Below-cloud scavenging of size-segregated aerosols and its effect on rainwater acidity and nutrient deposition: A long-term (2009-2018) and real-time observation over eastern Himalaya.

The major removal pathway of atmospheric aerosols is the below cloud scavenging. The present study is the first-ever in the world, where long-term (2009-2018) as well as real-time observations on the below-cloud scavenging of ultrafine (<0.4 μm), superfine (0.4-1.0 μm) and coarse mode (>1 μm) aerosols have been made. The study was conducted with 919 rain events over a high altitude Himalayan station (27.01 °N, 88.15 °E, 2200 m amsl) in India. The other factors were normalized in order to investigate the "rain only" effect and therefore 919 rain events were screened and finally 165 events were studied. We determined threshold values of the rain rate (and duration) above which aerosols are scavenged in very high proportion (>75%) irrespective of the duration (and rain rate). These threshold values decrease as the aerosol size increases. For example, threshold rain rate decreases from ~17 mm h to ~8 mm h as the aerosol size increases from ultrafine to coarse mode. We also showed that how the rainwater acidity and the deposition flux of major inorganic nutrients (NH + NO + SO) vary with the rain rate and duration. We observed that the rains either >12 mm h or >80 min are all acidic. Maximum nutrients were accumulated in the ultrafine aerosols and hence the spectrum of the deposition flux of the nutrients (with rain rate and duration) was similar to the scavenging spectrum of ultrafine aerosol. Such long-term database enables us to quantitatively predict the aerosol scavenging, acid rains and nutrient deposition which showed excellent agreement with the observed results. Such quantitative prediction would in turn help the researchers to predict the rain-induced changes in air quality as well as any bio-geo chemical parameter. The present study bears paramount importance in Himalayan context as well as any ecologically-rich regions.