New Insights Into the Atmospheric Role and Fate of Reactive Intermediates from Chemical Ionization Mass Spectrometry

Abstract

Atmospheric radical and reactive intermediate species control the oxidative removal of many natural trace gases and anthropogenic compounds in the troposphere. This process results in formation of secondary pollutants, such as ozone and particulate matter, which have large impacts on climate and human health. High reactivity and rapid cycling of radicals and reactive intermediates determine their key importance in the atmosphere but also make quantification of these components challenging. Extremely sensitive detection systems are required to measure minute mixing ratio, typically a few parts per trillion or less, of these short-lived species. In this work, a new chemical ionization mass spectrometer (CIMS) was developed and built. This instrument can be operated in two ionization modes: hydronium reagent ions (H3O+ CIMS) are used to detect the broadest range of organic species, while the use of ammonia reagent ions (NH4+ CIMS) allows for quantification of a plethora of oxidized organic compounds without the need of their direct calibration. CIMS instrumentation was further implemeted to study photooxidation chemistry of several aromatic compounds and the fate of produced bicyclic peroxy radicals (RO2). It was shown that under relevant urban conditions these radicals can undergo unimolecular reactions leading to the formation of highly oxygenated organic molecules that contribute significantly to secondary organic aerosol formation. In addition, CIMS was coupled with spin trapping and chemical derivatization techniques to detect peroxy radicals and Criegee intermediates (CIs) produced via ozonolysis of several alkenes. Effective stabilization and scavenging of CIs and RO2 species using spin traps and chemical derivatization agents allow for suppression of the radical secondary chemistry, which eliminates potential interferences. This analytical method was used for integrated production measurements of CIs and RO2 in laboratory experiments and can be implemented for detecting these components in the ambient atmosphere in the future.