Atmospheric ice nucleators active ≥ −12 °C may be quantified on PM10 filters

Small number concentrations render it difficult to quantify ice nucleators (IN) in the atmosphere active at warm temperatures. A useful new method for IN measurement based around filter collections is proposed. It makes use of quartz filters used in 24 h PM10 monitoring (720m3 air sample). Small subsamples (1.8mm diameter) from the effective filter area and from the clean fringe (blank) are subjected to immersion freezing tests. We applied the method to eight filters from the High Alpine Research Station Jungfraujoch (3580m above sea level) in the Swiss Alps. All filters carried IN active at −7 °C and below. Number concentrations of IN active at −8, −10, and −12 °C were on average 3.3, 10.7, and 17.2m−3, respectively. Several-fold larger numbers of IN active at −12 °C per unit mass of PM10 were found in air masses influenced by Swiss and southern German atmospheric boundary layer air, compared to a Saharan dust event. In combination with data on PM10 mass, the method may be used to re-construct time series of IN number concentrations.