Insights into the complicated influence of NOx on the growth of new particles from monoterpene ozonolysis

Monoterpenes are one of the most important precursors of atmospheric secondary organic aerosol (SOA) globally, and nitrogen oxides (NO + NO2 = NOX) are known to perturb both the yields and the properties of this SOA. Recent advances in chemical ionization mass spectrometry (CIMS) have opened up new possibilities to study both the gas and aerosol phase constituents involved in the SOA formation process. We applied these techniques during a set of dedicated chamber experiments studying monoterpene ozonolysis in the CLOUD chamber in CERN, Switzerland, and in a Teflon chamber at the University of Helsinki, Finland. We also compared our findings with those from our boreal research station in Hyytiälä, Finland. The aim of these experiments was to deduce the impact of NOX on the composition and volatility of initial oxidation products and the subsequent effect on size-dependent growth of newly formed particles. Our results show that the addition of NOX impacts the peroxy radical (RO2) chemistry, as expected, by enhancing the RO2 + NO termination pathway, forming an increased amount of organic nitrates and decreasing the amount of peroxide dimers from RO2 cross reactions. While a detailed mechanistic understanding is still hampered by the large amount of products as well as the increased importance of both nitrate radicals and alkoxy radical pathways, several results become evident. We observe the formation of more volatile products as NOX concentration are enhanced, and this is subsequently reflected in the size-dependent growth rates of the new particles.