Time-averaged sampling

In this paper we have shown that there is no simple answer to the question posed in the title of this paper. Primary carbon particles dominate the carbonaceous aerosol under certain conditions while substantial secondary carbon may be present at other times. However, the importance of secondary carbon contributions is much less obvious when 24-h samples are examined. With shorter time averaged samples (e.g. 6-h or less) the increase in secondary carbon formation can be more easily detected. Secondary carbon appears to be more important in the summer rather than winter, in the afternoon father than the early morning, and in LA rather than St. Louis. It should be noted that these conditions of increased secondary carbon aerosol formation are also more favorable conditions for photochemical reactions. Our detailed emission inventory Indicates that much more primary carbon exists in the urban aerosol than was thought previously. This is in agreement with the data. Our analysis shows that even on the very smoggy days in the ACHEX study there were times when primary carbon dominated the carbonaceous aerosol. 

Water samplers can be either automatic or manually operated. Automatic samplers are used to collect samples at either fixed time-intervals or in proportion to the flow and then to retain the water sample in a separate container. These are commonly used, for example, in rivers or from a point source (effluent outfall). In addition, automatic samplers can be used to allow the collection of time-averaged samples or precipitation. In the case of the latter, the onset of rainfall triggers the collection mechanism. Manually operated samplers are essentially open tubes of known volume (typically 1 to 30 1) fitted with a closure mechanism at each end. They are constructed of stainless-steel or PVC. Manually operated samplers are particularly useful when sampling from open waters (e.g. oceans, seas, lakes, etc.) at specific depths. The sampling device is lowered on a calibrated line to the specific sampling depth, the sample is taken and then the top and bottom lids are closed and sealed. 

Calculating ensemble or time averages over a relatively small number of points (perhaps a few million) and a limited number of particles (perhaps a few hundred), instead of something which approaches a macroscopic sample of perhaps 10 ° molecules/configurations. 

Number of averages Typically, a minimum of three samples should be collected for an average. However, the factor that determines the actual number is time. One sample takes three to five seconds, a four-sample average takes 12 to 20 seconds, and a 1000-sample average takes 50 to 80 minutes to acquire. Therefore, the final... 

Another way of obtaining information on the time-averaged mobility of charge carriers on their way across a sample of arbitrary thickness is, to simultaneously measure the current and the stationary charge earners density via transient absorption and to infer the mobility from j-rnpF. Such experiments have been performed on blends of conjugated oligomers derived from /m-siilbene amine and a... 

Extraction technique Average solvent used per sample (ml) Average extraction time per sample... 

In static light scattering only the time average of the scattered intensity is recorded, while DLS measures the scattered intensity with a time resolution of milliseconds or even tenths of nanoseconds. These fluctuations in the scattered intensity can be related to dynamical processes within the sample. 

Area (time of sampling) Depth n Average VOC concentrations (hg C/l) Range (ggC/1) Average Range VOGTOC < (%) ratio (%) ... 

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