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Zhou J.,CAS Institute of Atmospheric Physics | Lei H.,CAS Institute of Atmospheric Physics | Lei J.,Beijing Weather Modification Office
Journal of Atmospheric and Oceanic Technology | Year: 2013

A new multilevel detection scheme for cloud tomography is developed. This scheme solves problems intrinsic to conventional single-level detection, such as the lateral boundary problem and thelow accuracy of liquid water content (LWC) retrieval for clouds without distinct liquid water cores. Sensitivity studies show that the new multilevel detection scheme can significantly enhance thewell posedness of the inverse problem and increases the accuracy of the retrieval. These improvements are achieved not only for clouds with distinct liquid water cores but also for clouds with weak or no liquid water cores, which are difficult to accurately reconstruct using a single-level detection scheme. The settlement of the lateral boundary problem also leads to a natural and easy wayof solving the detection time limit problem in cloud tomography. By using a multiaircraft flight(MAF) scheme, segmental retrieval can be applied to make the applicable scope of cloud tomography much broader. Considering the detection time limit and the cost in practice, the feasible flight scheme at present is MAF with two detection levels. Although only one detection level is added to the conventional single-level scheme, the accuracy of LWC retrieval can be improved by 1.4%-13.1%. © 2013 American Meteorological Society.

Quan J.,Beijing Weather Modification Office | Quan J.,Beijing Key Laboratory of Cloud | Tie X.,CAS Institute of Earth Environment | Tie X.,U.S. National Center for Atmospheric Research | And 10 more authors.
Atmospheric Environment | Year: 2014

A comprehensive measurement was carried out to analyze the heavy haze events during 2012-2013 winter in Beijing. The measured variables include some important meteorological parameters, such wind directions, wind speeds, relative humidity (RH), planetary boundary layer (PBL), solar radiation, and visibility. The aerosol composition and concentrations (including particular matters (PM2.5), nitrate (NO3), sulfate (SO4), ammonium (NH4)) as well as their gas-phase precursors (including nitrogen oxides (NOx) and sulfur dioxide (SO2)) were analyzed during the period between Nov. 16, 2012 and Jan. 15, 2013. The results show that the hourly mean concentrations of PM2.5 often exceeded 200μg/m3, with a maximum concentration of 600μg/m3 on Jan. 13, 2013. The relative humidity was increased during the haze events, indicating that both aerosol concentrations and RH had important effect on the reduction of visibility, causing the occurrence of the haze events. Because the wind speeds were generally low (less than 1m/s) during the haze event, the vertical dispersion and the PBL heights were very important factors for causing the strong variability of aerosol concentrations. This study also finds that under the lower visibility condition, the conversion from the gas-phase of NOx and SO2 to the particle phase of NO3 and SO4 were higher than the values under the higher visibility condition. Because the lower visibility condition was corresponding to the lower photochemical activity than the higher visibility condition, the higher conversion from gas phase to particle phase in the lower visibility condition indicated that there was important heterogeneous formation of NO3 and SO4 during the heavy haze events. © 2014 Elsevier Ltd.

Chen P.,Beijing Weather Modification Office | Chen P.,Institute of Urban Meteorology | Quan J.,Beijing Weather Modification Office | Quan J.,Beijing Key Laboratory of Cloud Precipitation and Atmospheric Water Resources | And 10 more authors.
Atmospheric Environment | Year: 2013

The vertical distributions of ozone (O3) over a mega city (Beijing, China), and the horizontal O3 distributions in the lower troposphere (2-3.6km) over Beijing and its surrounding areas located in the North China Plain (NCP), were analyzed based on the aircraft measurements from 159 flights during 2007-2010. The results are highlighted as follows: (1) There was a peak of O3 concentration occurring at ~1km over Beijing, and the peak values ranged between 60 and 120ppbv. (2) There was an O3 minimum at the surface. The minimum was largely caused by the chemical reaction of NO+O3. This process produced about 30ppbv of the O3 reduction below 0.5km in the morning (9:00-10:00). (3) There was a transition altitude (~1km), below which the ozone formation was in a VOC-limited condition, and above which the ozone formation was in a NOx-limited condition. (4) The analysis of the horizontal distribution shows that O3 concentrations were enhanced in the downwind of the city plumes. This result suggests that there was an important regional O3 chemical production in the NCP region. © 2013 Elsevier Ltd.

Zhang Q.,Beijing Weather Modification Office | Quan J.,Beijing Weather Modification Office | Tie X.,CAS Institute of Earth Environment | Tie X.,U.S. National Center for Atmospheric Research | And 4 more authors.
Science of the Total Environment | Year: 2015

The causes of haze formation in Beijing, China were analyzed based on a comprehensive measurement, including PBL (planetary boundary layer), aerosol composition and concentrations, and several important meteorological parameters such as visibility, RH (relative humidity), and wind speed/direction. The measurement was conducted in an urban location from Nov. 16, 2012 to Jan. 15, 2013. During the period, the visibility varied from >20km to less than a kilometer, with a minimum visibility of 667m, causing 16 haze occurrences. During the haze occurrences, the wind speeds were less than 1m/s, and the concentrations of PM2.5 (particle matter with radius less than 2.5μm) were often exceeded 200μg/m3. The correlation between PM2.5 concentration and visibility under different RH values shows that visibility was exponentially decreased with the increase of PM2.5 concentrations when RH was less than 80%. However, when RH was higher than 80%, the relationship was no longer to follow the exponentially decreasing trend, and the visibility maintained in very low values, even with low PM2.5 concentrations. Under this condition, the hygroscopic growth of particles played important roles, and a large amount of water vapor acted as particle matter (PM) for the reduction of visibility. The variations of meteorological parameters (RH, PBL heights, and WS (wind speed)), chemical species in gas-phase (CO, O3, SO2, and NOx), and gas-phase to particle-phase conversions under different visibility ranges were analyzed. The results show that from high visibility (>20km) to low visibility (<2km), the averaged PBL decreased from 1.24km to 0.53km; wind speeds reduced from 1m/s to 0.5m/s; and CO increased from 0.5ppmv to 4.0ppmv, suggesting that weaker transport/diffusion caused the haze occurrences. This study also found that the formation of SPM (secondary particle matter) was accelerated in the haze events. The conversions between SO2 and SO4 as well as NOx to NO3 - increased, especially under high humidity conditions. When the averaged RH was 70%, the conversions between SO2 and SO4 accounted for about 20% concentration of PM2.5, indicating that formation of secondary particle matter had important contribution for the haze occurrences in Beijing. © 2014 Elsevier B.V.

Zhang Q.,Beijing Weather Modification Office | Tie X.,CAS Institute of Earth Environment | Tie X.,U.S. National Center for Atmospheric Research | Lin W.,Chinese Academy of Meteorological Sciences | And 4 more authors.
Particuology | Year: 2013

A field experiment was conducted in an intensive fog event between November 5 and November 8, 2009, in a heavily SO2-polluted area in North China Plain (NCP), to measure SO2 and other air pollutants, liquid water content (LWC) of fog droplets, and other basic meteorological parameters. During the fog period, the concentrations of SO2 showed large variability, which was closely related to the LWC in the fog droplets. The averaged concentration of SO2 during non-fog periods was about 25 ppbv, while during the fog period, it rapidly reduced to about 4-7 ppbv. Such large reduction of SO2 suggested that a majority of SO2 (about 70%-80%) had reverted from gas to aqueous phase on account of the high solubility of SO2 in water in the fog droplets. However, the calculated gas to aqueous phase conversion was largely underestimated by merely using the Henry's Law constant of SO2, thus suggesting that aqueous reaction of SO2 in fog droplets might play some important role in enhancing the solubility of SO2. To simplify the phenomenon, an "effective solubility coefficient" is proposed in this study. This variability of SO2 measurement during the extensive fog event provides direct evidence of oxidation of SO2 in fog droplets, thus providing important implications for better understanding of the acidity in clouds, precipitation, and fogs in NCP, now a central environmental focus in China due to its rapid economic development. © 2012 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

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