Calibration of the 936Â nm water-vapor channel for the China aerosol remote sensing NETwork (CARSNET) and the effect of the retrieval water-vapor on aerosol optical property over Beijing, China

Based on measurements of the 936 nm water-vapor channel and the 870 nm, and 1020 nm atmospheric window band for the CE-318 sunphotometer of CARSNET, clear-sky direct solar radiation observational data and relative optical air masses of between 2 and 5, and considering sensitivity parameters such as pressure, altitude, wavelength, and AOD936nm, which may affect the calibration result, a method to calibrate the 936 nm water-vapor channel using a modified Langley method is presented. The daily average columnar water-vapor (CWV) over urban Beijing was retrieved using three methods (Methods A, B and C), and compared with the AERONET water-vapor product. Finally, the seasonal relationships of CWV with AOD500nm and α440-870 were analyzed. The calibration results showed the calibration value to differ from the original value by about 4%. The CWV retrieval result showed that AERONET and the three methods of retrieval for CWV were highly correlated. The monthly average variation trends of AOD500nm and CWV were similar, with the maximum occurring in July, and the minimum in December. The seasonal average variation trends of AOD500nm and CWV were also similar, but could be placed in the following order: summer > autumn > spring > winter. The seasonal variation of α440-870 versus AOD500nm, α440-870 versus CWV, and AOD500nm versus CWV, showed increasing AOD500nm with increasing CWV, which reflected the hygroscopicity of aerosol fine particle growth characteristics. Summer was mainly characterized by fine particles; while spring, autumn and winter were a mix of coarse and fine particles, showing typical urban aerosol properties.