1Aerosol and Haze Laboratory, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing, China
2Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Finland
3Joint International Research Laboratory of Atmospheric and Earth System Research, School of Atmospheric Sciences, Nanjing University, Nanjing, China
4State Key Joint Laboratory of Environment Simulation and Pollution Control, State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, School of Environment, Tsinghua University, Beijing, China
5Division of Environment and Sustainability, The Hong Kong University of Science and Technology (HKUST), Hong Kong SAR, China
6Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University (HKPolyU), Hong Kong SAR
7Department of Civil and Environmental Engineering, Faculty of Science and Technology, University of Macau, Taipa, Macau, China
8State Environmental Protection Key Laboratory of Formation and Prevention of Urban Air Pollution Complex, Shanghai Academy of Environmental Sciences, Shanghai, China
9Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen, Switzerland
1Aerosol and Haze Laboratory, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing, China
2Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Finland
3Joint International Research Laboratory of Atmospheric and Earth System Research, School of Atmospheric Sciences, Nanjing University, Nanjing, China
4State Key Joint Laboratory of Environment Simulation and Pollution Control, State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, School of Environment, Tsinghua University, Beijing, China
5Division of Environment and Sustainability, The Hong Kong University of Science and Technology (HKUST), Hong Kong SAR, China
6Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University (HKPolyU), Hong Kong SAR
7Department of Civil and Environmental Engineering, Faculty of Science and Technology, University of Macau, Taipa, Macau, China
8State Environmental Protection Key Laboratory of Formation and Prevention of Urban Air Pollution Complex, Shanghai Academy of Environmental Sciences, Shanghai, China
9Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen, Switzerland
Received: 06 Mar 2022 – Discussion started: 18 Mar 2022
Abstract. Oxygenated organic molecules (OOMs) are crucial for atmospheric new particle formation and secondary organic aerosol (SOA) growth. Therefore, understanding their chemical composition, temporal behavior, and sources is of great importance. Previous studies on OOMs mainly focus on environments where biogenic sources are predominant, yet studies on sites with dominant anthropogenic emissions, such as megacities, have been lacking. Here, we conducted long-term measurements of OOMs covering four seasons of the year 2019 in urban Beijing. The OOM concentration was found to be the highest in summer (1.6 × 108 cm-3), followed by autumn (7.9 × 107 cm-3), spring (5.7 × 107 cm-3) and winter (2.3 × 107 cm-3), suggesting that enhanced photo-oxidation together with the rise of temperature promote the formation of OOMs. Most OOMs contained 5 to 10 carbon atoms and 3 to 7 effective oxygen atoms (nOeff = nO–2 × nN). The average nOeff increased with increasing atmospheric photo-oxidation capacity, which was the highest in summer and the lowest in winter and autumn. By performing a newly developed workflow, OOMs were classified into four types: aromatic OOMs, aliphatic OOMs, isoprene OOMs, and monoterpene OOMs. Among them, aromatic OOMs (29–41 %) and aliphatic OOMs (26–41 %) were the main contributors in all seasons, indicating that OOMs in Beijing were dominated by anthropogenic sources. The contribution of isoprene OOMs increased significantly in summer (33 %), which is much higher than those in other three seasons (8–10 %). Concentrations of isoprene (0.2–5.3 × 107 cm-3) and monoterpene (1.1–8.4 × 106 cm-3) OOMs in Beijing were lower than those reported at other sites, and they possessed lower oxygen and higher nitrogen contents due to high NOx levels (9.5–38.3 ppbv) in Beijing. With regard to the nitrogen content of the two anthropogenic OOMs, aromatic OOMs were mainly composed of CHO and CHON species, while aliphatic OOMs were dominated by CHON and CHON2 ones. Such prominent differences suggest varying formation pathways between these two OOMs. By combining the measurements and an aerosol dynamic model, we estimated that the SOA growth rate through OOM condensation could reach 0.64 μg∙m-3∙h-1, 0.61 μg∙m-3∙h-1, 0.41 μg∙m-3∙h-1, and 0.30 μg∙m-3∙h-1 in autumn, summer, spring, and winter, respectively. Despite the similar concentrations of aromatic and aliphatic OOMs, the former had lower volatilities and, therefore, showed higher contributions (46–62 %) to SOA than the latter (14–32 %). By contrast, monoterpene OOMs and isoprene OOMs, limited by low abundances or high volatilities, had low contributions of 8–12 % and 3–5 %, respectively. Overall, our results improve the understanding of the concentration, chemical composition, seasonal variation and potential atmospheric impacts of OOMs, which can help formulate refined restriction policy specific to SOA control in urban areas.
This manuscript presents the results of a field study of oxygenated organic molecules (OOMs) and their contribution to secondary organic aerosol (SOA) in urban Beijing. The measurements of OOMs were conducted over the four seasons of the year 2019 using a nitrate-CIMS. The measured OOMs mainly contained 5-10 carbon atoms, 3-7 effective oxygen atoms and 0-2 nitrogen atoms and had 0-6 DBE values. The OOM concentration exhibited an obvious seasonal variation, ranging from ~1 ppt in winter to ~7 ppt in summer. Such seasonality was thought to be mainly driven by the seasonal variation of the intensity of photochemistry. According to the DBE value and the number of carbon, oxygen and nitrogen atoms in the molecules, ~1000 OOMs were classified into four groups, that is, isoprene, monoterpene, aliphatic and aromatic OOMs. Among them, aromatic (29-41%) and aliphatic (26-41%) OOMs were found to be the major contributors to OOMs in all seasons. The vapor condensation flux calculations further showed that these two classes of OOMs had largest contributions (46-62% and 14-32%, respectively) to SOA in urban Beijing throughout the year.
Up to date, field measurements of OOMs in urban areas are rare. This study provides valuable data on the concentration, chemical composition and seasonal variation of OOMs, as well as their potential contributions to SOA in polluted urban areas. Overall, the measurements and data analysis in this study are well performed, the results are appropriately discussed, and the manuscript is nicely written. I would recommend the publication of this manuscript in ACP after the following comments are fully addressed.
Line 63-65: It is described here that API-TOF provided the first direct measurements of OOMs. That is true for semi-volatile and low-volatility OOMs. However, OOMs literally mean organic compounds with oxygenated functional groups and also include the family of oxygenated volatile organic compounds, which had been measured, e.g., by GC-MS and PTR-MS before API-ToF or ToF-CIMS have been developed. The authors should provide a clear definition or specify the range of OOMs discussed in this study.
Line 119-121: Since OOMs can be detected either as a nitrate ion cluster (i.e., [M+NO3-] or [M+HNO3•NO3-]) or as a deprotonated ion by nitrate-CIMS, additional information as to how the product ions [CHON+NO3-] vs. [CHO+ HNO3•NO3-] and [CHON-] vs. [CHO•NO3-] were differentiated in this study should be provided in the manuscript.
Line 143-144: The selection of monoterpene OOMs excluded < C10 compounds, which have been shown to account for a considerable fraction of monoterpene oxidation products in laboratory studies. To what extent would this assignment affect the accuracy of the results regarding the contributions of monoterpene OOMs to total OOMs and SOA? if applying the binPMF results of Nie et al, 2022 to this study, what signal ratios of C10 compounds vs. < C10 compounds would be obtaned in the monoterpene OOM factor?
Line 174: According to the workflow shown in Figure 1, the species with DBE = 2-3 and Oeff ≥ 6 were considered as aromatics OOMs. However, recent laboratory studies (e.g., Wang et al., Commun Chem, 2021) have shown that alkanes (and oxygenated alkanes) can undergo efficient autoxidation to form highly oxygenated molecules with DBE = 1-3 and Oeff > 6, even at high concentrations of NOx. The authors should include a discussion about the uncertainty of this assignment in the manuscript.
Line 219-221 and 245-248: What was the influence of the seasonal variation of the NOx level on the seasonality of the oxygen content of OOMs?
Line 272-277: The authors concluded that the formation of epoxide group during isoprene oxidation was not favored based on their observations that most of isoprene OOMs had 0 or 1 DBE. This conclusion should be made with caution for two reasons: First, nitrate-CIMS is generally more sensitive to organic species containing -OOH and -OH groups than to epoxides and aldehydes/ketones, which might make isoprene OOMs with 0 or 1 DBE as the most abundantly detected species; Second, the epoxide species formed from isoprene photooxidation might undergo fast heterogeneous ring-opening reactions to form products with 0 to 1 DBE, resulting in low gas-phase epoxide concentrations.
Line 405-406: The two sentences here seem to convey the same information, that is, RO2 termination reactions with NOx are more important for aliphatic species than for aromatics. Did the authors mean to say in the second sentence that the branching ratio of the aliphatic RO2+NOx to form CHON species is higher than that of the aromatics?
Line 408-411: It is not clear how the correlation between CHON2 species and PM2.5 could lead to the conclusion that CHON2 species or their precursor VOCs originated from long-distance transport. Please clarify this.
Line 448-449: The authors stated that the condensation of isoprene OOMs had little contribution to SOA growth regardless of the season. However, Figure 11 shows that isoprene OOMs had a contribution of up to 5% to SOA, which is obviously non-negligible. Therefore, this statement needs to be rephrased.
Gaseous oxygenated organic molecules are able to form atmospheric aerosols, the suspended particles in the air. Those aerosols have significant influence on human health and climate change. Therefore, understanding the basic properties and aerosol formation potential of those organic molecules is of great importance.
Gaseous oxygenated organic molecules are able to form atmospheric aerosols, the suspended...
