Winter Haze Period Research Articles

Quantifying HONO Production from Nitrate Photolysis in a Polluted Atmosphere.

The photolysis of particulate nitrate (pNO3-) has been suggested to be an important source of nitrous acid (HONO) in the troposphere. However, determining the photolysis rate constant of pNO3- (jpNO3-) suffers from high uncertainty. Prior laboratory measurements of jpNO3- using aerosol filters have been complicated by the "shadow effect"─a phenomenon of light extinction within aerosol layers that potentially skews these measurements. We developed a method to correct the shadow effect on the photolysis rate constant of pNO3- for HONO production (jpNO3-→HONO) using aerosol filters with identical chemical compositions but different aerosol loadings. We applied the method to quantify jpNO3-→HONO over the North China Plain (NCP) during the winter haze period. After correcting for the shadow effect, the normalized average jpNO3-→HONO at 5 °C increased from 5.89 × 10-6 s-1 to 1.72 × 10-5 s-1. The jpNO3-→HONO decreased with increasing pH and nitrate proportions in PM2.5 and had no correlation with nitrate concentrations. A parametrization for jpNO3-→HONO was developed for model simulation of HONO production in NCP and similar environments.

Assessing PM2.5 Dynamics and Source Contributions in Southwestern China: Insights from Winter Haze Analysis

Despite enhancements in pollution control measures in southwestern China, detailed assessments of PM2.5 dynamics following the implementation of the Clean Air Action remain limited. This study explores the PM2.5 concentrations and their chemical compositions during the winter haze period of 2017 across four major urban centers—Chengdu, Chongqing, Guiyang, and Kunming. Significant variability in mean PM2.5 concentrations was observed: Chengdu (71.8 μg m−3) and Chongqing (53.3 μg m−3) recorded the highest levels, substantially exceeding national air quality standards, while Guiyang and Kunming reported lower concentrations, suggestive of comparatively milder pollution. The analysis revealed that sulfate, nitrate, and ammonium (collectively referred to as SNA) constituted a substantial portion of the PM2.5 mass—47.2% in Chengdu, 62.2% in Chongqing, 59.9% in Guiyang, and 32.0% in Kunming—highlighting the critical role of secondary aerosol formation. The ratio of NO3−/SO42− and nitrogen oxidation ratio to sulfur oxidation ratio (NOR/SOR) indicate a significant transformation of NO2 under conditions of heavy pollution, with nitrate formation playing an increasingly central role in the haze dynamics, particularly in Chengdu and Chongqing. Utilizing PMF for source apportionment, in Chengdu, vehicle emissions were the predominant contributor, accounting for 33.1%. Chongqing showed a similar profile, with secondary aerosols constituting 36%, followed closely by vehicle emissions. In contrast, Guiyang’s PM2.5 burden was heavily influenced by coal combustion, which contributed 46.3%, reflecting the city’s strong industrial base. Kunming presented a more balanced source distribution. Back trajectory analysis further confirmed the regional transport of pollutants, illustrating the complex interplay between local and distant sources. These insights underscore the need for tailored, region-specific air quality management strategies in southwestern China, thereby enhancing our understanding of the multifaceted sources and dynamics of PM2.5 pollution amidst ongoing urban and industrial development.

Influence of aerosol on photosynthetically active radiation under haze conditions

Photosynthetically active radiation (PAR, 400-700 nm), as a sub-band in solar shortwave radiation (280-2800 nm), has profoundly affected vegetation primary productivity and biogeochemical cycles of carbon. However, few previous studies focused on the quantitative effects of aerosols on PAR, a sub-band of the shortwave spectrum, during the winter haze period. Based on ground-based remote sensing observations, we have quantitatively investigated the role of aerosol in PAR variation under haze period through radiative transfer simulations at Wuhan, China. The results indicate that direct solar PAR irradiance is attenuated more dramatically and diffuse downward irradiance is enhanced weaklier as aerosol optical depth (AOD) increases in the haze period, which leads to a result of more significant surface aerosol radiative effect (ARE) in PAR spectrum (AREPAR). Simultaneously, the percentage of AREPAR in ARESW spectrum decreases by 3.33%, indicating that PAR is attenuated more than other sub-bands in SW as pollution enhances. Meanwhile, the ARE efficiency (REE) in PAR weakens from -122 W/m2 under the clear-air condition to -80 W/m2 during the severe-pollution period. These phenomena are attributed to stronger forward scattering in PAR due to the hygroscopic growth of fine-mode aerosol particles during haze. The results provides support for further studies on the effect of pollutants on PAR, and then vegetation productivity.

Hydroxymethanesulfonate formation as a significant pathway of transformation of SO2

The transformation mechanism of sulfur dioxide (SO2), especially during haze periods, is unclear, and hydroxymethanesulfonate (HMS) formation is recently recommended to be one of the important pathways for the conversion of SO2, while detailed laboratory studies under different ionic strength conditions are still lacking. Here we have experimentally discovered that the formation rate constant of HMS in bulk water and aerosol water is significantly influenced by the ionic strength, which is about 2–3 orders of magnitude larger in aerosol water but about 2 orders of magnitude smaller in cloud and fog than previously recognized. The HMS formation rate even exceeds the sulfate formation from oxidation of dissolved SO2 by H2O2 in aerosol water with a pH larger than 5.1. This study provides an explanation for the elevated particulate sulfur formation rate during winter haze periods.

A one-year study on black carbon in urban Beijing: Concentrations, sources and implications on visibility

A one-year field measurement was conducted at an urban atmospheric environment monitoring station in Beijing. The concentrations, sources and implications on visibility of black carbon (BC) during different periods were investigated. The average BC concentration was 2.8 ± 2.5, 3.2 ± 1.9, 4.4 ± 3.6 and 5.3 ± 4.5 μg m−3 in spring, summer, autumn and winter, respectively. The source apportionment presented liquid fuel (traffic) was the main source of BC in spring (77.4%), summer (86.8%) and autumn (70.8%), while solid fuel (coal and biomass burning) dominated in winter (64.3%), implying regional air transport occurred because of the limited combustion activities in urban Beijing. Notably, the contribution of solid fuel decreased from Ⅲ (4 ≤ Vis <6 km) to Ⅴ (0 < Vis <2 km), indicating the effect of regional air transport weakened during the wintertime severe haze pollution. Combining the results of PSCF, BC during the winter haze period was not only influenced by the air masses from southern regions but also western areas, e.g., Datong city, where coal is still widely used. The ratio of σabs/Bext (light absorption coefficient/atmospheric extinction coefficient) during the haze periods was higher than that of non-haze periods in spring, autumn and winter, proving higher BC concentration favored the decline in visibility. However, in all seasons, the decreasing ratio of σabs/Bext with the haze pollution getting worse, indicated that BC contributed less and other components (e.g., SO42−, NO3− and organics) contributed more to the visibility reduction when severe haze event occurred.

The effect of aerosol on downward diffuse radiation during winter haze in Wuhan, China

The effect of aerosol on diffuse radiation (DR) are examined using ground-based observation and radiative transfer simulation at Wuhan during the winter haze periods of 2015 and 2016. The results show that the DR values change with the severity of the pollution, from 156.51 W/m2 under clean air conditions to 157.52 W/m2 during severe pollution periods, while the ratio of DR to global radiation increases from 57% to 73% accordingly. Both DR and the ratio of DR to global radiation are significantly correlated with aerosol optical depth (AOD), with determination coefficient being 0.67 and 0.92, respectively. In addition, the aerosol forward scattering increased along with clean air to serious pollution, showing a logarithmic growth relationship with the effective radius of the fine-mode particles. The aerosol diffuse radiative effect (ADRE) was introduced to quantify aerosol's effect on DR, which increased from 122.83 W/m2 under clean air condition to 152.84 W/m2 during common pollution period and 180.99 W/m2 during severe pollution period. The sensitivity test of ADRE to aerosol optical properties show that the increase of ADRE is dominated by increase in AOD, and also related to increase in asymmetry factor (ASY) due to hygroscopic growth of fine mode particles under pollution conditions. The above findings enhance our understanding of the mechanisms governing ADRE variation during haze periods and favor the accurate evaluation of both the diffuse radiative effect of aerosols on the climate.

Rapid mass growth and enhanced light extinction of atmospheric aerosols during the heating season haze episodes in Beijing revealed by aerosol–chemistry–radiation–boundary layer interaction

Abstract. Despite the numerous studies investigating haze formation mechanism in China, it is still puzzling that intensive haze episodes could form within hours directly following relatively clean periods. Haze has been suggested to be initiated by the variation of meteorological parameters and then to be substantially enhanced by aerosol–radiation–boundary layer feedback. However, knowledge on the detailed chemical processes and the driving factors for extensive aerosol mass accumulation during the feedback is still scarce. Here, the dependency of the aerosol number size distribution, mass concentration and chemical composition on the daytime mixing layer height (MLH) in urban Beijing is investigated. The size distribution and chemical composition-resolved dry aerosol light extinction is also explored. The results indicate that the aerosol mass concentration and fraction of nitrate increased dramatically when the MLH decreased from high to low conditions, corresponding to relatively clean and polluted conditions, respectively. Particles having their dry diameters in the size of ∼400–700 nm, and especially particle-phase ammonium nitrate and liquid water, contributed greatly to visibility degradation during the winter haze periods. The dependency of aerosol composition on the MLH revealed that ammonium nitrate and aerosol water content increased the most during low MLH conditions, which may have further triggered enhanced formation of sulfate and organic aerosol via heterogeneous reactions. As a result, more sulfate, nitrate and water-soluble organics were formed, leading to an enhanced water uptake ability and increased light extinction by the aerosols. The results of this study contribute towards a more detailed understanding of the aerosol–chemistry–radiation–boundary layer feedback that is likely to be responsible for explosive aerosol mass growth events in urban Beijing.

Enhancing effect of NO2 on the formation of light-absorbing secondary organic aerosols from toluene photooxidation

Aromatic hydrocarbons are one of the major precursors of atmospheric brown carbon (BrC) and both abundantly co-exist with NOx in the urban atmosphere especially in winter haze period. However, the impact of NOx on the formation of BrC derived from aromatic hydrocarbons is still not fully understood. In this study, the yield and light absorption of secondary organic aerosols (SOA) from toluene photooxidation under various nitrogen oxides (NO2) levels were investigated by using a 5 m3 photooxidation smog chamber. A trend of increase at first and then decrease in the SOA yield with an increasing NO2 concentration was observed. The acid-catalyzed heterogeneous reactions lead to the increase of SOA yield in the low-NO2 regime. The formation of low-volatility species might be suppressed at high-NO2 conditions is responsible for the decreased SOA yield. In contrast, light absorption and mass absorption coefficient (MAC) of the toluene-derived SOA continuously increased with the increasing NO2 concentrations. HR-ToF-AMS results showed that nitrogen-containing organic compounds (NOCs) are the main species that lead to the increase of the SOA light absorption. The ratio of CHN family to the total NOCs, which are derived from the nitro compounds, also increased dominantly with the increasing NO2 levels and accounted for more than half of the total NOCs when the NO2 concentration increased to 495 ppbv, indicating that nitro compounds rather than organic nitrates are the major light-absorbing species and preferably formed in the toluene oxidation process.

Rapid sulfate formation from synergetic oxidation of SO2 by O3 and NO2 under ammonia-rich conditions: Implications for the explosive growth of atmospheric PM2.5 during haze events in China

Extremely high levels of atmospheric sulfate aerosols have still frequently occurred in China especially in winter haze periods and often been underestimated by models due to some missing formation mechanisms. Here we investigated the heterogeneous reaction dynamics of SO2 oxidation by the abundantly co-existing O3 and NO2 in the urban atmosphere of China by using a laboratory smog chamber simulation technique. Our results showed that with an increase of NH3 concentrations from 0.05 ppm to 1.5 ppm, SO2 oxidation by O3 can be greatly promoted and lead to an exponential increase of diameter growth factor (GF) of particles in the chamber from 1.29 to 1.98 for NaCl seeds and from 1.20 to 1.60 for (NH4)2SO4 seeds, along with an increasing uptake coefficient (γ) of SO2 from 4.47 × 10−5 to 1.52 × 10−4 on NaCl seeds and from 2.32 × 10−5 to 5.74 × 10−5 on (NH4)2SO4 seeds, respectively. The heterogeneous production of sulfate from oxidation of SO2 under NH3-rich conditions by O3 and NO2 mixture in the chamber was 2.0–3.5 times the sum of sulfate from SO2 oxidations by O3 and NO2, suggesting a strongly synergetic effect of the mixed oxidants on the heterogeneous oxidation of SO2, which can cause rapid formation of (NH4)2SO4 and NH4NO3 and is responsible for the explosive growth of PM2.5 in the winter haze period of China. Our chamber results further showed that such synergetic process is only efficient under NH3-rich conditions, clearly indicating that the combined controls on O3, NOx and NH3 are necessary for further mitigating the PM2.5 pollution in China.

TEMPORAL-SPATIAL CHANGES IN AEROSOLS DURING WINTER HAZE OF WUHAN： A TYPICAL HIGH-HUMID INLAND CITY IN CENTRAL CHINA

Abstract. Temporal and spatial changes of aerosols characteristics was investigated to improve understandings of haze in winters of Wuhan, based on measurements of a CIMEL sun-sky radiometer and a modified Dark Target (DT) method. A large increase in no-absorbing and fine-mode particle is the main characteristic of aerosol during winter haze periods. From no-haze to haze conditions, aerosol optical depth (AOD), fine-mode peak value of size distribution, and single scattering albedo increased respectively from 0.46, 0.47 µm3/µm2, and 0.84 to 0.93, 0.91 µm3/µm2, and 0.87. Based on above aerosol optical and microphysical properties, we re-calculated aerosol models and improved the DT method. Results showed that our improvement increased the expect error (%) from 17.9 to 70.9. Spatial distributions revealed that heavy aerosol loading often occurred in rural and suburban areas rather than city centre. With the haze developing, the AOD increased by ∼0.3 over north of Wuhan and showed a downward trend from north to south.

Simultaneous Measurements of Chemical Compositions of Fine Particles during Winter Haze Period in Urban Sites in China and Korea

We performed simultaneous measurements of chemical compositions of fine particles in Beijing, China and Gwangju, Korea to better understand their sources during winter haze period. We identified PM2.5 events in Beijing, possibly caused by a combination of multiple primary combustion sources (biomass burning, coal burning, and vehicle emissions) and secondary aerosol formation under stagnant conditions and/or dust sources under high wind speeds. During the PM2.5 events in Gwangju, the contribution of biomass burning and secondary formation of nitrate and organics to the fine particles content significantly increased under stagnant conditions. We commonly observed the increases of nitrogen-containing organic compounds and biomass burning inorganic (K+) and organic (levoglucosan) markers, suggesting the importance of biomass burning sources during the winter haze events (except dust event cases) at both sites. Pb isotope ratios indicated that the fraction of Pb originated from possibly industry and coal combustion sources increased during the PM2.5 events in Gwangju, relative to nonevent days.

Long‐Term Investigation of Aerosol Optical and Radiative Characteristics in a Typical Megacity of Central China During Winter Haze Periods

AbstractDecadal aerosol characteristics and related radiative effects (ARE) and efficiencies (AREE) at ultraviolet (UV), visible (VIS), near‐infrared (NIR), and shortwave wavelengths were investigated in Wuhan during winter haze periods based on observations from 2007 to 2016. Aerosols changed significantly under haze conditions; the average aerosol optical depth increased from 0.46 to 0.93, and the Ångström exponent increased from 1.12 to 1.23. Fine‐mode particles became dominant, and their peak radius also increased due to hygroscopic growth. The mean value of the UV single‐scattering albedo increased from 0.846 to 0.873, revealing the emergence of numerous fine‐mode, nonabsorbing aerosols. Backward trajectories suggested that local aerosols were partially affected by transported dust from Northwest China, especially in 2015. The interannual variation of the mean AREE at the top of atmosphere showed a gradually increasing tendency, revealing the enhanced potential cooling capacity of aerosols to the Earth‐atmosphere system over Wuhan. The increase in the AREE fraction at UV and VIS wavelengths, and its decrease at NIR wavelengths, indicated that this phenomenon was due to the increased proportion of fine‐mode scattering components in aerosols. The variation in AREE at top of atmosphere depended mainly on the single‐scattering albedo, while the same variation at the surface depended more on particle sizes. A detailed investigation of aerosols during haze periods can help us to further understand localized climate variations and haze‐inducing mechanisms.

Differences in Model Performance and Source Sensitivities for Sulfate Aerosol Resulting from Updates of the Aqueous- and Gas-Phase Oxidation Pathways for a Winter Pollution Episode in Tokyo, Japan

During the Japanese intercomparison study, Japan’s Study for Reference Air Quality Modeling (J-STREAM), it was found that wintertime SO42– concentrations were underestimated over Japan with the Community Multiscale Air Quality (CMAQ) modeling system. Previously, following two development phases, model performance was improved by refining the Fe- and Mn-catalyzed oxidation pathways and by including an additional aqueous-phase pathway via NO2 oxidation. In a third phase, we examined a winter haze period in December 2016, involving a gas-phase oxidation pathway whereby three stabilized Criegee intermediates (SCI) were incorporated into the model. We also included options for a kinetic mass transfer aqueous-phase calculation. According to statistical analysis, simulations compared well with hourly SO42– observations in Tokyo. Source sensitivities for four domestic emission sources (transportation, stationary combustion, fugitive VOC, and agricultural NH3) were investigated. During the haze period, contributions from other sources (overseas and volcanic emissions) dominated, while domestic sources, including transportation and fuel combustion, played a role in enhancing SO42– concentrations around Tokyo Bay. Updating the aqueous phase metal catalyzed and NO2 oxidation pathways lead to increase contribution from other sources, and the additional gas phase SCI chemistry provided a link between fugitive VOC emission and SO42– concentration via changes in O3 concentration.

Characterizing the level, photochemical reactivity, emission, and source contribution of the volatile organic compounds based on PTR-TOF-MS during winter haze period in Beijing, China

The atmospheric volatile organic compounds (VOCs) are the key participators in the formation of secondary organic aerosol and other atmospheric photochemical processes. However, it is still far from clear understanding of the VOCs in Northern China. In this study, VOCs were measured using a proton transfer reaction-time of flight-mass spectrometer (PTR-TOF-MS) in the downtown area of Beijing during a winter haze period in December 2016. The average mixing ratios of VOCs on the haze days were (ppb): methanol (51.75), formaldehyde (41.65), acetaldehyde (15.81), acetone (7.92), C8-aromatics (7.35), toluene (6.11), benzene (4.30), MEK (2.53), acetonitrile (2.05), C9-aromatics (1.54), benzaldehyde (1.12), isoprene (1.00), styrene (0.62). These concentrations represented the largest values among the cities around the world. VOC concentrations varied dramatically, with the largest value mainly appeared on days having the weakest diffusion condition. Due to the different solar radiation in this period, the photochemical reactivity of the VOCs showed the minimum and maximum values on haze and clear days, respectively. The primary anthropogenic emissions contributed more than half of the mixing ratios of most VOCs. However, formaldehyde received 56.6% contribution from the secondary or biogenic sources, and this contribution mainly increased at the noon hours when sunlight was stronger.

Contribution of regional transport to the black carbon aerosol during winter haze period in Beijing

The mass concentrations of atmospheric refractory black carbon (rBC), an important absorber of solar radiation, were continuously measured with a single particle soot photometer (SP2) during wintertime haze period to investigate the transport of pollution to Beijing. The average mass concentration of rBC was 6.1 ± 3.9 μg m−3 during hazy periods, which was 4.7 times higher than it during non-hazy periods. Cluster analysis showed that the air parcels arriving at Beijing mainly originated from the northwest, passed through the south and brought the most polluted air to Beijing. Concentration-weighted trajectory analyses indicated that the central North China Plain were the most likely source region for the rBC that impacted Beijing. Furthermore, the Weather Research and Forecasting-Black Carbon model showed that 71.4–82.0% of the rBC at Beijing was from regional transport during the high rBC episodes and that 47.9–56.8% of the rBC can be attributed to sources in the central North China Plain. These results suggest that regional transport from the central North China Plain, rather than local emissions, was a more important source for rBC pollution in Beijing.

Variation of polycyclic aromatic hydrocarbons in atmospheric PM2.5 during winter haze period around 2014 Chinese Spring Festival at Nanjing: Insights of source changes, air mass direction and firework particle injection

Daily PM2.5 samples were collected at a suburban site of Nanjing around 2014 Chinese Spring Festival (SF) and analyzed for 18 kinds of polycyclic aromatic hydrocarbons (PAHs) by GC–MS. Comparison of PAH concentrations during different periods, with different air mass origins and under different pollution situations was done. Sources were analyzed by diagnostics ratios and principal component analysis (PCA). The threat of PAHs was assessed by BaP equivalent concentrations (BaPeq) and incremental lifetime cancer risk (ILCR). The averaged PAHs for pre-SF, SF and after SF periods were 50.6, 17.2 and 29ngm−3, indicating the variations of PAH sources, with reduced traffic, industrial and construction activities during SF and gradually re-starting of them after-SF. According to PAH mass concentrations, their relative abundance to particles, ratio of PAHs (3-ring+4-ring)/PAHs(5-ring+6-ring), mass concentrations of combustion-derived and carcinogenic PAHs, fireworks burning is an important source for PAHs during SF. The ILCR values for Chinese New Year day were 0.68 and 3.3 per 100,000 exposed children and adults. It suggested the necessity of controlling fireworks burning during Chinese SF period which was always companied with serious regional haze pollution. PAH concentrations exhibited decreasing trend when air masses coming from the following directions as North China Plain (63.9ngm−3)>Central China (53.0ngm−3)>Shandong Peninsula (46.6ngm−3)>Northwest China (18.8ngm−3)>Sea (15.8ngm−3). For different pollution situations, they decreased as haze (44.5ngm−3)>fog-haze (28.4ngm−3)>clear (12.2ngm−3)>fog day (9.2ng m−3). Coal combustion, traffic emission, industrial processes and petroleum (only for non-SF holiday periodss) were the main sources of PM2.5 associated PAHs. Fireworks burning contributed 14.0% of PAHs during SF period. Directly measurement of PAHs from fireworks burning is urgently needed for source apportionment studies in the future.

The impacts of firework burning at the Chinese Spring Festival on air quality: insights of tracers, source evolution and aging processes

Abstract. To understand the impact of firework-burning (FW) particles on air quality and human health during the winter haze period, 39 elements, 10 water-soluble ions and 8 fractions of carbonaceous species in atmospheric PM2.5 in Nanjing were investigated during the 2014 Chinese Spring Festival (SF). Serious regional haze pollution persisted throughout the entire sampling period, with PM2.5 averaging at 113 ± 69 μg m−3 and visibility at 4.8 ± 3.2 km. The holiday effect led to almost all the chemical species decreasing during the SF, except for Al, K, Ba and Sr which were related to FW. The source contributions of coal combustion, vehicle emission and road dust decreased dramatically, whereas FW contributed to about half of the PM2.5 during the SF period. The intensive emission of FW particles on New Year's Eve accounted for 60.1% of the PM2.5. Fireworks also obviously modified the chemical compositions of PM2.5, with 39.3% contributed by increased organic matter, followed by steadily increased loadings of secondary inorganic ions. The aging processes of the FW particles lasted for about 4 days reflected by the variations of Ba, Sr, NH4+, NO3−, SO42− and K+, characterized by heterogeneous reactions of SO2 and NOx on crustal materials directly from FW, the replacement of Cl− by NO3− and SO42−, coating of NO3− and SO42− on soot, formation of secondary organic aerosols and metal-catalyzed formation of NO3− and SO42− at higher relative humidity. During aging, the main contributors to the extinction coefficient shifted from elemental carbon and organic matter to ammonium sulfate. The particles raised higher cancer risk of 1.62 × 10−6 by heavy metals (especially for Cd and As). This study provided detailed composition data and first comprehensive analysis of the aging processes of FW particles during the serious haze pollution period and their potential impact on human health.