Articles | Volume 16, issue 3
Atmos. Chem. Phys., 16, 1219–1244, 2016

Special issue: CHemistry and AeRosols Mediterranean EXperiments (ChArMEx)...

Atmos. Chem. Phys., 16, 1219–1244, 2016

Research article 03 Feb 2016

Research article | 03 Feb 2016

On the radiative impact of aerosols on photolysis rates: comparison of simulations and observations in the Lampedusa island during the ChArMEx/ADRIMED campaign

S. Mailler1,2, L. Menut1, A. G. di Sarra3, S. Becagli4, T. Di Iorio3, B. Bessagnet6, R. Briant1, P. Formenti5, J.-F. Doussin5, J. L. Gómez-Amo3,7, M. Mallet8, G. Rea1, G. Siour5, D. M. Sferlazzo9, R. Traversi4, R. Udisti4, and S. Turquety1 S. Mailler et al.
  • 1Laboratoire de Météorologie Dynamique, IPSL, CNRS, École Polytechnique, École Normale Supérieure, Université Paris 6, UMR8539 91128 Palaiseau CEDEX, France
  • 2École Nationale des Ponts et Chaussées – Paristech, Cité Descartes, 6–8 Avenue Blaise Pascal, 77455 Champs-sur-Marne, France
  • 3ENEA, Laboratory for Earth Observations and Analyses, Via Anguillarese 301, 00123 Roma, Italy
  • 4Department of Chemistry, University of Florence, Sesto Fiorentino, Florence, 50019, Italy
  • 5LISA (Laboratoire Inter-Universitaire des Systèmes Atmosphériques), UMR CNRS 7583, Université Paris Est Créteil et Université Paris Diderot, Institut Pierre Simon Laplace, Créteil, France
  • 6National Institute for Industrial Environment and Risks (INERIS), Parc Technologique ALATA, 60550 Verneuil-en-Halatte, France
  • 7Dpt. Earth Physics and Thermodynamics, University of Valencia, Dr. Moliner, 50, 46100, Burjassot (Valencia), Spain
  • 8Laboratoire d'Aérologie, Observatoire Midi-Pyrénées, 14 Avenue Edouard Belin, 31400 Toulouse, France
  • 9ENEA, Laboratory for Earth Observations and Analyses, Contrada Grecale, 92010, Lampedusa, Italy

Abstract. The Mediterranean basin is characterized by large concentrations of aerosols from both natural and anthropogenic sources. These aerosols affect tropospheric photochemistry by modulating the photolytic rates. Three simulations of the atmospheric composition at basin scale have been performed with the CHIMERE chemistry-transport model for the period from 6 June to 15 July 2013 covered by the ADRIMED campaign, a campaign of intense measurements in the western Mediterranean basin. One simulation takes into account the radiative effect of the aerosols on photochemistry, the second one does not, and the third one is designed to quantify the model sensitivity to a bias in the ozone column.

These simulations are compared to satellite and ground-based measurements, with a particular focus on the area of Lampedusa. Values of the aerosol optical depth (AOD) are obtained from the MODIS instrument on the AQUA and TERRA satellites as well as from stations in the AERONET network and from the MFRSR sun photometer deployed at Lampedusa. Additional measurements from instruments deployed at Lampedusa either permanently or exceptionally are used for other variables: MFRSR sun photometer for AOD, diode array spectrometer for actinic fluxes, LIDAR for the aerosol backscatter, sequential sampler for speciation of aerosol and Brewer spectrophotometer for the total ozone column. It is shown that CHIMERE has a significant ability to reproduce observed peaks in the AOD, which in Lampedusa are mainly due to dust outbreaks during the ADRIMED period, and that taking into account the radiative effect of the aerosols in CHIMERE considerably improves the ability of the model to reproduce the observed day-to-day variations of the photolysis rate of ozone to O2 and O(1D), J(O1D), and that of NO2 to NO and O(3P), J(NO2). While in the case of J(O1D) other variation factors such as the stratospheric ozone column are very important in representing correctly the day-to-day variations, the day-to-day variations of J(NO2) are captured almost completely by the model when the optical effects of the aerosols are taken into account.

Finally, it is shown that the inclusion of the direct radiative effect of the aerosols in the CHIMERE model leads to reduced J(O1D) and J(NO2) values over all the simulation domain, which range from a few percents over continental Europe and the north-east Atlantic Ocean to about 20 % close to and downwind from Saharan dust sources. The effect on the modelled ozone concentration is 2-fold: the effect of aerosols leads to reduced ozone concentrations over the Mediterranean Sea and continental Europe, close to the sources of NOx, but it also leads to increased ozone concentrations over remote areas such as the Sahara and the tropical Atlantic Ocean.

Short summary
We studied the impact of aerosols on tropospheric photolysis rates at Lampedusa during the CharMEx/ADRIMED campaign in June 2013. It is shown by using the CHIMERE chemistry-transport model (CTM) as well as in situ and remote-sensing measurements that taking into account the radiative effect of the tropospheric aerosols improves the ability of the model to reproduce the observed photolysis rates. It is hence important for CTMs to include the radiative effect of aerosols on photochemistry.
Final-revised paper