Source apportionments of black carbon induced by local and regional transport in the atmospheric boundary layer of the Yangtze River Delta under stable weather conditions.

Long-range transport (LRT) and local accumulation (LA) are key atmospheric physical processes affecting air pollution formation, and their impacts on surface air pollution have been extensively researched. Due to the lack of vertical observations and emphases on model simulations, the characteristics and regional sources of black carbon (BC) aerosol profiles have been relatively understudied. In this study, the chemistry-coupled Weather Research and Forecasting model (WRF-Chem) with a BC source-tagging method was used to quantify BC source contributions (considering 18 geographical regions over east Asia, including 3 subregions over the Yangtze River Delta (YRD)) during a November 2017 pollution event in the YRD, China. In this event, the YRD mainly experienced a uniform pressure field, stable weather and weak wind fields. During the uniform high-pressure period, the dominant contribution to surface BC in each sub-region was from that region itself (70.6 %-98.2 %), with little intra- and inter-regional transport. During the uniform low-pressure period, highly variable contributions to the surface BC from intra-regional transport within the YRD (0.05 %-65.9 %) and inter-regional transport outside the YRD (mostly from Anhui (AH) to the west of the YRD, 0.37 %-23.9 %) were simulated. In the vertical direction, the dominant contributors were local YRD sources (73.8 %-94.2 %) below the atmospheric boundary layer (ABL). The inter-transport contributions increased extensively above the ABL. As a westerly trough crossing, a long-range inter-regional transport from South China (SCHN, 3.3 %) and the North China Plain (NCP, 2.7 %) was simulated above the ABL. We found that when the surface experienced similar stable weather conditions, the weather conditions in the upper air may have been quite different, resulting in significant differences in the regional transport of BC in the upper ABL. This study provides a reference for improving air quality from the local scale to the regional scale.

[Source Apportionment and Optical Properties of Fine Particles Associated with Regional Pollution in the Yangtze River Delta].

In order to study the distribution and optical characteristics of fine particulate matter pollution in the Yangtze River Delta, PM2.5 samples were collected and analyzed from city (Suzhou), suburb (Nanjing), and regional background monitoring stations (Lin'an). The average concentrations of PM2.5 in Suzhou, Nanjing, and Lin'an were (169.8±56.5), (169.9±51.2), and (154.0±54.9) µg·m-3, respectively. There was little difference in meteorological conditions and the chemical composition of PM2.5 among the three sites, and PM2.5 pollution showed significant synchronization and regionalization characteristics. The extinction coefficients estimated using a chemical component method for Suzhou, Nanjing, and Lin'an are (561±223), (655±340), and (679±349) Mm-1, respectively. There is strong correlation between the extinction coefficients estimated by the chemical component method and those estimated by a visibility-based method (r 0.73-0.80). Using a PMF model to analyze the PM2.5 sources, secondary nitrate sources (32%) and secondary sulfate sources (25%) accounted for the largest proportions followed by biomass combustion (16%), incomplete combustion (7%), fuel combustion (7%), soil crusts (8%), and marine sources (5%). The primary sources of the extinction coefficients of PM2.5 are secondary nitrate and sulfate sources, incomplete combustion, and biomass combustion. Compared with the source contribution of mass concentrations, the proportion of secondary nitrate and sulfate sources decreased by approximately 4% and the proportion of the incomplete combustion source increased by 5%. These results show that there are differences in the contributions of various PM2.5 source according to mass concentrations and extinction coefficients.

Stable and transport indices applied to winter air pollution over the Yangtze River Delta, China.

Previous studies have developed a stable weather index (SWI) based on meteorological elements that adequately represent PM2.5 pollution over the North China Plain (NCP). However, the SWI performs poorly over the Yangtze River Delta (YRD) region because air pollution over this region is affected not only by stagnant weather (STAG) but also by transport (TRANS). For example, air pollutants can be transported from the NCP to the YRD by cold fronts. In this study, an obliquely rotated principal component analysis in the T-model is applied to classify the synoptic patterns of winter weather over the YRD region from 2013 to 2018. Among the four identified synoptic patterns, two of which cause TRANS, one pattern is most likely to cause STAG, and one pattern can lead to either STAG or TRANS depending on the location of high pressure around Shandong province. Due to the large contribution (63%) of TRANS to the total PM2.5 pollution events, a transport pollution index (TPI) is constructed to describe the transport features of PM2.5 pollution over the YRD region. Our results show that, when considering the SWI alone, the correlation coefficients between the SWI and ln(PM2.5) range from 0.50 to 0.57 in the main cities of the YRD. Excitingly, when considering both the TPI and SWI (TPI+SWI), the correlation coefficients increase significantly to 0.63-0.78, suggesting that TPI+SWI better reflects the wintertime PM2.5 pollution level over the YRD region. In addition, satisfactory performance in validation also suggests that TPI+SWI can increase the accuracy of evaluating and forecasting of PM2.5 pollution episodes over regions downstream of source emissions.

Quantifying Arctic lower stratospheric ozone sources in winter and spring.

The dynamical and chemical characteristics of unusually low Arctic ozone events in 2005 and 2011 have been well-studied. However, the quantitative identification of Arctic ozone sources is lacking. Here, we use tagged ozone tracers in a numerical simulation to quantify the contributions to Arctic lower stratospheric ozone (ARCLS_O3) at diverse latitudes in winter and spring from 2005-2011. We demonstrate that the northern mid-latitudinal stratosphere steadily contributes approximately half of ARCLS_O3. The absolute contributions during February have evident variations, which are smaller in cold years (151.3 ± 7.0 Dobson units (DU) in 2005 and 139.0 ± 7.4 DU in 2011) and greater in warm years (182.6 ± 7.3 DU in 2006 and 164.6 ± 7.4 DU in 2009). The tropical stratosphere is also an important source. During February, its absolute contributions are 66.5 ± 11.5 DU (2005), 73.1 ± 4.7 DU (2011), 146.0 ± 9.0 DU (2006), and 153.7 ± 7.0 DU (2009). Before and after stratospheric warming, variations in the tropical components of ARCLS_O3 (51.8 DU in 2006 and 77.0 DU in 2009) are significantly larger than those in the mid-latitudinal components (17.6 DU in 2006 and 18.1 DU in 2009). These results imply that although the mid-latitudinal components of ARCLS_O3 are larger, the tropical components control stratospheric temperature-induced ARCLS_O3 anomalies in winter and spring.

Spatial and Temporal Distributions of Air Pollutants and Size Distribution of Aerosols over Central and Eastern China.

The origins and spatial and temporal distributions of air pollutants (PM2.5, PM10, CO, SO2, NO2 and O3) during May to June of 2015 were investigated using data from 1490 monitoring sites in China. Aerosol number concentrations and meteorological data from Shijiazhuang, Nanjing, and Suzhou were combined with the MIX Asian emission data and the HYSPLIT model. Furthermore, the diurnal variation, size distribution, and main sources of air pollutants and aerosols were selectively characterized in the North China Plain (NCP) and the Yangtze River Delta (YRD). High values of particulate matter concentrations (PM), including PM2.5 and PM10, occurred in the northwestern and central regions of eastern China. Elevated PM2.5 and PM10 concentrations represented natural dust sources and anthropogenic resident, power plant, industry, and traffic emissions sources, respectively. The concentrated distributions of SO2 were similar to those of PM. The CO concentrations were distributed uniformly in China. High O3 values occurred above the Qinghai province. During the observation period, the air masses mainly originated from the northwest NCP and from the southwest or northeastern ocean in the YRD, resulting in high concentrations of PM2.5, PM10, SO2, and CO in the NCP, the average values of which were 61.8 ± 40.0, 118.8 ± 66.4, 24.1 ± 24.6 µg m-3, and 1.2 ± 0.9 mg m-3, respectively, and were 1.2, 1.4, 1.5, and 1.3 times larger than those in the YRD. NO2 had higher concentrations in the YRD with an average of 43.7 ± 24.8 µg m-3, which was 1.2 times larger than that in the NCP. The diurnal variations of PM, NO2 and CO had bimodal distributions and SO2 and O3 had unimodal distributions in the NCP and YRD. The aerosol number concentrations had average values of 12,661 ± 5266, 11,189 ± 5905, and 12,797 ± 5931 cm-3 in Shijiazhuang, Nanjing, and Suzhou. Their diurnal variations displayed trimodal peaks at 18:00-21:00, 11:00-14:00, and 06:00-08:00, and their spectra distributions were all unimodal with peaks at 60-70, 60-70, and 100-110 nm, respectively.

The Effect of Elevated Ozone Concentrations with Varying Shading on Dry Matter Loss in a Winter Wheat-Producing Region in China.

Surface-level ozone pollution causes crop production loss by directly reducing healthy green leaf area available for carbon fixation. Ozone and its precursors also affect crop photosynthesis indirectly by decreasing solar irradiance. Pollutants are reported to have become even more severe in Eastern China over the last ten years. In this study, we investigated the effect of a combination of elevated ozone concentrations and reduced solar irradiance on a popular winter wheat Yangmai13 (Triticum aestivum L.) at field and regional levels in China. Winter wheat was grown in artificial shading and open-top-chamber environments. Treatment 1 (T1, i.e., 60% shading with an enhanced ozone of 100±9 ppb), Treatment 2 (T2, i.e., 20% shading with an enhanced ozone of 100±9 ppb), and Control Check Treatment (CK, i.e., no shading with an enhanced ozone of 100±9 ppb), with two plots under each, were established to investigate the response of winter wheat under elevated ozone concentrations and varying solar irradiance. At the field level, linear temporal relationships between dry matter loss and cumulative stomatal ozone uptake were first established through a parameterized stomatal-flux model. At the regional level, ozone concentrations and meteorological variables, including solar irradiance, were simulated using the WRF-CMAQ model (i.e., a meteorology and air quality modeling system). These variables were then used to estimate cumulative stomatal ozone uptake for the four major winter wheat-growing provinces. The regional-level cumulative ozone uptake was then used as the independent variable in field data-based regression models to predict dry matter loss over space and time. Field-level results showed that over 85% (T1: R(2) = 0.85 & T2: R(2) = 0.89) of variation in dry matter loss was explained by cumulative ozone uptake. Dry matter was reduced by 3.8% in T1 and 2.2% in T2 for each mmol O3·m(-2) of cumulative ozone uptake. At the regional level, dry matter loss in winter wheat would reach 50% under elevated ozone concentrations and reduced solar irradiance as determined in T1, and 30% under conditions as determined in T2. Results from this study suggest that a combination of elevated ozone concentrations and reduced solar irradiance could result in substantial dry matter loss in the Chinese wheat-growing regions.

A case study of surface ozone source apportionment during a high concentration episode, under frequent shifting wind conditions over the Yangtze River Delta, China.

Surface ozone is an environmental issue occurring at several scales, ranging from local to continental. One of the most developed regions in China, the Yangtze River Delta (YRD), experiences severe tropospheric ozone problem. Hence, quantifying the contributions from various geographical source regions is helpful for better understanding the regional ozone problem. Ozone source apportionment studies can provide relevant information for designing suitable air pollution protection strategies. In the present work, the WRF-Chem model coupled with an online ozone tagging method is applied to a case study, with the objective of exploring the ozone contributions to the surface ozone from different source regions over the YRD region, during a frequent wind-shifting period. Our results show that the YRD was highly affected by the upwind source regions bearing high values both ozone and its precursors. The contribution from the source region outside the main air pollution zones in the Central Eastern China (super regional contribution) was also important, accounting for more than 30 ppb of daytime maximum mean ozone concentrations. Ozone arising from increased local and regional emissions during high-concentration events was more significant than super regional contribution. It reveals that the ozone from Anhui region was transported through vertical mixing and horizontal advection to receptor areas in the YRD during the study time focus. Chemical process contributed significantly at ground and high altitude levels of 500 and 1000 m. However, most of the ozone from the remote regions of Henan and Hubei provinces was transported to the receptor of Nanjing through physical processes. The vertical mixing process played a crucial positive role at super regional scales, with regard to the formation of surface ozone over the YRD region during the addressed time interval.

[Modeling Study of A Typical Summer Ozone Pollution Event over Yangtze River Delta].

WRF/Chem model was used to analyze the temporal and spatial distribution characteristics and physical and chemical mechanism of a typical summer ozone pollution event over Yangtze River Delta (YRD). The result showed that the model was capable of reproducing the temporal and spatial distribution and evolution characteristics of the typical summer ozone pollution event over YRD. The YRD region was mainly affected by the subtropical high-pressure control, and the weather conditions of sunshine, high temperature and small wind were favorable for the formation of photochemical pollution on August 10-18, 2013. The results of simulation showed that the spatial and temporal distribution of O3 was obviously affected by the meteorological fields, geographic location, regional transport and chemical formation over YRD. The sensitivity experiment showed that the O3 concentration affected by maritime airstream was low in Shanghai, but the impact of Shanghai emissions on the spatial and temporal distribution of O3 concentration over YRD was significant; The main contribution of the high concentration of O3 in Nanjing surface was chemical generation ( alkene and aromatic) and the vertical transport from high-altitude O3, whereas the main contribution of the high concentration of O3 in Hangzhou and Suzhou was physics process. The influence of the 15:00 peak concentration of O3 over YRD was very obvious when O3 precursor was reduced at the maximum O3 formation rate (11-13 h).

[Analysis of the impact of two typical air pollution events on the air quality of Nanjing].

Nanjing and the surrounding area have experienced two consecutive serious air pollution events from late October to early November in 2009. The first event was long-lasting haze pollution, and the second event was resulted from the mixed impact of crop residue burning and local transportation. The effects of regional transport and local sources on the two events were discussed by cluster analysis, using surface meteorological observations, air pollution index, satellite remote sensing of fire hot spots data and back trajectory model. The results showed that the accumulation-mode aerosol number concentrations were higher than those of any other aerosol modes in the two pollution processes. The peak value of aerosol particle number concentrations shifted to large particle size compare with the previous studies in this area. The ratio of SO4(2-)/NO3(-) was 1.30 and 0.99, indicating that stationary sources were more important than traffic sources in the first event and the reverse in the second event. Affected by the local sources from east and south, the particle counts below 0.1 microm gradually accumulated in the first event. The second event was mainly affected by a short-distance transport from northeast and local sources from southwest, especially south, the concentration of aerosol particles was higher than those in other directions, indicating that the sources of crop residue burning were mainly in this direction.

[Atmospheric particle formation events in Nanjing during summer 2010].

Feature of aerosol particle number concentration, condition and impact factor of new particle formation (NPF) were investigated in Nanjing during summer. In this study, aerosol particle number concentration and gaseous pollutants (O3, SO2 and NO2) measurements were carried out by Wide-Range Particle Spectrometer (WPS) and Differential Optical Absorption Spectroscopy (DOAS) in July 2010. Combining with observations from Automatic Weather Station and Backward Trajectory Simulation, the condition and impact factor of NPF were discussed. Results showed that the averaged 10-500 nm particle number concentration was 1.7 x 10(4) cm(-3), similar to some typical observation values in North American and Europe; the 10-25 nm particle number concentration accounted for 25% of the total number concentration. Six NPF events occurred during observation. We analyzed that stable wind speed and direction, strong solar radiation promoted the NPF. The humidity during NPF event varied from 50% to 70%. Results indicated that clean ocean air mass brought from easterly and southerly wind promoted the NPF by Backward Trajectory Model Simulation. During the NPF event, the 10 - 25 nm particle number concentration positively correlated with the concentration of SO2, and negatively correlated with O3, whereas poorly correlated with NO2.

Size distributions of aerosol and water-soluble ions in Nanjing during a crop residual burning event.

To investigate the impact on urban air pollution by crop residual burning outside Nanjing, aerosol concentration, pollution gas concentration, mass concentration, and water-soluble ion size distribution were observed during one event of November 4-9, 2010. Results show that the size distribution of aerosol concentration is bimodal on pollution days and normal days, with peak values at 60-70 and 200-300 nm, respectively. Aerosol concentration is 10(4) cm(-3) x nm(-1) on pollution days. The peak value of spectrum distribution of aerosol concentration on pollution days is 1.5-3.3 times higher than that on a normal day. Crop residual burning has a great impact on the concentration of fine particles. Diurnal variation of aerosol concentration is trimodal on pollution days and normal days, with peak values at 03:00, 09:00 and 19:00 local standard time. The first peak is impacted by meteorological elements, while the second and third peaks are due to human activities, such as rush hour traffic. Crop residual burning has the greatest impact on SO2 concentration, followed by NO2, O3 is hardly affected. The impact of crop residual burning on fine particles (< 2.1 microm) is larger than on coarse particles (> 2.1 microm), thus ion concentration in fine particles is higher than that in coarse particles. Crop residual burning leads to similar increase in all ion components, thus it has a small impact on the water-soluble ions order. Crop residual burning has a strong impact on the size distribution of K+, Cl-, Na+, and F- and has a weak impact on the size distributions of NH4+, Ca2+, NO3- and SO4(2-).