Stable Carbon Isotope Ratios and the Photochemical Age of Atmospheric Volatile Organic Compounds

Abstract

The dependence of ozone formation on the mixing ratios of volatile organic compounds (VOCs) and nitrogen oxides (NO x ) has been widely studied. In addition to the atmospheric levels of VOCs and NO x , the extent of photochemical processing of VOCs has a strong impact on ozone levels. Although methods for measuring atmospheric mixing ratios of VOCs and NO x are well established and results of those measurements are widely available, determination of the extent of photochemical processing of VOCs, known as photochemical age (PCA), is difficult. In this article a recently developed methodology for the determination of PCA for individual compounds based on the change in their stable carbon isotope composition is used to investigate the dependence between ozone and VOC or NO x mixing ratios at a rural site in Ontario, Canada, during fall and winter. The results show that under these conditions the variability in VOC mixing ratios is predominantly a result of the varying impact of local emissions and not a result of changes in the extent of atmospheric processing. This explains why the mixing ratio of ozone shows no systematic dependence on the mixing ratios of VOCs or NO x in this environment and at this time of the year. RÉSUMÉ [Traduit par la rédaction] La dépendance de la formation de l'ozone à l'égard des rapports de mélange des composés organiques volatiles (COV) et des oxydes d'azote (NO x ) a fait l'objet de nombreuses études. En plus des concentrations atmosphériques de COV et de NO x , le degré d'avancement du traitement photochimique des COV a un effet marqué sur les concentrations d'ozone. Même si les méthodes de mesure des rapports de mélange atmosphérique des COV et des NO x sont bien établies et que les résultats de ces mesures sont facilement disponibles, il reste difficile de déterminer le degré d'avancement du traitement photochimique des COV, que l'on nomme « âge photochimique ». Dans cet article, nous employons une méthodologie récemment mise au point pour la détermination de l'âge photochimique de différents composés, basée sur le changement dans la composition de leurs isotopes stables du carbone, pour étudier la dépendance de l'ozone à l'égard des rapports de mélange des COV ou des NO x à un site rural en Ontario, au Canada, durant l'automne et l'hiver. Les résultats montrent que dans ces conditions la variabilité dans les rapports de mélange des COV est principalement attribuable à l'effet variable des émissions locales et non à des changements dans le degré d'avancement du traitement atmosphérique. Cela explique pourquoi le rapport de mélange de l'ozone n'affiche pas de dépendance systématique à l'égard des rapports de mélange des COV ou des NO x dans cet environnement et à ce moment de l'année.

Dependence between the Photochemical Age of Light Aromatic Hydrocarbons and the Carbon Isotope Ratios of Atmospheric Nitrophenols

“…For the range of carbon 10 isotope ratios observed in ambient studies uncertainties of the rate constants and kinetic isotope 11 effects known from laboratory studies result in uncertainty of nitrophenol carbon isotope ratios 12 typically less than ± 0.5 ‰, which is similar to or below the estimated accuracy of ambient 13 measurements of nitrophenol carbon isotope ratios (Saccon et al, 2013;Saccon et al, 2015). 14 reports that 25 % of PCA derived from carbon isotope ratio measurements of benzene and 9 toluene are below 1.1×10 11 s molecules cm -3 and 1.6×10 10 s molecules cm -3 , respectively.…”

“…However, partitioning will influence the loss rate for SVOC since it is assumed that there 12 is little to no chemical loss in the PM phase. Furthermore, uncertainty in phase partitioning will 13 only result in minor uncertainties of the photochemical nitrophenol loss rate since it has been 14 reported that only approximately 20 % of the nitrophenols partition into the particle phase 15 (Saccon et al, 2013(Saccon et al, , 2015. 16 There are also some reaction rate constants which for which no laboratory measurements 17 are available.…”

“…Since differences in isotope ratios between species are 3 small, delta values are expressed in per mille notation. Limited studies using the isotope ratio of 4 atmospheric trace components, including several aromatic VOC and nitrophenols in SOA, have 5 been applied to differentiate between sources and to trace components back to the precursor, 6 respectively (Moukhtar et al, 2011; Kornilova et al, 2013;Saccon et al, 2013). The concept to 7 derive information on photochemical processing of trace constituents from isotope ratios is based 8 on the kinetic isotopic effect (KIE) which describes the dependence of the rate constant of a 9 reaction on the atomic mass of isotopologues.…”

A method for stable carbon isotope ratio and concentration measurements of ambient aromatic hydrocarbons

“…Details about the sampling site can be found in Kornilova et al, 2013. This paper also contains a detailed discussion of the use of the measured VOC carbon isotope ratios to quantitatively evaluate photochemical VOC processing.…”

“…The newly developed method was applied in an ambient air quality field study in 2009 to 2010 (Kornilova et al, 2013) at Egbert, a semi-rural location in southern Ontario. Details about the sampling site can be found in Kornilova et al, 2013.…”