Abstract

Abstract. The global aerosol extinction from the CALIOP space lidar was used to compute aerosol optical depth (AOD) over a 9-year period (2007–2015) and partitioned between the boundary layer (BL) and the free troposphere (FT) using BL heights obtained from the ERA-Interim archive. The results show that the vertical distribution of AOD does not follow the diurnal cycle of the BL but remains similar between day and night highlighting the presence of a residual layer during night. The BL and FT contribute 69 and 31 %, respectively, to the global tropospheric AOD during daytime in line with observations obtained in Aire sur l'Adour (France) using the Light Optical Aerosol Counter (LOAC) instrument. The FT AOD contribution is larger in the tropics than at mid-latitudes which indicates that convective transport largely controls the vertical profile of aerosols. Over oceans, the FT AOD contribution is mainly governed by long-range transport of aerosols from emission sources located within neighboring continents. According to the CALIOP aerosol classification, dust and smoke particles are the main aerosol types transported into the FT. Overall, the study shows that the fraction of AOD in the FT – and thus potentially located above low-level clouds – is substantial and deserves more attention when evaluating the radiative effect of aerosols in climate models. More generally, the results have implications for processes determining the overall budgets, sources, sinks and transport of aerosol particles and their description in atmospheric models.

Highlights

  • Aerosols influence the radiative budget of the Earth by absorbing or scattering solar radiation and by changing microphysical properties of clouds

  • This paper examines the vertical distribution of the aerosol optical depth (AOD) through its partitioning between the www.atmos-chem-phys.net/18/7709/2018/

  • The CALIOP aerosol extinction product is combined with estimates of boundary layer (BL) height from ERA-Interim over a 9-year period (2007–2015)

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Summary

Introduction

Aerosols influence the radiative budget of the Earth by absorbing or scattering solar radiation and by changing microphysical properties of clouds. Several studies have pointed out convective transport and in-cloud scavenging as the main processes determining the vertical distribution of aerosols (e.g., Cui and Carslaw, 2006; Ekman et al, 2006; Bourgeois and Bey, 2011; Weigel et al, 2011; Kipling et al, 2016). Other processes such as condensation, biomass burning injection height, deposition and BL mixing have been reported as important (Kipling et al, 2016; Peers et al, 2016). Results for the Antarctica region are likely spurious and excluded because high aerosol values are reported by CALIOP despite the region being known to be pristine

CALIOP observations
ERA-Interim data
The LOAC instrument
Daytime analysis
Nighttime analysis
Comparison with the LOAC in situ observations
Findings
Discussion and conclusion

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