Photochemical aging of Beijing urban PM2.5: Production of oxygenated volatile organic compounds

Significant contribution of carbonyls to atmospheric oxidation capacity (AOC) during the winter haze pollution over North China Plain

Atmospheric carbonyl compounds play significant roles in the cycling of radicals and have exhibited surprisingly high levels in winter that were well correlated to particulate matter, for which the reason have not been clearly elucidated. Here we measured carbonyl compounds and other trace gasses together with PM2.5 over urban Jinan in North China Plain during the winter. Markedly higher carbonyl concentrations (average: 14.63 ± 4.21 ppbv) were found during wintertime haze pollution, about one to three-times relative to those on non-haze days, with slight difference in chemical composition except formaldehyde (HCHO). HCHO (3.68 ppbv), acetone (3.17 ppbv), and acetaldehyde (CH3CHO) (2.83 ppbv) were the three most abundant species, accounting for ∼75% of the total carbonylson both haze and non-haze days. Results from observational-based model (OBM) with atmospheric oxidation capacity (AOC) indicated that AOC significantly increased with the increasing carbonyls during the winter haze events. Carbonyl photolysis have supplied key oxidants such as RO2 and HO2, and thereby enhancing the formation of fine particles and secondary organic aerosols, elucidating the observed haze-carbonyls inter-correlation. Diurnal variation with carbonyls exhibiting peak values at early-noon and night highlighted the combined contribution of both secondary formation and primary diesel-fuel sources. 1-butene was further confirmed to be the major precursor for HCHO. This study confirms the great contribution of carbonyls to AOC, and also suggests that reducing the emissions of carbonyls would be an effective way to mitigate haze pollution in urban area of the NCP region.

Summertime carbonyl compounds in an urban area in the North China plain: Identification of sources, key precursors and their contribution to O3 formation

Carbonyl compounds are critical components of volatile organic compounds. They significantly participate in the photochemical formation of atmospheric ozone and thus threaten human health. This study measured 15 C₁-C₈ carbonyl compounds at an urban site in Linyi, a typically industrialised city in the North China Plain (NCP). Formaldehyde (3.89 ppbv), acetaldehyde (1.66 ppbv) and acetone (2.03 ppbv) were found to be the top three carbonyl compounds, accounting for 76.11% of the total concentration of carbonyl compounds. Anthropogenic secondary formation was recognised as the main source of the top five carbonyl compounds, which included formaldehyde, acetaldehyde, acetone, butyraldehyde and benzaldehyde, and accounted for 46-54% of all sources. Alkenes were the most important precursors of formaldehyde and acetaldehyde, suggesting that reducing the emission of alkenes from anthropogenic sources is an effective way to control carbonyl compound pollution in Linyi. Furthermore, the photolysis of carbonyl compounds played a significant role (68-75%) as sources of HO₂• and RO₂• and thus made a significant contribution (14.6%) to the photochemical formation of O₃. This study highlights the importance of anthropogenic secondary formation as a source of carbonyl compounds and provides a scientific basis for O₃ pollution control in carbonyl compound-enriched cities in the NCP.

PM2.5-mediated photochemical reaction of typical toluene in real air matrix with identification of products by isotopic tracing and FT-ICR MS

The sight into photoconversion of toluene, a ubiquitous typical pollutant, attentively by the involvement of PM2.5 in the real air environment is crucial for controlling haze pollution. Compared with the large-size PM2.5 on normal day (PM2.5-ND), the PM2.5 on haze day (PM2.5-HD) formed of small particle agglomerates featured greater oxidation capability, evidenced by the valence distribution of sulfur species. Notably, PM2.5-HD had abundant O₂^-• and •OH and participated in the photochemical reaction of toluene, giving it a greater toluene conversion with a first-order kinetic rate constant of 0.4 d^-1 on haze day than on normal day (0.2 d^-1). During the toluene photoconversion, isotopic labelling traced small molecules including benzene and newfound pentane, ethylbenzene, 1,3,8-p-menthatriene and 4-methyl-1-pentanone benzene that could be formed by methyl breakage, ring opening, fragmentation reforming and addition reaction of toluene. Given ADMET properties, 1,3,8-p-menthatriene was assigned high priority since it had poor metabolism, low excretion and severe toxicity, while benzene and 4-methyl-1-pentanone benzene should also be noticeable. FT-ICR MS results indicated that toluene could create multiple macromolecular products that are more sensitive to SOA generation in haze air matrix with broader carbon number and O/C, more oxygenated substitution with CHO/CHON occupying by 81.4%, lower DBE_average at 4.66 and higher OS_C‾ at -1.60 than normal air matrix. Accordingly, a photochemical reaction mechanism for toluene in real air atmosphere was proposed. The stronger oxidation property of PM2.5 not only facilitated toluene to generate small molecules but also boosted the conversion of intermediates to oxygenated macromolecular products, contributing to the formation of SOA.

Fate of Oxygenated Volatile Organic Compounds in the Yangtze River Delta Region: Source Contributions and Impacts on the Atmospheric Oxidation Capacity

The Community Multiscale Air Quality model (CMAQv5.2) was implemented to investigate the sources and sinks of oxygenated volatile organic compounds (OVOCs) during a high O₃ and high PM_2.5 season in the Yangtze River Delta (YRD) region, based on constraints from observations. The model tends to overpredict non-oxygenated VOCs and underpredict OVOCs, which has been improved with adjusted emissions of all VOCs. The OVOCs in the YRD are dominated by ketones, aldehydes, and alcohols. Ketones and aldehydes mainly originate from direct emissions and secondary formation in the northern YRD, and primarily originate from secondary formation in the southern part influenced by biogenic emissions. The concentration of secondary organic aerosols (SOA) produced by OVOCs is 0.5-1.5 μg/m³, with 40-80% originated from organic nitrates, 20-70% originated from dicarbonyls, and 0-20% originated from isoprene epoxydiols. The influences of OVOCs on the atmospheric oxidation capacity are dominated by the OH^• pathway during the day (∼350%) and by the NO₃^• pathway at night (∼150%). Consequently, O₃ is enhanced by 30-70% in the YRD. Aerosols are also enhanced by 50-140%, 20-80%, and ∼20% for SOA, nitrate, and sulfate, respectively.

Field observations and quantifications of atmospheric formaldehyde partitioning in gaseous and particulate phases

Formaldehyde (HCHO) can possibly be taken by atmospheric particles due to its moderate solubility. Although previous model studies have proposed that uptake by particles was a large sink for HCHO, direct observation of HCHO partitioning and estimation of HCHO uptake coefficient (γ) for tropospheric conditions are still limited. In this work, online measurements of gaseous HCHO (HCHO_g) and particulate HCHO (HCHO_p) were carried out simultaneously at an urban site in Beijing in winter and spring. The results indicated that the average concentrations of HCHO_p ranged from 0.15 to 0.4 μg m^-3, accounting for 1.2% to 10% of the total HCHO (i.e., HCHO_g + HCHO_p). The median values of estimated γ based on the measured data were in the range of about 1.09 ∗ 10^-5-2.42 ∗ 10^-4, with lower values during PM_2.5 pollution episodes. Besides, the pH and liquid water content of aerosols that are mainly determined by ambient relative humidity (RH) and inorganic salt composition were identified as the main influencing factors of γ. We propose that the HCHO uptake process was mainly driven by hydrone and hydrogen ions in particles.