科研成果 by Year: 2010

We present an experimental investigation of the dependence of the production of large methane clusters on the cluster source conditions. The clusters were produced at room temperature through supersonic expansion of methane gas at the backing pressures P0 ranging from 10 to 84 bar using five conical nozzles of different geometries. The cluster size was characterized by Rayleigh scattering measurements and calibrated with Coulomb explosion of the clusters at P0 = 44 bar subjected to an ultraintense laser pulse. A quantitative evaluation of the performance of the conical nozzles against the nozzle geometry and the backing pressure was made by introducing a parameter δ. Differ from the idealized case where the performance of the conical nozzle can be described by the equivalent sonic nozzle of diameter deq, in the present work, the “effective equivalent sonic-nozzle diameter” of the conical nozzle defined by deq* = δdeq is introduced. δ represents the deviation of the performance in cluster formation of the conical nozzles from that predicted on the basis of the concept of the equivalent diameter deq = d/tan α, with d being the throat diameter, and α the half-opening angle of the conical nozzle. Experimental results show that the cluster growth process will be restricted when the gas backing pressure P0 is higher and/or d/tan α of the conical nozzle becomes larger, resulting in smaller δ. From the experimental data, δ can be expressed by an empirical relation δ = A/[P0B(d/tan α)1.36], where A = 8.4 and B = 0.26 for 24 bar ≤ P0 ≤ 54 bar (2.8 mm < d/tan α < 4.5 mm), and A = 72 and B = 0.80 for 54 bar ≤ P0 ≤ 84 bar (2 mm ≤ d/tan α ≤ 7 mm). For all the cases investigated in this work, δ was found to lie between about 0.2 and 1.0, and the average radii of the methane clusters were measured to be 1−7 nm, depending on the experimental conditions. For lack of the experimental data on methane cluster formation with sonic nozzles, the data from the “sonic-like” conical nozzles were applied. Consequently, the δ values provided in this work for the conical nozzles remain relative in nature.

The sampling artifacts (both positive and negative) and the influence of thermal-optical methods (both charring correction method and the peak inert mode temperature) on the split of organic carbon (OC) and elemental carbon (EC) were evaluated in Beijing. The positive sampling artifact constituted 10% and 23% of OC concentration determined by the bare quartz filter during winter and summer, respectively. For summer samples, the adsorbed gaseous organics were found to continuously evolve off the filter during the whole inert mode when analyzed by the IMPROVE-A temperature protocol. This may be due to the oxidation of the adsorbed organics during sampling (reaction artifact) which would increase their thermal stability. The backup quartz approach was evaluated by a denuder-based method for assessing the positive artifact. The quartz-quartz (QBQ) in series method was demonstrated to be reliable, since all of the OC collected by QBQ was from originally gaseous organics. Negative artifact that could be adsorbed by quartz filter was negligible. When the activated carbon impregnated glass fiber (CIG) filter was used as the denuded backup filter, the denuder efficiency for removing gaseous organics that could be adsorbed by the CIG filter was only about 30%. EC values were found to differ by a factor of about two depending on the charring correction method. Influence of the peak inert mode temperature was evaluated based on the summer samples. The EC value was found to continuously decrease with the peak inert mode temperature. Premature evolution of light absorbing carbon began when the peak inert mode temperature was increased from 580 to 650 degrees C; when further increased to 800 degrees C, the OC and EC split frequently occurred in the He mode, and the last OC peak was characterized by the overlapping of two separate peaks. The discrepancy between EC values defined by different temperature protocols was larger for Beijing carbonaceous aerosol compared with North America and Europe, perhaps due to the higher concentration of brown carbon in Beijing aerosol.