This paper explores the effect of aerosol acidity on alpha-pinene SOA formation under low- and high-NOx conditions. Although this system has been examined by prior studies, only a few studies (research groups) have really investigated this question but under varying reaction conditions (i.e., oxidant type, mixing ratios of alpha-pinene, NO, NO2, and NOx, as well as inorganic seed aerosol composition and RH). This has made the interpretation of the effect of acidity on alpha-pinene unclear. As the authors rightly point out in their reply to one of the initial reviewer comments, just because this topic has been examined before doesn't mean the scientific question under study is fully addressed. I think the most important results from this study include, (1) a stronger effect of aerosol acidity on alpha-pinene SOA formation under high-NOx conditions, especially early in the reaction (first hour), compared to low-NOx conditions. The authors suggest that since the SOA yields are larger under low-NOx conditions (even with less acidic aerosol present), it is possible that organic coatings form on the inorganic core and thus prevent later generation products from interacting with the acidic media to undergo acid-catalyzed particle-phase reactions; (2) Under the low-NOx condition, the authors did find if they initially conducted the photooxidation without seed aerosol they didn't see significant nucleation of SOA. This is important as they waited a few hours later to inject inorganic seed aerosol and found the more acidic inorganic seed aerosol yielded more SOA, suggesting that later-generation products produced under low-NOx conditions could in fact reactively uptake onto these acidic particles. To me, this is one of the most interesting and significant findings of these studies conducted here. I think the authors may want to highlight this effect in the abstract! One could imagine the potential implications of this. For example, if NOx conditions are low enough and alpha-pinene produces later-generation products from a forested area that are then transported near a power plant plume or urban area, you might have acid enhancement of alpha-pinene SOA as a result; and (3) I think the acid enhancement of organic nitrates in the aerosol phase in the high-NOx experiments is quite interesting and potentially important! For example, Surratt et al. (JPCA, 2008) showed the the presence of acidic sulfate aerosol yielded significant quantities of nitrated organosulfates. It is possible that high-volatility organic nitrate products are converted into lower volatility products (like organosulfates), especially if the nitrate group is on a favorable carbon type (i.e., primary, secondary, or tertiary). Professor Matthew Elrod's group at Oberlin College has published work on how stable different organic nitrates are stable in acidic aqueous media. These studies might be helpful in supporting your results or at least provide insights into which types of organic nitrates remain unhydrolyzed in the SOA particles. Overall, I think this is a well-written paper that warrants publication in ACP after the authors consider my specific comments below. I think the authors did a great job citing all of the relevant published work.
Specific Comments to Consider Before Publication:
1.) There is concern that H2O2 used in low-NOx chamber experiments may bias a larger production of organic hydroxyhydroperoxides than what might actually occur in the atmosphere. This has been recently discussed in recent results published on isoprene SOA by Liu et al. (2016, ES&T). The reason this matters is this may make SOA yields much higher than expected in the atmosphere since many of these hydroperoxides are multifunctional and are ELVOC like. Thus, you may make a lot of SOA in chamber experiments due to the high HO2 levels but maybe not a lot in the atmosphere through these compounds. This is not answered question, but I think the authors may want to put some word of caution on the use of H2O2 in chamber studies.
2.) Coatings - I agree that organic coatings could be a major reason for the observed effects you reported. I noticed that the authors probably didn't cite a recent paper that wasn't published at the time of this submission that demonstrated that a-pinene SOA coatings derived from a-pinene ozonolysis suppressed the reactive uptake of isoprene epoxydiols (IEPOX) (Riva et al, 2016, ES&T). Although only a minor point in this prior study, Lin et al. (2014, ES&T) did demonstrate that acid-catalyzed reactive uptake of IEPOX could be self-limiting depending on the initial inorganic seed aerosol. Specifically, when they injected the same amount of IEPOX into a chamber filled with either MgSO4 + H2SO4 or (NH4)2SO4 + H2SO4 they found that the SOA yield was MUCH MUCH lower for the former case due to the production of light-absorbing oligomers. It appeared that these oligomers may have formed a diffusion barrier for further uptake of IEPOX. Furthermore, Professor Faye McNeill's group at Columbia University demonstrated that the reactive uptake of alpha-pinene oxide is also self-limiting (Drozd et al., 2014, ACP). The point of me in raising these studies is I think it provides more credibility for the interpretation of your results.
3.) Vapor losses to Chamber - The time scale of your chamber experiments is quite long, which could be a problem for low vapor pressure products in a small chamber like yours. How might wall losses of ELVOC-like species (Ehn et al., 2014, Nature) affect the SOA yields you report here? It is becoming more common practice now to use CIMS instruments to measure wall losses of gaseous species in order to provide more accurate estimates of SOA yields (Zhang et al., PNAS, 2015). The question I'm posing to the authors is since you focus on reporting SOA yields, couldn't the yields you report actually be an underestimate due to wall loss issues related to "sticky" vapors? I think this has to be acknowledged in the text.
4.) Something remains unclear to me. In the experiments using pure ammonium sulfate seed aerosol, do the authors think these particle effloresced? If so, how can you accurately calculate the acidiies of these particles? Further, if they did efflorescece, do you think the lack of aerosol water on these particles could have affected the potential multiphase chemistry?
5.) Lack of acidity effect under low-NOx conditions: The lack of an acidity effect is interesting! Recent work by Liu et al. (2015, PCCP) demonstrated the hydroperoxides might react on acidic particles and off gas more volatile products? Do you think this could be happening here? One way to check is to examine your data where you conduct the photooxidaiton experiments without seed aerosol but later add in the acidic aerosol. Does your PTR-MS reveal enhanced volatile product formation after this introduction of seed aerosol under low-NOx conditions? Further, I wonder if you see changes in the aerosol composition with the AMS? That might reveal something about potential multiphase chemistry? It's possible with the AMS you see very different OA mass spectra when you add in the seed aerosol, suggesting something is occuring. This kind of connection to the chemical data you have could provide more insights into the SOA yields you report.
6.) The authors say the particles were coated with SOA. How is this known? Are you assuming this based on prior studies that related phase separation to O:C ratios (which is fine, but wanting to make sure I'm clear on how you know it is a coating versus mixture). Or did you conduct microscopy measurements to know this?
7.) Is it possible to show average mass spectra from each condition in the SI? I'm curious to know if the authors find any marker ions at higher m/zs that could potentially be used as tracers? For example, Lin et al. (2012, ES&T) and Budisulistiorini et al. (2013, ES&T) showed that m/z 82 could be a direct marker ion for the acid-catalyzed reactive uptake of IEPOX. This has been later confirmed by Hu et al. (2015, ACP).
Minor Comments:
1.) Page 2, Line 35: Need citation to literature here for this sentence.
2.) Page 2, Line 38: Do the authors mean the use of "e.g." instead of "i.e." when citing these prior studies on acid-catalyzed chemistry?
3.) Page 2, Line 39: The authors may want to cite Lin et al. (2012, ES&T) from the Surratt group here.
4.) Page 2, Lines 41-42: The authors mean to say the following?
"Furthermore, the enhanced formation of SOA and organosulfates has been reported from the acid-catalyzed reactive uptake of epoxide compounds in ambient aerosols that are acidic enough to promote this multiphase chemistry."
The studies you cite are fine, but you may want to report recent kinetics studies showing that this is feasible, such as work by Matthew Elrod's group (Oberlin college) as well as Gaston et al. (2014, ES&T) and Riedel et al. (2015, ES&T Lett).
5.) Page 12, Line 68: Should "Noted" be changed to "It should be noted...." |
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