Nocturnal Low-levelWinds and Their Impacts on Particulate Matter over the Beijing Area

Three-month wind profiles, 260 m PM_1 concentrations [i.e., particulate matter(PM) with an aerodynamic diameter ≤1μm], and carrier-to-noise ratio data at two Beijing sites 55 km apart(urban and suburban) were collected to analyze the characteristics of low-level nocturnal wind and PM in autumn and winter. Three mountain-plain wind events with wind shear were selected for analysis. The measurements indicated that the maximum wind speeds of the northerly weak low-level jet(LLJ) below 320 m at the suburban site were weaker than those at the urban site, and the LLJ heights and depths at the suburban site were lower than those at the urban site. The nocturnal 140 m mean vertical velocities and the variations in vertical velocity at the urban site were larger than those at the suburban site. A nocturnal breeze with a weak LLJ of ～3 m s^(-1) noticeably offset nocturnal PM transport due to southerly flow and convergence within the northern urban area of Beijing. Characteristics of the nocturnal LLJ, such as start-up time, structure, intensity, and duration, were important factors in determining the decrease in the nocturnal horizontal range and site-based low-level variations in PM.

Satellite estimates and subpixel variability of rainfall in a semi-arid grassland

卫星估雨精度的不确定性受到当地降雨类型和像元内降雨非均匀性影响,而结合这两个关键因素开展半干旱草原卫星估雨的研究有限.2009年夏,我们在中国锡林郭勒半干旱草原用多部微雨雷达和雨量计构建了9 km卫星像元降雨观测网,观测了像元内降雨非均匀性(空间变异系数CV),并评估了卫星估雨精度.结果表明:(1) CV值受像元内平均降雨量,降雨类型,降雨云面积及移向等影响,如高CV值的降雨过程大多为平均降雨量小,对流性降雨过程,降雨云边缘像元CV值较高;(2) TRMM 3B42V7卫星估雨产品适用性较好,CMORPH和PERSIANN次之,但TRMM3B42V7易在半干旱草原湖泊处高估降雨.

Characteristics of one-year observation of VOCs,NOx, and O_3 at an urban site in Wuhan, China

A continuous online observation of ozone and its precursors(NOx, VOCs) was carried out in central urban Wuhan from September 2016 to August 2017. The concentration levels of ozone,NOx, VOCs and their variations in urban Wuhan were analyzed, as well as effects of VOCs on ozone photochemical generation and the main controlling factors for ozone production. During the observation period, the average concentrations of ozone and NOx in Wuhan was 22.63 and30.14 ppbv, respectively, and the average concentration of VOCs was 32.61 ppbv(42.3% alkanes,13.0% alkenes, 10.0% aromatics, 7.3% acetylene, 9.9% OVOCs, and 10.5% halohydrocarbons).Ozone concentration was higher in spring and summer as compared with autumn and winter,wheras VOCs and NOx concentratios were lower in spring and summer but higher in autumn and winter. Aromatics and alkenes, two of VOCs species, showed the highest contributions to ozone formation potential in Wuhan(35.7% alkenes, 35.4 aromatics, 17.5% alkanes, 8.6% OVOCs,1.6% halogenated hydrocarbons, and 1.4% acetylene). Among all VOCs species, those with the highest contribution were ethylene, m-xylene, toluene, propylene and o-xylene. The contribution of these five compounds to the total ozone formation potential concentration was 43.90%.Ozone-controlling factors in Wuhan changed within one day; during the early morning hours(6:00–9:00), VOCs/NOx was low, and ozone generation followed a VOCs-limited regime.However, during the peak time of ozone concentration(12:00–16:00), the ratio of VOCs/NOx was relatively high, suggesting that ozone generation followed a NOx-limited regime.

Effect of potential HONO sources on peroxyacetyl nitrate(PAN)formation in eastern China in winter

As an important secondary photochemical pollutant,peroxyacetyl nitrate(PAN)has been studied over decades,yet its simulations usually underestimate the corresponding observations,especially in polluted areas.Recent observations in north China found unusually high concentrations of PAN during wintertime heavy haze events,but the current model still cannot reproduce the observations,and researchers speculated that nitrous acid(HONO)played a key role in PAN formation.For the first time we systematically assessed the impact of potential HONO sources on PAN formation mechanisms in eastern China using the Weather Research and Forecasting/Chemistry(WRF-Chem)model in February of 2017.The results showed that the potential HONO sources significantly improved the PAN simulations,remarkably accelerated the RO x(sum of hydroxyl,hydroperoxyl,and organic peroxy radicals)cycles,and resulted in 80%–150%enhancements of PAN near the ground in the coastal areas of eastern China and 10%–50%enhancements in the areas around 35–40°N within 3 km during a heavy haze period.The direct precursors of PAN were aldehyde and methylglyoxal,and the primary precursors of PAN were alkenes with C>3,xylenes,propene and toluene.The above results suggest that the potential HONO sources should be considered in regional and global chemical transport models when conducting PAN studies.

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 PM1.8mass concentrations were 166.0 ± 120.5 and 91.6 ± 69.7 μg/m^3, 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 Ca^2＋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, Ca SO4, 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.

Characteristics and formation mechanism of regional haze episodes in the Pearl River Delta of China

To investigate the characteristics and the specific mechanism of continuous haze,comprehensive measurements were conducted from 15 October to 19 November in the Atmospheric Environment Monitoring Super-Station in Heshan of Guangdong province.Five haze episodes occurred in October and November 2014 in the Pearl River Delta（PRD）region. The meteorological parameters, gas data, chemical compositions, and optical parameters of the aerosols were obtained. Among these events, the second haze episode,with the highest concentration of PM2.5 of 187.51 μg/m^3, was the most severe. NO^3-was always higher than SO_4^（2-）, which indicated that motor vehicles played an important role in the haze, even though the oxidation rate from SO_2 to SO_4^（2-）was faster than that of NOXto NO_3^-. The difference between the hourly averages of Na＋and K＋during the haze episode and clean days was small, implying that straw combustion and sea salt had no significant effect on the occurrence of haze, and the backward trajectories of the air masses also conformed with this result. The air pollutants were difficult to disperse because of the significant decrease in the planetary boundary layer（PBL） height. Relative humidity played a crucial role in the formation of haze by leading to hygroscopic growth of the diameter of aerosols.

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.

Insights into the phenomenon of an explosive growth and sharp decline in haze:A case study in Beijing

A severe haze episode occurred in winter in the North China Plain(NCP),and the phenomenon of an explosive growth and sharp decline in PM2.5(particulate matter with an aerodynamic diameter equal to or less than 2.5μm)concentration was observed.To study the systematic causes for this phenomenon,comprehensive observations were conducted in Beijing from November 26 to December 2,2015;during this period,meteorological parameters,LIDAR data,and the chemical compositions of aerosols were determined.The haze episode was characterized by rapidly varying PM2.5 concentration,and the highest PM2.5 concentration reached 667μg/m3.During the haze episode,the NCP was dominated by a weak high-pressure system and continuously low PBL(planetary boundary layer)heights,which are unfavorable conditions for the diffusion of pollutants.The large increases in the concentrations of SNA(SO42-,NO3-and NH4+)during the haze implied that the formation of SNA was the largest contribution.Water vapor also played a vital role in the formation of haze by promoting the chemical transformation of secondary pollutants,which led to higher PM2.5 concentrations.The spatial distributions of PM2.5 in Beijing at different times and the backward trajectories of the air masses also indicated that pollutants from surrounding provinces in particular,contributed to the higher PM2.5concentration.

Observation of Wind Shear during Evening Transition and an Estimation of Submicron Aerosol Concentrations in Beijing Using a Doppler Wind Lidar

The wind speed and direction measured over six months by a Doppler wind lidar（Windcube-8）were compared with wind cup anemometers mounted on the 325-m Beijing meteorological tower（BMT）.Five mountain–plain wind cases characterized by wind direction shear were selected based on the high-frequency（1.1 s）wind profile of the Windcube-8 and analyzed with 1-h mesoscale surface weather charts.Also analyzed was the relationship between insitu PM_1（aerodynamic diameter≤1μm）concentrations measured at 260 m on BMT and the carrier-to-noise ratio（CNR）of the co-located Windcube-8.The results showed that the 10-min averaged wind speed and direction were highly correlated（R=0.96–0.99）at three matched levels（80,140,and 200 m）.The evening transition duration was1–3 h,with an average wind speed of 1 m s^（–1） at 80 m above the ground.In addition,there was a zero horizontalwind-speed zone along the wind direction shear line,and in one case,the wind speed was characterized by a Kelvin–Helmholtz gravity wave.The variability of the PM_1 concentrations was captured by the CNR of the Windcube-8 in a fair weather period without the long-range transport of dust.

Synergistic impacts of anthropogenic and biogenic emissions on summer surface O_3 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 O3（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 （1×1） 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 O3 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 O3 control measures obtained from episodic studies could be limited for long-term applications.

Long-term winter observation of nitrous acid in the urban area of Beijing

The particulate matter(PM)pollution has been significantly improved by carrying out various valid emission control strategies since 2013 in China.Meanwhile the variation trend of nitrous acid(HONO)is worthy to investigate due to its vital role in the atmospheric oxidation process.In this study,field observation in the winter is conducted to investigate the concentration of HONO in an urban area of Beijing.In the winter of 2019,the mean HONO concentration is 1.38 ppbV during the whole winter.Photo-enhanced NO_(2)heterogeneous reactions on the ground and aerosol surfaces were found as the possible daytime sources of HONO.Compared to O_(3),photolysis of HONO dominates the primary OH sources during the winter time.To understand the HONO pollution patterns by years variation,multi-year data is summarized and finds that primary pollutants including CO and NO decreased,but secondary pollutants i.e.,HONO(mostly generated via secondary process)increased.Our study highlights the requirement to mitigate secondary pollution by control HONO concentration.

Aerosol optical properties under different pollution levels in the Pearl River Delta(PRD) region of China

To clarify the aerosol hygroscopic growth and optical properties of the Pearl River Delta(PRD)region,integrated observations were conducted in Heshan City of Guangdong Province from October 19 to November 17,2014.The concentrations and chemical compositions of PM2.5,aerosol optical properties and meteorological parameters were measured.The mean value of PM2.5 increased from less than 35(excellent)to 35-75μg/m^3(good)and then to greater than 75μg/m^3(pollution),corresponding to mean PM2.5 values of 24.9,51.2,and 93.3μg/m^3,respectively.The aerosol scattering hygroscopic growth factor(f(RH=80%))values were 2.0,2.12,and 2.18 for the excellent,good,and pollution levels,respectively.The atmospheric extinction coefficient(σext)and the absorption coefficient of aerosols(σap)increased,and the single scattering albedo(SSA)decreased from the excellent to the pollution levels.For different air mass sources,under excellent and good levels,the land air mass from northern Heshan had lower f(RH)andσsp values.In addition,the mixed aerosol from the sea and coastal cities had lower f(RH)and showed that the local sources of coastal cities have higher scattering characteristics in pollution periods.

‘Aerosol optical properties under different pollution levels in the pearl river delta(PRD)region of China’J.Environ.Sci.87(2020)49–59

The authors regret<In Abstract"The mean value of PM 2.5 increased from less than 35(excellent)to 35–75μg/m^(3)(good)and then to greater than 75μg/m^(3)(pollution),corresponding to mean PM 2.5 values of 24.9,51.2 and 93.3μg/m^(3),respectively.The aerosol scattering hygroscopic growth factor(f(RH=80%))values were 2.0,2.12 and 2.18 for the excellent,good and pollution levels,respectively."should be revised to"The mean value of PM 2.5(24-hr)increased from less than 35μg/m^(3)(excellent)to 35–75μg/m^(3)(good)and then to greater than 75μg/m^(3)(pollution),corresponding to mean PM 2.5 values of 24.9,51.2 and 93.3μg/m^(3),respectively.The aerosol scattering hygroscopic growth factor(f(RH=80%))values were 2.69,2.33 and 2.09 for the excellent,good and pollution levels,respectively."