Browsing by Subject "radiative forcing"

Sort by: Order: Results:

Now showing items 1-2 of 2
  • Ding, A. J.; Huang, X.; Nie, W.; Sun, J. N.; Kerminen, V. -M.; Petäjä, T.; Su, H.; Cheng, Y. F.; Yang, X. -Q.; Wang, M. H.; Chi, X. G.; Wang, J. P.; Virkkula, A.; Guo, W. D.; Yuan, J.; Wang, S. Y.; Zhang, R. J.; Wu, Y. F.; Song, Y.; Zhu, T.; Zilitinkevich, S.; Kulmala, M.; Fu, C. B. (2016)
    Aerosol-planetary boundary layer (PBL) interactions have been found to enhance air pollution in megacities in China. We show that black carbon (BC) aerosols play the key role in modifying the PBL meteorology and hence enhancing the haze pollution. With model simulations and data analysis from various field observations in December 2013, we demonstrate that BC induces heating in the PBL, particularly in the upper PBL, and the resulting decreased surface heat flux substantially depresses the development of PBL and consequently enhances the occurrences of extreme haze pollution episodes. We define this process as the dome effect of BC and suggest an urgent need for reducing BC emissions as an efficient way to mitigate the extreme haze pollution in megacities of China.
  • Zhou, You (Helsingfors universitet, 2015)
    Long wave (LW) radiation in the Earth's atmosphere is defined as the radiation at wavelengths longer than 4 µm (infrared). The short wave (SW) radiation wavelengths are less than 4 µm (visible light, ultraviolet). SW radiation is usually from solar origin. The absorbed solar SW radiation is closely balanced by the outgoing LW radiation in the atmosphere. This radiation balance keeps the global average temperature stable. The main cause of the current global warming trend is human expansion of the 'greenhouse effect'. Atmospheric greenhouse gases absorb the thermal LW radiation from a planetary surface. The absorbed radiation is re-emitted to all directions. Some of the energy is transferred back to the surface and the lower atmosphere since part of the re-radiation is directed towards the surface, resulting in increased surface temperature. The local radiation balance is also affected by clouds and aerosols in the atmosphere since they too can absorb and scatter radiation. The effects of clouds and greenhouse gases on the global radiative balance and surface temperature are well known. The aerosols, however, are one of the greatest sources of uncertainty in the interpretation and projection of the climate change. Natural aerosols such as those due to large eruptions of volcanoes and wind-blown mineral dust are recognised as significant sources of climate forcing. In addition, there are several ways in which humans are altering atmospheric aerosols. These include industrial emissions to the lower atmosphere as well as emissions to as high as lower stratosphere by aircraft. In this thesis the effect of aerosols on LW radiation was studied based on narrowband LW calculations in a reference mid-latitude summer atmosphere with and without aerosols. Aerosols were added to the narrowband LW scheme based on their typical schematic observed spectral and vertical behaviour over European land areas. This was found to agree also with spectral aerosol data from the Lan Zhou University Semi-Arid Climate Observatory and Laboratory measurement stations in north-western China. A volcanic stratospheric aerosol load was found to induce local LW warming with a stronger column “greenhouse effect” than a doubled CO2 concentration. A heavy near-surface aerosol load was found to increase the downwelling LW radiation to the surface and to reduce the outgoing LW radiation, acting very much like a thin low cloud in increasing the LW greenhouse effect of the atmosphere. The short wave reflection of white aerosol has, however, stronger impact in general, but the aerosol LW greenhouse effect is non-negligible under heavy aerosol loads.