Environ. Sci. Technol. 2009, 43, 4884–4889
Mass Spectra Deconvolution of Low, Medium, and High Volatility Biogenic Secondary Organic Aerosol E V A N G E L I A K O S T E N I D O U , †,‡ BYONG-HYOEK LEE,§ GABRIELLA J. ENGELHART,§ JEFFREY R. PIERCE,§ AND S P Y R O S N . P A N D I S * ,†,‡,§ Institute of Chemical Engineering and High Temperature Chemical Processes, ICE-HT, Patras, Greece, Department of Chemical Engineering, University of Patras, Patras, Greece, and Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania
Received December 26, 2008. Revised manuscript received April 16, 2009. Accepted May 3, 2009.
Secondary organic aerosol (SOA) 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 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. R-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 byanAMS.Thesignalfractionofm/z44,andthustheconcentration 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 R-and β-pinene SOA. Combining 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.
Introduction SOA is formed by condensation of low-volatility products of the oxidation of volatile organic compounds (VOCs) on preexisting particles and is an important contributor to ambient particulate matter in urban, rural, and remote areas (1, 2). Monoterpene SOA can contribute as much as half of the * Corresponding author e-mail:
[email protected]. † Institute of Chemical Engineering and High Temperature Chemical Processes. ‡ University of Patras. § Carnegie Mellon University. 4884
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total organic aerosol (3), with SOA from R-pinene, β-pinene, and limonene often dominating the system as they account for 30-70% of the overall terpene emissions (4-9). SOA formation has been studied in environmental chambers for more than 30 years (10-13) focusing mainly on the produced aerosol concentration and yield. In the beginning only organic volume or mass concentration was measured with later studies focusing more on the speciation of the products (14-16), but a complete picture of the SOA was missing. High concentrations of precursors have often been used in smog chamber experiments so that the organic aerosol concentration and composition can be monitored with the existing instrumentation, samples can be obtained for chemical analysis, contamination effects can be minimized, and reactions can be accelerated so that the experiment can take place in a reasonable time scale, etc. The recent development of the Aerodyne AMS (17, 18) provides real-time size and composition analysis of aerosol particles and has allowed the characterization of the mass spectra of all the SOA produced in smog chamber experiments (19, 20). Zhang et al. (21) introduced a new technique to deconvolve AMS mass spectra in two components: hydrocarbon-like and oxygenated organic aerosol (HOA and OOA). This approach indicated that the AMS mass spectrum of OOA is similar to that of aged organic aerosol in rural areas, but rather different from that of smog chamber SOA. The OOA mass spectrum is dominated by CO2+ (m/z 44), but laboratory SOA mass spectra from R-pinene, β-pinene, limonene, and β-caryophyllene have a greater signal at m/z 43 than at m/z 44. This discrepancy potentially puts into question the relevance of the smog chamber experiments for the study of atmospheric SOA and indicates gaps in our understanding of the system. Organic aerosol consists of species with a wide range of volatilities, with the less volatile dominating at low concentrations and the more volatile dominating at high (22, 23). The wide range of volatilities reflects the wide range of products that are formed during the oxidation of these rather large organic molecules. The volatility distribution of the resulting SOA is affected by temperature, the precursor, oxidant, and NOx levels, and is determined by the oxidation pathways for each precursor. The species of different volatility can be partially separated using a thermodenuder operating at the appropriate temperature (24). In a recent study Lee at al (25). investigated the volatility of R-pinene, β-pinene, and limonene SOA using a thermodenuder and a scanning mobility particle sizer (SMPS). They showed that limonene SOA was less volatile, the relative humidity (RH) had a small effect on the SOA volatility, and that the high NOx conditions produced more volatile R-pinene and β-pinene SOA. The results of their study will be combined here with AMS measurements to investigate the changes in composition of the SOA as its more volatile components are removed by the thermodenuder. In this work we use medium precursor concentrations to produce SOA of both high and low volatility. Using the volatility basis-set (22) we then analyze and compare the AMS mass spectra as a function of the volatility of the products.
Experimental Approach SOA was produced by ozonolysis of R-pinene, β-pinene, limonene, and β-caryophyllene in the Carnegie Mellon University smog chamber. The experiments were conducted at low (
