Abstract
Abstract. We describe a thermal dissociation cavity ring-down spectrometer (TD-CRDS) for measurement of ambient NO2, total peroxy nitrates (ΣPNs) and total alkyl nitrates (ΣANs). The spectrometer has two separate cavities operating at ∼ 405.2 and 408.5 nm. One cavity (reference) samples NO2 continuously from an inlet at ambient temperature, the other samples sequentially from an inlet at 473 K in which PNs are converted to NO2 or from an inlet at 723 K in which both PNs and ANs are converted to NO2, difference signals being used to derive mixing ratios of ΣPNs and ΣANs. We describe an extensive set of laboratory experiments and numerical simulations to characterise the fate of organic radicals in the hot inlets and cavity and derive correction factors to account for the bias resulting from the interaction of peroxy radicals with ambient NO and NO2. Finally, we present the first measurements and comparison with other instruments during a field campaign, outline the limitations of the present instrument and provide an outlook for future improvements.
Highlights
Reactive nitrogen oxides are centrally important trace gases in atmospheric chemistry as they affect air quality, climate and ecosystem nutrients
Beside inorganic NOx (NOx ≡ NO + NO2) there are several classes of organic nitrogen oxides including peroxy nitrates (RO2NO2) and alkyl nitrates (RONO2) which have an important influence on atmospheric composition
Peroxy nitrates and alkyl nitrates are produced as by-products in the photochemical oxidation of volatile organic compounds (VOCs) in the presence of NOx, the same processes and reactions that produce the pollutant and greenhouse gas ozone
Summary
Reactive nitrogen oxides are centrally important trace gases in atmospheric chemistry as they affect air quality, climate and ecosystem nutrients. These considerations led to the development of instruments (Day et al, 2002) which make use of the thermal instability of RO2NO2 and RONO2 at elevated temperatures and which report measurements of the sum of peroxy nitrates ( PN) or the sum of alkyl nitrates ( AN) by monitoring the NO2 product of the thermal decomposition of PNs and ANs at different temperatures These studies have helped to confirm that ANs and PNs represent a significant fraction of atmospheric NOy and confirm their role in e.g. HOx radical chain termination, or as indicators of photochemical O3 generation (see e.g. Day et al, 2003; Rosen et al, 2004; Perring et al, 2013). Our instrument operates under similar conditions of pressure flow, oven temperature and laser wavelength to those described in Paul et al (2009) and Paul and Osthoff (2010)
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