Usage of aerosol mass spectrometry for the measurement of the physical and chemical properties of the atmospheric nanoparticles

Abstract

The overall objective of this dissertation is to improve our understanding of the organic and inorganic atmospheric particles combining laboratory and field studies and state-of-the-art instrumentation. The recent development of on-line aerosol mass spectrometry and other continuous techniques allows for the first time detailed high time resolution investigations of atmospheric particulate matter and its chemical and physical properties (e.g. volatility, density etc). Organic aerosol (OA) consists of compounds with a wide range of volatilities and its ambient concentration is sensitive to this volatility distribution. Recent field studies have shown that the typical mass spectrum of ambient oxygenated organic aerosol (OOA) as measured by the Aerodyne Aerosol Mass Spectrometer (AMS) is quite different from the secondary organic aerosol (SOA) mass spectra reported in smog chamber experiments. Part of this discrepancy is due to the dependence of SOA composition on the organic aerosol concentration. High precursor concentrations lead to higher concentrations of the more volatile species in the produced SOA while at lower concentrations the less volatile compounds dominate the SOA composition. α-pinene, β-pinene, d-limonene and β-caryophyllene ozonolysis experiments were performed at moderate concentration levels. Using a thermodenuder the more volatile SOA species were removed achieving even lower SOA concentration. The less volatile fraction was then chemically characterized by an AMS. The signal fraction of m/z 44, and thus the concentration of CO2+, is significantly higher for the less volatile SOA. High NOx conditions result in less oxidized SOA than low NOx conditions, while increasing relative humidity levels results in more oxidized products for limonene but has little effect on α-and β-pinene SOA. Combing a smog chamber with a thermodenuder model employing the volatility basis-set framework the AMS SOA mass spectrum for each experiment and for each precursor is deconvoluted into low, medium and high volatility component mass spectra. The spectrum of the surrogate component with the lower volatility is quite similar to that of ambient OOA. The density of organic aerosol is another poorly-characterized property and there is still no general method for its estimation in aerosol systems with size-dependent composition where the organic-inorganic fraction changes fast. An algorithm for the calculation of organic aerosol density in mixed organic-inorganic particles combining measurements by the AMS and the Scanning Mobility Particle Sizer (SMPS) was developed. The approach is applicable to particles with size-dependent composition. The estimated density of secondary organic aerosol (SOA) formed by α-pinene, β-pinene and d-limonene ozonolysis was in the range of 1.4-1.65 g cm-3. However, in two cases the SOA had much lower density (0.9-1.0 g cm-3) indicating that there may be changes in particle morphology depending on the conditions of SOA formation. The high estimated density for these systems suggests that SOA particles may be solid or waxy. Based on our results, SOA yields in smog chamber experiments may be a lot higher (up to 50%) than the currently assumed values. Most of the literature results have been calculated by measuring the SOA number distribution with an SMPS and then multiplying the volume concentration with a density equal to 1 or 1.2 g cm-3. This method is also applicable to the primary organics and to ambient particles assuming that these particles are close to spherical and can also provide the collection efficiency (CE) of the AMS. For example this approach was used for field data during the Finokalia Aerosol Measurement Experiments (FAMEs). Real-time measurements of non-refractory submicron aerosols (NR-PM1) were conducted during the FAME-08 (early summer 2008) and FAME-09 (winter 2009) field studies. Both campaigns were part of the intensive EUCAARI Pan-European campaigns. The Finokalia site, located at a remote coastal place on Crete (Greece), is isolated and away from anthropogenic sources of pollution and this allows the study of aged organic aerosol coming from different directions. In order to measure the size-resolved chemical composition of the NR-PM1 particles an AMS was used. During FAME-08 ammonium sulfate and ammonium bisulfate were the largest components of the NR-PM1, followed by organics and water. There was practically zero nitrate. The organic aerosol was mainly regional, since the total organic aerosol mass changed little with source region. During FAME-09 organics, ammonium sulfate and ammonium bisulfate had almost equal fractions with the organics, but the nitrate was very low. The aerosol concentrations were almost a factor of 3 lower in FAME-09 than in FAME0-08 but more variable. A thermodenuder coupled with the Q-AMS and an SMPS were operated as well. We used the algorithm that combines AMS and SMPS measurements to estimate the AMS CE and organic particle density both for ambient and thermodenuder samples. Using these values the concentrations measured by the AMS for the ambient particles were in a good agreement with the other independent concentration measurements (filters, steam sampler). The Finokalia Station is ideal for studying organic aerosol of different photochemical age. The site is isolated, away from human activities and the wind may come from various directions: Europe, Balkans, Africa and sea. Further, the oxidation levels can vary, being stronger in the summer and weaker during winter. This allows the investigation of particles originating from the same sources but transferred under different metrological and photochemical conditions. The AMS-SMPS thermodenuder system was employed as well, for organic aerosol volatility measurements. For the volatility corrections we used the organic density and the CE of the AMS both of ambient and thermodenuded aerosol as calculated by the algorithm that combines AMS and SMPS data. The organic aerosol sampled during FAME-08 is quite uniform and highly oxidized more than one order of magnitude less volatile than laboratory-generated α-pinene SOA. The final goal of this research is to expand methods for measurement of the semi-volatile inorganic species of atmospheric aerosol and the gases that are in equilibrium with them. Modifying the steam jet aerosol collector (SJAC) technique both particulate phase (e.g. sulfate, nitrate, chloride, ammonium) and gas phase (e.g. ammonia, nitric acid, nitrous acid) can be measured. For this approach a denuded (from the gas species) and an undenuded line are used. From their difference the gas phase contribution can be estimated. Evaluating this system in an agricultural environment the concentrations of NH3, HONO, and HCl were measured. The HNO3 concentration was close to the detection limit (0.5 ppb) of our system.