NO increases direct aerosol precursor yields from aromatic carbonyl compounds

Ambient oxidation of volatile organic compounds (VOCs) is the route to condensable oxygenated molecules that form ambient secondary organic aerosol (SOA). It is generally accepted that NOx (=NO and NO₂) considerably hinders, even prevents, the formation of highly condensable products, and thus cuts short the SOA production. However, in certain chemical systems the involvement of NOx, or rather NO, can increase the yield of condensable chemicals by converting relatively unreactive peroxy radicals (RO₂) into much more reactive alkoxy radicals (RO) that contrary to previous reports can propagate the oxidation sequence through mechanistic bottlenecks. In select oxidation systems this leads to remarkably enhanced generation of highly condensable matter, an observation which carries an important message to polluted air chemistry.

In this work we studied three aromatic carbonyl oxidation systems benzaldehyde, acetophenone and phenylacetaldehyde by a joint experimental-computational approach. In the lab the reactions were studied in flow reactor setups under variable short reaction times and NOx additions, and the products were quantified utilizing nitrate ion based chemical ionization mass spectrometry. Computations and kinetic modelling were performed to strengthen the hypotheses originating from the experimental work. We find significant differences between the systems, with 2/3 studied aromatics showing much pronounced condensable product generation upon addition of NO, and the remaining 1/3 channeling the yield into a single nitro hydroxy product channel. Importantly the results show that even unrealistically large NO addition of 1 ppm does not shut down the highly efficient oxidation cascade but instead leads to several condensable products in higher yields than in absence of NO. This is in stark contrast to insights from the frequent monoterpene chamber oxidation experiments, in which practically invariably NO has been implied to severely hinder SOA generation.