Effects of aerosols on the solar radiation in mid-hill of Nepal: a case study in the Kathmandu Valley

Abstract

Routinely surface solar radiation and aerosol optical depth (AOD) were measured at Kathmandu Valley during all possible days of the year from December 2011 to March 2013 to investigate the effects of aerosols on the surface solar radiation in clear sky. The analysis shows that the solar radiation changes with maximum (15.64 MJ/m2) in pre monsoon season to minimum (7.08 MJ/m2) in winter season whereas the relative humidity changes with minimum in pre monsoon to maximum in winter and monsoon seasons. The temperature in the valley varies from 31.4 °C in pre monsoon to 2.7 °C in winter with correlation coefficient, R = 0.72 between solar radiation and temperature. The relative humidity was found to increase AOD and Angstrom’s coefficient β. Calculated from AOD, the values of β found to vary from 0.16 ± 0.03 to 0.35 ± 0.15 with high values in summer and low values in winter. The higher values of β (>0.1) show that Kathmandu Valley is one of the most polluted cities. This study shows that Angstrom’s exponent α and Angstrom’s coefficient β vary with an anti correlation, R = −0.59. This variation is affected by humidity, temperature, wind speed, and transboundary transport of aerosols as well as by rainout and washout. It is found that solar radiation and β vary in opposite way which is consistent with the fact that higher the values of β, greater is the aerosols that produces more extinction of energy in solar radiation reaching the earth’s surface.