Browsing by Author "Murto, Sonja"

In recent decades, rapid urbanization together with industrialization has led to an increase in anthropogenic emissions, resulting in high air pollution concentrations and poor air quality particularly in developing countries, such as in China. Due to both the enhanced environmental and severe public health risks poor air quality is causing and the climate impacts of aerosols, it is of great interest to study and understand aerosol particles and their impact on our surroundings. Aerosols affect the radiative properties of the atmosphere and the surface energy balance. The impact of aerosols on the surface radiative fluxes of the urban surface energy balance is widely known, but the impact on the turbulent energy fluxes, which are important components in the energy balance, has until now remained unclear. To extend the knowledge of aerosol impacts on all the energy balance components, a simple urban land surface model (SUEWS) during the period of 2006-2009 is used, together with aerosol data, Aerosol Optical Depth (AOD), received from an AERONET station located in Beijing. With the use of commonly measured meteorological variables together with parameters defined for the study area of 1 km radius around a meteorological tower, the components of the urban surface energy balance are simulated by the model. For further data analysis, the data are divided into thermal seasons and pollutant categories according to the available AOD-data. Extreme polluted conditions are achieved during 24 % for the time of available AOD-data, additionally showing relatively less situations with poor air quality (8 %) in winter compared to 27 % observed in summer. The aim of this study is to analyse how much aerosol particles can modify the different surface energy balance components, particularly focusing on the turbulent fluxes. The model is evaluated against observed turbulent fluxes in the same tower, showing an overestimation of the sensible heat flux and an underestimation and a better model performance of the latent heat flux. Still, the diurnal behaviour of the fluxes is shown to be well reproduced by the model. The behaviour of the modelled components is further investigated, showing a clear monthly variation for almost all the fluxes contributing to the surface energy balance. The behaviour of the total energy balance is in general controlled by the wet (occurring from May to October) and dry periods, distinguishing the climate in Beijing. The sensible heat flux is the dominant flux in March, accounting for 59 % of the available energy, whereas during the wet periods, higher portion of the available energy is consumed by the turbulent latent heat flux (61 % in August). Adding the effect of aerosols, the results clearly show how the net radiative flux is decreased in poor air quality conditions, giving differences of 138 W/m2 in the median flux due to aerosol loading in the atmosphere. The main finding of this study is that aerosols also influence the turbulent fluxes, with largest aerosol impact on the sensible heat flux occurring during thermal spring (66 W/m2 difference between clean and polluted air conditions). Likewise, in summer, when the latent heat flux is the largest contributor for consuming the available energy, the influence of aerosols is most visible (25 W/m2 difference). This study highlights the importance of maintaining measurements of aerosol concentrations and characteristics of the pollutants over urban areas due to their influence not only on the radiative fluxes, but all the components of the surface energy balance, which can further alter the water circulation and give rise to other environmental risks. These findings can therefore be used in urban planning and issues related to water management and air pollution regulations.