Abstract
Abstract. To investigate new particle formation (NPF) events in urban and forest environments, number size distributions of ultrafine particles were measured at an urban site and a deciduous forest site in Sapporo, Northern Japan, during the summers of 2011 and 2010, respectively. The burst of nucleation mode particles at the urban site typically started in the morning (07:00–11:30 local time, LT) with simultaneous increases in SO2 and O3 concentrations and the UV index under clear (sunny) weather conditions. The growth rates of nucleated particles at the urban site ranged from 5.0 to 7.8 nm h−1 with an average of 6.3 ± 1.1 nm h−1. NPF events at the urban site were separated into events with or without subsequent particle growth after the burst of nucleation mode particles. This division was found to relate to prevailing wind direction because the subsequent growth of freshly nucleated particles typically occurred when wind direction shifted to northwesterly (from residential and public park areas), whereas it did not occur under southeasterly winds (from the downtown area). During the periods with NPF events, elevated concentrations of non-methane hydrocarbons (NMHC) were obtained under conditions of northwesterly winds when compared to southeasterly winds, whereas no difference in SO2 levels was recorded. These results suggest that variations in NMHC concentration may play an important role in the growth of freshly nucleated particles at the urban site. The burst of nucleation mode particles at the forest site typically started around noon (10:30–14:30 LT), which was 3–4 h later than that at the urban site. Interestingly, at the forest site the burst of nucleation mode particles usually started when air masses originating from urban Sapporo arrived at the forest site. The present study indicates that the inflow of these urban air masses acted as a trigger for the initiation of the burst of nucleation mode particles in the deciduous forest.
Highlights
New particle formation (NPF), an important source of ambient aerosols, is frequently observed occurring worldwide (Kulmala et al, 2004; Mikkonen et al, 2006; Holmes, 2007; Bzdek and Johnston, 2010; Hallar et al, 2011; Pryor et al, 2011)
On the basis of these criteria, 10 new particle formation (NPF) events were characterized at the urban site (Table 2), corresponding to 53 % of the measurement days (n = 19), whereas seven NPF events were observed at the forest site (Table 3), corresponding to 47 % of the measurement days (n = 15)
Diel variations in gas-phase pollutants and meteorological parameters suggest that increases in SO2 and O3 concentrations and the UV index in the morning under clear conditions are needed for the burst of nucleation mode particles at the urban site
Summary
New particle formation (NPF), an important source of ambient aerosols, is frequently observed occurring worldwide (Kulmala et al, 2004; Mikkonen et al, 2006; Holmes, 2007; Bzdek and Johnston, 2010; Hallar et al, 2011; Pryor et al, 2011). NPF and subsequent particle growth from biogenic emissions of volatile organic compounds (VOCs) in forested areas are an important source of CCN Such NPF events have been widely observed, their formation mechanisms over forests are poorly understood. NPF events have been observed in European coniferous forests, including Finland’s boreal forest (e.g., Kulmala et al, 2004) and coniferous forest in Germany (Held et al, 2004), NPF events have not been reported in the mixed deciduous forests of the Amazon and the northern United States, where emissions of biogenic isoprene are abundant (Martin et al, 2010; Kanawade et al, 2011). To better understand NPF events and the mechanisms of subsequent particle growth, we continuously measured particle number size distributions using a scanning mobility particle sizer (SMPS) at two atmospheric sites: one in an urban area and the other in a broadleaf deciduous forest in Sapporo, northern Japan. We report NPF events at the two sites and discuss in detail the impact of anthropogenic emissions on NPF in a deciduous forest
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