Xuejiao Deng

Design and Application of an Unattended Multifunctional H-TDMA System

The hygroscopic properties of aerosols have a significant impact on aerosol particle number size distributions (PNSD), formation of cloud condensation nuclei, climate forcing, and atmospheric visibility, as well as human health. To allow for the observation of the hygroscopic growth of aerosols with long-term accuracy, an unattended multifunctional hygroscopicity-tandem differential mobility analyzer (H-TDMA) system was designed and built by the Institute of Tropical and Marine Meteorology (ITMM), China Meteorological Administration (CMA), in Guangzhou, China. The system is capable of measuring dry and wet PNSD, hygroscopic growth factor by particle size, and mixing states. This article describes in detail the working principles,components, andcalibrationmethodsofthesystem.Standardpolystyrenelatex(PSL)sphereswithfive different diameters were chosen to test the system’s precision and accuracy of particle size measurement. Ammonium sulfate was used to test the hygroscopic response of the system for accurate growth factor measurement. The test results show that the deviation of the growth factor measured by the system is within a scope of 20.01 to 20.03 compared to K€ theoretical curves. Results of temperature and humidity control performancetestsindicatethatthesystemisrobust.Aninternaltemperaturegradientoflessthan0.2 Kforasecond differential mobility analyzer (DMA2) makes it possible to reach a set-point relative humidity (RH) value of 90% and with a standard deviation of 60.44%, sufficient for unattended field observation.

A vertical sounding of severe haze process in Guangzhou area

We detected a severe haze process in Guangzhou area with lidar and microwave radiometer, performed an inversion to get boundary layer height by wavelet covariance transform, and analyzed the correlation between meteorological factors of boundary layer and visibility from the perspective of dynamical and thermodynamic structures. Our results indicate that the boundary layer height shows significant daily changes, consistent with ground visibility variation. During the cleaning process, the boundary layer height exceeded 1 km; during severe haze, the height was only 500 m. Temperature gradient of 50–100 m, which was 30 h lag, was remarkably correlated with visibility, with the correlation coefficient of 0.77. High layer visibility (255 m) and low layer stability were significantly anticorrelation, and the maximum anticorrelation coefficient was up to −0.76 in cleaning days and −0.49 in haze days. In the related boundary layer meteorological factors, surface ventilation coefficient was linearly correlated with ground visibility, with the greatest correlation coefficient of 0.88. The correlation coefficients of boundary layer height, ground wind velocity, relative humidity and ground visibility were 0.76, 0.67, and −0.77, respectively. There was a strong correlation between different meteorological factors. The dominant meteorological factor during this haze process was surface ventilation coefficient. In the area without boundary layer height sounding, ground visibility and wind velocity could be used to estimate boundary layer height.

Characterization of Secondary Aerosol and its Extinction Effects on Visibility over the Pearl River Delta Region, China

Aerosol samples collected from July 2007 to March 2008 were used to obtain major aerosol constituents in an urban location in the Pearl River Delta Region (PRD), China. The minimum organic carbon (OC)/elemental carbon (EC) ratio was used to calculate the primary and secondary organic carbon and the extinction effect of the secondary aerosol on visibility was estimated. As indicated in the analysis, the mass of secondary aerosol takes up 50% of the total mass of PM2.5; the OC/EC ratio is larger than 2 and there are significant characteristics of secondary aerosol generation; the levels of secondary OC are comparable with those of sulfate; and there is obvious enrichment of secondary aerosol on more polluted days. In a dry environment, the extinction weight is 59% for the secondary aerosol, while it is as high as 82% if the environment is highly humid (relative humidity [RH] = 95%). The hygroscopic growth of the aerosol can reduce visibility greatly; the secondary aerosol shares much larger quotas on more polluted days. For the Pearl River Delta (PRD), secondary aerosol and carbonaceous aerosol, especially secondary organic carbon (SOC), are a very acute problem; the study of the generating mechanism and sources for secondary aerosol is the key to the effort of controlling visibility in this region. The equation set forth in IMPROVE experiments can only be referenced but is not applicable to evaluate the extinction effect of individual aerosol components on visibility in the PRD region. Implications: The extinction effects on visibility by different constituents are studied in this work using compositional data derived from the measurements, with special efforts on examining the extinction of secondary aerosol and the enrichment and extinction contributions of the constituents with the variation of pollution level and relative humidity, so as to provide a scientific basis for the mitigation of atmospheric aerosol pollution and improving visibility in the PRD of China.

Assessment of regional air quality resulting from emission control in the Pearl River Delta region, southern China

To evaluate the impact of emission control measures on the air quality in the Pearl River Delta (PRD) region of South China, statistic data including atmospheric observations, emissions and energy consumptions during 2006-2014 were analyzed, and a Weather Research and Forecasting - Community Multi-scale Air Quality (WRF-CMAQ) model was used for various scenario simulations. Although energy consumption doubled from 2004 to 2014 and vehicle number significantly increased from 2006 to 2014, ambient SO2, NO2 and PM10 were reduced by 66%, 20% and 24%, respectively, mainly due to emissions control efforts. In contrast, O3 increased by 19%. Model simulations of three emission control scenarios, including a baseline (a case in 2010), a CAP (a case in 2020 assuming control strength followed past control tendency) and a REF (a case in 2020 referring to the strict control measures based on recent policy/plans) were conducted to investigate the variations of air pollutants to the changes in NOx, VOCs and NH3 emissions. Although the area mean concentrations of NOx, nitrate and PM2.5 decreased under both NOx CAP (reduced by 1.8%, 0.7% and 0.2%, respectively) and NOx REF (reduced by 7.2%, 1.8% and 0.3%, respectively), a rising of PM2.5 was found in certain areas as reducing NOx emissions elevated the atmospheric oxidizability. Furthermore, scenarios with NH3 emission reductions showed that nitrate was sensitive to NH3 emissions, with decreasing percentages of 0-10.6% and 0-48% under CAP and REF, respectively. Controlling emissions of VOCs reduced PM2.5 in the southwestern PRD where severe photochemical pollution frequently occurred. It was also found that O3 formation in PRD was generally VOCs-limited while turned to be NOx-limited in the afternoon (13:00-17:00), suggesting that cutting VOCs emissions would reduce the overall O3 concentrations while mitigating NOx emissions in the afternoon could reduce the peak O3 levels.

An analysis of aerosol liquid water content and related impact factors in Pearl River Delta.

Aerosol liquid water content (ALWC) has an important effect on atmospheric visibility as well as heterogeneous chemical reactions. In this paper, we used the data size-resolved particle hygroscopic growth factor, and particle number size distribution (PNSD) obtained from H-TDMA and SMPS to compute ALWC at the Guangzhou Panyu site from the winter of 2014 and the spring of 2015. The corresponding results were relatively consistent with the trend for ALWCISO calculated from the ISORROPIA II thermodynamic equilibrium model based on the measurement of aerosol water-soluble ionic compositions obtained from MARGA, with a linear fit yielding an R2 value of 0.76. The fact that ALWCHTDMA was somewhat higher than ALWCISO at low RH values was at least partially attributable to the fact that effects resulting from organic matter hygroscopicity were not taken into account when computing ALWCISO. In sensitivity testing, ambient relative humidity, PNSD and particle hygroscopicity were all found to affect ALWC, in that order. Particles of different modes made different contributions to ALWC with the contributions of nuclear, Aitken, accumulation and coarse modes assessed at <1%, 3%, 85% and 12%, respectively, indicating that the contribution of accumulation mode particles to ALWC dominated among all the aerosol particle modes. During clean processes, decreases in relative humidity and PM2.5 both resulted in a decrease in ALWC. During the pollution processes, calm winds caused local particle accumulation, with ALWC increasing as RH increased. Intraday trends in ALWC and relative humidity were consistent, with minimum mean values observed in the afternoon due to low ambient relative humidity inhibiting an increase in ALWC. However, diurnal variation of aerosol hygroscopicity and ALWC tended to be somewhat anti-correlated, indicating that diurnal changes in aerosol hygroscopicity are not a primary factor resulting in ambient AWLC changes.

Study on aerosol optical properties and radiative effect in cloudy weather in the Guangzhou region.

Currently, Guangzhou region was facing the problem of severe air pollution. Large amount of aerosols in the polluted air dramatically attenuated solar radiation. This study investigated the vertical optical properties of aerosols and inverted the height of boundary layer in the Guangzhou region using the lidar. Simultaneously, evaluated the impact of different types of clouds on aerosol radiation effects using the SBDART. The results showed that the height of the boundary layer and the surface visibility changed consistently, the average height of the boundary layer on the hazy days was only 61% of that on clear days. At the height of 2km or lower, the aerosol extinction coefficient profile distribution decreased linearly along with height on clear days, but the haze days saw an exponential decrease. When there was haze, the changing of heating rate of atmosphere caused by the aerosol decreased from 3.72K/d to 0.9K/d below the height of 2km, and the attenuation of net radiation flux at the ground surface was 97.7W/m(2), and the attenuation amplitude was 11.4%; when there were high clouds, the attenuation was 125.2W/m(2) and the attenuation amplitude was 14.6%; where there were medium cloud, the attenuation was 286.4W/m(2) and the attenuation amplitude was 33.4%. Aerosol affected mainly shortwave radiation, and affected long wave radiation very slightly.

A Modeling Study of Impact of Emission Control Strategies on PM2.5 Reductions in Zhongshan, China, Using WRF-CMAQ

A WRF-CMAQ modeling system is used to assess the impact of emission control strategies and weather conditions on haze pollution in Zhongshan, Guangdong Province, China. One-month simulations for January 2014 are completed and evaluated with the observational data. The simulations show reasonable agreement with the observations. Several sensitivity studies are completed to quantify the percentage contributions of local emissions versus regional emissions to the PM2.5 concentrations under different weather conditions. The results indicate that the contributions from local emission is higher than those of the emissions from regional transport when there is no intrusion of cold front (i.e., 58% contribution from local emission versus 42% contribution from the regional transport). The contribution of regional transport is increased to 76% when a strong cold front appears. Furthermore, the sensitivity study demonstrates that PM2.5 concentrations on the first, second, and third days are reduced by 47%, 52%, and 58%, respectively, after the local emissions are turned off when there is no intrusion of cold front. Finally, a case study shows that industrial, residential, and mobile emissions account for 24%, 22%, and 15% of the change of PM2.5, respectively, during a heavy haze pollution event in Zhongshan.

Transport paths and vertical exchange characteristics of haze pollution in Southern China

Transport paths and vertical exchange characteristics are important factors for understanding the long-term transport, dispersion capability for haze prediction. Many previous studies revealed that the Pearl River Delta (PRD) region, one of the major polluted areas in China, is largely affected by the long-range pollution transport. However, mostly of these studies focused on the source apportionment or horizontal transport path of pollutants by using short-term data, and the vertical exchange characteristics had been rarely analyzed. In this study, using HYSPLIT model, the transport paths and the vertical exchange characteristics of haze episodes over four sub-region of Guangdong (GD) Province in southern China of dry season and wet season were analyzed by using 10years data from 2005 to 2014. Three major transport paths can be statistically summarized based on the long-term data. The haze episodes in PRD and North-GD were distinguished by the characteristics of high frequency and long duration, while the West-GD and East-GD are relatively clean. The haze over North-GD and PRD were mainly influenced by the airflows from northern path, which could bring the pollution from Jiangxi, Anhui, and also influenced by the airflows from coastal path, which could bring the pollution of eastern coastal from Zhejiang and Fujian to Guangdong, while regional transport contributions from Guangdong province and adjacent areas can also be clearly observed. The haze pollution from the identified two major transport paths were mainly transported within the mixing layer (>80% trajectories, <500m), whereas the probability of haze trajectories across mixing layer was relatively low and generally associated with much longer transport distance and higher terrain height over Western China. Combing the vertical exchange analysis, results also show that Wuyi Mountains and Nanling Mountains played a role as barrier to obstruct the haze airflows from other regions of China to the Guangdong province.

Column-integrated aerosol optical properties of coarse- and fine-mode particles over the Pearl River Delta region in China

The sun-photometer data from 2011 to 2013 at Panyu site (Panyu) and from 2007 to 2013 at Dongguan site (Dg) in the Pearl River Delta region, were used for the retrieving of the aerosol optical depth (AOD), single scattering albedo (SSA), Ångström exponent (AE) and volume size distribution of coarse- and fine-mode particles. The coarse-mode particles presented low AOD (ranging from 0.05±0.03 to 0.08±0.05) but a strong absorption property (SSA ranged from 0.70±0.03 to 0.90±0.02) for the wavelengths between 440 and 1020nm. However, these coarse particles accounted for <10% of the total particles. The AOD of fine particles (AODf) was over 3 times as large as that of coarse particles (AODc). The fine particles SSA (SSAf) generally decreased as a function of wavelength, and the relatively lower SSAf value in summer was likely to be due to the stronger solar radiation and higher temperature. More than 70% of the aerosols at Panyu site were dominated by fine-mode absorbing particles, whereas about 70% of the particles at Dg site were attributed to fine-mode scattering particles. The differences of the aerosol optical properties between the two sites are likely associated with local emissions of the light-absorbing carbonaceous aerosols and the scattering aerosols (e.g., sulfate and nitrate particles) caused by the gas-phase oxidation of gaseous precursors (e.g., SO2 and NO2). The size distribution exhibited bimodal structures in which the accumulation mode was predominant. The fine-mode volume showed positive dependence on AOD (500nm), and the growth of peak value of the fine-mode volume was higher than that of the coarse volume. Both the AOD and SSA increased with increasing relative humidity (RH), while the AE decreased with increasing RH. These correlations imply that the aerosol properties are greatly modified by condensation growth.

Source Contributions to PM2.5 under Unfavorable Weather Conditions in Guangzhou City, China

Historical haze episodes (2013–16) in Guangzhou were examined and classified according to synoptic weather systems. Four types of weather systems were found to be unfavorable, among which “foreside of a cold front” (FC) and “sea high pressure” (SP) were the most frequent (>75% of the total). Targeted case studies were conducted based on an FC-affected event and an SP-affected event with the aim of understanding the characteristics of the contributions of source regions to fine particulate matter (PM2.5) in Guangzhou. Four kinds of contributions—namely, emissions outside Guangdong Province (super-region), emissions from the Pearl River Delta region (PRD region), emissions from Guangzhou–Foshan–Shenzhen (GFS region), and emissions from Guangzhou (local)—were investigated using the Weather Research and Forecasting–Community Multiscale Air Quality model. The results showed that the source region contribution differed with different weather systems. SP was a stagnant weather condition, and the source region contribution ratio showed that the local region was a major contributor (37%), while the PRD region, GFS region and the super-region only contributed 8%, 2.8% and 7%, respectively, to PM2.5 concentrations. By contrast, FC favored regional transport. The super-region became noticeable, contributing 34.8%, while the local region decreased to 12%. A simple method was proposed to quantify the relative impact of meteorology and emissions. Meteorology had a 35% impact, compared with an impact of -18% for emissions, when comparing the FC-affected event with that of the SP. The results from this study can provide guidance to policymakers for the implementation of effective control strategies.摘要我们收集、调研了2013~2016年期间广州的灰霾日,并将其按照主导天气型进行了分类,得到四类不利的天气型. 其中,“冷锋前部型”(FC)和“海上高压型”(SH)发生的频率是最高的(发生概率大于75%). 因此,为了了解广州PM2.5可能的贡献来源,我们开展了针对性的案例研究来解析FC和SH下PM2.5的来源可能. 通过利用WRF-CMAQ模式,解析了不利天气条件下四类潜在的来源贡献,即,广东省以外的贡献(超远距离输送)、珠三角区域的贡献、广州-佛山-深圳的贡献(广佛深)输送和广州本地的贡献. 结果表明,不同天气型主导的条件下PM2.5的污染来源也不同. “海上高压型”是一种静稳的天气,解析表明本地排放是占据主导的贡献(37%),珠三角区域的贡献、广佛深的贡献和外省的贡献仅占据8%,2.8%和7%. 相反,“冷锋前部型”是利于区域输送的天气,外省的贡献(超远距离输送)变得十分显著,贡献量为34.8%,而在这类天气条件下,本地的贡献将降低为12%. 此外,我们提出了一个简化的方法来评估排放和天气条件的影响. 同“海上高压型”相比,天气条件在“冷锋前部型”的影响为35%,而排放的贡献为-18%. 本文研究成果有助于政府相关部门减排施测.