La profesión de la guerra

A2.3.2.1 Aqueous phase reactions

Prediction of the SOA mass in the atmosphere has been mostly based on the implementation of the gas-to-particle partitioning approach in models, implying that SOA compounds are formed in the gas phase and then partition to an organic phase. However, discrepancies in predicting organic aerosol oxidation state, size and product (molecular mass) distribution, relative humidity (RH) dependence, and vertical profile suggest that additional SOA sources and aging processes may be important or that a reversible equilibrium partitioning approach is not applicable and that condensing components are “locked away” during growth of solid particle (see discussion in 2.1.2.1 about condensed phase diffusional limitation). The formation of SOA in cloud and aerosol water is not considered in these models even though water is an abundant medium for atmospheric chemistry and such chemistry can form dicarboxylic acids and “humic-like substances” (oligomers, high-molecular-weight compounds), i.e. compounds that do not have any gas phase sources but comprise a significant fraction of the total SOA mass (Decesari et al., 2000; Gelencser et al., 2000a; 2000b; Kiss et al., 2002). There is increasing direct evidence from both laboratory (Carlton et al., 2007) and _Þeld measurements (Sorooshian et al., 2007; 2010;Hennigan et al., 2009) for aqueous phase formation of SOA, however the importance of this pathway to the global source of organic aerosol is currently unclear. Fu et al. (2008) estimated a global source of 11 Tg yr−1 of SOA formed through the irreversible uptake of dicarbonyls in clouds and aerosols, comparable to the source of SOA from the gas phase pathway. Inclusion of this additional source of SOA in the GEOS-Chem model eliminated the low bias previously seen during a specific model/measurement comparison study, with a small improvement in the ability of the model to capture the observed variability in water soluble organic carbon (Fu et al., 2009). Similarly, Carlton et al. (2008) found that including a detailed aqueous phase SOA mechanism in the CMAQ model modestly improved the correlation of the same model/measurement study. Comparing the GEOS-Chem global model simulation of the vertical profile of organic aerosol with 17 measurement datasets including remote, polluted and fire-influenced conditions, Heald et al., (2011) found that

aqueous phase SOA made up more than 20 % of total organic aerosol at all altitudes, with a pronounced enhancement from 2–6 km, where contributions vary from 40–80 % of total simulated OA. This mid-tropospheric enhancement was not supported by the measured vertical pro_Þles, nor was the model-observation discrepancy (when aqueous SOA was not included) larger in this region of the troposphere. The authors stated that the addition of the aqueous phase source, as it was described in the GEOS-Chem model, was unlikely to improve the model simulation, beyond a mean reduction in bias. They concluded that their analysis did not offer de_Þnitive evidence for the importance of aqueous phase SOA, but it did not preclude it and recommended that additional chemical constraints were clearly required to investigate the importance of this source to the global OA budget.

Understanding of the aqueous phase process has been increasing based on conducted laboratory investigations determining kinetic and mechanistic parameters of reactions in aqueous solutions (see Zellner and Herrmann, 1995; Herrmann, 2003). Ervens et al., (2011) reviewed the current knowledge on aqueous phase organic reactions and combined evidence that pointed to a significant role of aqueous phase SOA formation in the atmosphere. In their review, a modelling-based comparison was made between aqueous and gas phase SOA yields and mass predictions for selected conditions. These simulations suggested that the aqueous phase SOA might contribute almost as much mass as gas phase SOA to the SOA budget, with highest contributions from biogenic emissions of volatile organic compounds (VOC) in the presence of anthropogenic pollutants (i.e. NOx) at high

relative humidity and cloudiness. It was previously hypothesised that organic compounds are oxidised in the aqueous phase of cloud and fog droplets and products remain in the particle phase upon water evaporation (Blando and Turpin, 2000). More recently, it was hypothesised that chemical processes in the aqueous phase of hygroscopic particles (aerosol water) can also efficiently contribute to aqueous phase SOA mass (Volkamer et al., 2007). Aqueous phase processes start with water-soluble, polar precursors and form SOA that is more oxygenated than gas phase SOA as they lead to functionalisation or accretion (e.g. acid formation, oligomerisation) rather than to breakage of the carbon structure as in gas phase reactions. Laboratory studies have shown that at low, cloud-relevant aqueous phase concentrations, organic acids (e.g. oxalate) are formed from small aldehydes and related compounds. At the higher solute concentrations associated with aerosol water, “high- molecular weight compounds” (HMWC) tend to preferentially form (Lim et al., 2010). It has been shown that the organic fractions of atmospheric aerosol particles have properties consistent with such aqueous laboratory studies (Ervens et al., 2011). A host of organic acids has been observed in ambient particles, with oxalate globally ubiquitous throughout the tropospheric column (e.g. (Kawamura and Sakaguchi, 1999; Kawamura et al., 2003)). The atmospheric dynamics of oxalate suggests that it is predominantly secondary, and yet its abundance has not been explained by gas phase chemistry. Laongsri and Harrison (2013) have recently reported measurements of oxalate at urban and rural sites in the UK and inferred that it had a predominantly secondary source with advection from mainland Europe being important.

A2.3.2.2 Macromolecules, HULIS and Oligomers

There is additional interest in condensed phase reactions because they might help to explain the formation of high molecular weight products detected in atmospheric aerosol particles (Gelencser et al., 2002; Limbeck et al., 2003). Studies have reported that macromolecular species contribute significantly to the mass of organic compounds present in atmospheric aerosol particles (Decesari et al., 2000; Gelencser et al., 2000a; 2000b;Kiss et al., 2002). The largest fraction of these species showed considerable similarities in structural properties to humic and fulvic acids and were as a result termed humic-like substances (HULIS). HULIS is an operationally defined fraction of the aerosol and its quantification depends to some extent on the applied extraction, isolation and detection method. This fraction consists of polyacidic compounds of aliphatic and aromatic structures with additional substituted

functional groups. Their molecular mass was determined to be between 150–500 Da (Kiss et al., 2003). In a comprehensive review, Graber and Rudich (2006) concluded that, although it is difficult to distinguish atmospheric HULIS from terrestrial and aquatic humic substances on a chemical level, there are significant differences in physical properties such as hygroscopicity and cloud condensation nuclei (CCN) activity (e.g. Dinar et al., 2006a; 2006b; Asa-Awuku and Nenes, 2007). There is indication that HULIS can be of primary origin, e.g., wood combustion, but may also be associated with secondary particle-phase production (Gelencser et al., 2002; Surratt et al., 2007a; 2008). Higher-MW reaction products (i.e., products with MWs higher than those of first- and higher-generation oxidation products) have been identified in laboratory SOA produced from the atmospheric oxidation of a wide range of compounds, including 1,3,5-trimethylbenzene, cycloalkenes, -pinene and isoprene (Gao et al., 2004a; 2004b; Iinuma et al., 2004; Kalberer et al., 2004; Tolocka et al., 2004; Dommen et al., 2006; Hamilton et al., 2006), as well as the hydration of glyoxal (Hastings et al., 2005). In addition to these classical SOA precursor systems, other compounds which are present in atmospheric aerosols have also been investigated as possible contributors to oligomers, such as levoglucosan (Holmes and Petrucci, 2006).

Numerous laboratory studies have shown evidence for the reactive uptake of volatile organic species during SOA formation as well as the formation of higher-MW organics in SOA. The first studies were based on indirect evidence, i.e., increased SOA mass concentration observed in the presence of acidic seed aerosol (Tobias and Ziemann, 2000; Jang et al., 2002; Jang et al., 2004; Limbeck et al., 2003). Although it was proposed that polymerisation reactions of volatile carbonyls may account for the observed SOA mass increase, there was no direct evidence to support this at the time. Subsequent studies utilised mass spectrometry for the direct detection of higher-MW products, and showed that oligomers, characterized by a highly regular mass difference pattern of 12, 14, 16 or 18 Da, constituted a considerable portion of the SOA (Kalberer et al., 2004; Tolocka et al., 2004; Gao et al., 2004a; 2004b). It should be noted that the terms oligomer and oligomerisation, as opposed to polymer and polymerisation, are normally used in this context because, in the reaction systems studied to date, the MW range of the products is limited (IUPAC, 1996).