Online measurement of highly oxygenated compounds from organic aerosol
Abstract. Highly oxygenated compounds are important contributors to the formation and growth of atmospheric organic aerosol, and thus have an impact on Earth’s radiation balance and global climate. However, knowledge of the contribution of highly oxygenated compounds to organic aerosol and their fate after condensing into the particle phase has been limited by the lack of suitable detection techniques. Here, we present a new online method for measuring highly oxygenated compounds from organic aerosol. The method includes thermal evaporation of particles in a new inlet, Vocus inlet for aerosols (VIA), followed by identification of the evaporated highly oxygenated compounds by a nitrate chemical ionization mass spectrometer (NO3-CIMS). The method does not require sample collection, enabling highly time-resolved measurements of particulate compounds. We evaluate the performance of the method by measuring the detection limit and performing background measurements. We estimate a detection limit of below 1 ng m−3 for a single compound and below 1 μg m−3 for SOA with the sampling set-up used here. These detection limits can be improved upon by optimizing the flow setup. Furthermore, we detect hundreds of particulate highly oxygenated compounds from organic aerosol generated from different precursors. Our results are consistent with previous studies showing that the volatility of organic compounds decreases with increasing m/z ratio and higher level of oxygenation, and that organic aerosol consists of monomers and oligomeric compounds. By comparing the gas- and particle-phase compounds, we found indications of potential particle-phase reactions occurring in organic aerosol. Future work will focus both on further improving the sampling design, as well as on better understanding the evaporation dynamics of the system, but already these initial tests show that VIA coupled to the NO3-CIMS is a promising method for investigating the transformations and fate of the compounds after condensing into the particle phase.
Ella Häkkinen et al.
Ella Häkkinen et al.
Ella Häkkinen et al.
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Review of Hakkinen et al., “Online measurement of highly oxygenated compounds from organic aerosol”
The authors characterize a new Vocus Inlet for Aerosols (VIA) inlet, which thermally evaporates particles for online sampling. Coupled with an NO3-CIMS, they demonstrate the use of the VIA for measuring the chemical composition of several different aerosol types. These measurements show that the VIA could be useful for making online speciated particle composition measurements. The authors also discuss several artifacts and possible unknowns that need to be further investigated before the VIA can really be trusted to provide accurate speciation of highly oxygenated organic compounds. It would be best if the authors could provide direct comparisons of speciated VIA-NO3CIMS measurements with one or more of the other particle phase measurements discussed in the introduction (see my last comment below), which could give the reader a better idea of how well this method works in comparison to other methods, but that may not have been feasible. After addressing my comments below, I recommend for publication.
Line 25 and throughout: Why have you chosen to use the term “highly oxygenated compounds” throughout the manuscript instead of HOMs? Is there a technical difference between highly oxygenated compounds and HOMs that you can give in the text to clarify this, or can you instead refer to them as HOMs everywhere?
Line 32: I’m not sure what you mean by “form larger components of SOA”. I think you mean form larger molecules through oligomerization, but this could also mean they just accumulate until they comprise a large component of the SOA by mass. Please clarify.
Line 136: This paragraph is a bit confusing. First you say the internal temperature is not known, then you say you measured it and it internally reaches the setpoint by the outlet, then you go back to using the external temps and saying you use the setpoints and they aren’t necessarily the same as actual internal temps. I think it would be best if you show/state the internal measurements, but it would also be fine to just replace this paragraph with a simple sentence saying the temps are controlled externally so the internal temps may vary slightly.
Line 160: By “decreasing trend”, do you mean the fact that the linear regression slopes are less than 1? This language could probably be clarified. But my main question here is what role could wall losses of H2SO4 or highly oxidized organics to the tubing between the VIA and the NO3-CIMS be playing? Could that be causing this effect at higher concentrations?
Line 163: How does this calibration value of 2x10^10 cm^-3 compare to calibrations previously determined for NO3-CIMS instruments? Is it perhaps lower due to sampling line losses between the VIA and the NO3-CIMS in this setup? Please comment on this.
Line 173: Can you apply your calibration factor to the organic compounds here to give a rough estimate of mass concentration measured by the NO3-CIMS?
Line 174: The detection limit will depend on the dilution amount relative to sample flow, right? Since you’re diluting here by a factor of 4 (2 lpm sample flow and 8 lpm dilution flow), does that mean the limit of detection should be stated as 0.25 ug m-3 mass concentration at the NO3-CIMS inlet?
Line 222: Does this sentence refer to the data in Fig. 9 and the discussion in Sect. 4.1.1? If so, I suggest removing this statement from here. You are already discussing this concept of comparing the oxygen number between gas and particle measurements in 4.1.1. If you are actually referring to something else in this sentence, please provide a citation or show data to back up this assertion.
Line 243: Is there any literature on how fast these particle phase reactions could be at room temperature? That could help inform whether these particle phase reactions are occurring before the particles enter the VIA, or if the heat in the VIA is the real cause of particle phase changes. If the latter, that’s not so great for trying to identify specific compounds in the particle phase.
General comment: I’m wondering if you have tried sampling the gas phase (without seed aerosol) through the VIA with heating, to compare with normal gas phase sampling without heating. Does the heat lead to gas phase reactions?
Line 257: Can you compare this 10-20% number with previous literature or expectations?
Line 271: Could the thermal decomposition be not just breaking C=C bonds, but loss of water, CO, CO2, etc leaving you with less oxygenated species that are no longer clustering with NO3-? I’d suggest adding a sentence discussing this here.
Another general comment: In the introduction, you summarized well the existing particle phase speciation techniques. I am wondering if there are opportunities to directly compare measurements taken with the VIA and NO3-CIMS to some of these other techniques. For example, are there any published spectra of e.g. alpha pinene ozonolysis SOA that measure similar compounds, that you could show a direct comparison with? The ionization techniques might be different, but it would be interesting to see such a comparison. Sampling the same aerosol with multiple techniques would of course be the best way to directly compare, but may not be feasible depending on the authors’ access to the other techniques.