| Popis: |
A field experiment studying aerosol scavenging in winter precipitation is presented. Events of frontal precipitation (rain and snow) are investigated. Emphasis is put on the temporal evolution of aerosol and hydrometeor size distributions, and mesometeorological conditions. The simultaneous observation of the microphysics (for scavenging processes) and the mesometeorology (for air mass changes) is crucial for the quantitative understanding of pollutant concentrations in air and precipitation. Our studies suggest that microphysical conditions govern pollutant levels during the initial stage (approx. first mm) of precipitation, while effects from air mass changes dominate at later time intervals. Regarding scavenging, nucleation scavenging has been found to be the major removal mechanism for particles with diameter > 0.25 μm (mass reduction of > 95%), whereas below-cloud scavenging is much less effective (10% reduction of large particles, 30% of giant particles). Scavenging coefficients (λ) as a function of aerosol diameter have been determined for rain in below-cloud conditions. It is recommended that scavenging coefficients should be calculated and expressed in units of mm−1 rather than of s−1. Values determined in an individual rain event show a very large scatter of more than a factor of 10, a manifestation of the complexity of scavenging mechanisms. Values for large particles observed in this study range from 3 × 10−4 to 5 × 10−6s−1, or 0.05–20 mm−1. A dependence of the scavenging coefficient on rainfall rate of the form λ ∗ = a ∗ × R b ∗ is found, with a ∗ between 0.10 and 0.77 mm−1, and b ∗ between −0.57 and −0.91, for large particles. |
