Ion-induced sulfuric acid-ammonia nucleation drives particle formation in coastal Antarctica

Formation of new aerosol particles from trace gases is a major source of cloud condensation nuclei (CCN) in the global atmosphere, with potentially large effects on cloud optical properties and Earth's radiative balance. Controlled laboratory experiments have resolved, in detail, the different nucleation pathways likely responsible for atmospheric new particle formation, yet very little is known from field studies about the molecular steps and compounds involved in different regions of the atmosphere. The scarcity of primary particle sources makes secondary aerosol formation particularly important in the Antarctic atmosphere. Here, we report on the observation of ion-induced nucleation of sulfuric acid and ammonia-a process experimentally investigated by the CERN CLOUD experiment-as a major source of secondary aerosol particles over coastal Antarctica. We further show that measured high sulfuric acid concentrations, exceeding 10(7) molecules cm(-3), are sufficient to explain the observed new particle growth rates. Our findings show that ion-induced nucleation is the dominant particle formation mechanism, implying that galactic cosmic radiation plays a key role in new particle formation in the pristine Antarctic atmosphere.Peer reviewe

On Model Selection, Bayesian Networks, and the Fisher Information Integral

We study BIC-like model selection criteria and in particular, their refinements that include a constant term involving the Fisher information matrix. We perform numerical simulations that enable increasingly accurate approximation of this constant in the case of Bayesian networks. We observe that for complex Bayesian network models, the constant term is a negative number with a very large absolute value that dominates the other terms for small and moderate sample sizes. For networks with a fixed number of parameters, d, the leading term in the complexity penalty, which is proportional to d, is the same. However, as we show, the constant term can vary significantly depending on the network structure even if the number of parameters is fixed. Based on our experiments, we conjecture that the distribution of the nodes’ outdegree is a key factor. Furthermore, we demonstrate that the constant term can have a dramatic effect on model selection performance for small sample sizes.Peer reviewe

Comprehensive analysis of particle growth rates from nucleation mode to cloud condensation nuclei in boreal forest

Growth of aerosol particles to sizes at which they can act as cloud condensation nuclei (CCN) is a crucial factor in estimating the current and future impacts of aerosol–cloud–climate interactions. Growth rates (GRs) are typically determined for particles with diameters (dP) smaller than 40&thinsp;nm immediately after a regional new particle formation (NPF) event. These growth rates are often taken as representatives for the particle growth to CCN sizes (dP&thinsp;&gt;&thinsp;50–100&thinsp;nm). In modelling frameworks, the concentration of the condensable vapours causing the growth is typically calculated with steady state assumptions, where the condensation sink (CS) is the only loss term for the vapours. Additionally, the growth to CCN sizes is represented with the condensation of extremely low-volatility vapours and gas–particle partitioning of semi-volatile vapours. Here, we use a novel automatic method to determine growth rates from below 10&thinsp;nm to hundreds of nanometres from a 20-year-long particle size distribution (PSD) data set in boreal forest. With this method, we are able to detect growth rates also at times other than immediately after a NPF event. We show that the GR increases with an increasing oxidation rate of monoterpenes, which is closely coupled with the ambient temperature. Based on our analysis, the oxidation reactions of monoterpenes with ozone, hydroxyl radical and nitrate radical all are capable of producing vapours that contribute to the particle growth in the studied size ranges. We find that GR increases with particle diameter, resulting in up to 3-fold increases in GRs for particles with dP&thinsp; ∼ &thinsp;100&thinsp;nm in comparison to those with dP&thinsp; ∼ &thinsp;10&thinsp;nm. We use a single particle model to show that this increase in GR can be explained with aerosol-phase reactions, in which semi-volatile vapours form non-volatile dimers. Finally, our analysis reveals that the GR of particles with dP&thinsp;&lt;&thinsp;100&thinsp;nm is not limited by the condensation sink, even though the GR of larger particles is. Our findings suggest that in the boreal continental environment, the formation of CCN from NPF or sub-100&thinsp;nm emissions is more effective than previously thought and that the formation of CCN is not as strongly self-limiting a process as the previous estimates have suggested.</p

Major contribution of neutral clusters to new particle formation at the interface between the boundary layer and the free troposphere

The formation of new aerosol particles in the atmosphere is a key process influencing the aerosol number concentration as well as the climate, in particular at high altitude, where the newly formed particles directly influence cloud formation. However, free tropospheric new particle formation (NPF) is poorly documented due to logistic limitations and complex atmospheric dynamics around high-altitude stations that make the observation of this day-time process challenging. Recent improvements in measurement techniques make now possible the detection of neutral clusters down to similar to 1 nm sizes, which opens new horizons in our understanding of the nucleation process. Indeed, only the charged fraction of clusters has been reported in the upper troposphere up to now. Here we report day-time concentrations of charged and neutral clusters (1 to 2.5 nm mobility diameter) recorded at the interface between the boundary layer (BL) and the FT as well as in the FT at the altitude site of Puy de Dome (1465 m a.s.l.), central France, between 10 and 29 February 2012. Our findings demonstrate that in the FT, and especially at the interface between the BL and the FT, the formation of 1.5 nm neutral clusters significantly exceeds the one of ionic clusters during NPF events, clearly indicating that they dominate in the nucleation process. We also observe that the total cluster concentration significantly increases during NPF events compared to the other days, which was not clearly observed for the charged cluster population in the past. During the studied period, the nucleation process does not seem to be sulfuric acid-limited and could be promoted by the transport of pollutants to the upper troposphere, coupled with low temperatures.Peer reviewe

High concentrations of sub-3nm clusters and frequent new particle formation observed in the Po Valley, Italy, during the PEGASOS 2012 campaign

The concentrations of neutral and charged sub3nm clusters and their connection to new particle formation (NPF) were investigated during the PEGASOS campaign (7 June-9 July 2012) at the San Pietro Capofiume measurement station in the Po Valley, Italy. Continuous high concentrations of sub-3nm clusters were detected during the measurement period, although the condensation sink was relatively high (median value 1.1 x 10(-2) s(-1)). The median cluster concentrations were 2140 and 7980 cm 3 in the size bins of 1.5-1.8 and 1.8-3 nm, and the majority of them were electrically neutral. NPF events were observed during the measurement period frequently, on 86% of the days. The median growth rates of clusters during the events were 4.3, 6.0 and 7.2 nm h(-1) in the size ranges of 1.5-3, 3-7 and 720 nm. The median formation rate of 1.6 nm clusters was high, 45 cm 3 s(-1), and it exceeded the median formation rate of 2 nm clusters by 1 order of magnitude. The ion-induced nucleation fraction was low; the median values were 0.7% at 1.6 nm and 3.0% at 2 nm. On NPF event days the neutral cluster concentration had a maximum around 09: 00 (local winter time), which was absent on a non-event day. The increase in the cluster concentrations in the morning coincided with the increase in the boundary layer height. At the same time radiation, temperature and SO2 concentration increased, and RH and condensation sink decreased. The concentrations of neutral and charged clusters were observed to have a positive correlation with sulfuric acid proxy, indicating the significance of sulfuric acid for the cluster formation in San Pietro Capofiume. The condensation sink had a negative correlation with the concentration of charged clusters but no clear relation to the neutral cluster concentration. This finding, together with back-trajectory analysis, suggests that the precursor vapors of the clusters and background aerosol particles, acting as their sink, have possibly originated from the same sources, including e.g., power plants and industrial areas in the Po Valley.Peer reviewe

Biogenic particles formed in the Himalaya as an important source of free tropospheric aerosols

Aerosols of biogenic and anthropogenic origin affect the total radiative forcing of global climate. Poor knowledge of the pre-industrial aerosol concentration and composition, in particular of particles formed directly in the atmosphere from gaseous precursors, constitutes a large uncertainty in the anthropogenic radiative forcing. Investigations of new particle formation at pre-industrial-like conditions can contribute to the reduction of this uncertainty. Here we present observations taken at the remote Nepal Climate Observatory Pyramid station at 5,079 m above sea level, a few kilometres from the summit of Everest. We show that up-valley winds funnel gaseous aerosol precursors to higher altitudes. During this transport, these are oxidized into compounds of very low volatility, which rapidly form a large number of aerosol particles. These are then transported into the free troposphere, which suggests that the whole Himalayan region may act as an 'aerosol factory' and contribute substantially to the free tropospheric aerosol population. Aerosol production in this region occurs mainly via organic precursors of biogenic origin with little evidence of the involvement of anthropogenic pollutants. This process is therefore likely to be essentially unchanged since the pre-industrial period, and may have been one of the major sources that contributes to the upper tropospheric aerosol population during that time. Newly formed biogenic particles in the Himalaya increase free-tropospheric background aerosol concentration by a factor of up to two.Peer reviewe