On the importance of aerosol nitrate over Europe : data analysis and modelling

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 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.