How much of the global aerosol optical depth is found in the boundary layer and free troposphere?

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.

High Summertime Aerosol Loadings Over the Arabian Sea and Their Transport Pathways

“…Dust aerosols can be lifted up to 5 km over the Arabian Peninsula and then transported to the Arabian Sea at a higher altitude of up to 6 km. The dust layers at high altitude over the Arabian Sea in this case study are consistent with findings of Bourgeois et al () that the dust aerosols in the free troposphere dominate the total AOD over the Arabian Sea in the boreal summer. This could be attributed to the confluence of the strong southwesterly summer monsoon flow over ocean and the northwesterly Shamal winds, which could result in strong uplift and accumulation of dust aerosols over the Arabian Sea.…”

“…The AOD over the Arabian Sea has strong seasonal variations: it peaks in the boreal summer, followed by fall, spring, and winter in the decreasing order, which is consistent with the sea salt seasonal variations but different from the dust seasonal variations documented by Tindale and Pease (). It should be noted that CALIPSO AOD is larger during night time than during day time, which is probably due to high signal‐to‐noise ratio during night time (Bourgeois et al, ).…”

Novel aerosol extinction coefficients and lidar ratios over the ocean from CALIPSO–CloudSat: evaluation and global statistics

“…One of the few global satellite-based estimates of lidar ratio is reported in Bréon (2013), who estimated LR utilizing the retrieved scattering phase function at a 180 • angle derived from the Polarization and Directionality of the Earth's Reflectances (POLDER) satellite instrument and a prescribed aerosol model. POLDER LR and CALIOP-SODA (Figs.…”

“…In addition, both POLDER and CALIOP-SODA produce LR < 30 sr over the open ocean. Conversely, LRs from POLDER tend to be slightly higher, with a typical range between 30 and 70 sr. Bréon (2013) also indicates that because POLDER retrievals rely on scattered photon measurements, LR might be biased low in regions dominated by absorbing aerosol, such as the southeast Atlantic. A somewhat different method of retrieving LR from SODA AOD, documented in Josset et al (2011), consists of analytically solving the lidar equation.…”

Validation of MAX-DOAS retrievals of aerosol extinction, SO&lt;sub&gt;2&lt;/sub&gt;, and NO&lt;sub&gt;2&lt;/sub&gt; through comparison with lidar, sun photometer, active DOAS, and aircraft measurements in the Athabasca oil sands region

“…Error bars on the Pandora VCDs are root-sum-square errors of the standard error of the average values and the reported instrumental precision. Deming fit linear regressions were performed using the Monte Carlo method, which included the errors on the x and y data, with the "linfitxy" function in MATLAB (Browaeys, 2017). The 23 August and 3 September AERONET and Pandora data were also correlated with MAX-DOAS and lidar data by subtracting 30 min from the Oski-Ôtin data to account for the time of air mass transport between the Fort McKay South and Oski-Ôtin given the wind speeds.…”