Isoprene is a significant source of secondary organic aerosol (SOA) in the atmosphere. This study investigates the physicochemical properties of isoprene-derived SOA formed through ozonolysis and photooxidation under varying NOx conditions in an environmental chamber. SOA produced by dark ozonolysis and under low NOx conditions had a density of 1.35–1.38 g cm−3 and an organic-to-carbon (O:C) ratio of 0.89–0.97. It was relatively volatile, consisting of semi-volatile organic compounds (SVOCs, 40%) and low-volatility organic compounds (LVOCs, 52%), with a small fraction of extremely low-volatility organic compounds (ELVOCs, ~7%); its vaporization enthalpy (ΔHvap) was 90–106 kJ mol−1. Under high NOx conditions (isoprene/NOx ratios = 1.2–6.8, with isoprene units in ppbC), SOA exhibited lower density (1.26–1.29 g cm−3) and lower O:C ratios (0.62–0.72). It was also less volatile than SOA formed under dark ozonolysis and low NOx conditions; volatility decreased with decreasing isoprene/NOx ratio, while ΔHvap increased from 65 to 95 kJ mol−1. SOA formed under very high NOx conditions (isoprene/NOx ratio = 0.6) was characterized by a higher density (1.34 g cm−3) and O:C ratio (0.88). However, it was the least volatile, comprising 68% LVOCs and 32% ELVOCs, and had the highest ΔHvap of 114 kJ mol−1. At low isoprene/NOx ratios (0.6–1.2) yields were suppressed (0.6%) in comparison to those (6.8%) at higher isoprene/NOx ratios (5–7).
Tzouvaras et al. (Sun,) studied this question.