The contributions of non-methane hydrocarbon emissions by different fuel type on-road vehicles based on tests in a heavily trafficked urban tunnel.

Automobile exhaust is a major source of volatile organic compounds (VOCs) in metropolitan areas, yet it is difficult to accurately determine the contributions of different types of on-road vehicles. Tunnel tests are an effective way to measure real-world vehicle emissions, and the data collected are also suitable for receptor modeling to analyze the contributions of non-methane hydrocarbons (NMHCs) from different types of vehicles, as the closed environment ensures good mixing and minimal aging. In this study, tunnel tests were conducted inside a heavily trafficked city tunnel in Guangzhou in south China, and the positive matrix factorization (PMF) model was applied to the inlet-outlet incremental NMHC data. The results revealed that gasoline vehicles (GVs), Liquefied Petroleum Gas vehicles (LPGVs), and diesel vehicles (DVs) were responsible for 39 %, 45 % and 16 % of NMHCs, and 52 %, 23 %, and 24 % of the ozone formation potentials, respectively. LPGVs were the largest contributor of (56 %) alkanes, and GVs were the largest contributor of aromatics (61 %) and C2-C4 alkenes (55 %). With the video-recorded traffic counts the emissions of different fuel types are further compared on a per-vehicle-per-kilometer basis, and the results reveal that LPGVs and GVs were comparable in the OFPs of NMHCs emitted per kilometer, while on average a DV emitted 2.0 times more NMHCs than a GV with 2.4 times more OFPs. This study highlights substantial contribution of reactive alkenes and aromatics by DVs and the benefits of strengthening diesel exhaust control in terms of preventing ozone pollution.

Emissions and light absorption of PM2.5-bound nitrated aromatic compounds from on-road vehicle fleets.

Vehicle emissions are an important source of nitrated aromatic compounds (NACs) in particulate size smaller 2.5 µm (PM2.5), which adversely affect human health and biodiversity, especially in urban areas. In this study, filter-based PM2.5 samples were collected during October 14-19, 2019, in a busy urban tunnel (approximately 35,000 vehicles per day) in south China to identify PM2.5-bound NACs. Among them, 2,8-dinitrodibenzothiophene, 3-nitrodibenzofuran and 2-nitrodibenzothiophene were the most abundant nitrated polycyclic aromatic hydrocarbons (NPAHs), while 2-methyl-4-nitrophenol, 2,4-dinitrophenol, 3-methyl-4-nitrophenol and 4-nitrophenol were the most abundant nitrophenols (NPs). The observed mean fleet emission factors (EFs) of NPAHs and NPs were 2.2 ± 2.1 and 7.7 ± 4.1 µg km-1, and were 2.9 ± 2.7 and 10.2 ± 5.4 µg km-1 if excluding electric and liquefied petroleum gas vehicles, respectively. Regression analysis revealed that diesel vehicles (DVs) had NPAH-EFs (55.3 ± 5.3 µg km-1) approximately 180 times higher than gasoline vehicles (GVs) (0.3 ± 0.2 µg km-1), and NP-EFs (120.6 ± 25.8 µg km-1) approximately 30 times higher than GVs (4.1 ± 0.2 µg km-1), and thus 89% NPAH emissions and 56% NP emissions from the onroad fleets were contributed by DVs although DVs only accounted for 3.3% in the fleets. Methanol solution-based light absorption measurements demonstrated that the mean incremental light absorption for methanol-soluble brown carbon at 365 nm was 6.8 ± 2.2 Mm-1, of which the 44 detected NACs only contributed about 1%. The mean EF of the 7 toxic NACs was approximately 3% that of the 16 priority PAHs; However, their benzo(a)pyrene toxic equivalence quotients (TEQBaP) could reach over 25% that of the PAHs. Moreover, 6-nitrochrysene mainly from DVs contributed 93% of the total TEQBaP of the NACs. This study demonstrated that enhancing DV emission control in urban areas could benefit the reduction of exposure to air toxins such as 6-nitrochrysene.

Decrease in ambient volatile organic compounds during the COVID-19 lockdown period in the Pearl River Delta region, south China.

During the COVID-19 lockdown, ambient ozone levels are widely reported to show much smaller decreases or even dramatical increases under substantially reduced precursor NOx levels, yet changes in ambient precursor volatile organic compounds (VOCs) have been scarcely reported during the COVID-19 lockdown, which is an opportunity to examine the impacts of dramatically changing anthropogenic emissions on ambient VOC levels in megacities where ozone formation is largely VOC-limited. In this study, ambient VOCs were monitored online at an urban site in Guangzhou in the Pearl River Delta region before, during, and after the COVID-19 lockdown. The average total mixing ratios of VOCs became 19.1% lower during the lockdown than before, and those of alkanes, alkenes and aromatics decreased by 19.0%, 24.8% and 38.2%, respectively. The levels of light alkanes (C < 6) decreased by only 13.0%, while those of higher alkanes (C ≥ 6) decreased by 67.8% during the lockdown. Disappeared peak VOC levels in morning rush hours and the drop in toluene to benzene ratios during the lockdown suggested significant reductions in vehicle exhaust and industrial solvent emissions. Source apportioning by positive matrix factorization model revealed that reductions in industrial emissions, diesel exhaust (on-road diesel vehicles and off-road diesel engines) and gasoline-related emissions could account for 48.9%, 42.2% and 8.8%, respectively, of the decreased VOC levels during the lockdown. Moreover, the reduction in industrial emissions could explain 56.0% and 70.0% of the reductions in ambient levels of reactive alkenes and aromatics, respectively. An average increase in O3-1 h by 17% and a decrease in the daily maximum 8-h average ozone by 11% under an average decrease in NOx by 57.0% and a decrease in VOCs by 19.1% during the lockdown demonstrated that controlling emissions of precursors VOCs and NOx to prevent ambient O3 pollution in megacities such as Guangzhou remains a highly challenging task.

Contribution of Vehicle Emission and NO2 Surface Conversion to Nitrous Acid (HONO) in Urban Environments: Implications from Tests in a Tunnel.

Nitrous acid (HONO) is an important photochemical precursor to hydroxyl radicals particularly in an urban atmosphere, yet its primary emission and secondary production are often poorly constrained. Here, we measured HONO and nitrogen oxides (NOx) at both the inlet and the outlet in a busy urban tunnel (>30 000 vehicles per day) in south China. Multiple linear regression revealed that 73.9% of the inlet-outlet incremental HONO concentration was explained by NO2 surface conversion, while the rest was directly emitted from vehicles with an average HONO/NOx ratio of 1.31 ± 0.87%, which was higher than that from previous tunnel studies. The uptake coefficient of NO2, γ(NO2), on the tunnel surfaces was calculated to be (7.01 ± 0.02) × 10-5, much higher than that widely used in models. As tunnel surfaces are typical of urban surfaces in the wall and road materials, the dominance of HONO from surface reactions in the poorly lit urban tunnel demonstrated the importance of NO2 conversion on urban surfaces, instead of NO2 conversion on the aerosol surface, for both daytime and night-time HONO even in polluted ambient air. The higher γ(NO2) on urban surfaces and the elevated HONO/NOx ratio from this study can help explain the missing HONO sources in urban areas.

Emissions and light absorption of carbonaceous aerosols from on-road vehicles in an urban tunnel in south China.

With changing numbers, compositions, emission standards and fuel quality of on-road vehicles, it is imperative to accordingly characterize and update vehicular emissions of carbonaceous aerosols for better understanding their health and climatic effects. In this study, a 7-day field campaign was conducted in 2019 in a busy urban tunnel (>30,000 vehicles day-1) in south China with filter-based aerosol samples collected every 2 h at both the inlet and the outlet for measuring carbonaceous aerosols and their light absorbing properties. Observed fleet average emission factor (EF) of total carbon (TC) was 13.4 ± 8.3 mg veh-1 km-1, and 17.4 ± 11.3 mg veh-1 km-1 if electric and LPG-driven vehicles were excluded; and fleet average EF of organic carbon (OC) and elemental carbon (EC) was 8.5 ± 6.6 and 4.9 ± 2.6 mg veh-1 km-1 (11.0 ± 8.8 and 6.3 ± 3.6 mg veh-1 km-1 if excluding electric and LPG vehicles), respectively. Regression analysis revealed an average TC-EF of 319.8 mg veh-1 km-1 for diesel vehicles and 2.1 mg veh-1 km-1 for gasoline vehicles, and although diesel vehicles only shared ~4% in the fleet compositions, they still dominate on-road vehicular carbonaceous aerosol emissions due to their over 150 times higher average TC-EF than gasoline vehicles. Filter-based light absorption measurement demonstrated that on average brown carbon (BrC) could account for 19.1% of the total carbonaceous light absorption at 405 nm, and the average mass absorption efficiency of EC at 635 nm and that of OC at 405 nm were 5.2 m2 g-1 C and 1.0 m2 g-1 C, respectively.

Secondary organic aerosols produced from photochemical oxidation of secondarily evaporated biomass burning organic gases: Chemical composition, toxicity, optical properties, and climate effect.

Biomass burning (BB) is an important source of primary organic aerosols (POA). These POA contain a significant fraction of semivolatile organic compounds, and can release them into the gas phase during the dilution process in transport. Such evaporated compounds were termed "secondarily evaporated BB organic gases (SBB-OGs)" to distinguish them from the more studied primary emissions. SBB-OGs contribute to the formation of secondary organic aerosols (SOA) through reactions with atmospheric oxidants, and thus may influence human health and the Earth's radiation budget. In this study, tar materials collected from wood pyrolysis were taken as proxies for POA from smoldering-phase BB and were used to release SBB-OGs constantly in the lab. OH-initiated oxidation of the SBB-OGs in the absence of NOx was investigated using an oxidation flow reactor, and the chemical, optical, and toxicological properties of SOA were comprehensively characterized. Carbonyl compounds were the most abundant species in identified SOA species. Human lung epithelial cells exposed to an environmentally relevant dose of the most aged SOA did not exhibit detectable cell mortality. The oxidative potential of SOA was characterized with the dithiothreitol (DTT) assay, and its DTT consumption rate was 15.5 ± 0.5 pmol min-1 µg-1. The SOA present comparable light scattering to BB-POA, but have lower light absorption with imaginary refractive index less than 0.01 within the wavelength range of 360-600 nm. Calculations based on Mie theory show that pure airborne SOA with atmospherically relevant sizes of 50-400 nm have a cooling effect; when acting as the coating materials, these SOA can counteract the warming effect brought by airborne black carbon aerosol.