Fog-Haze Transition and Drivers in the Coastal Region of the Yangtze River Delta.

Low-visibility events (LVEs) are severe weather phenomena that are closely linked with anthropogenic pollution, which negatively affects traffic, air quality, human health, and the environment. This study conducted a two-month (from October to December 2019) continuous measurement campaign on Chongming Island in Shanghai to characterize the LVEs transition and its drivers. The LVEs accounted for 38% of the time during the campaign, of which mist accounted for 14%, fog-haze for 13%, haze for 6%, and fog for 5%. The fog and mist mainly occurred from midnight to early morning, while haze mostly occurred during the daytime. Different LVEs were interdependent and transitioned from one to another. Fog generally turned into haze after sunrise, while haze turned into fog after sunset. Their formation and evolution were caused by the combined impacts of meteorological conditions and aerosol particles. It was found that temperature difference was the dominant meteorological factor driving the evolution of LVEs. Within the short term, cooling led to a greater increase in relative humidity than humidification. Radiative cooling during the night promoted the formation of fog and mist. During fog and mist events, cloud condensation nuclei (CCN) were mainly internally mixed due to the impact of fog droplet removal and aqueous/heterogeneous aerosol reactions occurring under high humidity. Increased CCN concentration appeared to increase the fog droplet number and liquid water content in fog events. Overall, conditions of high humidity and high particle loading were conducive to LVEs, whereas conditions of sufficient water vapor at a low particle level and sufficient particles at a low humidity level also caused LVEs. This study provided insights into LVEs classification, evolution scheme, and aerosol roles from a micro point of view. The findings could be useful for improving forecasts of local radiative fog and other LVEs.

Effects of shipping emissions on cloud physical properties over coastal areas near Shanghai.

To investigate the effects of shipping aerosols on radiation, cloud physical properties, and near-surface PM2.5, four sensitive experiments with the WRF-Chem model were performed over coastal areas near Shanghai for July 2014. In general, the direct effect of shipping aerosols resulted in negative shortwave (SW) radiation forcing at the land surface. However, when considering the indirect effect, the downward SW radiation at the sea surface declined significantly. By the direct effect, shipping aerosols could modify cloud structure, resulting in a higher cloud base, lower cloud top, and shallower cloud depth. With the indirect effect included, both the cloud base and cloud top showed a declining trend over sea areas. The indirect effect of shipping aerosols was relatively more significant in influencing clouds. For example, the results revealed a 1.2% change of low cloud coverage from the indirect effect but only a 0.1% change due to the direct effect. Through their direct and indirect effects, shipping aerosols cause non-negligible impacts on precipitation, which are concentrated within light precipitation (<0.1 mm h-1). Finally, we concluded that after considering the shipping aerosols, the peak of the cloud droplet spectrum increases by about 50 cm-3/µm. It can be found that when the average volume radius of the cloud droplet is less than 2 µm, the number concentration of cloud droplets increases sharply, and when the average radius of the cloud drop is greater than 2 µm and less than 5 µm, the cloud droplet number concentration drops sharply.

Spatiotemporal changes of surface solar radiation: Implication for air pollution and rice yield in East China.

The changes of surface solar radiation (SSR) have significant implication for air pollution and rice yield. In this study, gridded SSR data, derived from multi-platform datasets and radiation model, were used to analyze its spatiotemporal changes over East China during 2000-2016. The results show SSR experiences dimming during 2000-2005, then turns into brightening till 2016. Both aerosol optical depth (AOD) and single scattering albedo (SSA) contribute to SSR trend. AOD dominates the spatiotemporal changes of SSR in East China, and this impact is higher in the North than the South. SSA has little impact on SSR with low AOD, but its contribution to SSR becomes important as AOD increases. Moreover, gridded planet boundary layer (PBL) was simulated by the Weather Research and Forecasting Model (WRF) and SSR-PBL relationship was also explored. Long-term evidence indicates PBL has a regulatory effect on SSR in the air pollution. Additionally, aerosol-induced radiation reduction can influence rice yield in East China, and it can result in about mean 6.74% reduction in rice yield over East China. Province-level changes of aerosol-induced reduction in rice production were also evaluated and it suggests the impact of aerosols on rice production is non-negligible, especially in Jiangsu and Anhui Province. Our study underscores the importance of aerosol pollution on surface radiation and the mitigation of aerosols is beneficial for crop production under climate change.

The accuracy improvement of clear-sky surface shortwave radiation derived from CERES SSF dataset with a simulation analysis.

Towards the Xiaotang region along the northern margin of the China's largest desert, a quantitative assessment of the precision of clear-sky satellite observations (the Single Scanner Footprint TOA/Surface Fluxes and Clouds downward surface shortwave radiation product of Clouds and the Earth's Radiant Energy System (CERES), DSSRCER) is conducted, the localized inversion mode of "absolutely clear-sky" downward surface shortwave radiation (DSSR) is established, and the "absolutely clear-sky" DSSR in Xiaotang during 2005-2018 is simulated by the Santa Barbara Discrete Atmospheric Radiative Transfer (SBDART) model. In general, under the "absolutely clear-sky" condition of Xiaotang region, there is a significant error in DSSRCER, and the simulated results of SBDART (DSSRSBD) with same input parameters as DSSRCER is better and more comparable. Single scattering albedo (SSA), asymmetry parameter (ASY) and aerosol optical depth (AOD) play crucial roles in deciding the accuracy of DSSR, and after parameter adjustment, the DSSRSBD is better than the initial, which is improved remarkably with all indexes of the fitting results greatly improved. The temporal variation of the DSSR during 2005-2018 indicates that the highest annual average value is found in 2008 (770.00 W·m-2), while the lowest appears in 2010 (600.97 W·m-2). Besides, the highest seasonal mean DSSR appears in summer, which between 860.6 and 935.07 W·m-2, while reaches the lowest in winter (403.79-587.53 W·m-2). Moreover, the monthly average DSSR changes as a curve with a single peak and is close to normal distribution, the highest appears in June (934.61 W·m-2), while the minimum with the value of 390.34 W·m-2 is found in December. All of the solar elevation angle, the characteristics of climate and aerosol particles in different seasons may contribute to the temporal variation.

Long-term variation of satellite-based PM2.5 and influence factors over East China.

With the explosive economic development of China over the past few decades, air pollution has attracted increasing global concern. Using satellite-based PM2.5 data from 2000 to 2015, we found that the available emissions of atmospheric compositions show similar yearly variation trends to PM2.5, even if the synchronization is not met for each composition, implying that the intensity of anthropogenic emissions dominates the temporal variation of PM2.5 in East China. Empirical orthogonal function analysis demonstrates that the dominant variability in the seasonal PM2.5 is closely associated with climate circulation transformation, incarnated as the specific climate index such as the Asia Polar Vortex intensity in spring, the Northern Hemisphere Subtropical High Ridge Position for the leading mode and the Kuroshio Current SST for the second mode in summer, the Asia Polar Vortex Area for the leading mode and the Pacific Polar Vortex Intensity for the second mode in autumn, the NINO A SSTA for the leading mode and the Pacific Decadal Oscillation for the second mode in winter. Therefore, apart from anthropogenic emissions effects, our results also provide robust evidence that over the past 16 years the climate factor has played a significant role in modulating PM2.5 in eastern China.

Influence of pollutants on activity of aerosol cloud condensation nuclei (CCN) during pollution and post-rain periods in Guangzhou, southern China.

Atmospheric pollutions have an important impact on aerosol, condensation nuclei (CN) and cloud condensation nuclei (CCN) loadings near the ground through disturbing particle size, number, chemical composition and reactions, mixing state, hygroscopicity, and so on. Aerosols and CCN were measured in urban Guangzhou during pollution and post-rain periods to examine effects of particulate pollutants on aerosol CCN activity and compare their mechanisms between summer and winter. In contrast with different levels of pollutions, particle matter (PM2.5) and number (CN) and CCN almost showed an opposite trend to aerosol activity (CCN/CN). In summer, new particle formation (NPF) events triggered by photochemical reactions (e.g. O3) always occurred in no-pollution daytime, and increased significantly CN and CCN as a dominant contributor to secondary aerosols. Under pollution conditions, the gas-to-particle transition driven by photochemical reactions guided the formation and aging processes of particles in daytime, especially in changing soluble species, whereas atmospheric oxidation and heterogeneous reactions dominated at night. In winter, stagnant weather conditions, high pollutant levels and relatively high RH were in favor of particle growing and aging through enhancing secondary particle formation and heterogeneous reactions. The wet scavenging of precipitation reduced greatly CCN amount by scouring pre-existing particles in winter, and during post-rain period the photochemical reactions did not promote the burst of secondary particle formation in the absence of ozone, compared with summer. The results may provide insights into the relationship between aerosol moisture absorption and pollution that may be useful for improving air quality.

Impact of meteorological factors on the incidence of childhood hand, foot, and mouth disease (HFMD) analyzed by DLNMs-based time series approach.

BACKGROUND: Hand, foot, and mouth disease (HFMD) has become an emerging infectious disease in China in the last decade. There has been evidence that meteorological factors can influence the HFMD incidence, and understanding the mechanisms can help prevent and control HFMD. METHODS: HFMD incidence data and meteorological data in Minhang District, Shanghai were obtained for the period between 2009 and 2015. Distributed lag non-linear models (DLNMs) were utilized to investigate the impact of meteorological factors on HFMD incidence after adjusting for potential confounders of long time trend, weekdays and holidays. RESULTS: There was a non-linear relationship between temperature and HFMD incidence, the RR of 5th percentile compared to the median is 0.836 (95% CI: 0.671-1.042) and the RR of 95th percentile is 2.225 (95% CI: 1.774-2.792), and the effect of temperature varied across age groups. HFMD incidence increased with increasing average relative humidity (%) (RR = 1.009, 95% CI: 1.005-1.015) and wind speed (m/s) (RR = 1.197, 95% CI: 1.118-1.282), and with decreasing daily rainfall (mm) (RR = 0.992, 95% CI: 0.987-0.997) and sunshine hours (h) (RR = 0.966, 95% CI: 0.951-0.980). CONCLUSIONS: There were significant relationships between meteorological factors and childhood HFMD incidence in Minhang District, Shanghai. This information can help local health agencies develop strategies for the control and prevention of HFMD under specific climatic conditions.

Long-term characteristics of satellite-based PM2.5 over East China.

With the explosive economic development of China over the past few decades, air pollution has become a serious environmental problem and has attracted increasing global concern. Using satellite-based PM2.5 data from 2000 to 2015, we found that the temporal-spatial variation of PM2.5 in East China is characterized by high concentrations in the northern part and low concentrations in the southern part of East China, and by being seasonally high in autumn and winter but low in spring and summer. We also found that the regional average PM2.5 concentration shows an approximative peak pattern over the last 16years, with the highest, 60.13µgm-3, and the lowest, 46.18µgm-3, occurring in 2007 and 2000, respectively. Despite obviously diminishing heavy polluted regions with a PM2.5 of >80µgm-3 after 2011, those cells dominated by natural background have still not recovered back to the clean level of 2000. These characteristics are valuable information to analyze the relative contributions of anthropogenic emissions and atmospheric conditions to the temporal-spatial variation characteristics of PM2.5.

Comparison of aerosol and cloud condensation nuclei between wet and dry seasons in Guangzhou, southern China.

Cloud condensation nuclei (CCN), condensation nuclei (CN) and aerosol chemical composition were measured simultaneously at an urban site of Guangzhou from July to August 2015 and in January 2016, and the seasonal variations of aerosol activated fractions (NCCN/NCN) as well as their relevant influence factors were further studied accordingly. NCN is generally higher in winter (dry season), whereas NCCN and NCCN/NCN are mostly higher in summer (wet season) instead. In particular, NCCN and NCCN/NCN are much lower at smaller supersaturation levels (SS<0.2) in winter. In spite of similar diurnal variations for NCCN and NCN, NCCN/NCN indicates an opposite tendency, relatively lower at midday, dusk and before midnight. Other than the size of particles as well as their chemical composition, some other factors, such as mass, gas precursors, pollutant transportation, meteorological conditions, etc., also contribute to the variations of NCCN and NCCN/NCN. Particles from the local source or local-oceanic combination source cast influence on CN and CCN significantly, while the pollutants originating from and crossing over distant polluted areas contribute largely to CCN/CN. NCN and NCCN are relatively higher under pollution-free conditions in summertime and polluted conditions in wintertime, but NCCN/NCN is just the opposite. On various polluted conditions, aerosol CCN activities are greatly discrepant between summer and winter, especially during mist or heavy haze periods. The results imply that anthropogenic pollutants exert critical impacts on aerosol CCN activation.

Vertical distribution of ambient aerosol extinctive properties during haze and haze-free periods based on the Micro-Pulse Lidar observation in Shanghai.

Ambient aerosols make a significant contribution to the environment and climate through their optical properties. In this study, the aerosol extinction coefficient and Aerosol optical depth (AOD) retrieved using the Fernald Method from the ground-based Micro-Pulse Lidar (MPL) were used to investigate the characteristics of aerosols during haze and haze-free periods in Shanghai. There were 216 haze days including 145 dry haze days, 39 damp haze days and 32days of both dry and damp haze in Shanghai from March 2009 to February 2010. During the haze periods, aerosols were concentrated mainly below 600m resulting in the most severe pollution layer in Shanghai. In contrast to the aerosol optical properties during haze-free periods, aerosol extinction coefficients and AOD were larger in the lower altitude (below 1km) during haze periods. The lowest 1km contributed 53-72% of the Aerosol optical depth (AOD) below 6km for the haze periods and <41% of that for the haze-free periods except summer. According to the analysis of influencing factors, although atmospheric convection was strong in summer which led to reduce the extinction, the highest occurrence of haze with relatively low aerosol extinction most of time was in summer, which resulted from the factors such as higher relative humidity, temperature and more solar radiation causing hygroscopic growth of particles and formation of secondary aerosols; in spring and autumn, there was less haze occurrences because the boundary layer was relatively higher, which allowed pollutants to diffuse more easily, but spring was the second most frequency season of haze due to frequent dust transport from the north; in winter high concentrations of particles and low boundary layer height were not beneficial to the diffusion of pollutants near the surface and caused haze occurrence rather high with high aerosol extinction.

[Analysis of characteristics of black carbon concentration in Shanghai from 2008 to 2012].

The variations of black carbon (BC) mass concentration in Shanghai are analyzed and discussed by using hourly averaged data monitored continuously at Shanghai Urban Environmental Meteorological Center from January 2008 to December 2012. The results show that the annual mean mass concentration of BC decreased from (4 045.3 +/- 3 375.4) ng x m(-3) in 2008 to (2 766.2 +/- 2 078.9) ng x m(-3) in 2012, and the annual changes are 2.3%, - 6.5%, -18.7% and -12.1%, respectively. The yearly averaged BC mass concentration in Shanghai is on the low side of middle-level compared with other mega-cities of China. According to the test data,the highest monthly averaged concentration of BC appeared in November and December,which were 5 426.6 ng x m(-3) and 5 365.3 ng x m(-3), respectively,and then in January, June and October, which were 4402.5, 3763.3 and 3412.7 ng x m(-3), respectively. The diurnal cycles of the BC mass concentration show that there are two obvious peaks during morning 07:00-10:00 and during evening 18:00-22:00 Beijing time (BT), and the first peak was higher than the second on weekdays, but opposite on weekends and holidays. The daily mean mass concentration of BC was 9% higher on weekdays than on weekends and holidays. In addition, an empirical equation is obtained for daily BC concentration estimation and prediction using all the effective test data during the 5-year observation period and employing the regression analysis.