Comment on acp-2022-147

Abstract

Atmospheric aerosol particle concentrations are strongly affected by various wet processes, including wet scavenging (below and in-cloud) and aqueous phase oxidation in-cloud. This study employs airmass history analysis and observational data to investigate how wet scavenging and cloud processes affect particle concentrations and composition during transport to a rural boreal forest site in northern Europe. Long term particle size distribution (~ 15 years) and composition measurements (~ 8 years) were utilized in combination with airmass trajectories with relevant variables (e.g. rainfall rate, relative humidity, mixing layer height) from reanalysis data. Additional observational data sets (e.g. temperature, trace gases) were used to further evaluate the wet processes along trajectories with mixed effects models. All investigated chemical species (sulfate, black carbon and organics) showed an exponential decrease in the particle mass concentration as a function accumulated precipitation along the airmass route. Clear seasonal differences in wet removal were observed for sulfate (SO4) aerosols, whereas organics (Org) and black carbon (eBC) showed more minor differences. The removal efficiency varied slightly among the different reanalysis datasets (ERA-Interim and GDAS) used for the trajectory calculations, due to the difference in the average occurrence of precipitation events along the airmass trajectories between the reanalysis datasets. Aqueous phase processes were investigated by using a proxy for airmasses travelling inside clouds. Significant increases in total SO4 mass concentration were observed for airmasses recently been inside non-precipitating clouds when compared to airmasses that had no experience of clouds or precipitation during the last 24 hours before arrival to SMEAR II. Mixed effects model, in which other contributing factors (e.g. trace gases, local meteorology, diurnal variation) affecting particle mass concentrations in SMEAR II were considered, also indicated in-cloud production of SO4. Aqueous phase formation of SO4 was observed despite the reanalysis dataset used in the trajectory calculations. Investigation of the particle size distribution measurements revealed that most of the SO4 formed in-cloud can be attributed to particle sizes larger than 200 nm (electrical mobility diameter). No significant formation of aqueous phase secondary organic aerosol (aqSOA) was observed.