Abstract
The central theme of this thesis is the nitrate content of aerosols (or
particulate matter (PM)). Aerosols play an important role in the climate
system by scattering and/or absorbing solar radiation. In the last decades
research has been devoted to quantify the radiative forcing of aerosols on
climate. However, little is known about the
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forcing of aerosol nitrate. The
large uncertainties around the nitrate forcing are directly related to a
lack of reliable measurement data. In this thesis a study devoted to assess
the importance of nitrate for the radiation balance over Europe is
presented.
The first step in this study was to search for data on aerosol nitrate.
However, sampling aerosol nitrate is subject to evaporation losses and
adsorption of nitric acid. Therefore, an analysis of sampling artefacts is
presented first. Main results of this analysis are that quartz filters loose
nitrate above 20ºC and that cellulose filters sample both nitrate and nitric
acid quantitatively. Using the knowledge of artefacts we constructed a
nitrate field over Europe for the winter. High nitrate levels are projected
over north western, central and eastern Europe. There the nitrate
concentrations exceed 4 µg/m3. Maximum levels are found in the Po valley. In
Scandinavia nitrate levels trail off from 2.5 µg/m3 in the south to less
than 0.5 µg/m3 in the north.
In addition, we developed a chemistry-transport model (CTM) to describe the
formation, dispersion and removal of nitrate and other aerosol components in
the atmosphere. The model is able to reproduce the general features of the
wintertime distribution derived from the observations. In winter nitric
acid, the precursor for aerosol nitrate, is formed through heterogeneous
reactions on the surface of aerosols. Appreciable ammonium nitrate
concentrations in summer are limited to those areas with high ammonia
emissions, e.g. the Netherlands. Over large parts of eastern and southern
Europe low ammonium nitrate concentrations are modelled. Averaged over all
stations the model reproduces the measured concentrations for NO3, SO4, NH4,
TNO3, TNH4 and SO2 within 20%. Special attention is given to the uncertainty
of the results towards ammonia emissions.
A new emission database for black carbon (BC) and other primary aerosols was
used to model the fine aerosol distribution over Europe. After sulphate,
nitrate and carbonaceous aerosols are the most important contributors to the
fine aerosol mass over Europe. The modelled concentrations for BC and total
primary material are underestimated. An extended discussion on the
discrepancy between model and observations for BC is presented.
The nitrate fields over Europe, determined within this thesis, enable to
estimate the radiative forcing by nitrate. Based on our simulations the
annual forcing by nitrate is calculated to be 25 % of that by sulphate. In
summer nitrate is found to be only regionally important, e.g. in the
Netherlands, where the forcing of nitrate equals that by sulphate. In winter
the nitrate forcing over Europe is about half the sulphate forcing. Overall,
nitrate forcing is significant and should be taken into account to estimate
the impact of regional climate change in Europe.
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