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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
15 Jul 2016
Review status
This discussion paper is a preprint. It has been under review for the journal Atmospheric Chemistry and Physics (ACP). The revised manuscript was not accepted.
Temperature-dependence of aerosol optical depth over the southeastern US
Tero Mielonen1, Anca Hienola2, Thomas Kühn1,3, Joonas Merikanto2, Antti Lipponen1, Tommi Bergman1,a, Hannele Korhonen2, Pekka Kolmonen2, Larisa Sogacheva2, Darren Ghent4, Antti Arola1, Gerrit de Leeuw2,5, and Harri Kokkola1 1Finnish Meteorological Institute, Kuopio, 70211, Finland
2Finnish Meteorological Institute, Helsinki, 00560, Finland
3Department of Applied Physics, University of Eastern Finland, Kuopio, 70211, Finland
4Department of Physics & Astronomy, University of Leicester, LE1 7RH, UK
5Department of Physics, University of Helsinki, 00560, Finland
anow at: Royal Netherlands Meteorological Institute, De Bilt, 3731 GA, the Netherlands
Abstract. Previous studies have indicated that summer-time aerosol optical depths (AOD) over the southeastern US are dependent on temperature but the reason for this dependence and its radiative effects have so far been unclear. To quantify these effects we utilized AOD and land surface temperature (LST) products from the Advanced Along-Track Scanning Radiometer (AATSR) with observations of tropospheric nitrogen dioxide (NO2) column densities from the Ozone Monitoring Instrument (OMI). Furthermore, simulations of the global aerosol-climate model ECHAM-HAMMOZ have been used to identify the possible processes affecting aerosol loads and their dependence on temperature over the studied region. Our results showed that the level of AOD in the southeastern US is mainly governed by anthropogenic emissions but the observed temperature dependent behaviour is most likely originating from non-anthropogenic emissions. Model simulations indicated that biogenic emissions of volatile organic compounds (BVOC) can explain the observed temperature dependence of AOD. According to the remote sensing data sets, the non-anthropogenic contribution increases AOD by approximately 0.009 ± 0.018 K−1 while the modelled BVOC emissions increase AOD by 0.022 ± 0.002 K−1. Consequently, the regional direct radiative effect (DRE) of the non-anthropogenic AOD is −0.43 ± 0.88 W/m2/K and −0.17 ± 0.35 W/m2/K for clear- and all-sky conditions, respectively. The model estimate of the regional clear-sky DRE for biogenic aerosols is also in the same range: −0.86 ± 0.06 W/m2/K. These DRE values indicate significantly larger cooling than the values reported for other forested regions. Furthermore, the model simulations showed that biogenic emissions increased the number of biogenic aerosols with radius larger than 100 nm (N100, proxy for cloud condensation nuclei) by 28 % per one degree temperature increase. For the total N100, the corresponding increase was 4 % which implies that biogenic emissions could also have a small effect on indirect radiative forcing in this region.

Citation: Mielonen, T., Hienola, A., Kühn, T., Merikanto, J., Lipponen, A., Bergman, T., Korhonen, H., Kolmonen, P., Sogacheva, L., Ghent, D., Arola, A., de Leeuw, G., and Kokkola, H.: Temperature-dependence of aerosol optical depth over the southeastern US, Atmos. Chem. Phys. Discuss.,, 2016.
Tero Mielonen et al.
Tero Mielonen et al.


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Short summary
We studied the temperature dependence of AOD and its radiative effects over the southeastern US. We used spaceborne observations of AOD, LST and tropospheric NO2 with simulations of ECHAM-HAMMOZ. The level of AOD in this region is governed by anthropogenic emissions but the temperature dependency is most likely caused by BVOC emissions. According to the observations and simulations, the regional clear-sky DRE for biogenic aerosols is −0.43 ± 0.88 W/m2/K and −0.86 ± 0.06 W/m2/K, respectively.
We studied the temperature dependence of AOD and its radiative effects over the southeastern US....