copyright c 2011 Royal Meteorological Society Key Words: transport index; Bultynck-Malet scheme; stability classification; RASS sodar; CO 2 statistics.