PM2.5 impacts the atmospheric temperature structure through scattering or absorbing solar radiation, whose concentration and composition can affect the impact. This study calculated the effect of PM2.5 on the temperature structures in the urban centre and the suburbs of Nanjing, as well as their differences. The results show that the optical parameters, atmospheric heating rate, radiative forcing, and temperature are all impacted by the concentration and composition of PM2.5. The uneven distribution of PM2.5 influences the differences in those factors between the urban centre and suburbs. In spring, summer, autumn, and winter, surface temperatures in the urban centre were approximately 283 K, 285 K, 305 K, and 277 K, while those in the suburbs were approximately 282 K, 283 K, 304 K, and 274 K. The urban heat island intensity has been reduced by 0.1-0.4 K due to the presence of PM2.5 in Nanjing. Due to the black carbon component's warming effect on the top of the boundary layer, the impact of PM2.5 on the urban heat island intensity profile drops quickly at the 0.75-1.25 km. PM2.5 may mask the warm city problem and have a more complex impact on the urban climate.

A box model has been used to compare the burdens, optical depths and direct radiative forcing from anthropogenic PM2.5 aerosol constituents over the Indian subcontinent. A PM2.5 emission inventory from India for 1990, compiled for the first time, placed anthropogenic aerosol emissions at 12.6 Tg yr(-1). The contribution from various aerosol constituents was 28% sulphate, 25% mineral (clay), 23% fly-ash, 20% organic matter and 4% black carbon. Fossil fuel combustion and biomass burning accounted for 68% and 32%, respectively, of the combustion aerosol emissions. The monthly mean aerosol burdens ranged from 4.9 to 54.4 mg m(-2) with an annual average of 18.4 +/- 22.1 mg m(-2). The largest contribution was from fly-ash from burning of coal (40%), which has a high average ash content of 30%. This was followed by contributions of organic matter (23 %) and sulphate (22%). Alkaline constituents of fly-ash could neutralise rainfall acidity and contribute to the observed high rainfall alkalinity in this region. The estimated annual average optical depth was 0.08 +/- 0.06, with sulphate accounting For 36%, organic matter for 32% and black carbon for 13%, in general agreement with those of Satheesh et al. (1999). The mineral aerosol contribution (5%) was lower than that from the previous study because of wet deposition from high rainfall in the months of high emissions and the complete mixing assumption in the box model. The annual average radiative forcing was - 1.73 +/- 1.93 W m-2 with contributions of 49% from sulphate aerosols, followed by organic matter (26%), black carbon (11%) and fly-ash (11%). These results indicate the importance of organic matter and fly-ash to atmospheric optical and radiative effects. The uncertainties in estimated parameters range 80-120% and result largely from uncertainties in emission and wet deposition rates. Therefore, improvement is required in the emissions estimates and scavenging ratios, to increase confidence in these predictions. (C) 2000 Elsevier Science Ltd. All rights reserved.