Comment on acp-2021-722

Abstract

We investigated the distribution of polycyclic aromatic hydrocarbons (PAH) on individual ambient aerosol particles at the Swedish west coast in a pristine environment for ten days in October 2019. The measurements were carried out using new technology in single-particle mass spectrometry (SPMS) that reveals both the inorganic particle composition as well as the particle-bound PAHs. More than 290,000 particles were characterized; 4,412 of them reveal PAH signatures. Most of the PAH-containing particles were internal mixtures of carbonaceous material, secondary nitrate, and metals from distant sources in Central and Eastern Europe. We characterize the aerosol with respect to the inorganic composition, comparable to conventional SPMS before we discuss the distribution of PAHs within this particle ensemble. Vice versa, we analyze the single-particle PAH spectra for characteristic patterns and discuss the inorganic composition, origin, and atmospheric processing of the respective particles. The study period comprised different meteorological situations: clean air conditions with winds from the North Sea/Kattegat and little terrestrial air pollution, long-range transport from Eastern Europe and southern Sweden as well as transport of aerosols from Central Europe over the sea. For all meteorological conditions, PAHs were detected in particles whose inorganic content indicates traffic emissions, such as soot, iron, and calcium as well as in particles with biomass burning signatures. However, there were variations in their amounts, dependent on the geographic origin. Because of strong mixing, rapid degradation, and speciation limits, e.g. for PAHs of the same nominal mass, the application of diagnostic ratios for source apportionment is limited under the conditions of our study. Nevertheless, the combination with the inorganic content and meteorological data provide unique insight into the particles’ origin, aging, and mixing state. We exemplarily show how the observation of PAH profiles and inorganic secondary components on a single-particle level can open a new door to investigate aerosol aging processes. To our best knowledge, we herewith present the first comprehensive study on the single-particle distribution of PAHs in ambient air as well as the first set of combined data on PAHs and inorganic composition on a single-particle level.