Modeling of photolysis rates over Europe: impact on chemical gaseous species and aerosols CEREA, Joint laboratory Ecole des Ponts ParisTech/EDF R&D, Université Paris-Est, 77455 – Champs sur Marne, France
Received: 13 April 2010 – Published in Atmos. Chem. Phys. Discuss.: 05 July 2010 Abstract. This paper evaluates the impact of photolysis rate
calculation on simulated European air composition and air quality.
In particular, the impact of the cloud
parametrisation and the impact of aerosols on photolysis rates are analysed.
Photolysis rates are simulated using the Fast-JX photolysis
scheme and gas and aerosol concentrations over Europe are simulated
with the regional chemistry-transport model Polair3D of the Polyphemus platform.
The photolysis scheme is first used to update the clear-sky tabulation
of photolysis rates used in the previous Polair3D version. Important differences in
photolysis rates are simulated, mainly due to updated cross-sections and
quantum yields in the Fast-JX scheme.
In the previous Polair3D version, clouds were taken into account by
multiplying the clear-sky photolysis rates by a correction factor. In
the new version, clouds are taken into account more
accurately by simulating them directly in the photolysis scheme.
Differences in photolysis rates inside clouds can be large but
outside clouds, and especially at the ground, differences are small.
Revised: 24 November 2010 – Accepted: 18 January 2011 – Published: 23 February 2011
To take into account the impact of aerosols on photolysis rates, Polair3D and
Fast-JX are coupled. Photolysis rates are updated every hour. Large
impact on photolysis rates is observed at the ground, decreasing with
altitude. The aerosol specie that impact the most
photolysis rates is dust especially in
south Europe. Strong impact is
also observed over anthropogenic emission regions (Paris, The Po
and the Ruhr Valley) where mainly nitrate and sulphate reduce the incoming
Differences in photolysis rates lead to changes in gas
concentrations, with the largest impact simulated on OH and NO concentrations. At the
ground, monthly mean concentrations of both species are reduced over
Europe by around 10 to 14% and their tropospheric burden by around 10%. The decrease
in OH leads to an increase of the life-time of several species such as
VOC. NO2 concentrations are not strongly impacted and O3
concentrations are mostly reduced at the ground (−3%). O3 peaks are
systematically decreased because of the NO2 photolysis rate
Not only gas are impacted but
also secondary aerosols, due to changes in gas precursors concentrations.
However changes in aerosol species concentrations often compensate each
other resulting in a low impact on PM10 and PM2.5 concentrations
(lower than 2%).
The changes in gas concentrations at the ground induced by the
modification of photolysis rates (by aerosols and clouds) are compared to changes
induced by 29 different model parametrisations in
Roustan et al. (2010). Among the 31 model parametrisations, "including
aerosols on photolysis rates calculation" has the strongest impact on OH
concentrations and on O3 bias in July.
In terms of air quality, ground concentrations (NO2, O3,
PM10) are compared with
measurements. Changes arising
from cloud parametrisation are small. Simulation performances are often
slightly better when including aerosol in photolysis rates calculation. The
systematic O3 peak reduction leads to large differences in the exceedances of
the European O3 standard as calculated by the model, in better
agreement with measurements. The number of
exceedances of the information and the alert threshold is divided by 2
when the aerosol impact on photochemistry is simulated. This shows
the importance of taking into account aerosols impact on photolysis
rates in air quality studies.
Citation: Real, E. and Sartelet, K.: Modeling of photolysis rates over Europe: impact on chemical gaseous species and aerosols, Atmos. Chem. Phys., 11, 1711-1727, doi:10.5194/acp-11-1711-2011, 2011.