Long-range transport influence on wintertime submicron aerosol chemical composition from simultaneous measurements in Lithuania and Southern Sweden

Due to the large extent of seawater surface over the globe, aerosol particles spend significant time being transported over this type of surface. Therefore, global aerosol circulation is largely impacted by the processes particles undergo during long-range transport overseas. In this study, we investigated submicron aerosol (PM₁) chemical composition changes during long-range transport events over the southern Baltic Sea across two sites in Sweden (Hyltemossa) and Lithuania (Preila). The chemical composition was determined simultaneously using mass spectroscopy (time-of-flight aerosol chemical speciation monitor (ToF-ACSM)) in Hyltemossa and quadrupole ACSM (Q-ACSM) in Preila and filter-based optical (AE33 in Hyltemossa and AE31in Preila) methods. The aim of the study was to investigate the effect of long-range air mass transport over the sea on PM₁ chemical composition at the two remote background sites in the southern Baltic Sea region. The campaign average PM₁ concentration in Preila (11.2 μg/m³) was higher than in Hyltemossa (6.3 μg/m³). Yet, during long-range transport events, PM₁ concentration in Preila, downwind, was 40% lower (2.5 μg/m³) than in Hyltemossa (4.3 μg/m³), upwind, suggesting aerosol removal and dilution processes over the sea. We used the meteorological and spatial parameters from backward trajectory analysis to categorise the long-range air mass transport events across the two sites. Then the combinations of dilution, wet and dry deposition, emissions over land and other processes were labelled as the sea transport effect, wet deposition processes and land-atmosphere interaction. The results showed that during the events when air mass trajectories stretched from west to east from over Hyltemossa to Preila without precipitation along the path, the PM₁ concentration decreased on average by 50%. The addition of precipitation resulted in a larger decrease (70%) of PM₁ concentration. However, during the events with precipitation when air mass trajectories passed over a continental area before arriving at the downwind site, the net decrease was the lowest (27%). The lowest relative change in PM₁ concentration during those events suggests the potential uptake of pollutants into the air mass over land with more significant anthropogenic pollution sources. Our approach allows us to quantitively assess the mitigating effects of sea transport, wet deposition processes and the opposite effect when air mass is advected over land on PM₁ concentration using the proposed classification based on meteorological and spatial parameters of HYSPLIT backward trajectories.