Spatiotemporal variations and reduction of air pollutants during the COVID-19 pandemic in a megacity of Yangtze River Delta in China.

In recent decades, air pollution has become an important environmental problem in the megacities of eastern China. How to control air pollution in megacities is still a challenging issue because of the complex pollutant sources, atmospheric chemistry, and meteorology. There is substantial uncertainty in accurately identifying the contributions of transport and local emissions to the air quality in megacities. The COVID-19 outbreak has prompted a nationwide public lockdown period and provides a valuable opportunity for understanding the sources and factors of air pollutants. The three-month period of continuous field observations for aerosol particles and gaseous pollutants, which extended from January 2020 to March 2020, covered urban, urban-industry, and suburban areas in the typical megacity of Hangzhou in the Yangtze River Delta in eastern China. In general, the concentrations of PM2.5-10, PM2.5, NOx, SO2, and CO reduced 58%, 47%, 83%, 11% and 30%, respectively, in the megacity during the COVID-Lock period. The reduction proportions of PM2.5 and CO were generally higher in urban and urban-industry areas than those in suburban areas. NOx exhibited the greatest reduction (>80%) among all the air pollutants, and the reduction was similar in the urban, urban-industry, and suburban areas. O3 increased 102%-125% during the COVID-Lock period. The daytime elevation of the planetary boundary layer height can reduce 30% of the PM10, PM2.5, NOx and CO concentrations on the ground in Hangzhou. During the long-range transport events, air pollutants on the regional scale likely contribute 40%-90% of the fine particles in the Hangzhou urban area. The findings highlight the future control and model forecasting of air pollutants in Hangzhou and similar megacities in eastern China.

Seasonal variation of atmospheric vertical extinction and its interaction with meteorological factors in the Yangtze River Delta region.

Based on ground-based lidar and microwave radiometer observations in Hangzhou from 1 January 2013 to 31 December 2015, the monthly characteristics of diurnal extinction as well as atmospheric boundary layer (ABL) were studied. The interactions between temperature (T), humidity fields including relative humidity (RH) and specific humidity (SH) and atmospheric stratification (AS) were analyzed to discuss the meteorological factors in the Yangtze River Delta region during the study period. The top of ABLMPL varied from 0.8 km to 1.0 km throughout in January with higher extinction intensity close to the surface combined to the largest PM2.5 about 100-120 µg/m3. Then the ABLMPL could develop up to 1.5 km in the spring due to the weaker extinction during the daytime. The RH in the whole column in January and December was lower than the mean value (ranging from 5% to 20%) distributed from the ground to 3 km. From May to September, the RH anomaly profiles became positive contributed to larger extinction by strengthened the particle scattering ability. In January and December, the AS was stable from the surface to 3 km coincided with the extinction distribution; while in July and August, the gradient of Δθse decreased which favored the diffusion of particle in the air. Moreover, April and October presented turning points in the variation of θse. The humidity field reveals a stable condition in January and December which favored particles suppressed from the near surface to 3 km; the temperature field has tended towards a neutral state in most months except for February. The first change-point of the meteorological fields was found in April possibly attributable to the abnormal abrupt in the subtropical high. This study could have important reference for understanding regional air quality and governing air control.

[Column-integrated Aerosol Optical Properties Determined Using Ground-based Sun Photometry Measurements in the Hangzhou Region].

To investigate the optical properties of aerosols in the Hangzhou region (Hangzhou, Tonglu, Jiande, and Chun'an), the aerosol optical depth (AOD), Ångström exponent (AE), single scattering albedo (SSA), and aerosol size distribution (ASD) were measured using CIMEL sun-photometers in 2012. The results showed that the annual average values of AOD440nm in Hangzhou, Tonglu, Jiande, and Chun'an were 0.94±0.16, 0.84±0.17, 0.82±0.22, and 0.71±0.20, respectively. The values generally decreased from the northeast to the southwest, and represented one of highest AOD districts in the Yangtze River Delta, China. The annual average values of AE440-870nm were 1.24±0.12, 1.19±0.17, 1.06±0.04, and 1.04±0.10, respectively, indicating that particles with small average effective radii were predominant. The relatively lower AE values in March and April were generally attributed to the long-range transport of dust aerosols from Northwest China. Obvious diurnal variations in the AOD were found in Hangzhou, Tonglu, and Jiande, but not in Chun'an. An average fine-mode effective radius of~0.15 µm was observed in spring, autumn, and winter, while a value of~0.25 µm was observed in summer, in conjunction with aerosol hygroscopic growth. An average coarse-mode effective radius of~2.94 µm was observed in summer, autumn, and winter, which was higher than the value in spring. The annual average values of SSA440nm were 0.91±0.01, 0.92±0.03, 0.92±0.02, 0.93±0.02, respectively, indicating that the particles had relatively strong to moderate absorption. Characterization of the aerosol types showed the predominance of biomass burning and urban industrial type aerosols in Hangzhou, while mixed type aerosols were observed in Tonglu, Jiande, and Chun'an.

Mortality burden attributable to PM1 in Zhejiang province, China.

BACKGROUND: Limited evidence is available on the health effects of particulate matter with an aerodynamic diameter of <1 µm (PM1), mainly due to the lack of its ground measurement worldwide. OBJECTIVES: To identify and examine the mortality risks and mortality burdens associated with PM1, PM2.5, and PM10 in Zhejiang province, China. METHODS: We collected daily data regarding all-cause (stratified by age and gender), cardiovascular, stroke, respiratory, and chronic obstructive pulmonary disease (COPD) mortality, and PM1, PM2.5, and PM10, from 11 cities in Zhejiang province, China during 2013 and 2017. We used a quasi-Poisson regression model to estimate city-specific associations between mortality and PM concentrations. Then we used a random-effect meta-analysis to pool the provincial estimates. To show the mortality burdens of PM1, PM2.5, and PM10, we calculated the mortality fractions and deaths attributable to these PMs. RESULTS: Daily concentrations of PM1, PM2.5, and PM10 ranged between 0-199 µg/m3, 0-218 µg/m3, and 0-254 µg/m3, respectively; Mortality effects were significant in lag 0-2 days. The relative risks for all-cause mortality were 1.0064 (95% CI: 1.0034, 1.0094), 1.0061 (95% CI: 1.0034, 1.0089), and 1.0060 (95% CI: 1.0038, 1.0083) associated with a 10 µg/m3 increase in PM1, PM2.5, and PM10, respectively. Age- and gender-stratified analysis shows that elderly people (aged 65+) and females are more sensitive to PMs. The mortality fractions of all-cause mortality were estimated to be 2.39% (95% CI: 1.28, 3.48) attributable to PM1, 2.53% (95% CI: 1.42, 3.63) attributable to PM2.5, and 3.08% (95% CI: 1.95, 4.19) attributable to PM10. The ratios of attributable cause-specific deaths for PM1/PM2.5, PM1/PM10, and PM2.5/PM10 were higher than the ratios of their respective concentrations. CONCLUSIONS: PM1, PM2.5 and PM10 are risk factors of all-cause, cardiovascular, stroke, respiratory, and COPD mortality. PM1 accounts for the vast majority of short-term PM2.5- and PM10-induced mortality. Our analyses support the notion that smaller size fractions of PM have a more toxic mortality impacts, which suggests to develop strategies to prevent and control PM1 in China, such as to foster strict regulations for automobile and industrial emissions.

Effects of auxiliary atmospheric state parameters on the aerosol optical properties retrieval errors of high-spectral-resolution lidar.

A detailed assessment is carried out in relation to the influence of the uncertainties associated with the input auxiliary atmospheric state parameters on retrieving aerosol optical properties from high-spectral-resolution lidar (HSRL) observations. The study starts from a review of the main spectral structure of the Rayleigh backscattering followed by evaluating the temperature effects on a backscattering cross section of atmospheric molecules based on numerical simulation. It shows that the transmittance of the background interference filter should be taken into account, depending on the full width at half maximum, although overall temperature dependence is negligible. Based on the Taylor expansion of the Tenti S6 model, the systematic errors arising from input temperature and pressure profiles are analyzed. It is demonstrated that the atmospheric pressure profiles have limited effects on the inversion results of aerosol optical parameters, as the atmospheric pressure is usually quite stable. The relative errors of the aerosol backscatter coefficient mainly stem from temperature profile errors and highly depend on the aerosol concentration. Quantitatively, the aerosol backscatter coefficient error could be larger than 5% with a 3 K deviation of temperature when the backscatter ratio is larger than 1.1. The accuracy of aerosol extinction coefficient retrieval is affected not only by the error in temperature, but also by the error in temperature lapse rate; the retrieval accuracy is more sensitive to the latter than the former. Further analysis based on the sounding temperature data shows that the variation of the temperature inversion layer during the night could induce a bias larger than 0.04 km-1 on the aerosol extinction coefficient retrieval. Therefore, the time resolution of temperature measurement from sounding balloons twice per day is too low to obtain an accurate retrieval of the aerosol optical properties from the HSRL.