Our measurements show that in the summertime conditions experienced in Beijing the ratio of (1-OH, 2-ONO2)-IHN to (4-OH, 3-ONO2)-IHN (the numbers indicate the carbon atom in the isoprene chain to which the radical is added) increases at NO mixing ratios below 2 ppb. This provides observational field evidence of the redistribution of the peroxy radicals derived from OH oxidation of isoprene away from the kinetic ratio towards a new thermodynamic equilibrium consistent with box model calculations. The observed amounts of δ-ICN demonstrate the importance of daytime addition of NO3 to isoprene in Beijing but suggest that the predominant source of the δ-ICN in the model (reaction of NO with δ-nitrooxy peroxy radicals) may be too large. Our speciated measurements of the four δ-ICN exhibit a mean C1 : C4 isomer ratio of 1.4 and a mean trans : cis isomer ratio of 7 and provide insight into the isomeric distribution of the δ-nitrooxy peroxy radicals. Together our measurements and model results indicate that propanone nitrate was formed from the OH oxidation of δ-ICN both during the day and night, as well as from NO3 addition to propene at night.
This study demonstrates the value of speciated IN measurements in testing understanding of the isoprene degradation chemistry and shows how more extensive measurements would provide greater constraints. It highlights areas of the isoprene chemistry that warrant further study, in particular the impact of NO on the formation of the IHN and the NO3-initiated isoprene degradation chemistry, as well as the need for further laboratory studies on the formation and the losses of IN, in particular via photolysis of δ-ICN and hydrolysis.
We are grateful for funding provided by the UK Natural Environment Research Council (NERC), UK Medical Research Council, and the Natural Science Foundation of China (NSFC) under the framework of the Newton Innovation Fund. Eloise Slater and Robert Woodward-Massey are grateful to the NERC SPHERES Doctoral Training Programme for funding their PhD studentships. Claire E. Reeves acknowledges Andrew Rickard (NCAS, University of York) for providing information on the MCM.
This research has been supported by the UK Natural Environment Research Council (NERC; grant nos. NE/N006909/1, NE/N006895/1, NE/N006976/1, and NE/N00700X/1), UK Medical Research Council, the Natural Science Foundation of China (NSFC) under the framework of the Newton Innovation Fund (grant no. 41571130031) and the NERC SPHERES Doctoral Training Programme.
ASJC Scopus subject areas
- Atmospheric Science