Development of synergy photometer/lidar methodologies for retrieving vertical aerosol extinction

The Fernald-Klett method is commonly used to derive aerosol optical properties profiles from elastic lidars together with AOD measurements from a collocated photometer. This method assumes a range independent lidar ratio, which involves an aerosol vertical distribution with range invariant physico-chemical properties. However this approximation may be unrealistic when we find different aerosol types at different heights. This situation can be improved with additional measurements and previous knowledge about the aerosol vertical distribution.

We analyzed a decade of lidar and photometric data at two heights from the observatories of Izaña and Santa Cruz. This region is characterized by a quasi-permanent thermal inversion which separates the troposphere into two layers: the marine boundary layer and the free troposphere. As a result, two layers with different lidar ratios have been considered in this study (Two-layer method). The results are compared with the classical one-layer analysis (One-layer method).

Figure 1.- Lidar ratio at 523 nm from one-layer (left) and two-layer methods (right) for each year between 2007 and 2017. The central rectangles extend from the first quartile to the third quartile and the median is represented by a horizontal line. The whiskers are defined as the upper and lower quartiles ±1.5IQR (inter-quartile range). Reprinted from Berjón et al. (2019).

Our results suggest the suitability of the Two-layer method to estimate aerosol backscatttering and extinction profiles βaer(r) and σaer(r), respectively, in those situations where different aerosol layers exist in the vertical. The lidar ratio obtained by means of the Two-layer method was also validated with the value derived independently from the sun/sky photometer 15 at IZO, with a good agreement between the two techniques (mean discrepancies about 1 sr). This project was developed during 2017-2018 and the results were published by Berjón et al. (2019).