Calibration of the 936 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

Özet

Based on measurements of the 936 nm water-vapor channel and the 870 nm, and 1020 nm atmosphericwindow band for the CE-318 sunphotometer of CARSNET, clear-sky direct solar radiation observational dataand relative optical air masses of between 2 and 5, and considering sensitivity parameters such as pressure,altitude, wavelength, and AOD936nm, whichmay affect the calibration result, amethod to calibrate the 936 nmwater-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 theAERONET water-vapor product. Finally, the seasonal relationships of CWV with AOD500nm and a440e870 wereanalyzed. 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 highlycorrelated. The monthly average variation trends of AOD500nm and CWV were similar, with the maximumoccurring in July, and the minimum in December. The seasonal average variation trends of AOD500nm andCWV were also similar, but could be placed in the following order: summer > autumn > spring > winter. Theseasonal variation of a440e870 versus AOD500nm, a440e870 versus CWV, and AOD500nm versus CWV, showedincreasing AOD500nm with increasing CWV, which reflected the hygroscopicity of aerosol fine particle growthcharacteristics. Summer was mainly characterized by fine particles; while spring, autumn and winter were amix of coarse and fine particles, showing typical urban aerosol properties.