Chemical composition and sources of amines in PM2.5 in an urban site of PRD, China.

Atmospheric amines have attracted increasing attention due to their significant impact on new particle formation, particle hygroscopicity and particle optical properties. In this study, four low-molecule-weight amines were detected from PM2.5 filter samples collected at an urban site of Pearl River Delta (PRD) region of China in 2018 autumn. During the campaign, the mass concentrations of ambient particulate methylamine (MA, CH3NH2), dimethylamine (DMA, (CH3)2NH), trimethylamine (TMA, (CH3)3N), and diethylamine (DEA, (C2H5)2NH) were quantified at daily or 12-h resolution using an optimized Ion Chromatograph (IC) method. The total measured amine concentration was 297 ± 209 ng/m3, which can account for 0.76 ± 0.33% of PM2.5 mass concentrations. The particulate amines in PRD urban area were dominated by MA (243 ± 179 ng/m3), accounting for over 80% of total amines, then followed by DMA (49 ± 30 ng/m3, 16.5%), TMA (4 ± 2 ng/m3) and DEA (1 ± 1 ng/m3). Based on the correlation analysis, MA and DMA mainly presented as nitrate and sulfate salts. We speculate the amines tend to react with gas-phase HNO3 or particle-phase nitrate to form particulate amine salts via local process in Guangzhou. As the relative humidity (RH) increased, enhanced partitioning of amine towards the particle phase was observed. Using approach of multiple linear regression, 71% of the particulate amines in PRD urban site could be explained by acid-base process and the rest by primary emissions from combustion sources (29%).

High Concentrations of Atmospheric Isocyanic Acid (HNCO) Produced from Secondary Sources in China.

Isocyanic acid (HNCO) is a potentially toxic atmospheric pollutant, whose atmospheric concentrations are hypothesized to be linked to adverse health effects. An earlier model study estimated that concentrations of isocyanic acid in China are highest around the world. However, measurements of isocyanic acid in ambient air have not been available in China. Two field campaigns were conducted to measure isocyanic acid in ambient air using a high-resolution time-of-flight chemical ionization mass spectrometer (ToF-CIMS) in two different environments in China. The ranges of mixing ratios of isocyanic acid are from below the detection limit (18 pptv) to 2.8 ppbv (5 min average) with the average value of 0.46 ppbv at an urban site of Guangzhou in the Pearl River Delta (PRD) region in fall and from 0.02 to 2.2 ppbv with the average value of 0.37 ppbv at a rural site in the North China Plain (NCP) during wintertime, respectively. These concentrations are significantly higher than previous measurements in North America. The diurnal variations of isocyanic acid are very similar to secondary pollutants (e.g., ozone, formic acid, and nitric acid) in PRD, indicating that isocyanic acid is mainly produced by secondary formation. Both primary emissions and secondary formation account for isocyanic acid in the NCP. The lifetime of isocyanic acid in a lower atmosphere was estimated to be less than 1 day due to the high apparent loss rate caused by deposition at night in PRD. Based on the steady state analysis of isocyanic acid during the daytime, we show that amides are unlikely enough to explain the formation of isocyanic acid in Guangzhou, calling for additional precursors for isocyanic acid. Our measurements of isocyanic acid in two environments of China provide important constraints on the concentrations, sources, and sinks of this pollutant in the atmosphere.

Haze insights and mitigation in China: an overview.

The present article provides an overview of the chemical and physical features of haze in China, focusing on the relationship between haze and atmospheric fine particles, and the formation mechanism of haze. It also summarizes several of control technologies and strategies to mitigate the occurrence of haze. The development of instruments and the analysis of measurements of ambient particles and precursor concentrations have provided important information about haze formation. Indeed, the use of new instruments has greatly facilitated current haze research in China. Examples of insightful results include the relationship between fine particles and haze, the chemical compositions and sources of particles, the impacts of the aging process on haze formation, and the application of technologies that control the formation of haze. Based on these results, two relevant issues need to be addressed: understanding the relationship between haze and fine particles and understanding how to control PM2.5.

Ozone and its precursors at an urban site in the Yangtze River Delta since clean air action plan phase II in China.

In response to regional ozone (O3) pollution, Chinese government has implemented air pollution control measures in recent years. Here, a case study was performed at an O3-polluted city, Wuhu, in Yangtze River Delta region of China to investigate O3 variation trend and the relationship to its precursors after implementation of Clean Air Action Plan Phase II, which aims to reduce O3 pollution. The results showed that peak O3 concentration was effectively reduced since Clean Air Action Plan Phase II. Due to significant NOx reduction, O3 formation tended to shift from volatile organic compound (VOC)-limited regimes to NOx-limited regimes during 2018-2022. VOC/NOx ratios measured in 2022 revealed that peak O3 concentration tended to respond positively to NOx. Apart from high-O3 period, Wuhu was still in a VOC-limited regime. The relationship of maximum daily 8-h ozone average and NO2 followed a lognormal distribution with an inflection point at 20 µg m-3 of NO2, suggesting that Wuhu should conduct joint control of VOC and NOx with a focus on VOC reduction before the inflection point. Alkenes and aromatics were suggested to be preferentially controlled due to their higher ozone formation potentials. Using random forest meteorological normalization method, meteorology had a positive effect on O3 concentration in 2018, 2019 and 2022, but a negative effect in 2020 and 2021. The meteorology could explain 44.0 ± 19.1% of the O3 variation during 2018-2022. High temperature favors O3 production and O3 pollution occurred more easily when temperature was over 25 °C, while high relative humidity inhibits O3 generation and no O3 pollution was found at relative humidity above 70%. This study unveils some new insights into the trend of urban O3 pollution in Yangtze River Delta region since Clean Air Action Plan Phase II and the findings provide important references for formulating control strategies against O3 pollution.

Emission factors of ammonia for on-road vehicles in urban areas from a tunnel study in south China with laser-absorption based measurements.

Vehicle emission is an important source of ammonia (NH3) in urban areas. To better address the role of vehicle emission in urban NH3 sources, the emission factor of NH3 (NH3-EF) from vehicles running on roads under real-world conditions (on-road vehicles) needs to update accordingly with the increasingly tightened vehicle emission standards. In this study, laser-absorption based measurements of NH3 were conducted during a six-day campaign in 2019 at a busy urban tunnel with a daily traffic flow of nearly 40,000 vehicles in south China's Pearl River Delta (PRD) region. The NH3-EF was measured to be 16.6 ± 6.3 mg km-1 for the on-road vehicle fleets and 19.0 ± 7.2 mg km-1 for non-electric vehicles, with an NH3 to CO2 ratio of 0.27 ± 0.09 ppbv ppmv-1. Multiple linear regression revealed that the average NH3-EFs for gasoline vehicles (GVs), liquefied petroleum gas vehicles, and heavy-duty diesel vehicles (HDVs) were 18.8, 15.6, and 44.2 mg km-1, respectively. While NH3 emissions from GVs were greatly reduced with enhanced performance of engines and catalytic devices to meet stricter emission standards, the application of urea selective catalytic reduction (SCR) in HDVs makes their NH3 emission an emerging concern. Based on results from this study, HDVs may contribute over 11% of the vehicular NH3 emissions, although they only share ∼4% by vehicle numbers in China. With the updated NH3-EFs, NH3 emission from on-road vehicles was estimated to be 9 Gg yr-1 in the PRD region in 2019, contributing only 5% of total NH3 emissions in the region, but still might be a dominant NH3 source in the urban centers with little agricultural activity.

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.

Decadal changes in emissions of volatile organic compounds (VOCs) from on-road vehicles with intensified automobile pollution control: Case study in a busy urban tunnel in south China.

In the efforts at controlling automobile emissions, it is important to know in what extent air pollutants from on-road vehicles could be truly reduced. In 2014 we conducted tests in a heavily trafficked tunnel in south China to characterize emissions of volatile organic compounds (VOC) from on-road vehicle fleet and compared our results with those obtained in the same tunnel in 2004. Alkanes, aromatics, and alkenes had average emission factors (EFs) of 338, 63, and 42 mg km-1 in 2014 against that of 194, 129, and 160 mg km-1 in 2004, respectively. In 2014, LPG-related propane, n-butane and i-butane were the top three non-methane hydrocarbons (NMHCs) with EFs of 184 ± 21, 53 ± 6 and 31 ± 3 mg km-1; the gasoline evaporation marker i-pentane had an average EF of 17 ± 3 mg km-1; ethylene and propene were the top two alkenes with average EFs of 16 ± 1 and 9.7 ± 0.9 mg km-1, respectively; isoprene had no direct emission from vehicles; toluene showed the highest EF of 11 ± 2 mg km-1 among the aromatics; and acetylene had an average EF of 7 ± 1 mg km-1. While EFs of total NMHCs decreased only 9% from 493 ± 120 mg km-1 in 2004 to 449 ± 40 mg km-1 in 2014, their total ozone formation potential (OFP) decreased by 57% from 2.50 × 103 mg km-1 in 2004 to 1.10 × 103 mg km-1 in 2014, and their total secondary organic aerosol formation potential (SOAFP) decreased by 50% from 50 mg km-1 in 2004 to 25 mg km-1 in 2014. The large drop in ozone and SOA formation potentials could be explained by reduced emissions of reactive alkenes and aromatics, due largely to fuel transition from gasoline/diesel to LPG for taxis/buses and upgraded vehicle emission standards.

Airborne submicron particulate (PM1) pollution in Shanghai, China: chemical variability, formation/dissociation of associated semi-volatile components and the impacts on visibility.

Hourly mass concentrations of water-soluble ions in PM1 and gasses (NH3, HNO3, HCl) were on-line measured with a Monitor for AeRosols and Gases Analyzer (MARGA) in Shanghai from Oct. 1 to Nov. 16, 2012. During the field campaign, 7 haze episodes (total 157 h) were identified. 845 h were identified as non-haze periods, excluding fog events and wet precipitation. The average mass concentration of PM1 and total water-soluble ions (TWSI) in PM1 in haze episodes were 78.9 ± 29.9 µg/m(3) and 47.2 ± 17.2 µg/m(3), 3.11 times (from 1.49 to 4.06 times) and 3.28 times (1.96 to 4.34 times) as those in non-haze periods, respectively. TWSI accounted for 60.4 ± 18.8% of PM1 mass loading in the whole campaign. With the ascending PM1 mass concentration from 2.5 to 125.0 µg/m(3) from non-haze periods to haze episodes, average contribution of TWSI to PM1 mass loading decreased from 86.1% to 54.2%, while different species altered. Contribution of NO3(-) increased from 14.0% to 26.8%, while SO4(2-) decreased from 39.5% to 15.0% and NH4(+) remained around 13.7%. Relationship of visibility with PM1 and TWSI was addressed in specific RH ranges. It was found that hourly TWSI mass concentration showed better correlation with visibility. Formation/dissociation of semi-volatiles (NH4NO3 and NH4Cl) was also investigated and demonstrated. NH4NO3 and NH4Cl tended to partition into gas phase in non-haze periods. Particularly, strong dissociation from 11:00 LT to 17:00 LT was observed. In haze episodes, HNO3 and HCl tended to react with NH3 to form particulate matters. Interestingly, we found that formation/dissociation of NH4NO3 and NH4Cl exerted great impacts on visibility. Excluding the strong dissociation hours (11:00 LT to 17:00 LT) in correlation analysis of PM1 and visibility, correlation coefficients (R(2)) increased from 0.5762 to 0.7738 at RH<50%. No significant difference was observed in other RH ranges. In addition, Strong NH3 and HNO3 reaction resulted in the enhancement of NH4NO3 mass fraction, therefore increased associated water content in PM1 under high RH condition and contributed to visibility degradation.