Interannual relationship between displacement and intensity of East Asian jet stream and haze over eastern China in winter

A recent case study indicated that the weakening of the East Asian subtropical jet was an important cause of the severe haze in North China in the winter of 2015. However, the interannual relationship between two key features, the displacement and the intensity of the East Asian jet stream (EAJS) and the haze days over eastern China (HD_EC), remains unclear. Observed data, ERA-Interim reanalysis, and Community Earth System Model Large Ensemble Numerical Simulation(CESM-LENS) were used to investigate the interannual relationship between the EAJS and HD_EC during the winter season from 1980 to 2017 and its possible associated atmospheric mechanisms. The results show that the northward movement of the EAJS is conducive to more HD_EC by weakening synoptic-scale transient eddy activities (STEA) and baroclinicity, forming an upper-level anticyclonic anomaly over eastern China (EC). The local meteorological conditions (e.g., stronger temperature inversion potential, higher relative humidity, descending motion) are favorable for the accumulation of HD_EC, showing consistent variations in more haze in the entire region of EC. The southward movement of the EAJS has the opposite effect. The strong East Asian subtropical jet (weak polar-front jet) could result in the distribution of the meridional dipole with less haze in north EC and more haze in south EC. The mean flow loses energy to the STEA over north EC and increases the baroclinicity, which is favorable for dispersing HD_EC. However, the configuration of upper-level cyclonic and low-level southwest wind anomalies that appeared in south EC weakened the STEA, which favored the accumulation of HD_EC. The observed results were further verified by CESM-LENS.

Risk evaluation of environmentally persistent free radicals in airborne particulate matter and influence of atmospheric factors

Environmentally persistent free radicals (EPFRs) was considered unrecognized composition of air pollutants and might help explain the long-standing medical mystery of why non-smokers develop tobacco-related diseases like lung cancer. EPFRs in airborne fine particulate matter (PM_2.5) can induce oxidative and DNA damage when inhaled. We assessed the inhalation risk of EPFRs in PM_2.5 and factors influencing this risk in Beijing as a large city with frequent haze events. The average concentration of EPFRs in PM_2.5 was 6.00 × 10¹⁷ spins/m³ in spring, autumn, and winter; lower concentrations were recorded in the summer. To estimate the daily inhalation risk of EPFRs in PM_2.5, we used the equivalent EPFRs in cigarette tar. The average daily inhalation exposure of EPFRs in PM_2.5 was estimated to be the equivalent of 33.1 cigarette tar EPFRs per day (range: 0.53-226.9) during both haze and non-haze days. The major factors influencing EPFR concentrations in the atmosphere were precipitation and humidity, which reduced airborne concentrations. Levels of PM_2.5 and carbon monoxide were positively correlated with EPFR concentrations. The health risks of inhaling airborne EPFRs could be significant and should be recognized and quantified.

Pollution characteristics and source difference of gaseous elemental mercury between haze and non-haze days in winter

The distribution characteristics and sources of gaseous elemental mercury (GEM) on haze and non-haze days are still not clear. During the winter heating period in 2017, the GEM concentrations in Qingdao were studied for their differences, sources, and pollution characteristics on haze and non-haze days. The GEM concentration on haze days (2.81 ± 2.23 ng/m³) was higher than that on non-haze days (1.90 ± 1.21 ng/m³) and the difference was significant (p < 0.01) during the period of artificial heating. The average concentration of GEM was 2.27 ng/m³ in the heating period, but lower than that before heating (3.30 ng/m³). However, the mercury to carbon monoxide ratio (GEM/CO) on haze days was lower than that on non-haze days. The ratio of GEM/CO in this study was lower than that in other studies reported from China. There was a positive correlation between the GEM/CO ratio and the air temperature (p < 0.01), suggesting that the mercury released from the Earth's surface was important. The environmental policies of China also contributed to decrease of the GEM/CO ratio. Similar diurnal patterns appeared on both haze and non-haze days, with one GEM peak at 14:00-15:00. This pattern was different from the bimodal pattern of other atmospheric pollutants in the morning and evening rush hours and was controlled by GEM from the Earth's surface (mostly re-emission of legacy Hg) whether on haze or non-haze days. Principal component analysis showed that the contribution of GEM directly from anthropogenic sources was relatively small. The main influencing factor on haze days was air temperature. GEM concentrations showed large spatial differences in air masses from different places. The GEM concentration in air masses from southern and the western Shandong Province was higher than from the north on haze days.

Investigating the impact of Glyoxal retrieval from MAX-DOAS observations during haze and non-haze conditions in Beijing

This study presents the Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements for Glyoxal (CHOCHO) in Beijing, China (39.95°N, 116.32°E). CHOCHO is the smallest compound of di-carbonyl group. As a primary sink of CHOCHO, its photolysis with NOx (oxides of nitrogen) results in the production of tropospheric ozone. Therefore, the focus of CHOCHO DOAS measurements is increasing in trend. We did the measurements from 09 May 2017 to 09 September 2017. The study was conducted to compare different retrieval settings in order to reveal best DOAS fit settings for CHOCHO; furthermore, effect of haze and non-haze days on CHOCHO concentration was examined. The root mean square of residual and Differential Slant Column density (dSCD) error was reduced when measurements were done with lower wavelength limit around 432-438 nm and upper intervals around 455-460 nm. Thus, lower wavelength intervals around 432-438 nm and upper intervals around 457-460 nm were best for the retrieval of dSCDs for CHOCHO. Meteorological conditions like haze or non-haze days did not have significant effect on DOAS fit parameters. The CHOCHO vertical column densities range from 1.33E+14 to 9.77E+14 molecules/cm² during the study period with average of 6.16E+14 molecules/cm². The results indicated that during haze days CHOCHO concentration was higher because of lower rate of photolysis and atmospheric oxidation potential. Our results did not show any significant weekend effect on CHOCHO atmospheric concentration.

Gas-particle phase partitioning and particle size distribution of chlorinated and brominated polycyclic aromatic hydrocarbons in haze

Chlorinated and brominated polycyclic aromatic hydrocarbons (Cl/Br-PAHs) are emerging semi-volatile organic pollutants in haze-associated particulate matter (PM). Their gas-particle phase partitioning and distribution among PM fractions have not been clarified. Clarification would increase understanding of atmospheric behavior and health risks of Cl/Br-PAHs. In this study, samples of the gas phase and 4 PM phases (aerodynamic diameters (d_ae) > 10 μm, 2.5-10 μm, 1.0-2.5 μm, and <1.0 μm) were collected simultaneously during haze events in Beijing and analyzed. Normalized histogram distribution indicated that the Cl/Br-PAHs tended to adhere to fine particles. Over 80% of the Cl-PAHs and 70% of the Br-PAHs were associated with fine PM (d_ae < 2.5 μm). The gas-particle phase partitioning and PM distribution of Cl/Br-PAHs when heating of buildings was required, which was associated with haze events, were obviously different from those when heating was not required. The relationship between the logarithmic geometric mean diameters of the Cl/Br-PAH congeners and reciprocal of the temperature (1/T) suggested that low air temperatures during the heating period could lead to high proportions of Cl/Br-PAHs in the fine particles. Increased coal burning during the heating period also contributed to high Cl/Br-PAH loads in the fine particles.

Role of climate anomalies on decadal variation in the occurrence of wintertime haze in the Yangtze River Delta, China

The wintertime haze day (HD) in the Yangtze River Delta (YRD) region of China shows a significant upward trend during the past decades due to the rapid industrialization and urbanization. Besides the enhanced anthropogenic emission, climate change also plays the important role in the long term HD variations. In this study, the significant decadal variation of wintertime HD during the period 1960-2012 in YRD is examined by the empirical orthogonal function (EOF) analysis, featured as less HD occurrence before 1980 and more occurrence after 2000. The numerical simulations by the global transport and chemical model (Model for Ozone and Related chemical Tracers, MOZART) with the same emission inventory suggest 8.4% enhancement of wintertime PM_2.5 (particulate matter with the equivalent diameter of air dynamics less than or equal to 2.5μm) mass concentration in YRD during 2001-2009 compared with that during 1971-1979 attributed to meteorological changes, indicating the significant effect of climate anomaly on the decadal variations of wintertime HD. Through the composite analysis on the atmospheric dynamical and thermal conditions based on the reanalysis data, the faster warming in the lower and middle troposphere over the continent in the recent decade is suggested to be important for the out-of-phase decadal HD variation in YRD. The thermal anomaly not only reverses the zonal thermal difference of land-sea to stimulate the anomalous southerlies over YRD leading to reduced prevailing north wind in winter, but also develops the deep inversion below the mid-troposphere to enhance the atmospheric stability. As a result, more frequent and persistent air stagnations in recent decade are expected for the reduction of atmospheric horizontal dispersion and vertical diffusion capacity leading to more occurrence of wintertime HD in YRD.

Clearance capacity of the atmosphere: the reason that the number of haze days reaches a ceiling

China has experienced rapid development in the past 30 years but, alongside and associated with this growth, increased levels of pollution too. However, despite the continued increase in emissions of haze-forming aerosols in the twenty-first century, the annual number of haze days in some megacities has not risen in tandem. Various mechanisms have been proposed for "city dimming", but the cause of the hiatus remains unclear. We found that the number of haze days in Taiyuan experienced a sharp increase during 1980-1998, with a growth rate 51.6 days/10a, and then exhibited fluctuating variation around a stable high level from 1998 to 2014, while at the same time the average visibility during haze days started to decrease. We present a novel method to explain the long-term variation in the number of haze days via a temporal-piecewise function of human activities and atmospheric cleaning processes: the number of haze days increases with the level of human activity before reaching the upper limit and then remains at a high level due to the restriction of a relatively stable number of strong cleaning days.