Haze Pollution Events Research Articles

Characteristics of Chemical Speciation in PM1 in Six Representative Regions in China.

A better knowledge of aerosol properties is of great significance for elucidating the complex mechanisms behind frequently occurring haze pollution events. In this study, we examine the temporal and spatial variations in both PM1 and its major chemical constituents using three-year field measurements that were collected in six representative regions in China between 2012 and 2014. Our results show that both PM1 and its chemical compositions varied significantly in space and time, with high PM1 loadings mainly observed in the winter. By comparing chemical constituents between clean and polluted episodes, we find that the elevated PM1 mass concentration during pollution events should be largely attributable to significant increases in organic matter (OM) and inorganic aerosols like sulfate, nitrate, and ammonium (SNA), indicative of the critical role of primary emissions and secondary aerosols in elevating PM1 pollution levels. The ratios of PM1/PM2.5 are found to be generally high in Shanghai and Guangzhou, while relatively low ratios are seen in Xi’an and Chengdu, indicating anthropogenic emissions were more likely to accumulate in forms of finer particles. With respect to the relative importance of chemical components and meteorological factors quantified via statistical modeling practices, we find that primary emissions and secondary aerosols were the two leading factors contributing to PM1 variations, though meteorological factors also played important roles in regulating the dispersion of atmospheric PM.

Evaluation of WRF-Chem simulations on vertical profiles of PM2.5 with UAV observations during a haze pollution event

Accurate simulation of PM2.5 vertical distribution is crucial to understand the formation, transport, and dispersion mechanisms of aerosols, and protect the public from exposure to high levels of air pollutants. While most of the current model evaluations are limited to near-surface variables, vertical profiles of simulated PM2.5 are rarely evaluated. In this study, the measurements with unmanned aerial vehicle (UAV) are used to evaluate the WRF-Chem simulated vertical profiles of PM2.5, potential temperature, water vapor, and winds while surface meteorological fields and air pollutants are evaluated with various observational data during a heavy aerosol pollution episode that occurred on December 22–25, 2017 in Nanjing, China. Results show that the simulated surface meteorology and air quality agree well with the observations. Vertical profile shapes and magnitude of PM2.5 are reasonably captured by the model under the convective conditions during the daytime, but not in the nighttime. It is noticed that a well-mixed vertical structure of PM2.5 concentration, a typical feature of the daytime convective boundary layer is observed in the nocturnal boundary layer (NBL) but not captured by the WRF-Chem model and PM2.5 concentrations are over-predicted significantly during the nighttime. The over-predictions of surface PM2.5 and vertical profiles are improved substantially by increasing the minimum value of eddy diffusivity. This study indicates the importance of refinement of parameterization schemes in reproducing the ABL vertical structures and PM2.5 concentrations, especially for the NBL with heavy pollution conditions.

Summertime atmospheric dicarboxylic acids and related SOA in the background region of Yangtze River Delta, China: Implications for heterogeneous reaction of oxalic acid with sea salts

The diacid chemistry of summertime PM2.5 and the size-segregated aerosols (9-stages) in Chongming Island, a coastal site in the Yangtze River Delta (YRD), China, were investigated. Our results showed that oxalic acid (C2) was the dominant dicarboxylic acid, followed by succinic acid (C4), malonic acid (C3), adipic acid (C6) and phthalic acid (Ph). Two types of haze pollution events were identified during the sampling period, i.e., Event I, which was mainly caused by the local biomass burning emission, and Event II, which was caused by a long-distance transport of the YRD urban pollution. C2 linearly correlated with SO42− and NO3− in Event I but only with O3 in Event II, indicating that oxalic acid formation was dominated by the aerosol aqueous phase oxidation in Event I and by the gaseous phase oxidation in Event II, respectively.65.5% of Cl− in sea salts at the site in the clean period was depleted and robustly correlated with oxalic acid (R2 = 0.74). We proposed a mechanism to explain such a significant Cl− depletion, in which anthropogenic VOC oxidize into oxalic acid and its precursors such as glyoxal and methyglyoxal by a photochemical oxidation, and then oxalic acid and the related compounds subsequently react with sea salts and release HCl into the troposphere. The significant Cl− depletion of sea salts related with the organic acid (C2) in coastal China was found for the first time and should be considered in future studies, because oxalic acid and related SOA in the country are abundant and the released HCl may effectively enhance the oxidation capacity of the atmosphere by photolytically producing Cl radicals.

A homogenized daily in situ PM<sub>2.5</sub> concentration dataset from the national air quality monitoring network in China

Abstract. In situ PM2.5 concentration observations have long been used as critical data sources in haze-related studies. Due to the frequently occurring haze pollution events, China started to regularly monitor PM2.5 concentration nationwide from the newly established air quality monitoring network in 2013. Nevertheless, the acquisition of these invaluable air quality samples is challenging given the absence of a publicly available data download interface. In this study, we provided a homogenized in situ PM2.5 concentration dataset that was created on the basis of hourly PM2.5 data retrieved from the China National Environmental Monitoring Center (CNEMC) via a web crawler between 2015 and 2019. Methods involving missing value imputation, change point detection, and bias adjustment were applied sequentially to deal with data gaps and inhomogeneities in raw PM2.5 observations. After excluding records with limited samples, a homogenized PM2.5 concentration dataset comprising of 1309 5-year long PM2.5 data series at a daily resolution was eventually compiled. This is the first attempt to homogenize in situ PM2.5 observations in China. The trend estimations derived from the homogenized dataset indicate a spatially homogeneous decreasing tendency of PM2.5 across China at a mean rate of about −7.6 % per year from 2015 to 2019. In contrast to raw PM2.5 observations, the homogenized data record not only has complete data integrity but is more consistent over space and time. This homogenized daily in situ PM2.5 concentration dataset is publicly accessible at https://doi.org/10.1594/PANGAEA.917557 (Bai et al., 2020a) and can be applied as a promising dataset for PM2.5-related studies such as satellite-based PM2.5 mapping, human exposure risk assessment, and air quality management.

Associations between winter atmospheric teleconnections in drought and haze pollution over Southwest China

In the early 21st century, Southwest China (SWC) frequently experienced extreme droughts and severe haze pollution events. Although the meteorological causes of these extreme droughts have been widely investigated, previous studies have yet to understand the causes of haze pollution events over SWC. Moreover, the associations between winter atmospheric teleconnections during drought and haze pollution event across SWC has received negligible attention and therefore warrants investigation. This study examines the associations between the atmospheric teleconnections with respect to winter droughts and winter haze pollution over SWC. Our main conclusions are as follows. (1) Winter precipitation and winter haze days (WHD) over SWC had three major fluctuations from 1959 to 2016. (2) The atmospheric circulation pattern over the Eurasian (EU) continent associated with WHD over SWC resembled that of winter droughts over SWC, where both can be characterized by an EU teleconnection pattern. The Arctic Oscillation (AO) mainly induced the atmospheric circulation pattern over the EU continent that is associated with WHD over SWC. (3) The sea surface temperature (SST) and low circulation anomalies in the Pacific and north Atlantic associated with WHD were similar to those associated with winter droughts over SWC. La Niña events and negative phases of the North Atlantic Oscillation (NAO) may induce winter drought and increase the WHD over SWC. (4) Compared with winter drought over SWC, the variation in the WHD was more complex and the factors affecting WHD were more diverse, and winter drought and its related atmospheric circulations were important factors that induced haze pollution over SWC. Overall, this study not only fills a gap in the literature with respect to the associations between the atmospheric teleconnections of winter drought and winter haze pollution over SWC, but also provides an important scientific basis for the development of potential predictions of local monthly haze pollution, which improves the forecast accuracy of local short-term haze pollution and enriches the theoretical understanding of the meteorological causes of local haze pollution.

Turbulence barrier effect during heavy haze pollution events

Under low wind speed conditions, the frequent intermittent turbulence phenomenon in the atmospheric boundary layer (ABL) greatly weakens the turbulent diffusion of pollutants to cause the heavy haze events. Turbulence may disappear at certain heights forming a laminar flow as if there is a barrier layer hindering the transmission up and down during the heavy haze periods. The turbulent data at five layers and PM2.5 (fine particular matter with a diameter smaller than 2.5 μm) concentration at two levels were used to discuss the barrier that is called the barrier effect vividly. The results revealed that the changes in the PM2.5 concentration at different heights corresponded excellently with the change in vertical turbulence barrier effect. This work explains the physical mechanism responsible for the accumulation of pollutants in heavy pollution events and the influence of turbulent diffusion conditions on the distribution of the PM2.5 concentration.

Rainwater chemistry of acid precipitation occurrences due to long-range transboundary haze pollution and prolonged drought events during southwest monsoon season: climate change driven

The purposes of this research were to study the characteristics chemistry of pH, anions and cations in rainwater, and to identify the possible sources that contributing to the acid precipitation during southwest monsoon season with occurrence of extreme drought event. During the southwest monsoon season, it normally occurs along with haze phenomenon that every year will hit Southeast Asia. This condition will aggravate with high acidic particles in the atmosphere due to the prolonged drought. The analysed parameters which involved pH, anions (NO3-, SO42- and Cl−) and cations (Ca2+, Mg2+, Na+ and K+) were analysed using pH meter, Hach DR 2800, argentometric method and ICP-OES. From the findings, it showed that acid rain occurred during the southwest monsoon season with the range of pH values from 4.95 ± 0.13 to 6.40 ± 0.03 and the total average of pH 5.71 ± 0.32. Anions NO3-, SO42- and Cl− were found to be the dominant compositions of the acid rain occurrences with higher concentrations detected. In overall, rural area recorded with higher acidity of precipitation at total average of pH 5.54 ± 0.39 compared to urban area at pH 5.77 ± 0.26. Rural area surprisingly recorded higher frequency occurrences of acid rain with pH lesser than 5.6 and below compared to urban area. As for public health and safety, all rainwater samples during the acid rain event were found exceeded the allowable limits of NWQS and WHO standards, that shown not suitable for skin contact, recreational purposes even for drinking purposes.

Characteristics of PM2.5-bound secondary organic aerosol tracers in a coastal city in Southeastern China: Seasonal patterns and pollution identification

Abstract Secondary organic aerosol (SOA) plays an important role in global climate change and air quality, and SOA tracers most directly characterize the sources and formation mechanisms of SOA. Four seasons of observation of SOA tracers was carried out in a coastal city of southeastern China. Fourteen PM2.5-bound SOA tracers, including isoprene (SOAI), α/β-pinene (SOAM), β-caryophyllene (SOAC), and toluene (SOAA), were measured using the GC–MS method. The concentrations of SOA tracers in Fuzhou were 25.9 ± 19.9 ng m−3 (SOAM), 7.45 ± 8.53 ng m−3 (SOAI), 3.15 ± 1.99 ng m−3 (SOAC), and 2.63 ± 1.54 ng m−3 (SOAA). The elevated SOAI concentration in summer was mainly controlled by high biogenic isoprene emission and strong oxidation, and biomass burning contributed strongly to DHOPA (SOAA) in fall. SO42− and H+insitu had an increased impact on later-generation SOAI products and low-NOx SOAM products. Based on the ratio of MGA/MTLs and MBTCA/(PA + PNA), atmospheric oxidation capacity (Ox, =NO2+O3) had a significant impact on the aerosol aging of SOA tracers. The increased proportions of low-NOx SOAI products were 3.23–7.21 times higher than those of high-NOx products from non-haze to haze periods, suggesting the influence of high SO42− concentration and RH on the reaction channel for SOAI formation. The percentages of later-generation SOAM products during RT were 2.46 times higher than those of first-generation products, suggesting the influence of aerosol aging during the regional transport. Both continental Asian outflow and biomass-burning plumes promoted precursor emissions and the formation process of SOA during the haze pollution events. These related findings help to understand the occurrence, sources and formation of SOA in coastal areas.

A comparison of haze pollution variability in China using haze indices based on observations.

Defining the occurrence and degree of haze pollution (HP) in China has been extremely controversial in recent years. This study elaborates on the meanings of ten most frequently used HP indices based on observation and compares their performance in measuring the HP variability in China concerning the annual cycle, interannual variations and interdecadal trends for the period of 1980-2015. It is found that these indices exhibit a high consistency in South China, while apparent discrepancies exist in North Chinam, Sichuan Basin and the Yangtze River Delta. Observed surface PM2.5 concentration is utilized to assess the performance of the ten HP indices. Results show that some of these indices can well capture primary features of the HP in some specific area, only these defined by a daily averaged value or 14:00 single threshold value have relatively good performance in the whole Eastern China. Such four indices (HPI2/HPI3/HPI4/HPI5) exhibit high consistence in depicting spatial-temporal variations (from annual cycle to interannual variability) of surface PM2.5 concentration over China, meaning they can highlight the significance of high aerosol concentration in triggering HP events. Moreover, further analysis suggest that the contributions of weather conditions to the occurrence and degree of the HP is becoming more in China, especially in the NC, SCB, YRD regions. These results provide useful guidance for monitoring variations of the HP in China and for making long-term environmental regulatory and controlling policy.

Fast oxidation of sulfur dioxide by hydrogen peroxide in deliquesced aerosol particles.

Atmospheric sulfate aerosols have important impacts on air quality, climate, and human and ecosystem health. However, current air-quality models generally underestimate the rate of conversion of sulfur dioxide (SO2) to sulfate during severe haze pollution events, indicating that our understanding of sulfate formation chemistry is incomplete. This may arise because the air-quality models rely upon kinetics studies of SO2 oxidation conducted in dilute aqueous solutions, and not at the high solute strengths of atmospheric aerosol particles. Here, we utilize an aerosol flow reactor to perform direct investigation on the kinetics of aqueous oxidation of dissolved SO2 by hydrogen peroxide (H2O2) using pH-buffered, submicrometer, deliquesced aerosol particles at relative humidity of 73 to 90%. We find that the high solute strength of the aerosol particles significantly enhances the sulfate formation rate for the H2O2 oxidation pathway compared to the dilute solution. By taking these effects into account, our results indicate that the oxidation of SO2 by H2O2 in the liquid water present in atmospheric aerosol particles can contribute to the missing sulfate source during severe haze episodes.

Emergency Response Measures to Alleviate a Severe Haze Pollution Event in Northern China during December 2015: Assessment of Effectiveness

ABSTRACT Using the WRF-Chem model, we simulated the surface PM2.5 concentrations on the North China Plain (NCP) during a severe winter haze episode (December 6–10, 2015) with the goal of assessing the effectiveness of the implemented emergency response measures (ERMs) in alleviating the pollution. We estimated that the ERMs decreased the anthropogenic pollutant emissions, with the exception of NH3, by 8–48% during this event. Inputting these reduced emission estimates, our simulations reproduced the observed PM2.5 concentrations and compositions. Stagnant regional meteorological conditions increased the lifetime of the PM2.5 in the NCP boundary layer from 1 day during the clean period to 5 days during the haze episode. Additionally, local emissions accounted for approximately only 20% of the surface PM2.5 in Beijing but more than 62% over the rest of the NCP. We found that the ERMs achieved a modest reduction in the mean surface PM2.5 concentrations during the event, decreasing them by 7% and 4% in Beijing and across the rest of the NCP, respectively. The limited effect was due to the duration of the ERMs being much shorter than the lifetime of the PM2.5, which prevented the concentrations of the latter from fully reflecting the reduction in emissions. We conclude that anthropogenic emissions on the NCP during severe winter haze episodes must be reduced by a much larger percentage to substantially abate the PM2.5 concentrations.

Vertical distribution of PM2.5 and interactions with the atmospheric boundary layer during the development stage of a heavy haze pollution event

Vertical profiles of PM2.5 (i.e., particulate matter with an aerodynamic diameter of 2.5 µm or less) and meteorological variables (e.g., potential temperature, specific humidity) are crucial to understand formation mechanism including accumulation and dispersion process of PM2.5, as well as interactions between aerosols and the atmospheric boundary layer (ABL). In this study, vertical distributions of PM2.5 are characterized through comprehensive analyses of vertical profiles measured by unmanned aerial vehicle (UAV), Micro Pulse LiDAR, and other surface observational data of a heavy aerosol pollution episode occurring on December 22–25, 2017 in Nanjing, China. Results show that PM2.5 profiles are characterized by a clear three-layer structure with near constant within the mixed layer, a transition layer with a large local gradient in the entrainment zone, and a layer with low concentration and small gradient in the free atmosphere, which shows a large similarity to that of specific humidity. The accumulation of aerosols is found near top of the ABL with the largest increase rate. Vertical distributions of PM2.5 and their evolution are largely constrained by the ABL thermodynamics during daytime, but show much less dependence on the ABL evolution at nighttime. PM2.5 provides an important feedback on the nocturnal boundary layer (NBL) leading to significant modification of vertical distributions of potential temperature and water vapor. Moreover, this study suggests that the current boundary layer parameterization scheme needs refinement with aerosol radiative effect included to further improve the ABL height (ABLH) and air quality predictions.

Numerical simulation of an extreme haze pollution event over the North China Plain based on initial and boundary condition ensembles

ABSTRACTThe North China Plain often suffers heavy haze pollution events in the cold season due to the rapid industrial development and urbanization in recent decades. In the winter of 2015, the meg...

Photochemical impacts of haze pollution in an urban environment

Abstract. Rapid economic growth in China over the past 30 years has resulted in significant increases in the concentrations of small particulates (PM2.5) over the city of Beijing. In addition to health problems, high aerosol loading can impact visibility and thus reduce photolysis rates over the city, leading to potential implications for photochemistry. Photolysis rates are highly sensitive not only to the vertical distribution of aerosols but also to their composition, as this can impact how the incoming solar radiation is scattered or absorbed. This study, for the first time, uses aerosol composition measurements and lidar optical depth to drive the Fast-JX photolysis scheme and quantify the photochemical impacts of different aerosol species during the Air Pollution and Human Health (APHH) measurement campaigns in Beijing in November–December 2016 and May–June 2017. This work demonstrates that severe haze pollution events (PM2.5 > 75 µg m−3) occur during both winter and summer, leading to reductions in O3 photolysis rates of 27 %–34 % (greatest in winter) and reductions in NO2 photolysis of 40 %–66 % (greatest in summer) at the surface. It also shows that in spite of much lower PM2.5 concentrations in the summer months, the absolute changes in photolysis rates are larger for both O3 and NO2. In the winter, absorbing species such as black carbon dominate the photolysis response to aerosols, leading to mean reductions in J[O1D] and J[NO2] in the lowest 1 km of 24 % and 23 %, respectively. In contrast, in the summer, scattering aerosol such as organic matter dominate the response, leading to mean decreases of 2 %–3 % at the surface and increases of 8 %–10 % at higher altitudes (3–4 km). During these haze events in both campaigns, the influence of aerosol on photolysis rates dominates over that from clouds. These large impacts on photochemistry can have significant implications for concentrations of important atmospheric oxidants such as the hydroxyl radical. Idealized photochemical box model studies show that such large impacts on photochemistry could lead to a 12 % reduction in surface O3 (3 % for OH) due to haze pollution. This highlights that PM2.5 mitigation strategies could have important implications for the oxidation capacity of the atmosphere both at the surface and in the free troposphere.

Possible Relationship between the Chukchi Sea Ice in the Early Winter and the February Haze Pollution in the North China Plain

AbstractHaze pollution is among the most serious disasters in the North China Plain, dramatically damaging human health and the social economy. The frequency of haze events in February typically varies from the number of haze days in the winter. To improve the understanding of haze pollution in February, this study not only showed the large-scale atmospheric circulations associated with the variation in the haze, but also analyzed its connection with Arctic sea ice. The observational and large ensemble model results both illustrated that the preceding increase in the early-winter Chukchi Sea ice might intensify the February haze pollution. The accumulated sea ice over the Chukchi Sea resulted in a steeper meridional sea surface temperature gradient and a significant and persistent westerly thermal wind. In February, the responsive pattern in the atmosphere developed into a Rossby wave–like pattern, linking the Chukchi Sea ice and the February haze pollution. Modulating by the induced large-scale atmospheric circulations, the horizontal and vertical atmospheric ventilation conditions and the hygroscopic growth conditions enhanced the frequency of haze pollution events.

Spatiotemporal Associations between PM2.5 and SO2 as well as NO2 in China from 2015 to 2018.

Given the critical roles of nitrates and sulfates in fine particulate matter (PM2.5) formation, we examined spatiotemporal associations between PM2.5 and sulfur dioxide (SO2) as well as nitrogen dioxide (NO2) in China by taking advantage of the in situ observations of these three pollutants measured from the China national air quality monitoring network for the period from 2015 to 2018. Maximum covariance analysis (MCA) was applied to explore their possible coupled modes in space and time. The relative contribution of SO2 and NO2 to PM2.5 was then quantified via a statistical modeling scheme. The linear trends derived from the stratified data show that both PM2.5 and SO2 decreased significantly in northern China in terms of large values, indicating a fast reduction of high PM2.5 and SO2 loadings therein. The statistically significant coupled MCA mode between PM2.5 and SO2 indicated a possible spatiotemporal linkage between them in northern China, especially over the Beijing–Tianjin–Hebei region. Further statistical modeling practices revealed that the observed PM2.5 variations in northern China could be explained largely by SO2 rather than NO2 therein, given the estimated relatively high importance of SO2. In general, the evidence-based results in this study indicate a strong linkage between PM2.5 and SO2 in northern China in the past few years, which may help to better investigate the mechanisms behind severe haze pollution events in northern China.

Investigation of the atmospheric boundary layer during an unexpected summertime persistent severe haze pollution period in Beijing

Persistent severe haze pollution seldom happens in summer in Beijing. In this study, a persistent severe haze pollution event, defined as daily PM2.5 concentration higher than 150 μg m−3 on 5 consecutive days, was observed in Beijing from 26 to 30 in July 2010. It was not caused by crop residue burning during the harvest period. Weak pressure systems dominated at surface and implicated weak advection. Regional weak southerly winds with speeds of 2–3 m s−1 consistently brought pollutants from southern large-emission regions to Beijing. Surface convergence resulted from northerly winds prevailing in northern regions induced pollutants remaining in Beijing, which contributed to the maximum daily PM2.5 concentration on 26 in July. A continental high-pressure system persisted in the northwest of Beijing at 500 hPa, which led to significant sinking motion. Elevated inversion was found in the form of subsidence inversion, and this was confirmed by a skew T-logp diagram and vertical velocity analysis. It is much different from the vertical boundary layer structure in wintertime pollution period that surface-based inversion dominates. The subsidence inversion was an extremely stable layer with an average depth of hundred meters and strength of 1.4 °C. The capping effect of the inversion layer at low altitude of average 720 m limited vertical diffusion of pollutants and trapped them in a shallow layer, and thus, extremely high concentration of PM2.5 remained. WRF-Chem model simulation demonstrated that about 70% of PM2.5 was transported to Beijing from its southern regions.

Characteristics and formation mechanism of persistent extreme haze pollution events in Chengdu, southwestern China

Extreme PM2.5 and nonmethane hydrocarbon (NMHC) pollution often occurs simultaneously during the winter. To study the formation mechanism of two pollution events in Chengdu from 23 December 2016 to 31 January 2017, we explored the weather conditions, chemical composition, secondary pollutant conversion, aerosol hygroscopic growth, and potential source contribution function (PSCF) during this period. During the study period, the humidity was high (67.9%), the wind speed was low (1.0 m s−1), the height of the planetary boundary layer was low (463.4 m), and the atmosphere remained stationary. The potential source regions of PM2.5 and NMHCs were locally polluted sites in the southwestern and southern regions of Chengdu, affected by the southwesterly air mass trajectories. PM2.5 and sulfur oxidation ratios (SOR), nitrogen oxidation ratios (NOR) and secondary organic aerosol formation potential (SOAP) showed a strong positive correlation. As pollution increased, the conversion from SO2, NOx and NMHCs to sulfate, nitrate and SOAs increased, resulting in an increase in the secondary aerosol concentration. As the relative humidity increases, aerosols begin to undergo rapid hygroscopic growth, which seriously affects the visibility of the atmosphere. In general, pollutant emissions, static weather, and secondary conversion, among other factors, lead to the occurrence of this persistent extreme haze pollution.

Increasing importance of nitrate formation for heavy aerosol pollution in two megacities in Sichuan Basin, southwest China

Secondary inorganic aerosols, including sulfate, nitrate, and ammonium contribute to a large extent to the severe haze pollution events in China. Understanding their formation mechanisms is critical for designing effective mitigation strategies to control haze pollution, especially as the role of nitrate seemed to become more important recently, especially in some megacities. In the present study, simultaneous observations were conducted in two megacities (Chengdu and Chongqing) in Sichuan Basin of southwest China, one of the regions suffering from severe aerosol pollution. One typical long-lasting pollution event in Chengdu and Chongqing was captured during wintertime from December 25, 2016 to January 5, 2017. The campaign-average of hourly concentrations of PM2.5, sulfate, and nitrate, measured by an Aerosol Analyzer (ZSF) were 101 ± 73.8 μg/m3, 15.9 ± 11.8 μg/m3, and 24.9 ± 20.6 μg/m3, respectively, in Chengdu, and were 87.7 ± 53.8 μg/m3, 19.7 ± 13.5 μg/m3, and 15.1 ± 10.1 μg/m3, respectively, in Chongqing. Nitrate contributed substantially to PM2.5 pollution when PM2.5 was lower than 150 μg/m3, largely due to the strong secondary transformation of NOX to nitrate during the occurrence of the pollution episode. Heterogeneous hydrolysis of N2O5 dominated nitrate formation during nighttime, while photochemical reactions and high-RH enhanced gas- to aqueous-phase dissolution of NH3 and HNO3 or cloud processes likely played important roles for nitrate formation during daytime. RH-dependent heterogeneous reactions contributed greatly to the formation of sulfate. NOX is confirmed to play an important role as an oxidant in accelerating the secondary transformation of SO2 to sulfate at high RH and low O3 levels under neutralization condition during heavy PM2.5 pollution episode. Results from this study identified the formation mechanism of nitrate, especially during the daytime, and addressed the importance of heterogeneous inorganic reactions in the formation of heavy aerosol pollution events.

Optimal Estimation Retrieval of Aerosol Fine-Mode Fraction from Ground-Based Sky Light Measurements

In this paper, the feasibility of retrieving the aerosol fine-mode fraction (FMF) from ground-based sky light measurements is investigated. An inversion algorithm, based on the optimal estimation (OE) theory, is presented to retrieve FMF from single-viewing multi-spectral radiance measurements and to evaluate the impact of utilization of near-infrared (NIR) measurements at a wavelength of 1610 nm in aerosol remote sensing. Self-consistency tests based on synthetic data produced a mean relative retrieval error of 4.5%, which represented the good performance of the OE inversion algorithm. The proposed algorithm was also performed on real data taken from field experiments in Beijing during a haze pollution event. The correlation coefficients (R) for the retrieved aerosol volume fine-mode fraction (FMFv) and optical fine-mode fraction (FMFo) against AErosol RObotic NETwork (AERONET) products were 0.94 and 0.95 respectively, and the mean residual error was 4.95%. Consequently, the inversion of FMFv and FMFo could be well constrained by single-viewing multi-spectral radiance measurement. In addition, by introducing measurements of 1610 nm wavelength into the retrieval, the validation results showed a significant improvement in the R value for FMFo (from 0.89–0.94). These results confirm the high value of NIR measurements for the retrieval of coarse mode aerosols.