Large-scale aircraft observations of ultra-fine and fine particle concentrations in the remote Siberian troposphere: New particle formation studies

Ultra-fine particle number concentrations were measured over Siberia during two large-scale airborne measurement campaigns in April and September 2006. During both campaigns, an aircraft flew between Novosibirsk and Yakutsk, collecting every 200 km vertical profiles up to 7 km. This dataset was completed by 5 years of monthly profiles above Novosibirsk. Particle number concentration was measured in the size ranges 3–70 and 70–200 nm, along with other tracers. Free troposphere (FT) particle concentrations (N_3–200) varied between 60 and 460 cm^?3, inferior to boundary layer concentrations (100–7000 cm^?3). In April, high concentrations of ～500 cm^?3 were observed in a polluted air mass recently uplifted at 5–6 km altitude over eastern Siberia, with no sign of significant new particle formation. In September, particle concentrations decreased with altitude, but with a steeper gradient in N_70–200 compared to N_3–70, the latter accounting for 90% of the total particle concentration in the free troposphere at 6–7 km altitude. Because ultra-fine particles presumably have short lifetimes, these observed particles could have been formed in situ in the clean Siberian atmosphere. Two cases of possible nucleation with high concentration and N_3–70/N_70–200 ratios are reported for the September campaign, in the upper troposphere and in cloud outflow in the mid-troposphere. In the seasonal analysis, a FT N_3–70 maximum is found in July–August between 6 and 7 km altitude, with N_3–70 accounting for ～90% of N_3–200 supporting the hypothesis of in situ formation in the FT. A secondary FT maximum of N_3–70 was identified later in autumn. In the boundary layer, seasonally maximum N_3–70 concentrations were found over Novosibirsk in May and September, but not in summer, possibly due to scavenging by precipitations and a large condensational sink from biomass burning aerosols. Our dataset has a limited size resolution and no speciation capability; more investigation is thus required to understand the conditions leading to in situ nucleation processes in the Siberian air shed.