Dominant contribution of combustion-related ammonium during haze pollution in Beijing.

Aerosol ammonium (NH4+), mainly produced from the reactions of ammonia (NH3) with acids in the atmosphere, has significant impacts on air pollution, radiative forcing, and human health. Understanding the source and formation mechanism of NH4+ can provide scientific insights into air quality improvements. However, the sources of NH3 in urban areas are not well understood, and few studies focus on NH3/NH4+ at different heights within the atmospheric boundary layer, which hinders a comprehensive understanding of aerosol NH4+. In this study, we perform both field observation and modeling studies (the Community Multiscale Air Quality, CMAQ) to investigate regional NH3 emission sources and vertically resolved NH4+ formation mechanisms during the winter in Beijing. Both stable nitrogen isotope analyses and CMAQ model suggest that combustion-related NH3 emissions, including fossil fuel sources, NH3 slip, and biomass burning, are important sources of aerosol NH4+ with more than 60% contribution occurring on heavily polluted days. In contrast, volatilization-related NH3 sources (livestock breeding, N-fertilizer application, and human waste) are dominant on clean days. Combustion-related NH3 is mostly local from Beijing, and biomass burning is likely an important NH3 source (∼15%-20%) that was previously overlooked. More effective control strategies such as the two-product (e.g., reducing both SO2 and NH3) control policy should be considered to improve air quality.

Tracer-based characterization of fine carbonaceous aerosol in Beijing during a strict emission control period.

Strict emission controls were implemented in Beijing and the surrounding regions in the North China Plain to guarantee good air quality during the 2014 Asia-Pacific Economic Cooperation (APEC) summit. Thus, the APEC period provides a good opportunity to study the sources and formation processes of atmospheric organic aerosol. Here, fine particles (PM2.5, particulate matter with a diameter of 2.5 µm or less) collected in urban Beijing before and during the APEC period were analyzed for molecular tracers of primary and secondary organic aerosol (SOA). The primary organic carbon (POC) and secondary organic carbon (SOC) were also reconstructed using a tracer-based method. The concentrations of biogenic SOA tracers ranged from 1.09 to 34.5 ng m-3 (mean 10.3 ± 8.51 ng m-3). Monoterpene oxidation products were the largest contributor to biogenic SOA, followed by isoprene- and sesquiterpene-derived SOA. The concentrations of biogenic SOA tracers decreased by 50 % during the APEC, which was largely attributed to the implementation of emission controls by the Chinese government. The increasing mass fractions of biogenic SOA tracers from isoprene and sesquiterpene during the pollution episodes implied that their photooxidation processes contributed to the poor air quality in urban Beijing. The reconstructed biogenic and anthropogenic SOC and POC concentrations were 89.6 ± 96.8 ng m-3, 570 ± 611 ng m-3, and 2.49 ± 2.08 µg m-3, respectively, accounting for 21.9 ± 11.4 % of OC in total. Biomass-burning derived OC was the largest contributor to carbonaceous aerosol over the North China Plain. By comparing the results before and during the APEC, the emission controls effectively mitigated about 34 % of the estimated OC and were more effective at reducing SOC than POC. This suggests that the reduction of the primary organic aerosol loading is harder than SOA over the North China Plain.

Biological and Nonbiological Sources of Fluorescent Aerosol Particles in the Urban Atmosphere.

Online detection of bioaerosols based on the light-induced fluorescence (LIF) technique is still challenging due to the complexity of bioaerosols and the external/internal mixing with nonbiological fluorescent compositions. Although many lab studies have measured the fluorescence properties of the biological and nonbiological materials, there is still a scarcity of knowledge of the sources of fluorescent aerosol particles (FAP) in the ambient atmosphere. Here, we fill this gap by combining the online measurement of an LIF-based instrument (wideband integrated bioaerosol sensor, WIBS, 0.8-20 µm) with the measurements of typical biological matter and the compositions related to major nonbiological FAP from May to July in the megacity Beijing. We find that fungal spores and pollen are widely observed in all types of FAP using a WIBS. Bacteria are suggested to be associated with the fine mode FAP (excitation/emission: 280 nm/310-400 nm; 0.8-3 µm). The FL-B and -BC particles (emission in 420-650 nm) contributing the most to FAP are strongly associated with humic-like substances, dust, burning and combustion emissions, and secondary organic aerosols (SOA). This study provides a guide for interpreting individual FAP measured by LIF instruments and points to the applicability of online LIF instruments to characterize nonbiological compositions including SOA.

Vertical profile of particle hygroscopicity and CCN effectiveness during winter in Beijing: insight into the hygroscopicity transition threshold of black carbon.

The hygroscopicity and ability of aerosol particles to act as cloud condensation nuclei (CCN) is important in determining their lifetime and role in aerosol-cloud interactions, thereby influencing cloud formation and climate. Previous studies have used the aerosol hygroscopic properties measured at the ground to evaluate the influence on cloud formation in the atmosphere, which may introduce uncertainty associated with aerosol hygroscopicity variability with altitude. In this study, the CCN behaviour and hygroscopic properties of daily filter collections of PM2.5 from three different heights (8, 120, 260 m) on a tower in Beijing were determined in the laboratory using water, water/methanol and methanol as the atomization solvents. Whilst there was substantial temporal variability in particle concentration and composition, there was little obvious difference in aerosol CCN and hygroscopic behaviour at different heights, although the planetary boundary layer height (PBLH) reduced to below the tower height during the nighttime, suggesting that use of surface hygroscopicity measurements is sufficient for the estimation of aerosol particle activation in clouds. Additionally, the critical coating thickness (in terms of mass ratio of coating/refractory BC, MRc) defining the BC transition between being hydrophobic to hydrophilic, was determined by combining hygroscopic tandem differential mobility analyser (H-TDMA), centrifugal particle mass analyzer (CPMA) and single particle soot photometer (SP2) measurements. The MRc of 250 nm BC-containing particles increased from a background value of between 0.8 and 1.6 to around 4.6 at the onset of the growth event of nanoparticles, decreasing monotonically back to the background level as the event progressed. This indicates that large particles do not act as an effective pre-existing condensation sink of the hygroscopic vapours during the nanoparticle growth events, leading to the 250 nm BC particles requiring more coating materials to transition between being hydrophobic and hydrophilic. These findings show that large particles may be less important in suppressing the new particle formation and subsequent growth in the atmosphere.

Molecular characterization of size-segregated organic aerosols in the urban boundary layer in wintertime Beijing by FT-ICR MS.

Organic aerosols, complicated mixtures of organic compounds, are important constituents of atmospheric particulate matter. However, little is known about the size distributions and vertical profiles of these constituents at a molecular level in the urban boundary layer. Here, we characterized the molecular compositions of size-segregated samples collected simultaneously at two heights (8 m and 260 m above ground level) in urban Beijing during the winter of 2018. The CHO, CHNO, CHOS, and CHNOS subgroups in water-soluble organic carbon were characterized using a 15-T ultrahigh-resolution Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometer. We found that both their numbers and magnitudes increased with a decrease in the particle size, especially for high molecular weight (HMW) compounds, except CHNOS. The number of CHNOS species also increased in the coarse mode, presumably because the alkalinity could inhibit their hydrolysis in the coarse mode. The compounds in small particles with higher O/C ratios and carbon oxidation state were possibly more aged, while the coarse particles with more lipid- and peptide-like compounds should originate from fresh emissions. Moreover, as the oxidation state increases in small particles, functionalization is enhanced for sulfur-containing compounds with fracturing of the benzene ring, while CHO and CHNO are potentially dominated by demethylation with ring-retaining products. It is worth noting that common compounds with the same molecular characteristics accounted for more than 86% of the total compounds between 260 m and ground level (8 m), demonstrating that the aerosols were well mixed in the urban boundary layer. Nonetheless, the relative content of the compounds was higher at ground level due to the impact of primary emissions, which increased with the particle size. In addition, the compounds in submicron particles were more oxidized at 260 m, while the opposite was observed in the coarse mode.

Summertime fluorescent bioaerosol particles in the coastal megacity Tianjin, North China.

Primary biological particles are an important subset of atmospheric aerosols. They have significant impacts on climate change and public health. Tianjin is a coastal megacity in the North China Plain, which is affected by both anthropogenic activities and marine air masses. To study the abundance and dynamic change of bioaerosols in Tianjin, fluorescent biological aerosol particles (FBAPs) in Tianjin were investigated by a wideband integrated bioaerosol sensor (WIBS-4A) in terms of number concentrations and size distributions in summer (11th -25th August 2018). Meanwhile, total suspended particles were collected and analyzed for chemical compounds to identify potential sources of bioaerosols. WIBS data showed that fluorescent biological particles exhibited two peaks at sunrise (~7:00) and in the evening (~20:00), which were probably caused by the enhancement of fungal spores and bacteria. Three rain events occurred during the observation period. Precipitation enhanced the abundance of biological particles, which were likely released from vegetation leaves, resuspended from soil surfaces, and/or carried by raindrops from high altitudes. The abundance of biological particles showed no significant correlation with Na+ (r = -0.17), indicating the air masses from marine areas carried limited biological particles compared to those from continental areas.

High daytime abundance of primary organic aerosols over Mt. Emei, Southwest China in summer.

Organic molecular composition of fine aerosols in the free troposphere is poorly understood. Here, PM2.5 (particles with aerodynamic diameters ≤ 2.5 µm) samples were collected at the summit of Mt. Emei (3080 m a.s.l.) in the Southwestern China on a daytime and nighttime basis during summer 2016 (June-July). The samples were analyzed by solvent-extraction followed by derivatization and gas chromatography/mass spectrometry (GC/MS). Four classes of organic compounds, i.e. n-alkanes, fatty acids, saccharides and lignin/resin acids were measured quantitatively. Fatty acids were found to be the most abundant species with an average concentration of 401 ±â€¯419 ng m-3 (range 25.7-1490 ng m-3) in the daytime, similar to the average concentration at night (399 ±â€¯447 ng m-3, 19.6-1970 ng m-3). However, the concentrations of biomass burning tracers (e.g., levoglucosan), primary biological aerosol tracers (e.g., mannitol and arabitol) and low molecular weight n-alkanes derived from fossil fuel combustion in daytime samples were obviously higher than those in nighttime samples. The results suggest that valley breezes transported a large number of aerosols and their precursors from the ground surface to the summit of Mt. Emei in the daytime. Estimated with tracer-based methods, the contributions of biogenic primary sources (plant debris, fungal spore, and biomass burning) to organic carbon was in the range of 3.28-83.5% (22.0 ±â€¯17.5%) in the daytime and 3.45-37.4% (10.9 ±â€¯8.97%) at night. As the largest contributor, biomass burning was an important anthropogenic/natural source of aerosol particles in the free troposphere over Mt. Emei. CAPSULE: Valley/mountain breeze is an important constraint to the temporal variations in organic aerosols over Mt. Emei.

Temporal characteristics and vertical distribution of atmospheric ammonia and ammonium in winter in Beijing.

To understand the temporal characteristics and vertical distributions of ammonia (NH3) and ammonium (NH4) in urban Beijing, we conducted ground-based and tower-based measurements of gaseous NH3 and submicron aerosol composition. The average mixing ratio of NH3 was 16.5 ±â€¯7.4 ppb, ranging from 3.8 to 36.9 ppb. Gas-to-particle partitioning of NHx (=NH3 + NH4) played a significant role on NH3 concentration as the molar ratio of NH3 to NHx decreased as a function of NH4 concentration. The NH3 concentrations increased as a function of PM1 at lower levels (<125 µg m-3), but remained relatively constant at higher PM and NH4 levels, indicating an enhanced gas-to-particle conversion of NH3 during highly polluted conditions. The potential sources of NHx were found to include fossil fuel combustion and biomass burning. Regional transport could also play an important role on NH3 concentration during the formation stage of haze episodes due to particle-to-gas conversion. Four distinctive types of vertical profiles (87% of the time) of both NH3 and fine particle light extinction coefficient (bext) were observed and they were associated with well-mixed atmosphere, fast accumulation of local emissions, regional transport aloft, and the formation of low urban boundary layer, respectively. However, the vertical profiles of NH3 typically (96% of the time) showed a more homogeneous characteristic than those of bext below 260 m, except periods with both strong temperature inversion and large aerosol gradient, the formation of urban boundary layer shall cause a significant transition in the vertical distribution of NH3 below 260 m. During highly polluted situations (PM1 > 125 µg m-3), the strong effect of gas-to-particle partitioning of NHx sometimes (7% of the time) caused opposite trends in vertical profiles of NH3 and bext.

Water-soluble low molecular weight organics in cloud water at Mt. Tai Mo Shan, Hong Kong.

Cloud-water samples collected at the summit of Mt. Tai Mo Shan (Mt. TMS, 957 m, a.s.l.), Hong Kong in autumn 2016 and spring 2017 were measured for molecular compositions and stable carbon isotope ratios (δ13C) of dicarboxylic acids, oxoacids and α-dicarbonyls. Oxalic acid (C2, 253-1680 µg L-1) was found as the most abundant diacid, followed by succinic acid (C4, 24-656 µg L-1) in autumn and phthalic acid (Ph, 27-363 µg L-1) in spring. Higher concentrations of Ph (192 ±â€¯197 µg L-1) and terephthalic acid (tPh, 31 ±â€¯15 µg L-1) were observed in autumn than those in spring, illustrating the enhanced contribution from fossil fuel combustion and plastic wastes burning. Stronger correlations for the shorter chain diacids (C2-C4) with NO3-, nss-SO42- and nss-K+ in autumn (R2 ≥ 0.7) than spring suggested that these diacids were mainly produced via atmospheric photooxidation following anthropogenic emissions. The δ13C values of C2 (mean - 14.7‰), glyoxylic acid (ωC2, -12.2‰), pyruvic acid (Pyr, -15.5‰), glyoxal (Gly, -13.5‰) were much higher than those in atmospheric aerosols from isoprene and other precursors, indicating that diacids, oxoacids and α-dicarbonyls in cloud at Mt. TMS were significantly influenced by photochemical formation during the long-range atmospheric transport.

The organic molecular composition, diurnal variation, and stable carbon isotope ratios of PM2.5 in Beijing during the 2014 APEC summit.

Organic tracers are useful for investigating the sources of carbonaceous aerosols but there are still no adequate studies in China. To obtain insights into the diurnal variations, properties, and the influence of regional emission controls on carbonaceous aerosols in Beijing, day-/nighttime PM2.5 samples were collected before (Oct. 15th - Nov. 2nd) and during (Nov. 3rd - Nov. 12th) the 2014 Asia-Pacific Economic Cooperation (APEC) summit. Eleven organic compound classes were analysed using gas chromatography/mass spectrometry (GC/MS). In addition, the stable carbon isotope ratios (δ13CTC) of total carbon (TC) were detected using an elemental analyser/isotope ratio mass spectrometry (EA/irMS). Most of the organic compounds were more abundant during the night than in the daytime, and their concentrations generally decreased during the APEC. These features were associated with the strict regional emission controls and meteorological conditions. The day/night variations of δ13CTC were smaller during the APEC than those before the APEC the summit, suggesting that regionally transported aerosols are potentially played an important role in the loading of organic aerosols in Beijing before the APEC summit. The source apportionment based on the organic tracers suggested that biomass burning, plastic and microbial emissions, and fossil fuel combustion were important sources of organic aerosols in Beijing. Furthermore, a similar contribution of biomass burning to OC before and during the APEC suggests biomass burning was a persistent contributor to PM2.5 in Beijing and its surroundings.

Stable sulfur isotope ratios and chemical compositions of fine aerosols (PM2.5) in Beijing, China.

Pervasive particulate pollution has been observed over large areas of the North China Plain. The high level of sulfate, a major component in fine particles, is pronounced during heavy pollution periods. Being different from source apportionments by atmospheric chemistry-transport model and receptor modeling methods, here we utilize sulfur isotopes to discern the potential emission sources. Sixty-five daily PM2.5 samples were collected at an urban site in Beijing between September 2013 and July 2014. Inorganic ions, organic/elemental carbon and stable sulfur isotopes of sulfate were analyzed. The "fingerprint" characteristics of stable sulfur isotopic composition, together with trajectory clustering modeled by HYSPLIT-4 (HYbrid Single-Particle Lagrangian Integrated Trajectory) and FLEXPART ("FLEXible PARTicle dispersion model"), was employed to identify potential aerosol sources in Beijing. Results exhibited a distinctive seasonality with sulfate, nitrate, ammonium, organic matter, and element carbon being the dominant species of PM2.5. Elevated concentrations of chloride with high organic matter were found in autumn and winter as a result of enhanced fossil fuel (mainly coal) combustion. The δ34S values of the Beijing aerosols ranged from 2.8‰ to 9.9‰ with an average of 6.0 ±â€¯1.8‰, further indicating that the major sulfur source was direct coal burning emission. Owing to the changing patterns between oxidation pathways of S(IV) in different seasons, δ34S values varied with a winter maximum (8.2 ±â€¯1.1‰) and a summer minimum (4.9 ±â€¯1.9‰). The results of trajectory clustering and FLEXPART demonstrated that higher concentrations of sulfate with lower sulfur isotope ratios (4.6 ±â€¯0.8‰) were associated with air masses from the south or east, whereas lower sulfate concentrations with heavier sulfur isotope ratios (6.7 ±â€¯1.6‰) were observed when the air masses were mainly from the north or northwest. These results suggested that the fine aerosol pollution in Beijing, especially sulfate pollution, was mainly due to coal combustion sources from regional and local regions.

Impacts of springtime biomass burning in the northern Southeast Asia on marine organic aerosols over the Gulf of Tonkin, China.

Fine particles (PM2.5) samples, collected at Weizhou Island over the Gulf of Tonkin on a daytime and nighttime basis in the spring of 2015, were analyzed for primary and secondary organic tracers, together with organic carbon (OC), elemental carbon (EC), and stable carbon isotopic composition (δ13C) of total carbon (TC). Five organic compound classes, including saccharides, lignin/resin products, fatty acids, biogenic SOA tracers and phthalic acids, were quantified by gas chromatography/mass spectrometry (GC/MS). Levoglucosan was the most abundant organic species, indicating that the sampling site was under strong influence of biomass burning. Based on the tracer-based methods, the biomass-burning-derived fraction was estimated to be the dominant contributor to aerosol OC, accounting for 15.7% ±â€¯11.1% and 22.2% ±â€¯17.4% of OC in daytime and nighttime samples, respectively. In two episodes E1 and E2, organic aerosols characterized by elevated concentrations of levoglucosan as well as its isomers, sugar compounds, lignin products, high molecular weight (HMW) fatty acids and ß-caryophyllinic acid, were attributed to the influence of intensive biomass burning in the northern Southeast Asia (SEA). However, the discrepancies in the ratios of levoglucosan to mannosan (L/M) and OC (L/OC) as well as the δ13C values suggest the type of biomass burning and the sources of organic aerosols in E1 and E2 were different. Hardwood and/or C4 plants were the major burning materials in E1, while burning of softwood and/or C3 plants played important role in E2. Furthermore, more complex sources and enhanced secondary contribution were found to play a part in organic aerosols in E2. This study highlights the significant influence of springtime biomass burning in the northern SEA to the organic molecular compositions of marine aerosols over the Gulf of Tonkin.

Springtime precipitation effects on the abundance of fluorescent biological aerosol particles and HULIS in Beijing.

Bioaerosols and humic-like substances (HULIS) are important components of atmospheric aerosols, which can affect regional climate by acting as cloud condensation nuclei and some of which can damage human health. Up to date, release of bioaerosols and HULIS initiated by precipitation is still poorly understood. Here we present different release processes for bioaerosols, non-bioaerosols and HULIS during a precipitation event in Beijing, China. Large fungal-spore-like aerosols were emitted at the onset and later weak stage of precipitation, the number concentration of which increased by more than two folds, while the number concentration of bacteria-like particles doubled when the precipitation strengthened. Besides, a good correlation between protein-like substances that were measured simultaneously by on-line and off-line fluorescence techniques consolidated their applications to measure bioaerosols. Furthermore, our EEM results suggest that the relative contribution of water-soluble HULIS to microbial materials was enhanced gradually by the rain event.