In-cloud and below-cloud scavenging of PAHs and VOCs

In the last decades, polycyclic aromatic hydrocarbons, PAHs, and volatile organic compounds, VOCs, concentration levels inside the atmosphere have been monitored in order to assess their effects on human health and to clarify their role in the climate change issue. Some substances belonging to these groups of organic compounds have been found to be carcinogenic to human health. and some others were found to be precursors of secondary aerosol particles, causing an increase of the overall number of aerosols in the atmosphere and thus affecting the global climate. Under these circumstances, understanding what affects the average time of residence inside the atmosphere of PAHs and VOCs is a key point. Among others, in-cloud and below-cloud scavenging phenomena are two well known wet deposition processes. Despite the vast scientific background, there are no models capable of predicting with a high accuracy their removal efficiency. For this reason, in the year 2015, a research was conducted in Japan with the aim of understanding the efficiency of the scavenging phenomenon as a wet deposition pathway of PAHs and VOCs. Measurement campaigns were conducted in three different sites, the urban site of Shinjuku, inside the Waseda University campus, the rural site at the foot of Mt. Fuji and the pristine site at the top of Mt. Fuji. Rainwater samples, cloud water samples and outdoor air samples were collected throughout the campaign, separately for PAHs analysis and VOCs analysis. PAHs samples were analysed by HPLC and VOCs samples were analysed by GCMS. PAHs scavenging was studied by calculating the deposition fluxes and the scavenging ratios while VOCs scavenging was studied by calculating the enrichment factor. All the results from this study were compared with other literature studies.