Volatile organic compound ratios as probes of halogen atom chemistry in the Arctic

A E Cavender,P B Shepson,J W Bottenheim,T A Biesenthal

doi:10.5194/acp-8-1737-2008

Abstract

Abstract. Volatile organic compound concentration ratios can be used as indicators of halogen chemistry that occurs during ozone depletion events in the Arctic during spring. Here we use a combination of modeling and measurements of [acetone]/[propanal] as an indicator of bromine chemistry, and [isobutane]/[n-butane] and [methyl ethyl ketone]/[n-butane] are used to study the extent of chlorine chemistry during four ozone depletion events during the Polar Sunrise Experiment of 1995. Using a 0-D photochemistry model in which the input of halogen atoms is controlled and varied, the approximate ratio of [Br]/[Cl] can be estimated for each ozone depletion event. It is concluded that there must be an additional source of propanal (likely from the snowpack) to correctly simulate the VOC chemistry of the Arctic, and further evidence that the ratio of Br atoms to Cl atoms can vary greatly during ozone depletion events is presented.

Highlights

It is well known that in Arctic regions periods of ozone depletion occur during the late winter and spring, corresponding with Polar Sunrise (Bottenheim et al, 1986, 2002; Oltmans and Komhyr, 1986)
Chlorine atom concentrations can be calculated based on decreases in measured alkanes, and relationships between volatile organic compounds (VOCs) can be utilized to further understand the role of Br and Cl (Jobson et al, 1994; Sander et al, 1997; Boudries et al, 2002; Evans et al, 2003)
We focus on four case study ozone depletion events from PSE95, indicated in the figure with the numbers 1 through 4

Summary

Introduction

It is well known that in Arctic regions periods of ozone depletion occur during the late winter and spring, corresponding with Polar Sunrise (Bottenheim et al, 1986, 2002; Oltmans and Komhyr, 1986). Cavender et al.: VOC ratios as probes of halogen atom chemistry in the Arctic This peroxy radical can go on to react with NO, HO2 or other peroxy radicals to form methyl ethyl ketone (MEK) and other products: CH3CH2CH(OO·)CH3 + NO. It has been observed that [isobutane]/[n-butane] is enhanced during ozone depletion events (Jobson et al, 1994; Solberg et al, 1996), which has been ascribed to rapid Cl-atom processing of n-butane By examining both [MEK]/[n-butane] and [isobutane]/[n-butane], the amount of processing that occurs by both Cl and OH radicals can be examined. Since [Br] is often quite large (>1×107 molec/cm3), relative to [Cl] and [OH] in the Arctic springtime boundary layer, and given the rate constant for bromine atom reaction with propanal, the ratio is substantially impacted by [Br]. A 0-D model has been constructed to support the analysis of VOC ratios to evaluate the relative importance of Cl, Br and OH radicals

Experimental

Ambient measurements

Evidence of chlorine atom chemistry

Evidence of bromine atom chemistry

Model and measurement comparison

Conclusions