An observation of physical and optical properties of aerosols was carried out in an urban atmosphere of Nagoya, Japan from 23 July to 4 September 2004. Such observed data were analyzed to understand urban aerosol effects on atmospheric heat budget. Aerosols over our observation area were found to be highly light absorptive with a very low mean single scattering albedo (ω) of 0.72. The presence of such light absorbing aerosols was capable to significantly influence the diurnal cycle of ω, indicating important roles of such aerosols on atmospheric heat budget. An algorithm was developed to estimate the refractive index and density of dry aerosols, and such algorithm produced the mean values of refractive index and density of 1.53-0.038i and 1.7 gcm-3, respectively. Sub micron (d < 1.0 mm) aerosols were observed to contribute total aerosol mass concentration and scattering coefficient for aerosols less than 10.0 μm in diameter by 81% and 95%, indicating the important roles of sub micron aerosols on atmospheric radiative transfer process. The 24 hours mean values of radiative forcing efficiency (RFE) at surface and top of the atmosphere (TOA), atmospheric forcing efficiency (AFE), and heating rate at surface were 71.8±12.3 Wm-2, ±2.5±6.8 Wm-2, 74.4±19.0 Wm-2 and 2.46±0.61 K/day, respectively. The study suggested that an increase of black carbon (BC) mass fraction could effectively increase the heating rate at surface by trapping more energy (or increasing AFE) in the atmosphere. It was observed that aerosols over our study area could have a positive RFE at TOA if BC mass fraction becomes greater than ̃0.05 (or ω becomes smaller than ̃0.75). Further, the study suggested that an increase of BC mass fraction by 0.1 could increase RFE at TOA by ̃15.8 Wm -2, decrease RFE at surface by ̃30.4 Wm-2 and increase heating rate at the surface ̃1.39 K/day, respectively.