Identification and classification of the formation of intermediate ions measured in boreal forest

[摘要] We have measured the size distributions of air ions (0.42–7.5 nm indiameter) with the Balanced Scanning Mobility Analyzer in boreal forest, inSouthern Finland since spring 2003. The size range covers the size range ofcluster ions (approximately 0.42–1.6 nm) and naturally charged nanometreaerosol particles (1.6–7.5 nm) or intermediate air ions. Based on themeasurements from April 2003 to March 2006 we studied the characteristics ofcharged aerosol particle formation by classifying each day either as aparticle formation event, undefined or non-event day. The principal of theclassification, as well as the statistical description of the chargedaerosol particle formation events are given. We found in total 270 (26%of the analysed days) and 226 (22% of the analysed days) particleformation days for negative and positive intermediate ions, respectively.For negatively charged particles we classified 411 (40% of the analyseddays) undefined and 348 (34% of the analysed days) non-event dayswhereas for positively charged particles 343 (33% of the analysed days)undefined and 460 (45% of the analysed days) non-event days. The resultswere compared with the ordinary classification based on the DifferentialMobility Particle Sizer (DMPS) measurements carried out at the same place.The above-presented values differed slightly from that found from the DMPSdata, with a lower particle diameter of 3 nm. In addition, we have found therain-induced intermediate ion bursts frequently. The rain effect wasdetected on 163 days by means of negative ions and on 105 days by positiveones. Another interesting phenomenon among the charged aerosol particles wasthe appearance and existence of intermediate ions during the snowfall. Weobserved this phenomenon 24 times with negatively charged particles and 21times with positively charged ones during winter months (October–April).These intermediate air ions were seen during the snowfall and may be causedby ice crystals, although the origin of these intermediate ions is unclearat the moment.

[学科分类] 大气科学