UV and aerosol climatology based on simulations and measurements by satellites and ground station
Doctoral thesis, Peer reviewed
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Solar radiation (200–4,000 nm) is very important for the climate system on Earth. It provides heat and drives the photosynthesis in plants to fuel all life. It plays a major role in the original synthesis of biomolecules and in the evolution of life on Earth. Many acute and chronic health damages to the skin, eyes, and immune system of humans are caused by prolonged exposure to solar UV radiation. The most well-known impacts on human health from exposure to UV radiation are skin aging, sunburn, and skin cancer. The atmosphere has a big impact on the transfer of energy between the Sun and the Earth’s surface. It serves to maintain thermal equilibrium and to control the climate. The UV irradiance decreases strongly at wavelengths below 330 nm due to absorption by atmospheric ozone. This absorption occurs in the UV-A and UV-B regions (280–400 nm), which is also called biologically active dose rate region. Although the absorption of UV-B radiation only is a small part of the total absorption in the UV spectral range, a small increment of UV-B radiation can lead to substantial biological effects because of higher exposure to radiation at shorter wavelengths. The UV index (UVI), the total ozone column amount (TOCA), and the cloud modification factor (CMF) at four sites on the Tibetan Plateau (TP) have been determined in this study by use of multichannel moderate-bandwidth filter instruments at the ground in the period 2008–2010. The geographical locations and altitudes of these sites are 29.66◦N, 94.37◦E, 2,995 m for Linzhi; 29.65◦N, 91.18◦E, 3,683 m for Lhasa; 28.66◦N, 87.13◦E, 4,335 m for Tingri; and 31.47◦N, 92.06◦E, 4,510 m for Nagchu. TP is located in a northern mid-latitude region and is the largest and highest plateau area in the world with an average elevation of more than 4,000 m. The UVI on the TP can reach up to 20.6 in one-minute measurement during summer time, while 10 is an exceptionally high value for northern mid-latitudes. The annual mean TOCA values were found to be similar around 260-264 Dobson units (DU) in Lhasa, Linzhi, and Nagchu, and 252 DU in Tingri. The CMF values were found to be 0.70 for Linzhi, 0.83 for Lhasa, 0.92 for Tingri, and 0.70 for Nagchu. The effects of altitude, latitude and TOCA values on the UVI have been investigated in this thesis through the use of a radiative transfer (RT) model. The result shows that the high altitude of the TP combined with low latitude and low TOCA values caused the high UVI values observed during our investigation. Atmospheric aerosols are suspensions of solid or liquid particles in air, and are often observed as dust, smoke, and haze. They have different compositions, sizes, shapes, and optical properties and varying atmospheric lifetimes. Aerosol particle sizes range from a few nanometers to a few tens of micrometers. The radiative climate forcing of the Earth is affected by aerosols. The direct effect of aerosols is caused by large aerosols particles, which can absorb part of the incoming solar radiation and scatter another part back to space. Absorption tends to heat the planet and backscattering tends to cool it. The indirect effect of aerosols is due to small aerosol particles, which can act as cloud condensation nuclei (CCN) and thus increase the amounts of clouds and modify their microphysical and radiative properties. Understanding optical properties of aerosols is important in investigations of climate change, since aerosols have a major impact on the radiative energy balance. In this study, we have compared aerosol loadings in Northern Norway and Svalbard based on data from AERONET (AErosol RObotic NETwork) stations at Andenes (69 ◦N, 16 ◦E, 379 m altitude) and Hornsund (77 ◦N, 15◦E, 10 m altitude) for the period 2008–2010. The three-year annual mean values for the aerosol optical thickness at 500 nm τ500 at Andenes and Hornsund were found to be 0.11 and 0.10, respectively. At Hornsund, there was less variation of the monthly mean value of τ500 than at Andenes. The annual mean values of the Ångström exponent α at Andenes and Hornsund were found to be 1.18 and 1.37, respectively. At Andenes and Hornsund α was found to be larger than 1.0 in 68% and 93% of the observations, respectively, indicating that fine-mode particles were dominating at both sites. Both sites had a similar seasonal variation of the aerosol size distribution although one site is in an Arctic area while the other site is in a sub-arctic area. We used information retrieved from AERONET measurements about aerosol microphysical properties, i.e. refractive index and size distribution of aerosol particles (which were assumed to be spherical), to obtain solutions of Maxwell’s equations for scattering by a size distribution of spherical particles (Mie scattering), and thus provide Inherent Optical Properties (IOPs) required for RT computations (AOT, single-scattering albedo, and scattering phase function). As additional inputs to RT computations, we used geometry parameters (e.g. altitude and solar zenith angle) from the AERONET site at Andenes (69 ◦N, 16 ◦E, 379 m altitude) together with ozone data from OMI and different types of surface albedo. We used a radiative transfer (RT) model for coupled atmosphere-ocean systems, in which the radiative coupling between the atmosphere and the ocean is accounted for. The mean value of the aerosol optical thickness at 500 nm derived from AERONET measurements was found to be very close to the mean value obtained from RT modeling with IOP inputs obtained from Mie-scattering computations in conjunction with aerosol size distributions and wavelength-dependent refractive index as derived from AERONET measurements. Changes in the aerosol radiative forcing (ARF) due to changes in the surface reflectance was studied by considering six different types of surfaces. Although the ARF at the BOA retrieved from the AERONET algorithm showed good agreement with that computed by C-DISORT, the spectral irradiances at BOA inferred from AERONET measurements did not agree so well with those obtained from C-DISORT computations. Overall, the ARF values derived from AERONET measurements were smaller than those obtained from C-DISORT computations at TOA, but were larger at BOA. We also analyzed UVI, TOCA, and aerosol index (AI) values based on measurements by satellite instruments. To that end, we used more than three decades of global data (1979–2012) for tropospheric aerosols from the Total Ozone Mapping Spectrometer (TOMS) and the Ozone Monitoring Instrument (OMI). These instruments provided backscattered radiance measurements in the wavelength range from 331 to 380 nm, which we used to determine the aerosol climatology and to investigate the effects of the AI on the UVI in two coastal land areas [Serrekunda (13.28◦N, 16.34◦W, at sea level), Gambia and Dar-es-Salaam (6.8◦S, 39.26◦W, at sea level), Tanzania] and one inland area [Kampala (0.19◦N, 32.34◦E, 1200 m altitude), Uganda]. We found heavy aerosol loadings to occur in the dry seasons at all these three locations. Also, we made a comparison of UVI values inferred from satellite measurements with those obtained from radiative transfer modeling. The RT code used for radiative transfer computations in this study is based on the multiple-stream, multiple scattering DIScrete Ordinate Radiative Transfer model (DISORT). This RT code, C-DISORT, applies to a coupled system, such as an atmosphereocean system or an atmosphere-snow-ice-ocean system, for use by researchers in the ocean optics, climate research, and remote sensing communities. The C-DISORT code allows for a user-specified number of layers in the coupled system to adequately resolve the vertical variation in the inherent optical properties (IOPs), and it computes apparent optical properties (AOPs), such as the upward and downward irradiances, the scalar irradiance, and the diffuse attenuation coefficients at user-specified optical depths in the coupled system. It also computes radiances in user-specified directions at userspecified optical depths in a coupled system. In future work, we plan to use the C-DISORT code to perform simultaneous retrieval of aerosol and marine parameters in waters along the Norwegian coast based on (i) data collected by hyperspectral radiance and irradiance sensors onboard ships of opportunity, such as the Norwegian Coastal Express (Hurtigruten), (ii) forward computations using C-DISORT, and (iii) inversions based on optimal estimation theory
Består avPaper I : G. Norsang, Y.-C. Chen, N. Pingcuo, A. Dahlback, Ø. Frette, B. Kjeldstad, B. Hamre, K. Stamnes, and J. J. Stamnes, Comparison of ground-based measurements of solar UV radiation at four sites on the Tibetan Plateau. Full text not available in BORA.
Paper II: Y.-C. Chen, B. Hamre, Ø. Frette, and J. J. Stamnes, Climatology of aerosol optical properties in Northern Norway and Svalbard. (2012) Atmospheric Measurement Techniques 5: 7619 – 7640, 2012. The article is available at: http://hdl.handle.net/1956/6713
Paper III: T. Ssenyonga, Y.-C. Chen, A. Dahlback, A. Steigen, W. Okullo, Ø. Frette, D. Muyimbwa, and J. J. Stamnes, Aerosols in coastal and inland areas in the equatorial African Belt. Full text not available in BORA.
Paper IV: Y.-C. Chen, B. Harmre, S. Stamnes, Ø. Frette, K. Stamnes, and J. J. Stamnes, Sensitivity analyses of aerosol radiative forcing in Northern Norway. Full text not available in BORA.