Measurement report: Air ion composition during new particle formation events in summertime Cyprus

Abstract. New particle formation is an important source of atmospheric aerosols, but the chemical factors controlling its frequent occurrence in the Eastern Mediterranean remain poorly constrained. We investigated summertime new particle formation and negative ion composition at the rural Cyprus Atmospheric Observatory site during July–September 2023 using ion and particle size distributions, molecular-level ion composition, trace gases, aerosol properties, and meteorological observations. New particle formation was observed on 19 of 59 measurement days and occurred less frequently in August than in September, consistent with the summertime minimum previously observed in Cyprus. Growth rates increased with particle size, while ion-induced nucleation contributed a median of ~4 % to the total particle formation rate. Events occurred under broadly similar global radiation, sulfur dioxide, ozone, nitrogen oxide, and meteorological conditions, but under lower particulate matter concentrations and condensation sink than non-event days. Molecular-level measurements revealed a pronounced diurnal evolution of atmospheric ion chemistry. Daytime ion composition was dominated by sulfuric acid monomers, dimers, and trimers together with several mixed sulfuric acid–organic clusters. Nighttime spectra were dominated by highly oxygenated organic molecules clustered with nitrate. Positive matrix factorization resolved four factors describing the diurnal changes, including an iodate-containing afternoon transition between sulfuric acid-dominated daytime chemistry and highly oxygenated organic molecule-dominated nighttime chemistry. These measurements provide molecular-level characterization of summertime atmospheric ion chemistry in the Eastern Mediterranean and new insight into the molecular composition of clusters involved in the earliest stages of atmospheric particle formation.