A New Approach to Quantify Total Organic Nitrate Yields from RO2 + NO Reactions: Application to Ethane-, Cyclohexane-, and Isoprene-Derived RO2 Radicals.

The fate of peroxy radicals (HO2 + RO2), in particular the competition between propagation and termination pathways, is critical for understanding the atmospheric oxidative capacity. A new methodology was developed to characterize an important RO2 termination pathway by measuring the total organic nitrate yields (YRONO2) in the reactions of RO2 with NO. This is based on the use of a chemical amplifier where YRONO2 is derived by measuring the fraction of RO2 that propagates to HO2. This offers an alternative to the common approach of determining YRONO2 from the formation of an organic nitrate oxidation product. This approach circumvents the need for precise calibration with nitrate products. Applications to ethane-, cyclohexane-, and isoprene-RO2 at room temperature and atmospheric pressure yield YRONO2 values (mean ± 1σ) of (4.1 ± 2.9)%, (30.3 ± 0.8)%, and (12.1 ± 1.9)%, respectively. A comparison to the Master Chemical Mechanism indicates a good agreement for isoprene-RO2 but higher experimental yields for ethane- and cyclohexane-RO2. The disagreement observed for ethane-RO2 is attributed to the formation of organic nitrite (RONO) within this chemical system, while the discrepancy observed for cyclohexane-RO2 is likely due to an underestimation of YRONO2 in the MCM mechanism. No dependence of YRONO2 with humidity was observed for water mixing ratios ranging from 0.2 to 1.0%.