Abstract
Abstract. In this work we present an analysis of the occurrence of nucleation events during more than three years of measurements at two different rural altitude sites, the puy de Dôme research station (1465 m a.s.l.) and the Opme station (660 m a.s.l.), central France. The collected database is a unique combination of scanning mobility particle sizer (10–400 nm), air ion spectrometers (from 0.8 to 42 nm for NTP-conditions), and, neutral clusters and air ion spectrometers (from 0.8 to 42 nm for NTP-conditions) measurements at these two different altitudes nearly located research stations, from February 2007 to June 2010. The seasonality of the frequency of nucleation events was studied at the puy de Dôme station and maximum of events frequency was found during early spring and early autumn. During the measurement period, neither the particle formation rates (J2= 1.382 ± 0.195 s−1) nor the growth rates (GR1.3−20 nm = 6.20 ± 0.12 nm h−1) differ from one site to the other on average. Hovewer, we found that, on 437 sampling days in common to the two sites, the nucleation frequency was higher at the puy de Dôme station (35.9 %, 157 days) than at the low elevation station of Opme (20.8 %, 91 days). LIDAR measurements and the evolution of the potential equivalent temperature revealed that the nucleation could be triggered either (i) within the whole low tropospheric column at the same time from the planetary boundary layer to the top of the interface layer (29.2 %, 47 events), (ii) above the planetary boundary layer upper limit (43.5 %, 70 events), and (iii) at low altitude and then transported, conserving dynamic and properties, at high altitude (24.8 %, 40 events). This is the first time that the vertical extent of nucleation can be studied over a long observational period, allowing for a rigorous statistical analysis of the occurrence of nucleation over the whole lower troposphere. This work highlights the fact that nucleation can occur over a large vertical extent, at least the whole low tropospheric column, and also the fact that it occurs twice as frequently as actually detected in the planetary boundary layer. The role of sulfuric acid and ions in the nucleation process was investigated at the altitude station and no correlation was found between nucleation events and the estimated sulfuric acid concentrations. However, the contribution of ion-induced nucleation was found to be relatively high (12.49 ± 2.03 % of the total nucleation rate).
Highlights
The formation of new ultrafine particles from the gas phase has been observed in various continental and marine locations indicating that nucleation followed by new particle formation (NPF) events is an ubiquitous phenomenon in the planetary boundary layer (PBL)
Fitting the PBL signal types of nucleation event can be observed, event days were classified into different classes (Ia, Ib, II and Bump) according to their quality and their applicability to a growth rate analysis (Hirsikko et al, 2005):
The puy de Dome research station (PdD) station is still in “in-cloud” conditions or in the vicinity of clouds (RH and liquid water content (LWC) between 07:00 and 16:00 are respectively 96.0 % and 0.024 g m−3 in average). This type of event do not inform us on the spatial extent of the nucleation but two hypotheses could be formulated: (i) the spatial extent of the nucleation is very low and it occurs only within a small geographical area before all reactive and condensable species have been completely consumed, (ii) the nucleation could occur within the whole column in the PBL but condensable vapors have been scavenged by cloud droplets at the high altitude site
Summary
The formation of new ultrafine particles from the gas phase has been observed in various continental and marine locations (see for a review Kulmala et al, 2004) indicating that nucleation followed by new particle formation (NPF) events is an ubiquitous phenomenon in the planetary boundary layer (PBL). During the intensive field campaign SATURN, Stratmann et al (2003) showed that nucleation could take place inside the residual layer and that it could be induced by the breakup of the nocturnal inversion They explained the ultrafine particle concentration at a ground-based measurement site as a result of the mixing down of the freshly. We propose the first long term study of the vertical extent of the new particle formation process based on a nearly 4-yr measurement period at two different altitudes: The puy de Dome station (1465 m a.s.l.) and the Opme station (660 m a.s.l.) Both sites were equipped with instruments which allow ions and neutral particle detection from 3.2 to 0.0013 cm V−1 s−1, corresponding to Stokes-Millikan mobility diameters (Makelaet al., 1996) between 0.8 and 42 nm in NTP-conditions. In addition to particle measurement devices, the atmospheric vertical structure and the boundary layer evolution was investigated using LIDAR measurements that were operated at about 11 km of each measurement sites
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