Influence of anthropogenic emissions on the composition of highly oxygenated organic molecules in Helsinki: a street canyon and urban background station comparison
Abstract. Condensable vapors, including highly oxygenated organic molecules (HOM), govern secondary organic aerosol formation and thereby impact the amount, composition, and properties (e.g. toxicity) of aerosol particles. These vapors are mainly formed in the atmosphere through the oxidation of volatile organic compounds (VOCs). Urban environments contain a variety of VOCs from both anthropogenic and biogenic sources, as well as other species, for instance nitrogen oxides (NOx), that can greatly influence the formation pathways of condensable vapors like HOM. During the last decade, our understanding of HOM composition and formation has increased dramatically, with most experiments performed in forests or in heavily polluted urban areas. However, studies on the main sources for condensable vapors and secondary organic aerosols (SOA) in biogenically influenced urban areas, such as suburbs or small cities, has been limited. Here, we studied the HOM composition, measured with two nitrate-based chemical ionization mass spectrometers and analyzed using positive matrix factorization (PMF), during late spring at two locations in Helsinki, Finland. Comparing the measured concentrations at a street canyon site and a nearby urban background station, we found a strong influence of NOx on the HOM formation at both stations, in agreement with previous studies conducted in urban areas. Even though both stations are dominated by anthropogenic VOCs, most of the identified condensable vapors originated from biogenic precursors. This implies that in Helsinki anthropogenic activities mainly influence HOM formation by the effect of NOx on the biogenic VOC oxidation. At the urban background station, we found condensable vapors formed from two biogenic VOC groups (monoterpenes and sesquiterpenes), while at the street canyon, the only identified biogenic HOM precursor was monoterpenes. At the street canyon, we also observed oxidation products of aliphatic VOCs, which were not observed at the urban background station. The only factors that clearly correlate (temporally and composition-wise) between the two stations contained monoterpene-derived dimers. This suggests that HOM composition and formation mechanisms are strongly dependent on localized emissions and the oxidative environment in these biogenically influenced urban areas, and they can change considerably also within distances of one kilometer within the urban environment.
Magdalena Okuljar et al.
Status: open (until 19 Jun 2023)
- RC1: 'Comment on egusphere-2023-524', Anonymous Referee #1, 24 May 2023 reply
- RC2: 'Comment on egusphere-2023-524', Yare Baker, 28 May 2023 reply
Magdalena Okuljar et al.
Magdalena Okuljar et al.
Viewed (geographical distribution)
Review of Okuljar et al., 2023
Okuljar et al. present measurements of HOMs and other condensable vapors from measurements at two sites in 1 km distance from another in Helsinki. The manuscript is well written and uses state-of-the-art scientific methods.
Given that there is still a lot that the atmospheric chemistry community does not know about the formation of HOMs/SOA, these observations in a suburban area provide an important addition to the literature. What I find the most interesting and striking point of this study is the strong difference between the composition and diurnal behavior of observed compounds during the same time, although the two sites are spatially so close to each other. This points to an important lesson for the atmospheric chemistry community regarding the (non-)representativeness of a single observation site for a whole metropolitan area, and generally to a very localized inhomogeneity of emissions, reaction processes and products. Therefore, I wish this point would be brought across more strongly, perhaps graphically. Thus see my comments below.
I recommend this article for publication in ACP after the following comments have been addressed:
l.44: apart from the listed sources, I suggest to mention cooking since it is also a strong source of anthropogenic VOCs/ condensable vapors.
Fig. 1: It would be helpful to add a wind rose to see which of the two stations is up-/downwind of each other.
Sect. 2.5 regarding transmission: I assume the signals are corrected for transmission/normalized to transmission - I think this should be mentioned for clarity that the signal strength is not impacted by the transmission issue between the two instruments.
Fig. 2: I think it would be helpful to add two panels showing the same data just for the time when there were simultaneous measurements at both sites. Otherwise, it is impossible to see how comparable the conditions (temperature etc.) are between both sites during this relevant time.
l.313: Since you mention monoterpene monomer factors with different diel variabilities – was there any diel variability in the monoterpene composition measured with sorbent tubes that could help to find out which monomer stems from which monoterpene or which part of the monoterpenes is anthropogenic/biogenic?
l.316-369: I think it would be interesting to mention the most important compounds/chemical formulas observed in each factor as you do for some, but not all that factors you describe here.
l.495: “This is clearly different from Helsinki” – I am not sure you can make that statement in the way you are making it, since you have only a few weeks of observations in one season and for China you are mentioning variability between seasons. It would be more appropriate to compare concrete cases with concrete numbers (which percentage of condensable vapors in Helsinki is biogenic, vs. which percentage is biogenic in the same season in a Chinese city?).
l.145: “only we can narrow it usage” – this phrase is unclear to me.
l.195: missing “the” before “identified”
Fig. 3 caption: It was not initially clear to me that “SC-1” stands for street canyon, factor 1. Please make the meaning of the numbering clearer in the caption.
l.412-413: This sentence is not clear to me, it seems as if both grammatically and content-wise something is mixed up in there?
l.482: lower resemblance than what?
l.490: “influenced” should be “influence”.
l.507: “particular” should be “particulate”.