On the increasing levels of NO2 in some cities. The role of primary emissions and shipping
This report is an attempt to investigate the background to the high NO2 levels in Gothenburg and the reason for the slowing decrease in NO2 observed during the last years. Two possible reasons for these observations are investigated: contribution from shipping to NO2 emissions, and increased fraction of NO2 in the NOX emissions from modern diesel engines. The issue was studied through emission measurement, passive sampling, dispersion modelling and atmospheric chemistry studies Two possible reasons for the high NO2 levels in Gothenburg were investigated: 1) increased fraction of NO2 in the NOX emissions from modern diesel engines, and 2) increasing total emission of NOX due to increasing contribution from shipping. The results also showed that local mixing conditions greatly influenced the dispersion of especially local and ground-based emissions This was mainly due to their main location within the Göta älv valleys where the dispersion becomes particularly poor during high pressure conditions. The effect of ship emissions in the Gothenburg area was very dominant along the harbour. At distances of about 1-2 km fromthe harbour area the ship contribution was still more than 30 % of the total NOX concentration level. The modelled concentration data was compared to measurement results from passive sampling performed mainly along the river but also with the continuous monitoring at the Femman site. In general, the NO2 concentrations were underestimated, the SO2 mainly coincided well and the O3 concentrations where somewhat overestimated in the calculations with the TAPM model. Variation in concentrations due to varying weather conditions were reproduced well but the modelled peaks are sometimes lower than the monitored concentration peaks. There are several explanations for the increased proportions of NO2 in the primary emissions of NOX. First the increase in the fraction of diesel vehicles by ca. 15% (as vehicle-km) during the last decade. Diesel vehicles generally have a higher fraction of NO2 in their NOX emissions than gasoline cars. On top of this, the large increase of diesel vehicles over the last decade was accompanied by a simultaneous increase of the NO2 fraction in NOX emissions from diesel trucks with Euro3 and Euro4 standards which became compulsory in 2000 and 2005, respectively. Measurements of NO2 and NOX concentrations in tunnels, and at sites largely dominated by primary emissions, indicated an increase in the NO2/NOX partitioning from 4-6% in the 1980s, and at the beginning of the 1990s, to today's 13%. The tunnel-model study indicated that the actual NO2/NOX fraction could be even larger if effects of the NO2 sinks in the tunnel are taken into account. The modelling results show that the increase in the NO2 share of the NO2 concentrations was greatest close to the sources since the NO in the primary emission reacted with ambient ozone forming NO2 on a time-scale of minutes and the NO2/NOX ratio quickly increased, approaching a photo-stationary state between NO, NO2 and ozone. The simultaneous measurements of NO2, NO (or NOX) and ozone indicated that the fraction of primary and secondary NO2 in the city varied largely depending on mixing and photochemical conditions. A sensitivity study with the city scale dispersion model was performed by raising the NO2/NOX emission ratio from 5 to 20%. The change in NO2 concentrations showed that the effect of the higher share of NO2 within the NOx emissions can affect the NO2 concentration level close to the source up to a distance of about 500-700m. The chemical development in ship plumes was studied with a detailed photochemical plume model to ensure that the simple chemistry treatment of the TAPM model accurately described the processes affecting the NO/NO2 distribution and NOX oxidation. Comparison of the detailed chemistry with the simplified version showed a significant similarity during day hours when chemistry is, to a large extent, driven by NO2 photolysis. The night-time chemistry of NOX, driven by nitrate radical and oxidation of N2O5 is not included in the TAPM chemical scheme which may lead to an underestimation of NOX oxidation during dark hours.
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