Abstract
A global 3-D chemistry and transport model, STOCHEM integrated with a detailed VOC oxidation scheme (CRI v2-R5) has been employed to study the important NOx reservoir compound, peroxyacetyl nitrate (PAN). Globally, PAN is produced entirely by the reaction of acetyl peroxy radicals (CH3CO3) with NO2 and up to 2.0 ppb of PAN is found over the polluted regions of North America during June- July-August for the present scenario. The imbalances between model and measurement data are noted, with STOCHEM-CRI overestimating PAN mixing ratios relative to the measurement data by +17 and +80 pptv for the lower and upper troposphere, respectively. The inclusion of additional HOx recycling mechanisms (e.g. related to isoprene oxidation) in STOCHEM-CRI causes a decrease in PAN in a present scenario by as much as 40% over sink regions and reduces the model-measurement disagreement by 90% for the lower troposphere and 40% for the upper troposphere. The lower NOx emissions and CH3CO3 formation upon including HOx recycling in a preindustrial scenario led to a decrease in PAN formation by as much as 40%. The decrease in PAN formation results in less nitrogen being transported to remote regions which in turn leads to the greatest percentage change in O3 concentration (9% decrease) in the equatorial regions.
Highlights
Peroxyacetyl nitrate (PAN) is formed via the oxidation or photolysis of oxygenated volatile organic compounds in the presence of NO2 [1,2]
The reduced total PAN global burden (346 Gg(N)) in the study are driven by the decreased production of CH3CO3 (e.g. 11% less CH3CO3 production in the ISOP scenario compared with the base case scenario) leading to the global decrease of PAN by 13% for the present scenario which given that the base simulation generally over-predicts PAN brings the model into closer agreement with measured values
The complexity of the VOC degradation in STOCHEM-CRI resulted in an increase of tropospheric PAN which act as nitrogen reservoir species before releasing and transporting reactive nitrogen (NOx) away from the pollution centers
Summary
Peroxyacetyl nitrate (PAN) is formed via the oxidation or photolysis of oxygenated volatile organic compounds (oVOCs) in the presence of NO2 [1,2]. PAN is phytotoxic in high concentrations [3,4], and is a powerful lachrymator formed in photochemical air pollution [5,6]. PAN has no known direct emission sources and is an excellent indicator of the photochemical processing of an air mass which plays a significant role in the atmospheric transport of reactive nitrogen on a regional and global scale. Quantification of the spatial distribution of PAN and its global budget can be a useful tool in determining the oxidative reactions involved in the formation of O3 as well as other secondary air pollutants (e.g. nitric acid). The measurements of the temporal variability of PAN are necessary to evaluate the global budget of O3, NOy, HOx, and the associated recycling of NOx. we present the global burden and the global distribution of PAN from the STOCHEM-CRI global chemistry transport model. The effects of additional HOx recycling mechanisms involving isomerisation of isoprene-derived peroxy radicals [57] and propagating channels for the reactions of HO2 with acyl peroxy radicals, RCO3 [58] in both present and preindustrial scenarios are shown in the study
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