A study of long-range transported smoke aerosols in the Upper Troposphere/Lower Stratosphere

Abstract

Long-range transported smoke aerosols in the UTLS (Upper Troposphere/Lower Stratosphere) over Europe were detected in Summer 2017. The measurements of ground-based instruments and satellite sensors indicate that the UTLS aerosol layers were originated from Canadian wildfires and were transported to Europe by UTLS advection. In this study, the observations of two multi-wavelength Raman Lidar systems in northern France (Lille and Palaiseau) are used to derive aerosol properties, such as optical depth of the UTLS layer, Lidar ratios at 355 and 532 nm and particle linear depolarization ratios 5 at 355, 532 and 1064 nm. The optical depth of the UTLS layers at 532 nm varies from 0.05 to above 0.20, with very weak spectral dependence between 355 and 532 nm. Lidar ratios at 355 nm are in 31 ± 15 sr to 45 ± 9 sr range and at 532 nm, the Lidar ratios are in the range of 54 ± 12 sr to 58 ± 9 sr. Such spectral dependence of Lidar ratio is known to be a characteristic feature of aged smoke. The typical particle depolarization ratios in the UTLS smoke layer are 25 ± 4% at 355 nm, 19 ± 3% at 532 nm and 4.5 ± 0.8% at 1064 nm. The relatively high depolarization ratios and such spectral dependence are an indication 10 of a complicated morphology of aged smoke particles. We found an increase of depolarization ratio versus transport time. The depolarization ratio at 532 nm increases from below 2 − 5% for fresh smoke to over 20% for smoke aged more than 20 days.The 3β + 2α observations of two cases at Palaiseau and Lille sites were inverted to the aerosol microphysical properties using regularization algorithm. The particles distribute in the 0.1-1.0 µm range with effective radius of 0.33 ± 0.10 µm for both cases. The derived complex refractive indices are 1.52(±0.05) + i0.021(±0.010) and 1.55(±0.05) + i0.028(±0.014) 15 for Palaiseau and Lille data. The retrieved aerosol properties were used to calculate the direct radiative forcing (DRF) effect specific to the UTLS aerosol layers. The simulations derive daily net DRF efficiency of -79.6 Wm −2 τ −1 at the bottom of the atmosphere for Lille observations. At the top of the atmosphere, the net DRF efficiency is -7.9 Wm −2 τ −1 . The results indicate that the UTLS aerosols strongly reduce the radiation reaching the terrestrial surface by absorption. The heating rate of the UTLS layers is estimated to be 3.7 Kday −1 . The inversion of Palaiseau data leads to similar results. The heating rate predicts 20 a temperature increase within the UTLS aerosol layer, which has been observed by the radiosonde temperature measurements.Atmos. Chem. Phys. Discuss., https://doi.

Scattering and Radiative Properties of Morphologically Complex Carbonaceous Aerosols: A Systematic Modeling Study

“…The LDRs were very close to zero for Model 9 particles when there was minimal or no coating, but could reach nearly 40% when the coating thickness was about seven times the spherule radius (i.e., ≈0.14 µm). The strong linear depolarization induced by Model 9 aerosols (as well as by other large particles of certain morphologies) can potentially explain the high LDR values identified in recent lidar observations [14,79,80].…”

Depolarization and lidar ratios at 355, 532, and 1064 nm and microphysical properties of aged tropospheric and stratospheric Canadian wildfire smoke

“…An optically dense stratospheric layer extended from 14 to 16 km over Leipzig and showed a 532 nm AOT of 0.6. As discussed by Ansmann et al (2018) and Hu et al (2018), record-breaking intensive fires combined with the formation of exceptionally strong pyrocumulonimbus clusters in the southern parts of British Columbia in western Canada and the northwestern United States in the afternoon of 12 August 2017 were most probably responsible for these unprecedented optically thick stratospheric smoke layers reaching Europe. Within cumulus towers, enormous amounts of smoke can be injected into the upper troposphere and lower stratosphere (Fromm and Servranckx, 2003;Fromm et al, 2010;Rosenfeld et al, 2007;Peterson et al, 2017).…”

“…To obtain the lidar ratios at 355 and 532 nm, the extinction profiles were combined with the respective backscatter profiles. In this procedure, we applied the optimum effective resolution concept (Iarlori et al, 2015;Mattis et al, 2016) and used a smoothing window length in the backscatter retrieval which was around 0.78 of the regression window length in the extinction retrieval.…”

“…3 and given in Table 1. To obtain the 1064 nm lidar ratio, we combined the extinction value for the 1250 m layer with the respective backscatter coefficient computed from signal profiles smoothed with a vertical window length of 937.5 m according to the effective resolution concept (Iarlori et al, 2015;Mattis et al, 2016).…”

Extreme levels of Canadian wildfire smoke in the stratosphere over central Europe on 21–22 August 2017

“…Naval Research Laboratory, Monterey, California, April 2017). This event obviously triggered the formation of an optically thick smoke layer (probably with an AOT of > 2-3) in the upper troposphere and lower stratosphere over British Columbia (see also Hu et al, 2018). According to the forward HYSPLIT trajectories started over southerncentral British Columbia on the afternoon of 12 August 2017, between a height of 5 and 12 km these dense smoke plumes traveled northward over the next two days.…”