Influence of semi-volatile species on particle hygroscopic growth
Introduction
Knowledge of the hygroscopic properties of atmospheric particles is important to better quantify their role in the climate. Uptake of water by atmospheric particles modifies the mass of aerosol particles, increases the particles size, and therefore their ability to diffuse solar radiation. An increase of the relative humidity from 40 to 80% for the global mean aerosol would, for example, result in an increase of the radiative forcing by a factor of 2.1 (Pilinis et al., 1995). Moreover, because hygroscopic growth is linked to the particle chemical composition, water uptake measurements have been used in the literature to infer the aerosol soluble fraction in various environments, which once parameterized, can be used in model calculations (Kandler and Schütz, 2008). Measurement of water uptake also reveals the degree of mixing of an aerosol population. Whether the various constituents are externally or internally mixed in turn influences the optical properties of atmospheric aerosols (Lesins et al., 2002). At last, hygroscopic growth also gives key information linked to the cloud condensation nuclei (CCN) properties of aerosols, a major parameter in the so-called indirect effect (Twomey, 1977). A number of studies have therefore investigated water uptake by atmospheric aerosols in order to provide an unambiguous and reliable link between aerosol hygroscopic properties and the number of particles of an aerosol population activating to cloud droplets in a humid environment (Rissler et al., 2004, Kreidenweis et al., 2005).
The Köhler theory (1936) is classically used to describe the critical supersaturation at which a CCN of known size is activated, i.e. grows into a cloud droplet. The reliability of the model calculation depends on the input parameters, e.g. the surface tension, the molality of the solution formed on the aerosol particle, the fraction of soluble material, the protonation constants, Van't Hoff factor, etc… Most of these properties are often not measurable in the field and input parameters are often simplified or reduced to a limited number of compounds, or compound families.
Approximated input parameters in the Köhler theory appear well suited to describe the activation of pure inorganic particulate matter (e.g., Ammonium Sulphate, Sodium Chloride etc.), but not always the behaviour of more complex mixture, that includes organic material (Wang et al., 2008). Organic compounds tend to have limited water solubility, which implies that they are less CCN active.
Water uptake/activity studies have been performed on many different organic species such as pure and internally mixed dicarboxylic acids (Prenni et al., 2003, Prenni et al., 2001), saccharides (Chan et al., 2008, Rosenoern et al., 2005), humic and fulvic acids/HULIS, high molecular weight organics (Brooks et al., 2004, Dinar et al., 2007, Dinar et al., 2006, Gysel et al., 2004, Petters et al., 2006, Wex et al., 2007, Ziese et al., 2008), polyols (Ekström et al., 2009, Marcolli and Peter, 2005), and secondary organic aerosols generated from various precursors (Baltensperger et al., 2005, Meyer et al., 2009, Prenni et al., 2007, Varutbangkul et al., 2006). While several studies have successfully predicted hygroscopic growth of some mixtures using the Zdanovskii–Stokes–Robinson (ZSR) mixing rule (Zdanovskii, 1936, Stokes and Robinson, 1966), deviation from this additive rule has also been encountered even for simple mixtures (Sjogren et al., 2007, Svenningsson et al., 2006, Zardini et al., 2008). Non-ideal mixing effects add to the difficulty of predicting the hygroscopic growth of complex aerosols.
Both volatility and hygroscopicity give indirect information on the aerosol chemical composition (Fletcher et al., 2007, Meyer et al., 2009). In particular, photooxidation of organic compounds in the gas phase may lead to semi-volatile secondary organic aerosols (SOA). These particles experience significant chemical changes while aging, which can be tracked with the study of their volatility properties. For instance, photooxidation of trimethylbenzenes in a smog chamber show that a substantial fraction of the organic aerosol mass is composed of oligomers, which, after aging for more than 20 h, result in a lower volatility SOA (Kalberer et al., 2004). An et al. (2007) also measured the volatility of SOA formed up to several hours of reaction between α-pinene and O3. However, SOA formed during the first hours of oxidation has usually been found to be volatile at 100 °C for gas-phase photooxidation processes (An et al., 2007), and in-cloud oxidation processes (Michaud et al., 2009).
In this study, we investigate how natural semi-volatile compounds affect the particle ability to grow in a humid environment using the Volatility-Humidity tandem differential mobility analyzer (VH-TDMA) (Johnson et al., 2004, Villani et al., 2008). The VH-TDMA technique has been used to study the hygroscopic behaviour of partially volatilized marine aerosols (Johnson et al., 2005) but it can also be used to investigate the effect of aging on the hygroscopic properties of aerosols by using low volatilization temperatures (lower than 100 °C).
Section snippets
Description of the VH-TDMA unit
The need to gain some insight into the chemical nature of sub-micrometre aerosols has previously lead to the development of hygroscopic growth and volatility methods for aerosol research. Since hygroscopic growth depends on the chemical properties of the aerosol, sized particles of different composition can be distinguished through their differing growth response to humidification. Frequently, these properties are investigated using tandem differential mobility analyser (TDMA) systems (Rader
Hygroscopic properties issued from H-TDMA measurements
A global view of the temporal variation of the average HGF distribution for each hygroscopic growth factor for each site is shown in Fig. 1. The aerosol population for a given diameter is composed of several hygroscopic classes, an indication of externally mixed particles, regardless of the location (Fig. 1). Within a HGF classes, HGF are rather stable (<0.2) over a 12-h period in all environments, except at Mace Head (Fig. 1d). At Mace Head, the HGF can vary from one limit of the HGF class to
Evidence of drastic changes in hygroscopicity due to gentle thermo-desorption
Volatility Hygroscopic Growth Factors (VHGF) have been measured during the campaigns, simultaneous to HGFs. As seen from the measurements of volatility summarised in Table 2 (VGF), thermal processing on the order of 100 °C leads to a change in particle diameter of 5% on average and comprised in absolute value from 3 to 9 nm.
Investigating the changes in HGF from thermal processing, we can see that VHGF and HGF are usually of the same order of magnitude for a given class and experiment (Table 1).
Conclusions
The HGF of aerosol particle have been measured in various environments (remote to urban). The results show that the aerosol population for a given diameter is composed of several hygroscopic classes, an indication of externally mixed particles, regardless of the location. Hydrophobic particles, likely composed of freshly formed carbonaceous material, are ubiquitous to all sites except for background sites during clean conditions. The moderately hygroscopic particles are also ubiquitous. This
Acknowledgements
The authors would like to the financial support of INSU-CNRS under the LEFE-CHAT program, ADEME and Ministère de l'Ecologie et du Développement. Durable under the PRIMEQUAL program, the scientific council of region Auvergne. P. Villani acknowledges financial support from CNRS and region Auvergne under BDI program. This is a contribution of ACCENT NoE and EUCAARI IP.
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