Yu Qu

Formation and evolution mechanism of regional haze: a case study in the megacity Beijing, China

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 (SO 2, NO-NO2-NOx, 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 atλ = 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 c se 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.

Enhancements of Major Aerosol Components Due to Additional HONO Sources in the North China Plain and Implications for Visibility and Haze

The Weather Research and Forecasting/Chemistry model (WRF-Chem) was updated by including photoexcited nitrogen dioxide (NO2) molecules, heterogeneous reactions on aerosol surfaces, and direct emissions of nitrous acid (HONO) in the Carbon-Bond Mechanism Z (CBM-Z). Five simulations were conducted to assess the effects of each new component and the three additional HONO sources on concentrations of major chemical components. We calculated percentage changes of major aerosol components and concentration ratios of gas NOy (NOyg) to NOy and particulate nitrates (NO3−) to NOy due to the three additional HONO sources in the North China Plain in August of 2007. Our results indicate that when the three additional HONO sources are included, WRF-Chem can reasonably reproduce the HONO observations. Heterogeneous reactions on aerosol surfaces are a key contributor to concentrations of HONO, nitrates (NO3−), ammonium (NH4+), and PM2.5 (concentration of particulate matter of ⩽2.5 μm in the ambient air) across the North China Plain. The three additional HONO sources produced a ∼5%–20% increase in monthly mean daytime concentration ratios of NO3− /NOy, a ∼15%–52% increase in maximum hourly mean concentration ratios of NO3−/NOy, and a ∼10%–50% increase in monthly mean concentrations of NO3− and NH4t+ across large areas of the North China Plain. For the Bohai Bay, the largest hourly increases of NO3− exceeded 90%, of NH4t+ exceeded 80%, and of PM2.5 exceeded 40%, due to the three additional HONO sources. This implies that the three additional HONO sources can aggravate regional air pollution, further impair visibility, and enhance the incidence of haze in some industrialized regions with high emissions of NOx and particulate matter under favorable meteorological conditions.

Role of secondary aerosols in haze formation in summer in the Megacity Beijing.

A field experiment from 18 August to 8 September 2006 in Beijing, China, was carried out. A hazy day was defined as visibility<l0 km and RH (relative humidity)<90%. Four haze episodes, which accounted for ~60% of the time during the whole campaign, were characterized by increases of SNA (sulfate, nitrate, and ammonium) and SOA (secondary organic aerosol) concentrations. The average values with standard deviation of SO4(2-), NO3-, NH4+ and SOA were 49.8 (±31.6), 31.4 (±22.3), 25.8 (±16.6) and 8.9 (±4.1)μg/m3, respectively, during the haze episodes, which were 4.3, 3.4, 4.1, and 1.7 times those in the non-haze days. The SO4(2-), NO3-, NH4+, and SOA accounted for 15.8%, 8.8%, 7.3%, and 6.0% of the total mass concentration of PM10 during the non-haze days. The respective contributions of SNA species to PM10 rose to about 27.2%, 15.9%, and 13.9% during the haze days, while the contributions of SOA maintained the same level with a slight decrease to about 4.9%. The observed mass concentrations of SNA and SOA increased with the increase of PM10 mass concentration, however, the rate of increase of SNA was much faster than that of the SOA. The SOR (sulfur oxidation ratio) and NOR (nitrogen oxidation ratio) increased from non-haze days to hazy days, and increased with the increase of RH. High concentrations of aerosols and water vapor favored the conversion of SO2 to SO4(2-) and NO2 to NO3-, which accelerated the accumulation of the aerosols and resulted in the formation of haze in Beijing.

An overview of emissions of SO2 and NOx and the long-range transport of oxidized sulfur and nitrogen pollutants in East Asia.

The long-range transport of oxidized sulfur (sulfur dioxide (SO2) and sulfate) and oxidized nitrogen (nitrogen oxides (NOx) and nitrate) in East Asia is an area of increasing scientific interest and political concern. This paper reviews various published papers, including ground- and satellite-based observations and numerical simulations. The aim is to assess the status of the anthropogenic emissions of SO2 and NOx and the long-range transport of oxidized S and N pollutants over source and downwind region. China has dominated the emissions of SO2 and NOx in East Asia and urgently needs to strengthen the control of their emissions, especially NOx emissions. Oxidized S and N pollutants emitted from China are transported to Korea and Japan, due to persistent westerly winds, in winter and spring. However, the total contributions of China to S and N pollutants across Korea and Japan were not found to be dominant over longer time scales (e.g., a year). The source-receptor relationships for oxidized S and N pollutants in East Asia varied widely among the different studies. This is because: (1) the nonlinear effects of atmospheric chemistry and deposition processes were not well considered, when calculating the source-receptor relationships; (2) different meteorological and emission data inputs and solution schemes for key physical and chemical processes were used; and (3) different temporal and spatial scales were employed. Therefore, simulations using the same input fields and similar model configurations would be of benefit, to further evaluate the source-receptor relationships of the oxidized S and N pollutants.

Chemical and optical properties of aerosols and their interrelationship in winter in the megacity Shanghai of China

A field campaign on air quality was carried out in Shanghai in winter of 2012. The concentrations of NO, NO2, NOx, SO2, CO, and PM2.5 increased during haze formation. The average masses of SO4(2-), NO3(-) and NH4(+) were 10.3, 11.7 and 6.7 μg/m(3) during the haze episodes, which exceeded the average (9.2, 7.9, and 3.4 μg/m(3)) of these components in the non-haze days. The mean values for the aerosol scattering coefficient (bsp), aerosol absorption coefficient (bap) and single scattering albedo (SSA) were 288.7, 27.7 and 0.91 Mm(-1), respectively. A bi-peak distribution was observed for the mass concentrations of CO, NO, NO2, and NOx. More sulfate was produced during daytime than that in the evening due to photochemical reactions. The mass concentration of NH4(+) achieved a small peak at noontime. NO3(-) showed lower concentrations in the afternoon and higher concentrations in the early morning. There were obvious bi-peak diurnal patterns for bsp and bap as well as SSA. bsp and bap showed a positive correlation with PM2.5 mass concentration. (NH4)2SO4, NH4NO3, organic mass, elemental carbon and coarse mass accounted for 21.7%, 19.3%, 31.0%, 9.3% and 12.3% of the total extinction coefficient during non-haze days, and 25.6%, 24.3%, 30.1%, 8.1% and 8.2% during hazy days. Organic matter was the largest contributor to light extinction. The contribution proportions of ammonium sulfate and ammonium nitrate to light extinction were significantly higher during the hazy time than during the non-haze days.

Chemical characterization of size-resolved aerosols in four seasons and hazy days in the megacity Beijing of China

Size-resolved aerosol samples were collected by MOUDI in four seasons in 2007 in Beijing. The PM10 and PM1.8 mass concentrations were 166.0±120.5 and 91.6±69.7 μg/m3, respectively, throughout the measurement, with seasonal variation: nearly two times higher in autumn than in summer and spring. Serious fine particle pollution occurred in winter with the PM1.8/PM10 ratio of 0.63, which was higher than other seasons. The size distribution of PM showed obvious seasonal and diurnal variation, with a smaller fine mode peak in spring and in the daytime. OM (organic matter=1.6×OC (organic carbon)) and SIA (secondary inorganic aerosol) were major components of fine particles, while OM, SIA and Ca2+ were major components in coarse particles. Moreover, secondary components, mainly SOA (secondary organic aerosol) and SIA, accounted for 46%-96% of each size bin in fine particles, which meant that secondary pollution existed all year. Sulfates and nitrates, primarily in the form of (NH4)2SO4, NH4NO3, CaSO4, Na2SO4 and K2SO4, calculated by the model ISORROPIA II, were major components of the solid phase in fine particles. The PM concentration and size distribution were similar in the four seasons on non-haze days, while large differences occurred on haze days, which indicated seasonal variation of PM concentration and size distribution were dominated by haze days. The SIA concentrations and fractions of nearly all size bins were higher on haze days than on non-haze days, which was attributed to heterogeneous aqueous reactions on haze days in the four seasons.

Chemical characteristics of size-resolved aerosols in winter in Beijing

Size-resolved aerosols were continuously collected by a Nano Sampler for 13 days at an urban site in Beijing during winter 2012 to measure the chemical composition of ambient aerosol particles. Data collected by the Nano Sampler and an ACSM (Aerodyne Aerosol Chemical Speciation Monitor) were compared. Between the data sets, similar trends and strong correlations were observed, demonstrating the validity of the Nano Sampler. PM₁₀ and PM₂.₅ concentrations during the measurement were 150.5 ± 96.0 μg/m³ (mean ± standard variation) and 106.9 ± 71.6 μg/m³, respectively. The PM₂.₅/PM₁₀ ratio was 0.70 ± 0.10, indicating that PM₂.₅ dominated PM₁₀. The aerosol size distributions showed that three size bins of 0.5-1, 1-2.5 and 2.5-10 μm contributed 21.8%, 23.3% and 26.0% to the total mass concentration (TMC), respectively. OM (organic matter) and SIA (secondary ionic aerosol, mainly SO₄(2-), NO₃(-) and NH₄(+)) were major components of PM₂.₅. Secondary compounds (SIA and secondary organic carbon) accounted for half of TMC (about 49.8%) in PM₂.₅, and suggested that secondary aerosols significantly contributed to the serious particulate matter pollution observed in winter. Coal burning, biomass combustion, vehicle emissions and SIA were found to be the main sources of PM₂.₅. Mass concentrations of water-soluble ions and undetected materials, as well as their fractions in TMC, strikingly increased with deteriorating particle pollution conditions, while OM and EC (elemental carbon) exhibited different variations, with mass concentrations slightly increasing but fractions in TMC decreasing.

Effects of NOx and VOCs from Five Emission Sources on Summer Surface O3 over the Beijing-Tianjin-Hebei Region

The impacts of emissions from industry, power plant, transportation, residential, and biogenic sources on daily maximum surface ozone (O3DM) over the Beijing-Tianjin-Hebei (BTH) region in North China in the summer of 2007 were examined in a modeling study. The modeling system consisted of the Weather Research and Forecasting (WRF) model and the photochemical dispersion model, CAMx. The factor separation technique (FST) was used to quantify the effect of individual emission source types and the synergistic interactions among two or more types. Additionally, the effectiveness of emission reduction scenarios was explored. The industry, power plant, and transportation emission source types were found to be the most important in terms of their individual effects on O3DM. The key contributor to high surface O3 was power plant emissions, with a peak individual effect of 40 ppbv in the southwestern BTH area. The individual effect from the biogenic emission category was quite low. The synergistic effects from the combinations of each pair of anthropogenic emission types suppressed O3 formation, while the synergistic effects for combinations of three were favorable for O3 formation when the industrial and power plant emission source types coexisted. The quadruple synergistic effects were positive only with the combination of power plant, transportation, residential, and biogenic sources, while the quintuple synergistic effect showed only minor impacts on O3DM concentrations. A 30% reduction in industrial and transportation sources produced the most effective impacts on O3 concentrations, with a maximum decrease of 20 ppbv. These results suggested that the synergistic impacts among emission source types should be considered when formulating emission control strategies for O3 reduction.

Impacts of uncertainty in AVOC emissions on the summer ROx budget and ozone production rate in the three most rapidly-developing economic growth regions of China

High levels of uncertainty in non-methane volatile organic compound (NMVOC) emissions in China could lead to significant variation in the budget of the sum of hydroxyl (OH) and peroxy (HO2, RO2) radicals (ROx = OH + HO2 + RO2) and the ozone production rate [P(O3)], but few studies have investigated this possibility, particularly with three-dimensional air quality models. We added diagnostic variables into the WRF-Chem model to assess the impact of the uncertainty in anthropogenic NMVOC (AVOC) emissions on the ROx budget and P(O3) in the Beijing-Tianjin-Hebei region, Yangtze River Delta, and Pearl River Delta of China. The WRF-Chem simulations were compared with satellite and ground observations, and previous observation-based model studies. Results indicated that 68% increases (decreases) in AVOC emissions produced 4%–280% increases (2%–80% decreases) in the concentrations of OH, HO2, and RO2 in the three regions, and resulted in 35%–48% enhancements (26%–39% reductions) in the primary ROx production and ∼ 65% decreases (68%–73% increases) of the P(O3) in Beijing, Shanghai, and Guangzhou. For the three cities, the two largest contributors to the ROx production rate were the reaction of O1D + H2O and photolysis of HCHO, ALD2, and others; the reaction of OH + NO2 (71%–85%) was the major ROx sink; and the major contributor to P(O3) was the reaction of HO2 + NO (∼ 65%). Our results showed that AVOC emissions in 2006 from Zhang et al. (2009) have been underestimated by ∼ 68% in suburban areas and by 〉 68% in urban areas, implying that daily and hourly concentrations of secondary organic aerosols and inorganic aerosols could be substantially underestimated, and cloud condensation nuclei could be underestimated, whereas local and regional radiation was overestimated.

Synergistic impacts of anthropogenic and biogenic emissions on summer surface O3 in East Asia

A factor separation technique and an improved regional air quality model (RAQM) were applied to calculate synergistic contributions of anthropogenic volatile organic compounds (AVOCs), biogenic volatile organic compounds (BVOCs) and nitrogen oxides (NOx) to daily maximum surface 03 (O3DM) concentrations in East Asia in summer (June to August 2000). The summer averaged synergistic impacts of AVOCs and NOx are dominant in most areas of North China, with a maximum of 60 ppbv, while those of BVOCs and NOx are notable only in some limited areas with high BVOC emissions in South China, with a maximum of 25 ppbv. This result implies that BVOCs contribute much less to summer averaged O3DM concentrations than AVOCs in most areas of East Asia at a coarse spatial resolution (1 degree x 1 degree) although global emissions of BVOCs are much greater than those of AVOCs. Daily maximum total contributions of BVOCs can approach 20 ppbv in North China, but they can reach 40 ppbv in South China, approaching or exceeding those in some developed countries in Europe and North America. BVOC emissions in such special areas should be considered when 03 control measures are taken. Synergistic contributions among AVOCs, BVOCs and NOx significantly enhance O3 concentrations in the Beijing-Tianjin-Tangshan region and decrease them in some areas in South China. Thus, the total contributions of BVOCs to O3DM vary significantly from day to day and from location to location. This result suggests that 03 control measures obtained from episodic studies could be limited for long-term applications.