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Li X.,Beijing Weather Modification Office | Li X.,Beijing Key Laboratory of Cloud | Zhang L.,Lanzhou University | Cao X.,Lanzhou University | And 5 more authors.
Atmospheric Research | Year: 2016

Precipitable water vapor (PWV) was retrieved using direct solar irradiance at 938 nm measured by a multifilter rotating shadowband radiometer (MFRSR) at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) located in the semi-arid area of northwest China from August 2007 to June 2010. Measurement also occurred at Zhangye, China, at the Atmosphere Radiation Measurements (ARM) Program's Ancillary Facility during the dust period from April to June 2008. The line-by-line radiative transfer model (LBLRTM) code combined with the HITRAN 2004 spectral database is used to model the water vapor spectral transmittance throughout the 938-nm spectral response of MFRSR in the retrieval algorithm. Gaussian fitting is proposed to determine the daily calibration constant at the top of atmosphere for a long-term series under an obvious annual change in solar radiation. PWV retrieved by MFRSR over SACOL shows that 90% of PWV values are smaller than 1.52. cm, and PWV distribution has a seasonal variation, with maximum in summer and minimum in winter. The comparisons between MFRSR and other measurements show a better agreement between MFRSR and sunphotometer (AERONET's Cimel) PWV retrievals with relative bias of 2.9% and RMS difference of 9.1% than between MFRSR and microwave radiometer (MWR) with relative bias of 10% and RMS difference of 23% over SACOL, and an excellent agreement between MFRSR and sunphotometer with relative bias of 0.56% and RMS difference of 6.1% over Zhangye. To verify satellite PWV products over the semi-arid area of northwest China, the comparisons of PWV from MODIS and AIRS with MFRSR suggest that the agreement between satellite and MFRSR PWV retrievals is not as good as that between MFRSR and other ground-based instruments. MODIS appears to slightly underestimate PWV in a dry atmosphere but overestimate PWV in a moist atmosphere against MFRSR. A method is proposed to correct MODIS PWV products. AIRS PWV products relative to MFRSR show a systematic underestimation. © 2015 Elsevier B.V. Source

Zhao D.,Beijing Weather Modification Office | Zhao D.,Beijing Key Laboratory of Cloud | Zhao D.,Institute of Urban Meteorology | Tie X.,CAS Institute of Earth Environment | And 8 more authors.
Atmosphere | Year: 2015

Due to rapid economic development in recent years, China has become a major global source of refractory black carbon (rBC) particles. However, surface rBC measurements have been limited, and the lower troposphere suffers from a complete lack of measurements, especially in heavily rBC-polluted regions such as China's capital, Beijing (BJ). In this study, we present the first concentration measurements using an airborne Single Particle Soot Photometer (SP2) instrument, including vertical distributions, size distributions, and the mixing state of rBC particles in the lower troposphere in BJ and its surrounding areas. The measurements were conducted from April to June 2012 during 11 flights. The results show that the vertical rBC distributions had noticeable differences between different air masses. When an air mass originated from the south of BJ (polluted OPEN ACCESS Atmosphere 2015, 6 714 region), the rBC particles were strongly compressed in the planetary boundary layer (PBL), and showed a large vertical gradient at the top of the PBL. In contrast, when an air mass originated from the north of BJ (clean region), there was a small vertical gradient. This analysis suggests that there was significant regional transport of rBC particles that enhanced the air pollution in BJ, and the transport not only occurred near the surface but also in the middle levels of the PBL (around 0.5 to 1 km). The measured size distributions show that about 80% of the rBC particles were between the diameters of 70 and 400 nm, and the mean diameter of the peak rBC concentrations was about 180-210 nm. This suggests that the rBC particles were relatively small particles. The mixing state of the rBC particles was analyzed to study the coating processes that occurred on the surface of these particles. The results indicate that the air mass strongly affected the number fraction (NF) of the coated particles. As for a southern air mass, the local air pollution was high, which was coupled with a lower PBL height and higher humidity. Consequently, hygroscopic growth occurred rapidly, producing a high NF value (~65%) of coated rBC particles. The correlation coefficient between the NF and the local relative humidity (RH) was 0.88, suggesting that the rBC particles were quickly converted from hydrophobic to hydrophilic particles. This rapid conversion is very important because it suggests a shorter lifetime of rBC particles under heavily polluted conditions. In contrast, under a northern air mass, there was no clear correlation between the NF and the local humidity. This suggests that the coating process occurred during the regional transport in the upwind region. In this case, the lifetime was longer than the southern air mass condition. © 2015 by the authors; licensee MDPI, Basel, Switzerland. Source

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. Source

Jin H.,Beijing Weather Modification Office | Jin H.,Beijing Key Laboratory of Cloud | He H.,Beijing Weather Modification Office | He H.,Beijing Key Laboratory of Cloud | And 8 more authors.
Journal of Tropical Meteorology | Year: 2013

Two field experiments were performed in order to dissipate the fog at Wuqing District of Tianjin in November and December of 2009. Hygroscopic particles were seeded to dissipate fog droplets on 6-7 November, 2009. Liquid nitrogen (LN) was seeded into the natural supercooled fog in the experiments of 30 November-1 December, 2009. Significant response was found after seeding. Significant changes were observed in the microstructure of fog in the field experiments. The of fog droplet changed dramatically; it increased first and then decreased after seeding. Remarkable variation also was found in the Liquid Water Content (LWC) and in the size of fog droplet. The Droplet Size Distribution (DSD) of fog broadened during the seeding experiments. The DSD became narrow after the seeding ended. After seeding, the droplets were found to be at different stages of growth, resulting in a transform of DSD between unimodal distribution and bimodal distribution. The DSD was unimodal before seeding and then bimodal during the seeding experiment. Finally, the DSD became unimodally distributed once again. Source

Liu K.,CAS Research Center for Eco Environmental Sciences | Quan J.,Beijing Weather Modification Office | Quan J.,Beijing Key Laboratory of Cloud | Mu Y.,CAS Research Center for Eco Environmental Sciences | And 10 more authors.
Atmospheric Environment | Year: 2013

A high sensitive method has been developed for measuring atmospheric BTEX (Benzene, Toluene, Ethylbenzene, Xylenes) by using Gas Chromatography equipped with Photo-Ionization Detector (GC-PID). The Method Detection Limits (MDLs) were: Benzene 0.66ngm-3, Toluene 2.03ngm-3, Ethylbenzene 5.91ngm-3, m,p-Xylene 6.49ngm-3 and o-Xylene 5.45ngm-3. Vertical distribution of BTEX from ground level to 3600m around Beijing city was measured during flight of aircraft in July and November, 2011. Remarkable decrease of each BTEX species with increasing height was observed, and the sum concentration of BTEX decreased from 21.7μgm-3 at ground level to 0.19μgm-3 at height of 3600m. The ratio of benzene to toluene (B/T) also increased pronouncedly with increasing altitude, but became flat above 1800m. The vertical distribution characters of BTEX and B/T were ascribed to diffusion and photochemical consumption of BTEX. © 2013 Elsevier Ltd. Source

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