Nucleation on soluble nuclei: organic aerosol formation . 55

As revealed by atmospheric measurements, aerosols often contain a considerable amount of organic matter [97]. The photo-oxidation of volatile organic com-pounds (VOCs), indeed, yields organic acids that are involved in the initial steps of new particle formation. VOCs are emitted in the atmosphere both from anthro-pogenic and biogenic sources, and are thus typical air constituents.

As detailed in Sec. 2.3, the Classical Nucleation Theory for the homogeneous nu-cleation of organic vapours predicts extremely large supersaturation of condens-able vapors, as high as 10⁴, in order to nucleate 1 nm particles. The estimated saturation ratio of these species in the atmosphere lies between 1 and 100 [13], therefore this pathway is thermodynamically forbidden.

photo-induced processes Conversely, organic vapors do condense, under tropospheric conditions, around nanometer-sized clusters, that typically result from the nucleation of the ternary H₂SO₄-NH₃-H₂O system [98]. This leads to the growth of the freshly nucleated particle, which are said to be ’activated’⁷, towards detectable size (3nm) and even further.

The activation of stable clusters by organic vapours has been described by Kul-mala et al. [13]; they provided a theoretical framework explaining the formation of new ultrafine particles via the activation of stable inorganic sulphate clusters in a supersaturated organic vapour. The growing cluster is considered as a solution of water, ammonia, sulfuric acid and the fully soluble organic compound. This model is analogous to the Köhler theory, that will be introduced in Sec. 2.5.2, which describes cloud formation in a supersaturated water vapor via the activa-tion of CCN, i.e. larger aerosols with diameters of∼100 nm. For this reason, it is commonly referred to asnano-Köhler theory.

The outcome of the aerosol dynamic simulations implementing the nano-Köhler theory (see details in Ref. [13]) prove that the cluster growth results from a dual-step process: prior to the activation, such growth is driven by the condensation of H₂SO₄, while above the threshold size the condensation of organic vapors dom-inates. Two requirements have to be met for organic aerosol formation to oc-cur: the concentrations of sulphuric acid and organic vapor must be of the order of 10⁶ cm⁻³ and 10⁷ cm⁻³, respectively. Such values are consistent with atmo-spheric observations for these species. The inset of Fig. 2.10 displays the temporal evolution of sulphate clusters size. If the thermodynamic equilibrium between the inorganic cluster and the organic vapour is not considered, the cluster growth ap-pears to be negligible even after 8 hours; conversely, when the new nano-Köhler theory is included in the model, activation of inorganic clusters starts already af-ter∼1.7 hours, for an organic vapor concentration of 12×10⁷cm⁻³, leading to particle as large as 90 nm after 8 hours. The resulting increase in concentration for particle sizes greater than 15 nm, displayed in the main plot, evidences that the particle yield is a strong function of the initial organic vapor concentration. At low organic vapor content, indeed, doubling the concentration results in a 400%

increase in the aerosol number concentration.

To experimentally assess the role of organic acids in the new particle formation, Zhang et al. [99] investigated the nucleation ofcis-pinonic acid (CPA) with H₂SO₄ and water vapor in a flow chamber. Pinonic acid is a chemical product that results from the oxidation of the biogenic terpeneα-pinene by ozone and OH radicals.

The nucleation of new particles was observed already in the absence of said or-ganic acid; however, adding CPA in the chamber at a saturation ratio much smaller

7This definition will arise again in the more appropriate context of Cloud Condensation Nuclei, in Sec. 2.11.

2.4 Heterogeneous nucleation

Figure 2.10: Time-dependent growth of sulphate clusters for different organic vapors (OV) concentrations. Main plot: Time evolution of the increase in concen-tration for particles larger than 15 nm. Insert: growth of sulphate clusters. The dots mark the activation point. The outcome of the model without nano-particle equilibrium is also displayed, showing no activation. Reprint from [13].

than unity, resulted in an 10-fold enhancement of the new particles formation rate.

Moreover, increasing the CPA concentration by only 20% yielded an increase in the particle concentration by more than 200%, proving that even a small fluctu-ation of the pinonic acid density in the air can cause a considerable effect in the organic aerosol formation.

Furthermore, the measurements in the flow chamber evidenced the key impor-tance of sulphuric acid in the overall process. In the absence of H₂SO₄, indeed, the formation of new particles from pure organic acid mixtures was inhibited even at saturation ratios of organic vapors as high as 30. Zhang and coauthors con-cluded their experimental study by discussing the competition between H₂SO₄ and CPA in the initial growth of freshly nucleated particles to the detectable size.

While the weakly soluble pinonic acid assists the formation of stable nuclei, the initial growth of such clusters is rather governed by sulphuric acid, consistently with theoretical predictions. The condensation of H₂SO₄ is indeed promoted by

photo-induced processes the simultaneous condensation of water molecules that prevent its evaporation.

2.5 Cloud physics

In the previous sections, the most relevant pathways for the formation of new at-mospheric particles are reviewed, in order to provide the knowledge necessary to identify the mechanisms underlying the laser-induced particle production, that will be discussed in the experimental part of this thesis work (Chapters 3-5).

However, another issue arises when artificially generating new aerosol by means of laser irradiation. Indeed, their behaviour when the air mass evolves towards supersaturated conditions must be considered, as they might be expected to offer a substrate for the condensation of water vapor, hence the formation of bigger droplets. This is the process by which clouds naturally form in the atmosphere, starting from a population of pre-existing aerosols.

In view of this potential laser-assisted cloud formation, the main aspects of cloud processing are reviewed in this section, such as the definition of Cloud Condensa-tion Nuclei (CCN), the assessment of the stability of such CCN, and the cooling of an air mass.