The role of highly oxygenated molecules (HOMs) in determining the composition of ambient ions in the boreal forest

Abstract. In order to investigate the negative ions in the boreal forest we have performed measurements to chemically characterise the composition of negatively charged clusters containing highly oxygenated molecules (HOMs). Additionally, we compared this information with the chemical composition of the neutral gas-phase molecules detected in the ambient atmosphere during the same period. The chemical composition of the ions was retrieved using an atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF-MS) while the gas-phase neutral molecules (mainly sulfuric acid and HOMs) were characterised using the same mass spectrometer coupled to a nitrate-based chemical ionisation unit (CI-APi-TOF). Overall, we divided the identified HOMs in two classes: HOMs containing only carbon, hydrogen and oxygen and nitrogen-containing HOMs or organonitrates (ONs). During the day, among the ions, in addition to the well-known pure sulfuric acid clusters, we found a large number of HOMs clustered with nitrate (NO 3 − ) or bisulfate (HSO 4 − ), with the first one being more abundant. During the night, the distribution of ions, mainly composed of HOM clustered with NO 3 − , was very similar to the neutral compounds that are detected in the CI-APi-TOF as adducts with the artificially introduced primary ion (NO 3 − ). For the first time, we identified several clusters containing up to 40 carbon atoms. These ions are formed by up to four oxidised α -pinene units clustered with NO 3 − . While we know that dimers (16–20 carbon atoms) are probably formed by a covalent bond between two α -pinene oxidised units, it is still unclear what bonding formed larger clusters. Finally, diurnal profiles of the negative ions were consistent with the neutral compounds revealing that ONs peak during the day while HOMs are more abundant at night-time. However, during the day, a large fraction of the negative charge is taken up by the pure sulfuric acid clusters causing differences between ambient ions and neutral compounds (i.e. less available charge for HOM and ON).