Organic aerosol source apportionment in Zurich using extractive electrospray ionization time-of-flight mass spectrometry (EESI-TOF): Part I, biogenic influences and day/night chemistry in summer

Abstract

<p><strong>Abstract.</strong> Improving the understanding of the health and climate impacts of PM<sub>1</sub> remains challenging and is restricted by the limitations of current measurement techniques. Detailed investigation of secondary organic aerosol (SOA), which is typically the dominating fraction of the organic aerosol (OA), requires instrumentation capable of real-time, in situ measurements of molecular composition. In this study, we present the first ambient measurements by a novel extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF). The EESI-TOF was deployed along with a high resolution time of flight aerosol mass spectrometer (HR-ToF-AMS) during summer 2016 at an urban location (Zurich, Switzerland). Positive matrix factorization (PMF), implemented within the Multilinear Engine (ME-2), was applied to the data from both instruments to quantify the primary and secondary contributions to OA. From the EESI-TOF analysis, a 6-factor solution was selected as the most representative and interpretable solution for the investigated dataset, including two primary and four secondary factors. The primary factors are dominated by cooking and cigarette smoke signatures while the secondary factors are discriminated according to their daytime (two factors) and nighttime (two factors) chemistry. All four factors showed strong influence by biogenic emissions but exhibited significant day/night differences. Factors dominating during daytime showed predominantly ions characteristic of monoterpene and sesquiterpene oxidation while the nighttime factors included less oxygenated terpene oxidation products, as well as organonitrates which were likely derived from NO<sub>3</sub> radical oxidation of monoterpenes. Overall, the signal measured by the EESI-TOF and AMS showed a good correlation. Further, the two instruments were in excellent agreement in terms of both the mass contribution apportioned to the sum of POA and SOA factors and the total SOA signal. However, while the OOA factors separated by AMS analysis exhibited a flat diurnal pattern, the EESI-TOF factors illustrated significant chemical variation throughout the day. The captured variability, inaccessible from AMS PMF analysis, was shown to be consistent with the variations in the physiochemical processes influencing chemical composition and SOA formation. The improved source separation and interpretability of EESI-TOF results suggest it to be a promising approach to source apportionment and atmospheric composition research.</p>

On the fate of oxygenated organic molecules in atmospheric aerosol particles

“…For the reasons identified above, most differences occur in the dimer region; 28% of the nonoverlapping chamber signal is attributed to C 16 -C 20 dimers and 4% to C 11 -C 15 dimers. The strong similarity of the chamber mass spectrum to the Hyytiälä measurements (discussed above) and, to a lesser extent, to a factor related to monoterpene oxidation retrieved from source apportionment of OA in Zurich (23) suggests that the overall aerosol composition is likewise similar, and therefore, the reactive processes observed in the chamber are highly likely to occur also in the atmosphere. Establishing a quantitative relationship between reactions observed in the chamber and those in ambient air (where conditions also vary considerably) is, of course, very challenging, and not attempted here.…”

Organic aerosol source apportionment in Zurich using an extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF-MS) – Part 2: Biomass burning influences in winter

“…In selecting the PMF solution that best represents the EESI-TOF-MS dataset, we considered both mathematical diagnostics (e.g., Q/Q exp and residuals) as a function of the number of factors, as well as the interpretability of the retrieved factors. Interpretability was judged according to 1. correlation of the time series and diurnal patterns between the AMS and EESI factors 2. comparison of factor profiles with mass spectra retrieved from less and more aged biomass burning exhaust from simulation chamber experiments at PSI (Bertrand et al, 2018) 3. similarities to EESI-TOF-MS factor mass spectra retrieved from summer measurements at the same site in Zurich (Stefenelli et al, 2019b) 4. identification of key ions in the factor profiles, including ions contributing a major fraction of the total factor signal, ions apportioned predominantly to a certain factor or related to a set of factors, and ions established in the literature as known tracers for specific sources/processes 5. interpretation of the temporal behavior in terms of meteorological data, including temperature, solar radiation, and wind speed/direction.…”

“…Here, we report on a study in Zurich, a midsize city in central Europe, utilizing the EESI-TOF-MS, complemented with AMS source apportionment results for a winter case. Summer measurement and source apportionment are presented in the companion paper (Stefenelli et al, 2019b). In both cases, due to the enhanced chemical resolution of the EESI-TOF-MS we are able to resolve more POA and SOA sources than in previous studies at the same site.…”

An extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF) for online measurement of atmospheric aerosol particles

“…This introduces considerable uncertainty into both the sensitivity and linearity of the EESI-TOF response to nicotine. However, the AMS nicotine tracer C 5 H 10 N + (Struckmeier et al, 2016) exhibits F. D. Lopez-Hilfiker et al: EESI-TOF for online measurements of atmospheric aerosol a strong linear correlation with the EESI-TOF nicotine measurement, suggesting that no significant nonlinearities are present under the conditions encountered in Zurich . Finally, the spectrum shows a significant contribution from [C x H y O z N 1−2 ]Na + ions, which are mostly assigned to organonitrates as discussed in detail elsewhere .…”