The Climatic Significance of Biogenic Aerosols in the Boreal Region Now
and in the Future
Abstract
The magnitude of aerosol radiative effects remains the single largest
uncertainty in current estimates of anthropogenic radiative forcing. One
of the key quantities needed for accurate estimates of anthropogenic
radiative forcing is an accurate estimate of the radiative effects from
natural aerosol. The dominant source of natural aerosols over Earth’s
forested regions is biogenic volatile organic compounds (BVOC) which,
following oxidation in the atmosphere, can participate in new particle
formation or condense onto aerosols to form secondary organic aerosol
(SOA). Consequently, BVOC emissions could introduce a regionally
relevant cooling feedback in a warming climate. The main objective of
this study is to provide a quantitative estimate of the regional aerosol
direct radiative effect caused by the temperature-dependent biogenic
emissions over the boreal forests in present day conditions and in a
warmer future. The study is done using a combination of climate modeling
and satellite data. The aerosol-chemistry climate model used is
ECHAM-HAMMOZ, which describes the relevant atmospheric aerosol
processes. The BVOC emissions are computed online using the MEGAN model,
which enables the simulation of the effects of temperature changes on
atmospheric aerosol load. Key remote sensing data used are the AATSR
based aerosol optical depth (AOD) and land surface temperature (LST)
products available from the Aerosol-CCI and GlobTemperature projects,
together with ancillary data, such as column concentrations of CO and
water vapour from AIRS, and NO2 column densities from OMI. Our analysis
shows that there could be a small temperature dependence in AOD over the
boreal forests but it cannot be reliably detected from the simulations
or observations. The only subregion with a clear temperature dependence
in AOD was found over western Russia. Anthropogenic emissions affect
this subregion more than the other regions analyzed thus, it is likely
that in addition to BVOC emissions hygroscopic sulfate aerosols affect
the temperature dependence of AOD. In a warmer future the clear-sky
radiative forcing caused by biogenic aerosols will increase, following
the increase of BVOC emissions, but if anthropogenic emissions will
decrease at the same time the total clear-sky forcing will also
decrease.