Significance: We illustrate the similarity and difference in particulate matter (PM) formation between Beijing and other world regions. The periodic cycle of PM events in Beijing is regulated by meteorological conditions. While the particle chemical compositions in Beijing are similar to those commonly measured worldwide, efficient nucleation and growth over an extended period in Beijing are distinctive from the aerosol formation typically observed in other global areas. Gaseous emissions of volatile organic compounds and nitrogen oxides from urban transportation and sulfur dioxide from regional industry are responsible for large secondary PM formation, while primary emissions and regional transport of PM are insignificant. Reductions in emissions of the aerosol precursor gases from transportation and industry are essential to mediate severe haze pollution in China.

Significance: Although black carbon (BC) represents a key short-lived climate forcer, its direct radiative forcing remains highly uncertain. The available results from available studies of absorption enhancement of BC particles during atmospheric aging are conflicting. Using a novel environmental chamber method, we have, for the first time to our knowledge, quantified the aging and variation in the optical properties of BC particles under ambient urban conditions representative of developed and developing countries. Our results indicate that BC under polluted urban environments could contribute significantly to both pollution development and large positive radiative forcing, implying that reduction of BC emissions under polluted environments achieves a cobenefit in simultaneously controlling air pollution and protecting climate, especially for developing countries.

To investigate the seasonal characteristics of submicron aerosol (PM₁) in Beijing urban areas, a high‐resolution time‐of‐flight aerosol‐mass‐spectrometer (HR‐ToF‐AMS) was utilized at an urban site in summer (August to September 2011) and winter (November to December 2010), coupled with multiple state of the art online instruments. The average mass concentrations of PM₁ (60–84 µg m⁻³) and its chemical compositions in different campaigns of Beijing were relatively consistent in recent years. In summer, the daily variations of PM₁ mass concentrations were stable and repeatable. Eighty‐two percent of the PM₁ mass concentration on average was composed of secondary species, where 62% is secondary inorganic aerosol and 20% secondary organic aerosol (SOA). In winter, PM₁ mass concentrations changed dramatically because of the different meteorological conditions. The high average fraction (58%) of primary species in PM₁ including primary organic aerosol (POA), black carbon, and chloride indicates primary emissions usually played a more important role in the winter. However, aqueous chemistry resulting in efficient secondary formation during occasional periods with high relative humidity may also contribute substantially to haze in winter. Results of past OA source apportionment studies in Beijing show 45–67% of OA in summer and 22–50% of OA in winter can be composed of SOA. Based on the source apportionment results, we found 45% POA in winter and 61% POA in summer are from nonfossil sources, contributed by cooking OA in both seasons and biomass burning OA (BBOA) in winter. Cooking OA, accounting for 13–24% of OA, is an important nonfossil carbon source in all years of Beijing and should not be neglected. The fossil sources of POA include hydrocarbon‐like OA from vehicle emissions in both seasons and coal combustion OA (CCOA) in winter. The CCOA and BBOA were the two main contributors (57% of OA) for the highest OA concentrations (>100 µg m⁻³) in winter. The POA/ΔCO ratios in winter and summer are 11 and 16 µg m⁻³ ppm⁻¹, respectively, similar to ratios from western cities. Higher OOA/O_x (= NO₂ + O₃) ratio (0.49 µg m⁻³ ppb⁻¹) in winter study than these ratios from western cities (0.03–0.16 µg m⁻³ ppb⁻¹) was observed, which may be due to the aqueous reaction or extra SOA formation contributed by semivolatile organic compounds from various primary sources (e.g., BBOA or CCOA) in Beijing. The evolution of oxygen to carbon ratio (O/C) with photochemical age allows to estimate an equivalent rate constant for chemical aging of OA in summer as k_OH ~ 4.1 × 10⁻¹² cm³ molecule⁻¹ s⁻¹, which is of the same order as obtained in other anthropogenic influenced areas and may be useful for OA modeling.

The main objective of this study is to investigate the formation and evolution mechanism of the regional haze in megacity Beijing by analyzing the process of a severe haze that occurred 20–27 September 2011. Mass concentration and size distribution of aerosol particles as well as aerosol optical properties were concurrently measured at the Beijing urban atmospheric environment monitoring station. Gaseous pollutants (SO₂, NO-NO₂-NO_x, O₃, CO) and meteorological parameters (wind speed, wind direction, and relative humidity) were simultaneously monitored. Meanwhile, aerosol spatial distribution and the height of planetary boundary layer (PBL) were retrieved from the signal of satellite and LIDAR (light detection and ranging). Concentrations of NO, NO₂, SO₂, O₃, and CO observed during 23–27 September had exceeded the national ambient air quality standards for residents. The mass concentration of PM_2.5 gradually accumulated during the measurement and reached at 220 μg m⁻³ on 26 September, and the corresponding atmospheric visibility was only 1.1 km. The daily averaged AOD in Beijing increased from ~ 0.16 at λ = 500 nm on 22 September and reached ~ 3.5 on 26 September. The key factors that affected the formation and evolution of this haze episode were stable anti-cyclone synoptic conditions at the surface, decreasing of the height of PBL, heavy pollution emissions from urban area, number and size evolution of aerosols, and hygroscopic growth for aerosol scattering. This case study may provide valuable information for the public to recognize the formation mechanism of the regional haze event over the megacity, which is also useful for the government to adopt scientific approach to forecast and eliminate the occurrence of regional haze in China.

The first wintertime in situ measurements of hydroxyl (OH), hydroperoxy (HO₂) and organic peroxy (RO₂) radicals (RO_x = OH + HO₂ + RO₂) in combination with observations of total reactivity of OH radicals, k_OH in Beijing are presented. The field campaign Beijing winter finE particle STudy – Oxidation, Nucleation and light Extinctions (BEST-ONE) was conducted at the suburban site Huairou near Beijing from January to March 2016. It aimed to understand oxidative capacity during wintertime and to elucidate the secondary pollutants' formation mechanism in the North China Plain (NCP). OH radical concentrations at noontime ranged from 2.4×10⁶ cm⁻³ in severely polluted air (k_OH ∼ 27 s⁻¹) to 3.6×10⁶ cm⁻³ in relatively clean air (k_OH ∼ 5 s⁻¹). These values are nearly 2-fold larger than OH concentrations observed in previous winter campaigns in Birmingham, Tokyo, and New York City. During this campaign, the total primary production rate of RO_x radicals was dominated by the photolysis of nitrous acid accounting for 46 % of the identified primary production pathways for RO_x radicals. Other important radical sources were alkene ozonolysis (28 %) and photolysis of oxygenated organic compounds (24 %). A box model was used to simulate the OH, HO₂ and RO₂ concentrations based on the observations of their long-lived precursors. The model was capable of reproducing the observed diurnal variation of the OH and peroxy radicals during clean days with a factor of 1.5. However, it largely underestimated HO₂ and RO₂ concentrations by factors up to 5 during pollution episodes. The HO₂ and RO₂ observed-to-modeled ratios increased with increasing NO concentrations, indicating a deficit in our understanding of the gas-phase chemistry in the high NO_x regime. The OH concentrations observed in the presence of large OH reactivities indicate that atmospheric trace gas oxidation by photochemical processes can be highly effective even during wintertime, thereby facilitating the vigorous formation of secondary pollutants.