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In January 2013, a long-lasting episode of severe haze occurred in central and eastern China, and it attracted attention from all sectors of society. The process and evolution of haze pollution episodes were observed by the ＂Forming Mechanism and Con- trol Strategies of Haze in China＂ group using an intensive aerosol and trace gases campaign that simultaneously obtained data at 11 ground-based observing sites in the CARE-China network. The characteristics and formation mechanism of haze pollu- tion episodes were discussed. Five haze pollution episodes were identified in the Beijing-Tianjin-Hebei （Jing-Jin-Ji） area; the two most severe episodes occurred during 9-15 January and 25-31 January. During these two haze pollution episodes, the maximum hourly PMz5 mass concentrations in Beijing were 680 and 530 ~tg m-3, respectively. The process and evolution of haze pollution episodes in other major cities in the Jing-Jin-Ji area, such as Shijiazhuang and Tianjin were almost the same as those observed in Beijing. The external cause of the severe haze episodes was the unusual atmospheric circulation, the depres- sion of strong cold air activities and the very unfavorable dispersion due to geographical and meteorological conditions. How- ever, the internal cause was the quick secondary transformation of primary gaseous pollutants to secondary aerosols, which contributed to the ＂explosive growth＂ and ＂sustained growth＂ of PM2.5. Particularly, the abnormally high amount of nitric ox- ide （NOx） in the haze episodes, produced by fossil fuel combustion and vehicle emissions, played a direct or indirect role in the quick secondary transformation of coal-burning sulphur dioxide （SO2） to sulphate aerosols. Furthermore, gaseous pollutants were transformed into secondary aerosols through heterogeneous reactions on the surface of fine particles, which can change the particle＇s size and chemical composition. Consequently, the proportion of secondary inorganic ions, such as sulphate and nitrate, gradually increased, which enhances particle hygroscopicity and thereby accelerating formation of the haze pollution.

Haze in China has been increasing in frequency of occurrence as well as the area of the affected region. Here, we report on a new mechanism of haze formation, in which coexistence with NOx can reduce the environmental capacity for SO 2 , leading to rapid conversion of SO 2 to sulfate because NO 2 and SO 2 have a synergistic effect when they react on the surface of mineral dust. Monitoring data from five severe haze episodes in January of 2013 in the Beijing-Tianjin-Hebei regions agreed very well with the laboratory simulation. The combined air pollution of motor vehicle exhaust and coal-fired flue gases greatly reduced the atmospheric environmental capacity for SO 2 and the formation of sulfate was found to be a main reason for the growth of fine particles, which led to the occurrence of haze. These results indicate that the impact of motor vehicle exhaust on the atmospheric environment might be underestimated.

Accurate determination of the atmospheric particulate matter mass concentration and chemical composition is helpful in exploring the causes and sources of atmospheric enthalpy pollution and in evaluating the rationality of environmental air quality control strategies. Based on the sampling and chemical composition data of PM 2.5 in different key regions of China in the CARE-China observation network, this research analyzes the environmental air quality data released by the China National Environmental Monitoring Centre during the studied period to determine the changes in the particulate matter mass concentration in key regions and the evolution of the corresponding chemical compositions during the implementation of the Action Plan for Prevention and Control of Air Pollution from 2013–2017. The results show the following. (1) The particulate matter mass concentration in China showed a significant downward trend; however, the PM 2.5 annual mass concentration in 64% of cities exceeds the New Chinese Ambient Air Quality Standard (CAAQS) Grade II (GB3095-2012). The region to the east of the Taihang Mountains, the Fenhe and Weihe River Plain and the Urumqi-Changji regions in Xinjiang, all have PM 2.5 concentration loading that is still high, and heavy haze pollution occurred frequently in the autumn and winter. (2) During the heavy pollution in the autumn and winter, the concentrations of sulfate and organic components decreased significantly. The mean S O 4 2 − concentration in PM 2.5 decreased by 76%, 12%, 81% and 38% in Beijing-Tianjin-Hebei (BTH), the Pearl River Delta (PRD), the Sichuan-Chongqing region (SC) and the Fenhe and Weihe River Plain, respectively. The mean organic matter (OM) concentration decreased by 70%, 44%, 48% and 31%, respectively, and the mean concentration of N H 4 + decreased by 68%, 1.6%, 38% and 25%, respectively. The mean elemental carbon (EC) concentration decreased by 84% and 20% in BTH and SC, respectively, and it increased by 61% and 11% in the PRD and Fenhe and Weihe River Plain, respectively. The mean concentration of mineral and unresolved chemical components (MI) dropped by 70%, 24% and 13% in BTH, the PRD and the Fenhe and Weihe River Plain, respectively. The change in the PM 2.5 chemical composition is consistent with the decrease of the PM 2.5 mass concentration. (3) In 2015, the mean OM concentration contributions to fine particles and coarse particles were 13–46% and 46–57%, respectively, and the mean MI concentration contributions to fine particles and coarse and particles were 31–60% and 39–73%, respectively; these values are lower than the 2013 values from the key regions, which is the most important factor behind the decrease of the particulate matter mass concentration. From 2013 to 2015, among the chemical components of different particle size fractions, the peak value of the coarse particle size fraction decreased significantly, and the fine particle size fractions of S O 4 2 − , N O 4 − , a n d N H 4 + decreased with the decrease of the particulate matter mass concentration in different particle size fractions. The fine-particle size peaks of S O 4 2 − , N O 4 − , a n d N H 4 + shifted from 0.65–1.1 μm to the finer size range of 0.43–0.65 μm during the same time frame.

Multiaxis differential absorption spectroscopy (MAX-DOAS) is a newly developed advanced vertical profile detection method, but the vertical nitrogen dioxide (NO 2 ) profiles measured by MAX-DOAS have not yet been fully verified. In this study, we perform MAX-DOAS and tower gradient observations to simultaneously acquire tropospheric NO 2 observations in the Beijing urban area from 1 April to 31 May 2019. The average values of the tropospheric NO 2 vertical column densities measured by MAX-DOAS and the tropospheric monitoring instrument are 15.8 × 10 15 and 12.4 × 10 15 molecules cm −2 , respectively, and the correlation coefficient R reaches 0.87. The MAX-DOAS measurements are highly consistent with the tower-based in situ measurements, and the correlation coefficients R from the ground to the upper air are 0.89 (60 m), 0.87 (160 m), and 0.76 (280 m). MAX-DOAS accurately measures the trend of NO 2 vertical profile changes, although a large underestimation occurs by a factor of two. By analyzing the NO 2 vertical profile, the NO 2 concentration reveals an exponential decrease with height. The NO 2 vertical profile also coincides with the evolution of the boundary layer height. The study shows that the NO 2 over Beijing mainly originates from local sources and occurs in the boundary layer, and its vertical evolution pattern has an important guiding significance to better understand nitrate production and ozone pollution.

We conducted the first real-time continuous vertical measurements of particle extinction (bext), gaseous NO2, and black carbon (BC) from ground level to 260 m during two severe winter haze episodes at an urban site in Beijing, China. Our results illustrated four distinct types of vertical profiles: (1) uniform vertical distributions (37 % of the time) with vertical differences less than 5 %, (2) higher values at lower altitudes (29 %), (3) higher values at higher altitudes (16 %), and (4) significant decreases at the heights of ∼ 100–150 m (14 %). Further analysis demonstrated that vertical convection as indicated by mixing layer height, temperature inversion, and local emissions are three major factors affecting the changes in vertical profiles. Particularly, the formation of type 4 was strongly associated with the stratified layer that was formed due to the interactions of different air masses and temperature inversions. Aerosol composition was substantially different below and above the transition heights with ∼ 20–30 % higher contributions of local sources (e.g., biomass burning and cooking) at lower altitudes. A more detailed evolution of vertical profiles and their relationship with the changes in source emissions, mixing layer height, and aerosol chemistry was illustrated by a case study. BC showed overall similar vertical profiles as those of bext (R2=0.92 and 0.69 in November and January, respectively). While NO2 was correlated with bext for most of the time, the vertical profiles of bext ∕ NO2 varied differently for different profiles, indicating the impact of chemical transformation on vertical profiles. Our results also showed that more comprehensive vertical measurements (e.g., more aerosol and gaseous species) at higher altitudes in the megacities are needed for a better understanding of the formation mechanisms and evolution of severe haze episodes in China.

Aerosol particles are of importance in the Earth's radiation budget since they scatter and absorb sunlight. While extensive studies of aerosol optical properties have been conducted at ground sites, vertical measurements and characterization are very limited in megacities. In this work, we present simultaneous real-time online measurements of aerosol optical properties at ground level and at 260 m on a meteorological tower from 16 November to 13 December in 2016 in Beijing along with measurements of continuous vertical profiles during two haze episodes. The average (±1σ) scattering and absorption coefficients (bsca and babs; λ=630 nm) were 337.6 (±356.0) and 36.6 (±33.9) Mm−1 at 260 m, which were 26.5 % and 22.5 % lower than those at ground level. Single scattering albedo (SSA), however, was comparable between the two heights, with slightly higher values at ground level (0.89±0.04). Although bsca and babs showed overall similar temporal variations between ground level and 260 m, the ratios of 260 m to ground varied substantially from less than 0.4 during the clean stages of haze episodes to > 0.8 in the late afternoon. A more detailed analysis indicates that vertical profiles of bsca, babs, and SSA in the low atmosphere were closely related to the changes in meteorological conditions and mixing layer height. The mass absorption cross section (MAC) of equivalent black carbon (eBC, λ=630 nm) varied substantially from 9.5 to 13.2 m2 g−1 in winter in Beijing, and it was strongly associated with the mass ratio of coating materials on refractory BC (rBC) to rBC (MR), and also the oxidation degree of organics in rBC-containing particles. Our results show that the increases in MAC of eBC in winter were mainly caused by photochemically produced secondary materials. Light absorption of organic carbon (brown carbon, BrC) was also important in winter, which on average accounted for 46 (±8.5) % and 48 (±9.3) % of the total absorption at 370 nm at ground level and 260 m, respectively. A linear regression model combined with positive matrix factorization analysis was used to show that coal combustion was the dominant source contribution of BrC (48 %–55 %) followed by biomass burning (17 %) and photochemically processed secondary organic aerosol (∼20 %) in winter in Beijing.

The vertical observation of volatile organic compounds (VOCs) is an important means to clarify the mechanisms of ozone formation. To explore the vertical evolution of VOCs in summer, a field campaign using a tethered balloon during summer photochemical pollution was conducted in Shijiazhuang from 8 June to 3 July 2019. A total of 192 samples were collected, 23 vertical profiles were obtained, and the concentrations of 87 VOCs were measured. The range of the total VOC concentration was 41–48 ppbv below 600 m. It then slightly increased above 600 m, and rose to 58 ± 52 ppbv at 1000 m. The proportion of alkanes increased with height, while the proportions of alkenes, halohydrocarbons and acetylene decreased. The proportion of aromatics remained almost unchanged. A comparison with the results of a winter field campaign during 8–16 January 2019 showed that the concentrations of all VOCs in winter except for halohydrocarbons were more than twice those in summer. Alkanes accounted for the same proportion in winter and summer. Alkenes, aromatics, and acetylene accounted for higher proportions in winter, while halohydrocarbons accounted for a higher proportion in summer. There were five VOC sources in the vertical direction. The proportions of gasoline vehicular emissions + industrial sources and coal burning were higher in winter. The proportions of biogenic sources + long-range transport, solvent usage, and diesel vehicular emissions were higher in summer. From the surface to 1000 m, the proportion of gasoline vehicular emissions + industrial sources gradually increased.

This study uses the WRF-Chem model combined with the empirical kinetic modeling method (EKMA curve) to study the compound pollution event in Beijing that happened in 13–23 May 2017. Sensitivity tests are conducted to analyze ozone sensitivity to its precursors, and to develop emission reduction measures. The results suggest that the model can accurately simulate the compound pollution process of photochemistry and haze. When VOCs and NO x were reduced by the same proportion, the effect of O 3 reduction at peak time was more obvious, and the effect during daytime was more significant than at night. The degree of change in ozone was peak time > daytime average. When reducing or increasing the ratio of precursors by 25% at the same time, the effect of reducing 25% VOCs on the average ozone concentration reduction was most significant. The degree of change in ozone decreased with increasing altitude, the location of the ozone maximum change shifted westward, and its range narrowed. As the altitude increases, the VOCs-limited zone decreases, VOCs sensitivity decreases, NO x sensitivity increases. The controlled area changed from near-surface VOCs-limited to high-altitude NO x -limited. Upon examining the EKMA curve, we have found that suburban and urban are sensitive to VOCs. The sensitivity tests indicate that when VOCs in suburban are reduced about 60%, the O 3 -1h concentration could reach the standard, and when VOCs of the urban decreased by about 50%, the O 3 -1h concentration could reach the standard. Thus, these findings could provide references for the control of compound air pollution in Beijing.

Photochemical smog characterized by high concentrations of ozone (O 3 ) is a serious air pollution issue in the North China Plain (NCP) region, especially in summer and autumn. For this study, measurements of O 3 , nitrogen oxides (NO x ), volatile organic compounds (VOCs), carbon monoxide (CO), nitrous acid (HONO), and a number of key physical parameters were taken at a suburban site, Xianghe, in the NCP region during the summer of 2018 in order to better understand the photochemical processes leading to O 3 formation and find an optimal way to control O 3 pollution. Here, the radical chemistry and O 3 photochemical budget based on measurement data from 1–23 July using a chemical box model is investigated. The daytime (0600–1800 LST) average production rate of the primary radicals referred to as RO x (OH + HO 2 + RO 2 ) is 3.9 ppbv h −1 . HONO photolysis is the largest primary RO x source (41%). Reaction of NO 2 + OH is the largest contributor to radical termination (41%), followed by reactions of RO 2 + NO 2 (26%). The average diurnal maximum O 3 production and loss rates are 32.9 ppbv h −1 and 4.3 ppbv h −1 , respectively. Sensitivity tests without the HONO constraint lead to decreases in daytime average primary RO x production by 55% and O 3 photochemical production by 42%, highlighting the importance of accurate HONO measurements when quantifying the RO x budget and O 3 photochemical production. Considering heterogeneous reactions of trace gases and radicals on aerosols, aerosol uptake of HO 2 contributes 11% to RO x sink, and the daytime average O 3 photochemical production decreases by 14%. The O 3 -NO x -VOCs sensitivity shows that the O 3 production at Xianghe during the investigation period is mainly controlled by VOCs.

Atmospheric nitrous acid (HONO) chemistry is of critical importance to air quality during polluted haze events, especially in China. However, current air quality models (such as WRF-CHEM, WRF-CMAQ, Box-MCM) generally underestimate the concentration of HONO, leading to a lack of fundamental understanding of haze pollution. Here, by combining field observations during haze events in Beijing and modeling results, we developed the new parameterization scheme for heterogeneous nitrogen dioxide (NO 2 ) reaction on aerosol surfaces with the synergistic effects of relative humidity and ammonia, which has not been considered in existing air quality models. Including NO 2 heterogeneous reactions into modeling significantly improves the estimation accuracy of HONO and OH levels, with the contribution reaching up to 91% and 78% during pollution episodes. The OH derived by HONO can partly explain high concentrations of particulate matter. Together, our work provides a new approach to illustrate the formation of HONO, OH, and haze with the consideration of heterogeneous NO 2  → HONO chemistry.