An improved free radical detector for tropospheric study using chemical amplification: Measurement and theory.

摘要

RO{dollar}sb{lcub}rm x{rcub}{dollar} radicals are important because their involvement in a variety of tropospheric chemistry issues ranging from ozone and acid rain formation to hydrocarbon oxidation. The determination of RO{dollar}sb{lcub}rm x{rcub}{dollar} concentrations is fundamental to an understanding of atmospheric chemistry, but the measurement has been fraught with difficulties, since RO{dollar}sb{lcub}rm x{rcub}{dollar} concentrations are low.; Chemical amplification, one of the techniques developed to make the atmospheric RO{dollar}sb{lcub}rm x{rcub}{dollar} radical measurement, was first proposed in the early 1980's. Some problems associated with this method are variable chain lengths and interferences from other atmospheric species resulting in zero modulation signals.; In this dissertation, an improved free radical detector using chemical amplification technique is reported. The reaction chamber in this version was equipped with a Teflon filter-nitrogen wall to maintain the nature of the wall constant and suppress the radical wall loss. The residence time of the radicals in the chamber was reduced from 3 s to 1.1 s by using a smaller reactor. Also, the procedure for the chain length determination and the chemical mechanism behind it were restudied in detail.; The results of laboratory studies were consistent with theoretical calculations assuming a radical wall loss coefficient of 1 s{dollar}sp{lcub}-1{rcub}{dollar}. Radical calibration gave relatively stable chain lengths, i.e., 26% variability over a period of three months. In addition, the zero modulation and the interferences from the ambient PAN and PNA decreased.; This improved free radical detection system was used in a field study to measure tropospheric RO{dollar}sb{lcub}rm x{rcub}{dollar} radicals for one month in the summer of 1993 in Denver, Colorado. The results were correlated with solar UV fluxes and ozone mixing ratios monitored during the same period. The RO{dollar}sb{lcub}rm x{rcub}{dollar} measurements were also compared to predictions from a computer simulation. Significant free radical levels were observed during the early evening and nighttime, and are believed to arise from the HO radicals produced by the reactions of ozone with ambient alkenes. Within the error bars of the measurements, and approximations and assumptions in the calculations, the data appear to be in accord with the theoretical model.