Aijun Ding

Chemical characterization of the boundary layer outflow of air pollution to Hong Kong during February–April 2001

[1] As a cooperative effort with the TRACE-P and ACE-Asia intensive in the spring of 2001, trace gases and aerosols were measured at a relatively remote coastal site (Hok Tsui) in southeastern Hong Kong. The main objective of the measurement program was to provide continuous ground-based data in the subtropical region of eastern Asia and to characterize the southward outflow of continental pollution that prevails in the lower atmosphere during early spring. In this paper, we present the results for ozone, CO, NO, NOy ,S O2, 222 Radon, methane and C2–C8 nonmethane hydrocarbons (NMHCs), C1–C2 halocarbons, and C1–C5 alkyl nitrate measurements obtained between 19 February and 30 April 2001. The average mixing ratios of O3, CO, SO2, and NOy were 45 ppbv, 404 ppbv, 1.8 ppbv, and 10.4 ppbv, respectively. The two dominant NMHCs were ethane (mean: 2368 pptv) and ethyne (mean: 1402 pptv), followed by propane (814 pptv), toluene (540 pptv), benzene (492 pptv), ethene (498 pptv), and n-butane (326 pptv). The most abundant halocarbon was CH3Cl (mean: 821 pptv), while 2-BuONO2 and i-PrONO2 were the two dominant alkyl nitrates species with a mean mixing ratio of 20 pptv and 19 pptv, respectively. The levels of trace gases were strongly influenced by the outflow of continental air masses initiated by the passage of cold fronts. The data are segregated into four air mass groups according to the levels of 222 Rn and wind direction, representing fresh continental outflow, coastal, perturbed maritime, and local urban air. Ozone and CO showed a moderate positive correlation (r 2 = 0.4) in the marine air group, characterized by low 222 Rn and CO levels, but they were poorly correlated in the other air mass groups. SO2 and NOy exhibited good correlations (r 2 > 0.6) with each other but were poorly correlated with CO, indicating differences in their emission sources and/or removal processes. CO very strongly correlated with ethyne and benzene (r 2 > 0.85) and also showed good correlations with several other NMHCs. Moreover, CO correlated moderately with a biomass burning tracer (CH3Cl) and an urban/industrial tracer (C2Cl4) indicating the impact of mixed pollution from urban and biomass burning sources. The relationship of CO, SO2, and NOy with the indicator of atmospheric processing, ethyne/ CO and propane/ethane, were also examined. The 2001 data were compared to the results obtained in the same period in 1994 during PEM-West B. The mean ozone level in the spring of 2001 was much higher than during PEM-West B. SO2 also had higher concentrations during TRACE-P, while CO and NOy were comparable during the two campaigns. The observed difference has been discussed in the context of emission changes and variations in meteorology. Although it is difficult to draw definitive conclusions about the extent of the influence of these two factors, it appears that clearer skies and drier conditions may have been responsible for the higher ozone concentrations during the TRACE-P period. INDEX TERMS: 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry;

Source origins, modeled profiles, and apportionments of halogenated hydrocarbons in the greater Pearl River Delta region, southern China

[1] We analyze 16-month data of 13 major halocarbons measured at a southern China coastal site in the greater Pearl River Delta (PRD). A total of 188 canister air samples were collected from August 2001 to December 2002. Overall inspection indicated that CH2Cl2 ,C 2Cl4, and C2HCl3 had similar temporal variations while CFC-11, CFC-12, and CFC-113 showed the same emission patterns during the sampling period. Diurnal variations of halocarbons presented different patterns during ozone episode days, mainly related to emission strength, atmospheric dispersion, and photochemical lifetimes. For further statistics and source appointment, Lagrangian backward particle release simulations were conducted to help understand the potential source regions of all samples and classify them into different categories, including local Hong Kong, inner PRD, continental China, and marineairmasses.WiththeexceptionofHCFC-142b,themixingratiosofallhalocarbonsin marine air were significantly lower than those in urban and regional air (p < 0.01), whereas no significant difference was found between urban Hong Kong and inner PRD regional air, reflecting the dominant impact of the greater PRD regional air on the halocarbon levels. Thehalocarbonlevelsinthisregionweresignificantlyinfluencedbyanthropogenicsources, causing the halocarbon mixing ratios in South China Sea air to be higher than the corresponding background levels, as measured by global surface networks and by airborne missions such as Transport and Chemical Evolution Over the Pacific. Interspecies correlation analysis suggests that CHCl3 is mainly used as a solvent in Hong Kong but mostly as a feedstock for HCFC-22 in the inner PRD. Furthermore, CH3Cl is often used as a refrigerant and emitted from biomass/biofuel burning in the inner PRD. A positive matrix factorization receptor model was applied to the classified halocarbon samples in the greater PRD for source profiles and apportionments. Seven major sources were identified and quantified. Emissions from solvent use were the most significant source of halocarbons (71 ± 9%), while refrigeration was the second largest contributor (18 ± 2%). By further lookingatsamplesfromtheinnerPRDandfromurbanHongKongseparately,wefoundthat more solvent was used in the dry cleaning industry in Hong Kong, whereas the contribution of cleaning solvent in the electronic industry was higher in the inner PRD. Besides the two common sources of solvent use and refrigeration, the contributions of biomass/biofuel burning and feedstock in chemical manufacturing was remarkable in the inner PRD but negligible in Hong Kong. These findings are of help to effectively control and phase out the emissions of halocarbons in the greater PRD region of southern China.

Enhanced haze pollution by black carbon in megacities in China

Aerosol-planetary boundary layer (PBL) interactions have been found to enhance air pollution in megacities in China. We show that black carbon (BC) aerosols play the key role in modifying the PBL meteorology and hence enhancing the haze pollution. With model simulations and data analysis from various field observations in December 2013, we demonstrate that BC induces heating in the PBL, particularly in the upper PBL, and the resulting decreased surface heat flux substantially depresses the development of PBL and consequently enhances the occurrences of extreme haze pollution episodes. We define this process as the “dome effect” of BC and suggest an urgent need for reducing BC emissions as an efficient way to mitigate the extreme haze pollution in megacities of China.

Transport of north China air pollution by midlatitude cyclones: Case study of aircraft measurements in summer 2007

Warm conveyor belts(WCBs) and frontal activity play important roles in the long-range transport of air pollutants by lifting them from the planetary boundary layer (PBL) into the free troposphere ( FT) in midlatitudes. In summer 2007, an aircraft study was carried out in northeast ( NE) China in order to understand the role of midlatitude cyclones in air pollution transport in north and east China in warm seasons. During a flight on 27 June, high concentrations of ozone and related trace gases were observed, with maximum concentrations (O(3) similar to 140 ppbv, SO(2) similar to 14.6 ppbv, CO similar to 1185 ppbv) recorded at an altitude of 2.6 km. In this paper we present a detailed analysis of this flight. The mesoscale meteorological model Weather Research and Forecasting (WRF) and a Lagrangian dispersion model called FLEXPART were used to aid the diagnostic analysis of the atmospheric dynamic structure and the understanding of the transport characteristics of regional and local air pollution. The flight took place in a region adjacent to a warm front associated with a weak cyclone in north China. The aircraft sampled both the WCB and warm air frontal zone of the cyclone. The simulations show that the observed high air pollution in the FT mostly originated from the North China Plain, especially the megacities Beijing and Tianjin. Their plumes were vented by a stagnant front, probably through, in part, topographic lifting by the mountains in the north, and then were quickly transported in the FT to the study region. Trajectory analysis and satellite data suggest that the observed air masses were further lifted by the WCB into the middle and upper troposphere and were exported from Asia toward North America and the Arctic.

Impact of aerosol?meteorology interactions on fine particle pollution during China?s severe haze episode in January 2013

In January 2013, a severe regional haze occurred over the North China Plain. An online-coupled meteorology-chemistry model was employed to simulate the impacts of aerosol?meteorology interactions on fine particles (PM2.5) pollution during this haze episode. The response of PM2.5 to meteorology change constituted a feedback loop whereby planetary boundary layer (PBL) dynamics amplified the initial perturbation of PM2.5. High PM2.5 concentrations caused a decrease of surface solar radiation. The maximal decrease in daily average solar radiation reached 53% in Beijing, thereby leading to a more stable PBL. The peak PBL height in Beijing decreased from 690 m to 590 m when the aerosol extinction was considered. Enhanced PBL stability suppressed the dispersion of air pollutants, and resulted in higher PM2.5 concentrations. The maximal increase of PM2.5 concentrations reached 140 ?g m?3 in Beijing. During most PM2.5 episodes, primary and secondary particles increased simultaneously. These results imply that the aerosol?radiation interactions played an important role in the haze episode in January 2013.

Enhanced air pollution via aerosol-boundary layer feedback in China

Severe air pollution episodes have been frequent in China during the recent years. While high emissions are the primary reason for increasing pollutant concentrations, the ultimate cause for the most severe pollution episodes has remained unclear. Here we show that a high concentration of particulate matter (PM) will enhance the stability of an urban boundary layer, which in turn decreases the boundary layer height and consequently cause further increases in PM concentrations. We estimate the strength of this positive feedback mechanism by combining a new theoretical framework with ambient observations. We show that the feedback remains moderate at fine PM concentrations lower than about 200 μg m−3, but that it becomes increasingly effective at higher PM loadings resulting from the combined effect of high surface PM emissions and massive secondary PM production within the boundary layer. Our analysis explains why air pollution episodes are particularly serious and severe in megacities and during the days when synoptic weather conditions stay constant.

A review of biomass burning: Emissions and impacts on air quality, health and climate in China.

Biomass burning (BB) is a significant air pollution source, with global, regional and local impacts on air quality, public health and climate. Worldwide an extensive range of studies has been conducted on almost all the aspects of BB, including its specific types, on quantification of emissions and on assessing its various impacts. China is one of the countries where the significance of BB has been recognized, and a lot of research efforts devoted to investigate it, however, so far no systematic reviews were conducted to synthesize the information which has been emerging. Therefore the aim of this work was to comprehensively review most of the studies published on this topic in China, including literature concerning field measurements, laboratory studies and the impacts of BB indoors and outdoors in China. In addition, this review provides insights into the role of wildfire and anthropogenic BB on air quality and health globally. Further, we attempted to provide a basis for formulation of policies and regulations by policy makers in China.

Polluted dust promotes new particle formation and growth

Understanding new particle formation and their subsequent growth in the troposphere has a critical impact on our ability to predict atmospheric composition and global climate change. High pre-existing particle loadings have been thought to suppress the formation of new atmospheric aerosol particles due to high condensation and coagulation sinks. Here, based on field measurements at a mountain site in South China, we report, for the first time, in situ observational evidence on new particle formation and growth in remote ambient atmosphere during heavy dust episodes mixed with anthropogenic pollution. Both the formation and growth rates of particles in the diameter range 15–50 nm were enhanced during the dust episodes, indicating the influence of photo-induced, dust surface-mediated reactions and resulting condensable vapor production. This study provides unique in situ observations of heterogeneous photochemical processes inducing new particle formation and growth in the real atmosphere, and suggests an unexpected impact of mineral dust on climate and atmospheric chemistry.

On the relationship between ozone and its precursors in the Pearl River Delta: application of an observation-based model (OBM)

Background, aim, and scopePhotochemical smog, characterized by high concentrations of O3 and fine particles, is of great concern in the urban areas, in particular megacities and city clusters like the Pearl River Delta.Materials, methods, and resultsAmbient ozone (O3) and its precursors were simultaneously measured at two sites in the Pearl River Delta, namely, Wan Qing Sha (WQS) in Guangzhou and Tung Chung (TC) in Hong Kong, from 23 October to 01 December 2007 in order to explore their potential relationship. Eight high O3 episode days were identified at WQS and two at TC during the sampling campaign, indicating a more serious O3 pollution in Guangzhou than in Hong Kong. An observation-based model was employed to determine the ozone–precursor relationship. At both sites, O3 production was found to be volatile organic compound (VOC)-limited, which is consistent with previous observations. Anthropogenic hydrocarbons played a key role in O3 production, while reducing nitric oxide emissions aided the buildup of O3 concentrations. Among VOC species, the summed relative incremental reactivity (RIR) of the top 12 compounds accounted for 89% and 85% of the total RIR at WQS and TC, respectively, indicating that local photochemical O3 formation can be mainly attributed to a small number of VOC species.Discussion and conclusionsA large increment in both simulated HO2 and O3 concentrations was achieved with additional input of hourly carbonyl data. This suggested that apart from hydrocarbons, carbonyls might significantly contribute to the O3 production in the Pearl River Delta.

Source of surface ozone and reactive nitrogen speciation at Mount Waliguan in western China : new insights from the 2006 summer study

measurement period of 2006 (55%) than that of 2003 (25%). The abnormally high values of NOy observed in 2003 were suspected to be due to the positive interference from ammonia (NH3) to the particular catalytic converter used in that study. Varied diurnal patterns were observed for the various NOy components. The ozone production efficiencies (DO3/DNOz), which were estimated from the slope of the O3‐NOz scatterplot, were 7.7–11.3 for the polluted plumes from central and eastern China. The speciation of reactive nitrogen was investigated for the first time in the remote free troposphere in western China. PAN and particulate NO3 − were the most abundant reactive nitrogen species at WLG, with average proportions of 32% and 31%, followed by NOx (24%) and HNO3 (20%). The relatively large contribution of particulate NO3 − to NOy was due to the presence of high concentrations of NH3 and crustal particles, which favor the formation of particulate nitrate. An analysis of backward trajectories for the recent 10 years revealed that air masses from central and eastern China dominated the airflow at WLG in summer, suggesting strong impact of anthropogenic forcing on the surface ozone and other trace constituents on the Plateau.