Abstract
Abstract. Reaction with the hydroxyl radical (OH) is the dominant removal mechanism for virtually all volatile organic compounds (VOCs) in the atmosphere; however, it can be difficult to reconcile measured OH reactivity with known sinks. Unresolved higher molecular weight VOCs contribute to OH sinks, of which monoaromatics are potentially an important sub-class. A method based on comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC × GC-TOFMS) has been developed that extends the degree with which larger VOCs can be individually speciated from whole air samples (WAS). The technique showed excellent sensitivity, resolution and good agreement with an established gas chromatography–flame ionisation (GC-FID) method, for compounds amenable to analysis on both instruments. Measurements have been made of VOCs within the UK east coast marine boundary layer and free troposphere, using samples collected from five aircraft flights in winter 2011. Ten monoaromatic compounds with an array of different alkyl ring substituents have been quantified, in addition to the simple aromatics, benzene, toluene, ethyl benzene and Σm- and p-xylene. These additional compounds were then included in constrained box model simulations of atmospheric chemistry occurring at two UK rural and suburban field sites in order to assess the potential impact of these larger monoaromatics species on OH reactivity; they have been calculated to contribute an additional 2–6% to the overall modelled OH loss rate, providing a maximum additional OH sink of ~0.9 s−1.
Highlights
It is well known that the hydroxyl radical (OH) controls the daylight oxidising capacity of the atmosphere (Heard and Pilling, 2003)
In the presence of NOx and volatile organic compounds (VOCs), reactions involving OH can contribute to the formation of a range of important secondary pollutants, including tropospheric ozone, NO2 and secondary organic aerosol (SOA) (Atkinson, 2000; Goldstein and Galbally, 2007)
This paper presents the development of a quantitative method for the analysis of atmospheric VOCs using GC × GC-TOFMS, tailored to clean boundary layer and free-tropospheric samples
Summary
It is well known that the hydroxyl radical (OH) controls the daylight oxidising capacity of the atmosphere (Heard and Pilling, 2003). This is typically performed either via a “pump and probe” (Sadanaga et al, 2004), discharge flow techniques (Kovacs and Brune, 2001; Di Carlo et al, 2004) or a comparative reactivity method (Sinha et al, 2008; Nölscher et al, 2012) The former uses a laser pulse to generate OH radicals within a reaction cell and, using laser-induced fluorescence (LIF), detect the OH concentration decay with time as ambient air is introduced. The first GC × GC analysis of VOCs in air was performed by Lewis et al (2000) with a flame ionisation detector (FID) This identified that much of the hydrocarbon loading in an urban atmosphere was unaccounted for using conventional one-dimensional GC-FID techniques. The impact of a small subset of monoaromatic species that are rarely reported in routine analysis has been investigated on OH reactivity using a constrained box model incorporating the Master Chemical Mechanism (MCMv3.1, http://mcm.leeds.ac.uk/MCM) to simulate summertime photochemistry occurring at two UK field sites: (1) on the rural edge of London and (2) on the north Norfolk coast
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