On the variability of tropospheric gases: Sampling, loss patterns, and lifetime

Abstract

[1] The relationship between the variability (relative standard deviation, s) in mixing ratio of a gas and its global mean lifetime (t) has been used to estimate the t of atmospheric gases. This can prove quite useful if it is a unique relationship. Here a three-dimensional chemical transport model is used to investigate the variability-lifetime relationship of tropospheric gases with two types of sources and three types of losses. The effects of sampling time and location are also explored. The relationship is best described in the form s = at Àb , where a and b are variable depending on the sources, sinks, and time and location of averaging. When spatially averaging over the troposphere and temporally averaging over 1 year, the model results give a b of 0.77-0.79 for t between 0.9 and 7.0 years. The variability of a CH 3 Br-like gas is also analyzed using different weightings of chemical sinks. Photochemical (OH), ocean mixed layer, and soil losses are scaled separately to maintain t % 1 year. These different scalings result in a ±17% spread in s, which translates into a ±20% spread in t inferred from the variability-lifetime relationship found in the model. In addition, the model is used to simulate conditions of Pacific Exploratory Mission (PEM) Tropics A and B field campaigns. The variability-lifetime relationships derived from the model do not compare to the field observations, except that both demonstrate a seasonal dependence of variability. This study identifies some factors controlling the variability of trace gases in the troposphere, estimates the error in using variability-lifetime analysis to determine an unknown t, and shows that the variability-lifetime relation is not universal among trace gases.

Volatile organic compound measurements at Trinidad Head, California, during ITCT 2K2: Analysis of sources, atmospheric composition, and aerosol residence times

“…The A and b parameters are indicative of the chemical and dynamic history of sampled air masses, and can be expected to display substantial seasonal as well as geographic variation [ Jobson et al , 1999; Johnston et al , 2002]. However, the s lnX ‐τ fit obtained in this study is consistent with results from other experiments in similar locations.…”

“…[48] The consistent s lnX -t trend among species with varying source types (combustion, evaporative, photochemical, biogenic, marine) is in contrast to modeling results which predict different s lnX -t trends for different source categories [Johnston et al, 2002]. It appears that sampling location plays a central role in determining the coherence of s lnX -t trends for different types of compounds.…”

NMHCs and halocarbons in Asian continental outflow during the Transport and Chemical Evolution over the Pacific (TRACE‐P) Field Campaign: Comparison With PEM‐West B

“…The change was less systematic for the 0–2 km altitude range, mostly the result of the relatively high variability in methyl bromide mixing ratios during PEM‐West B (Table 2). This high PEM‐West B variability may be a function of the previously larger emissions [e.g., Colman et al , 2001; Johnston et al , 2002]. The midlatitude location of the largest change in the 2–8 km altitude bin is consistent with the biggest use decrease having taken place in the highly populated midlatitude industrial regions.…”

Hydrocarbon source signatures in Houston, Texas: Influence of the petrochemical industry

“…Frequency distributions of hydrocarbon mixing ratios in these environments also appear to be lognormal, in particular for reactive species [ Jobson et al , 1999]. Chemical transport modeling studies have shown that species with similar source distributions and removal mechanisms define coherent variability‐lifetime trends [ Hamrud , 1983; Ehhalt et al , 1998; Johnston et al , 2002]. Lenschow and Gurarie [2002] have reviewed theoretical treatments and presented an analytical diffusion model as a conceptual framework for understanding the physical basis of variability lifetime relationships in the troposphere.…”