Aerosols serve as cloud condensation nuclei (CCN) and thus have a substantial effect on cloud properties and the initiation of precipitation. Large concentrations of human-made aerosols have been reported to both decrease and increase rainfall as a result of their radiative and CCN activities. At one extreme, pristine tropical clouds with low CCN concentrations rain out too quickly to mature into long-lived clouds. On the other hand, heavily polluted clouds evaporate much of their water before precipitation can occur, if they can form at all given the reduced surface heating resulting from the aerosol haze layer. We propose a conceptual model that explains this apparent dichotomy.

One of the goals of the East Pacific Investigation of Climate, year 2001 process study (EPIC2001), was to understand the mechanisms controlling the forcing of deep atmospheric convection over the tropical eastern Pacific. An intensive study was made of convection in a 4° × 4° square centered on 10°N, 95°W in September and October of 2001. This is called the intertropical convergence zone (ITCZ) study region because it encompasses the eastern Pacific intertropical convergence zone. Starting from an analysis of the theoretical possibilities and a plethora of dropsonde, in situ, radar, and satellite data, it is found that newly developing convection occurs where a deep layer of air (of order 1 km deep or deeper) is conditionally unstable with only weak convective inhibition. Shallower conditionally unstable layers are associated with numerous small clouds, but do not seem to produce deep convection. The occurrence of deep convection over the ITCZ study region is presumably related to the propensity ... Abstract One of the goals of the East Pacific Investigation of Climate, year 2001 process study (EPIC2001), was to understand the mechanisms controlling the forcing of deep atmospheric convection over the tropical eastern Pacific. An intensive study was made of convection in a 4° × 4° square centered on 10°N, 95°W in September and October of 2001. This is called the intertropical convergence zone (ITCZ) study region because it encompasses the eastern Pacific intertropical convergence zone. Starting from an analysis of the theoretical possibilities and a plethora of dropsonde, in situ, radar, and satellite data, it is found that newly developing convection occurs where a deep layer of air (of order 1 km deep or deeper) is conditionally unstable with only weak convective inhibition. Shallower conditionally unstable layers are associated with numerous small clouds, but do not seem to produce deep convection. The occurrence of deep convection over the ITCZ study region is presumably related to the propensity ...

The relationship between black carbon (BC) and carbon monoxide (CO) has been analyzed using measurements from two sites in Mexico City and five urban areas in Germany. The correlation coefficient between BC and CO is greater than 0.90 for all sites. The average slope of the linear regression line for BC versus CO is 2.2 μg mg⁻¹ for German sites and 1.1 μg mg⁻¹ in Mexico City. The most important factors that affect the BC to CO relationship appear to be the ratio of diesel to gasoline usage and the combustion efficiency of vehicles in a particular area. The results of this analysis suggest that CO measurements in urban areas can be used to estimate BC mass when direct measurements are not available.