| Abstract |
Teflon chambers are ubiquitous in studies of atmospheric chemistry. Secondary organic aerosol (SOA) formation can be underestimated, owing to deposition of SOA-forming vapors to the chamber wall. We present here an experimental protocol and a model framework to constrain the vapor–wall interactions in Teflon chambers. We measured the wall deposition rates of 25 oxidized organic compounds generated from the photooxidation of isoprene, toluene, ?-pinene, and dodecane in two chambers that had been extensively used and in two new unused chambers. We found that the extent of prior use of the chamber did not significantly affect the sorption behavior of the Teflon films. Among the 25 compounds studied, the maximum wall deposition rate is exhibited by the most highly oxygenated and least volatile compounds. By optimizing the model output to the observed vapor decay profiles, we identified that the dominant parameter governing the extent of wall deposition of a compound is its wall accommodation coefficient (?w,i), which can be correlated through its volatility with the number of carbons and oxygens in the molecule. By doing so, the wall-induced deposition rate of intermediate/semi-volatile organic vapors can be reasonably predicted based on their molecular constituency. However, it needs to be noted that vapor wall vapor deposition rates observed by Paul Ziemann and his group are larger than those measured here. At present, there is no consistent explanation for this discrepancy. Reconciliations of these differing experimental results is of high priority. One approach is to conduct matched experiments in different chambers. In this regard, we initiated a joint experiment between Caltech and Georgia Institute of Technology, in which matched chamber experiments were carried out in each laboratory. The goal of the experiments is to understand any differences in chamber performance for seemingly identical experiments of secondary organic aerosol formation in the dark ozone-alpha-pinene system. Initial analysis of the experiments carried out in the two chambers reveal significant differences in the amount and timing of SOA formation. Such experiments are of great value, as it is apparent that reconciliation and explanation of the data require analysis of all aspects of chamber operation, including chamber size, mixing, particle wall loss, and vapor wall loss, as well as the performance of the key instrumentation. This matched chamber study, the first of its type, to our knowledge, points to the potential importance of such inter-laboratory studies in diagnosing chamber results. As a result of this, we recommend that the CARB place a high priority on supporting future studies of this type. Completion of the analysis and preparation of an appropriate publication from the joint Caltech-Georgia Tech study will occur after the official termination of the present contract. Any publication will acknowledge support of the State of California Air Resources Board. |
