La explotación olivarera y las almazaras

The analytical techniques described above are suitable for time-integrated samples (using filters, impactors, etc.) analysed off line (i.e., in laboratory). This implies the risk of positive and negative artifacts due to adsorption, evaporation, and chemical reactions during sampling, storing or during analysis in laboratory (e.g., Turpin et al., 2000; Schauer et al., 2003; Subramanian et al., 2004;

Dzepina et al., 2007). Off-line techniques are particularly problematic when used on mobile observatories (e.g. ships and research aircrafts). Therefore, on-line techniques, which provide real-time measurements, have revolutionized the chemical analysis of aerosols. The two main types of on-line techniques currently in use are PILS-WSOC and AMS with a growing interest in TD-PTR-MS ones. Particle-into-liquid-samplers (PILS) collect particles into water for subsequent analysis (e.g., IC) (Weber et al., 2001; Sullivan et al., 2004; Sorooshian et al., 2006). A liquid TOC analyzer for continuous measurement of WSOC with a time resolution of a few minutes was coupled to a PILS instrument and deployed in several aircraft campaigns (e.g., Sorooshian et al.,2006, 2007; Peltier et al., 2007; Weber et al., 2007).

1.6.3.1 Aerosol Mass Spectrometer

The Aerodyne aerosol mass spectrometer (AMS) was developed for the on-line measurement of organic and inorganic components of aerosols. The principle of the instrument is to draw particles into a vacuum chamber (Liu et al., 1995) and impact them on a tungsten surface (molybdenum in early versions) heated to 600 °C prior to being analyzed by 70 eV EI-MS. Most of the gas material is removed by differential pumping. As explained above, while the 70 eV EI energy implies extensive

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molecular fragmentation, it ensures the ionization of nearly all molecules, organic and inorganic, and the compatibility of the data with existing mass spectral databases. The mass spectrum for the aerosol sample is derived by subtracting an internal background obtained from periodically blocking the particle beam. Yet as it contains contributions from both the non-refractory particles (those evaporating on the impaction plate) and the air that was not skimmed off, these contributions are removed by using a “fragmentation table” approach, where marker peaks in the mass spectra are used to quantify these components (Allan et al., 2004). Because of the two-stage desorption and ionization process, the AMS mass spectral data are a quantitative, linear combination of the contributions of the various particle components. AMS gain quantitative information on the overall organic composition and contributions from various sources. One of the simplest forms of analysis is the determination of the fractional contribution of key peaks to the total organic mass concentration. Because m/z 44 mainly corresponds to CO2+ from the thermal decomposition of dicarboxylic acids and multifunctional compounds, the fractional contribution of m/z 44 can be taken as a proxy for the oxygen content of the organic fraction (Aiken et al., 2008; Allan et al., 2004). When this is compared to the fractional contribution from m/z 43 (C3H7+ from aliphatic chains and C2OH3+ from alcohols, monocarboxylic acids, and carbonyls), correlations with the general level of atmospheric processing of the organic material can be seen. Similarly, the fractional contribution of m/z 60 (associated with anhydrosugars such as levoglucosan, mannosan, and galactosan) can be used in conjunction with m/z 44 to monitor the evolution of biomass burning aerosols (Cubison et al., 2011; Lack et al., 2013).

1.6.3.2 Aerosol Chemical Speciation Monitor

While the research grade AMS provides valuable information about trends in speciated aerosol mass concentrations for applications that require fast time resolution (1 min or less), it is not well suited for routine air quality monitoring applications because it is expensive to own and requires dedicated scientists to operate and analyze its multidimensional data. Aerosol Chemical Speciation Monitor (ACSM), is an instrument, developed since 2009 by Aerodyne Res., that has many of the capabilities of the AMS but is better suited for monitoring applications. The ACSM is designed and built around the same sampling and detection technology as the state-of-the art research grade AMS systems, but it has lower size, weight, cost, and power requirements than the AMS and is specifically designed to be a stand-alone monitor that is more easily transportable and can operate with minimal user intervention. In the Figure 1.9 below a schematic overview of the instrument is provided.

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Figure 1.9. Aerosol Chemical Speciation Monitor (ACSM) developed by Aerodyne Res.

(http://www.aerodyne.com/sites/default/files/ACSM%20Aerosol%20Chemical%20Speciation%20M onitor%20product%20sheet_0.pdf).

Much of the reduced complexity is due to the fact that the ACSM does not measure size distributions;

only aerosol mass spectra are reported. The quadrupole-ACSM measures mass concentrations of non-refractory submicron aerosol components (i.e., organic matter, nitrate, sulfate, ammonium and chloride). It uses lower cost components that reduce its sensitivity and time resolution compared to the AMS, the ACSM has sufficient sensitivity to operate as a monitoring instrument providing chemically speciated mass loadings and aerosol mass spectra at data rates up to 30 min for typical urban aerosol loadings (several μg/m³) (Ng et al. 2011).

Several papers on aerosol biomass burning, SOAs, seasonality in submicron aerosol and aerosol source apportionment have been published in last years (Bressi et al., 2016; Reyes-Villegas et al., 2016; Budisulistiorini et al., 2015). Shown in Figure 1.10 are the time series of organics, sulfate, nitrate, ammonium, and chloride as well as the average mass fraction of each species measured by the ACSM in the study reported by Ng et al. (2011).

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Figure 1.10. Time trends of non-refractory submicron aerosol species measured with the ACSM during the Queens, NY study. The average total loading is 7.2 μg/m³. The time resolution of these measurements is 30 min. The ACSM operated unattended during this study. (Ng et al., 2011).