Behavior of aerosol particles during dust- and pollen-affected time periods in southern Finland

Abstract. Natural dust and pollen episodes can alter aerosol particles, meaning tiny particles suspended in air, but their effects are not always easy to distinguish from optical measurements alone. We studied dust- and pollen-affected time periods at the Station for Measuring Ecosystem–Atmosphere Relations II (SMEAR II), a boreal forest field station in Hyytiälä, southern Finland. We combined in situ measurements of aerosol light scattering and absorption, size-resolved particulate matter mass, and aerosol particle size distributions. The merged optical and mass dataset contained 36 labelled event-affected sampling intervals: 15 dust-affected and 21 pollen-positive intervals. A separate event-level sensitivity analysis grouped the higher-resolution size-distribution data into 23 temporally separated episodes: 15 dust and 8 pollen episodes. Both dust- and pollen-affected periods increased aerosol scattering, showing that an optical enhancement can indicate that an atmospheric event occurred. However, bulk optical properties overlapped strongly and did not reliably identify whether the event was dust or pollen. Size-resolved mass and particle size-distribution measurements provided clearer physical information. Event–background differences increased with particle size and were strongest for particles larger than 10 micrometres, while accumulation-mode particle surface area showed the clearest temporally controlled dust–pollen tendency. These results show that, at this boreal forest site, dust and pollen can produce similar optical responses but different size-resolved particle responses. The study therefore helps clarify how long-term aerosol observations should be interpreted, and it cautions against assigning source type from optical enhancement alone. These findings support cautious use of monitoring records in boreal and other background environments.