Role of amospheric boundary layer processes in the diurnal evolution of CO2 concentration

Abstract

Atmospheric boundary layer (ABL) is the lower portion of the troposphere that links the biosphere and atmosphere by exchanging heat, moisture, carbon dioxide (CO2) and other constituents between them. Interpretation of observed CO2 mixing ratios in the ABL require information of the dynamical characteristics of the ABL. Horizontal advection and large scale subsidence are however rather difficult to measure and often their contribution on the ABL budget cannot be conclusively addressed. This causes significant uncertainties on the estimates of biosphere-atmosphere CO2 exchange that is inferred from the CO2 mixing ratio observations. This Master’s Thesis examines the influence of horizontal CO2 advection and large scale subsidence on the diurnal evolution of CO2 mixing ratio in a convective ABL. From a conceptual mixed-layer theory, a set of analytical sensitivity equations is derived to quantify the contribution of horizontal CO2 advection and other ABL variables on the bulk CO2 mixing ratio evolution. Similar equations are also derived for the inferred biosphere-atmosphere CO2 exchange. Measurements during two well characterized convective days at the Cabauw tower in the Netherlands are analyzed by using the derived relations. This analysis shows that errors in horizontal CO2 advection can lead to notable uncertainties in the diurnal evolution of the CO2 mixing ratio and the inferred CO2 surface exchange. Moreover, a systematic study is performed to investigate the behaviour of the derived sensitivities on different days by changing the initial vertical profiles of potential temperature and CO2 mixing ratio in the mixed-layer simulations. Influence of large scale subsidence on the diurnal evolution of CO2 mixing ratio and the inferred CO2 surface exchange is demonstrated by numerical mixed-layer simulations. These simulations indicate that an erroneous large scale subsidence can also cause non-negligible uncertainties in the evolution of the CO2 mixing ratio and the inferred CO2 surface exchange. The results presented in this thesis hence implicate that horizontal advection and large scale subsidence should be carefully regarded also when processes on diurnal time scales are considered.