Reply [to “Comment on ‘Indications of photochemical histories of Pacific air masses from measurements of atmospheric trace species at Point Arena, California’ by D. D. Parrish et al.”]

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. D8, PAGES 14,995-14,997, AUGUST Reply D. D. PARRISH, C. J. HAHN, 1 E. J. WILLIAMS, 1 R. B. NORTON, and F. C. FEHSENFELD Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado H. B. SINGH Earth System Science Division, NASA Ames Research Center, Moffett Field, California J. D. SHETTER,2 B. W. GANDRUD, and B. A. RIDLEY National Center for Atmospheric Research, Boulder, Colorado Chlorine atom chemistry in the marine boundary layer is a topic that has received considerable recent attention [see Finlayson-Pitts, this issue, and references therein]. In her comment, Finlayson-Pitts has pointed out that the observed slope of the n-butane to ethane versus propane to ethane correlation in Figure 9 of Parrish et al. [1992] is in closer agreement with C1 kinetics than the OH kinetics that was considered there and that this agreement may indicate that C1 atoms make a substantial contribution to the removal Fraserdale, Ontario (latitude 50 o N, longitude 82 o W), a remote boreal site in eastern central Canada; for this continental data set they found a slope of 1.44 with an r2 of 0.96. All of these slopes are in remarkable agreement, and all are significantly smaller than predicted by OH kinetics, which is 2.75 + 0.55 (confidence limit calculated from 2c• confidence limits of rate constants (R. Talukdar and A. R. Ravishankara, private communication, 1993) and a 250 to 298 K temperature range). of the These relatively small slopes are characteristic of the free light alkanes from the tropical and temperate marine troposphere and continental air masses as well as the marine troposphere. However, we believe that much more substantial boundary layer. Yet, the marine boundary layer is the only evidence must be presented for this conjecture before it can be region of the troposphere where the C1 atom sources cited by accepted. Moreover, evidence presented below suggests that C1 Finlayson-Pitts [this issue] could conceivably lead to a substantial contribution to alkane removal. atom chemistry is unlikely to play a significant role, at least in Second, there are correlation plots of alkane concentration the removal of light alkanes from either the continental or the ratios that can give more direct information regarding the marine troposphere in tropical and temperate regions. As discussed semiquantitatively in the appendix of Parrish et relative contributions of C1 atom and OH radical reactions to alkane removal, regardless of the effects of mixing. Table 1 al. [1992] and earlier by McKeen et al. [1990], the mixing of air masses of different ages leads to a systematic deviation from gives kinetic data for the reactions of C2-C4 alkanes with C1 and the slope expected from the kinetics of the removal processes. OH. The values of the relevant rate constants are such that the Recently, S. A. McKeen and S.C. Liu (Hydrocarbon ratios and C1 reactions will change the ratio of the two isomers of butane photochemical history of air masses, submitted to Geophysical but will leave the/-butane to propane ratio relatively constant. In contrast, OH reactions will leave the ratio of the butanes Research Letters, 1993) have quantitatively reproduced the observed slope from a three-dimensional model of the relatively constant but will change the /-butane to propane chemistry and mixing in the troposphere over the western ratio. Thus a log-log plot of the ratio of/-butane to n-butane Pacific region. This model included only OH removal of the versus /-butane to propane should provide two approximately alkanes. If the effects of mixing are not treated adequately, the orthogonal axes that will separate the C1 and OH contributions slopes of plots such as Figures 9 and 10 of Parrish et al. [1992] cannot be utilized to distinguish between the roles of different removal processes. Nevertheless, two arguments lead us to conclude that C1 atom chemistry plays no more than a minor role in the removal of the light alkanes from the tropical or temperate marine boundary layer. First, Figure 10 of Parrish et al. [1992] includes continental and free troposphere data as well as marine data. All of these data are reasonably well fit ( r 2 = 0.89) by a straight line with a slope (1.47) similar to that found in the Point Arena data alone (1.33) and in the North Atlantic (1.66) by Rudolph and Jobhen [1990]. Recently, Jobson et al. [1993] have presented a similar plot for data collected at 1Also at Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder. 2Presently at 841 Paragon Drive, Boulder, Colorado. Copyright 1993 by the American Geophysical Union. Paper number 93JD01416. 0148-0227/93/93 JD-01416502.00 to alkane removal. Furthermore, if one of these two ratios does indeed remain constant, then mixing of two air masses of different ages cannot affect that ratio since it would have the same value in both. Of course, this assumes that the source ratios are relatively constant; the data presented below suggest that this is indeed true. Figure 1 compares experimental data with the behavior expected from the reaction kinetics under five scenarios with differing contributions from C1 and OH reactions. (The format chosen for the plot is log-log rather than a linear plot of the logs of the ratios as in Figures 9 and 10 of Parrish et al. [1992]. The alkane present in the lowest concentrations and therefore subject to the greatest experimental uncertainties is placed in the numerator, and the most precisely determined ratio is used as the abscissa. Only data with/-butane concentrations greater than twice the detection limit are included in the plot, although the full data sets were used in the linear, least squares fits discussed below. ) The OH kinetics provide a mechanism for the evolution of the ratios characteristic of the urban emission source areas to the ratios observed in aged marine (Atlantic data) and continental (Boulder data) areas in Figure lb. These