Riemer Group

Hydrolysis of N2O5

The heterogeneous hydrolysis of N2O5 on the surface of deliquescent aerosols provides a removal path for nitrogen oxides and can therefore have an impact on photochemistry and nitrate deposition. However, recent measurements have shown that the values for the reaction probability N2O5 are smaller than formerly assumed and that there is a dependence of the reaction probability on the nitrate content of the aerosol ("nitrate effect"). These findings motivated us to investigate the impact of the hydrolysis of N2O5 on the surface of aerosols on a regional scale, focussing on the relevance for ozone, NO3 and nitrate. To elucidate the role of atmospheric transport, we carried out 1D- and 3D-simulations. In the 1D simulation only vertical turbulent diffusion acts as transport process. In the 3D simulation, horizontal diffusion as well as advection add to the transport.

Results

1-D simulations:
The left figure above displays the vertical profile of N2O5 and the surface density S. It is obvious that the two profiles do not coincide spacially. This spatial "mismatch" consequently leads to rather small reaction rates compared to a simple box model treatment. The middle figure shows that when the hydrolysis of N2O5 is included, less ozone is produced in the low NOx case, whereas in the high NOx case the ozone concentration increases. This finding is consistent with the fact that including the hydrolysis results in a removal of nitrogen oxides. The differences in ozone range on the order of 5% and 10%, respectively. In contrast to the relatively small changes in daytime ozone values, the vertical profile of NO3 is modified tremendously by including the N2O5 reaction (right figure). Here, differences of up to 80% occur.

3D simulation:

The horizontal distribution of the differences in the O3 concentration in the left figure below shows that ΔO3 at 14 CET is very small in the whole model domain and that there is no change of the sign reflecting high and low NOx conditions.
The reason for the latter observation is that there is a transport of air masses from areas where low NOx conditions apply into the plume areas where high NOx conditions exist. However, the horizontally averaged profiles for NO3 (right figure) demonstrate clearly that the profiles are significantly modified by the hydrolysis of N2O5.

Conclusion

Although the pure chemistry calculations may suggest that the hydrolysis of N2O5 has a strong impact on tropospheric chemistry, we have shown that transport effects reduce this impact significantly. For the representation in models it therefore appears to be important that the nocturnal boundary layer is adequately resolved. For summer smog episodes, the hydrolysis of N2O5 therefore has a minor impact on ozone concentrations. However, for other species the influence may be more important. This applies in particular for NO3, which constitutes the important radical in nighttime chemistry. Further studies indicate moreover that the hydrolysis is important for aerosol nitrate concentrations.

Current work

We are exploring the impact of organic coating on the N2O5 hydrolysis. This work is in collaboration with Heike and Bernhard Vogel, IMK, Tatu Antilla, Finnish Meteorological Institute, and Astrid Kiendler-Scharr and Thomas Mentel, FZ-Juelich.

Publications

N. Riemer, H. Vogel, B. Vogel, T. Anttila, A. Kiendler-Scharr, Th.F. Mentel [2009] The relative importance of organic coatings for the heterogeneous hydrolysis of N2O5, J. Geophys. Res., 114, D17307, DOI: 10.1029/2008JD011369.

N. Riemer, H. Vogel, B. Vogel, B. Schell, I. Ackermann, Ch. Kessler, H. Hass [2003] The impact of the heterogeneous hydrolysis of N2O5 on tropospheric chemistry and nitrate aerosol formation, Journal of Geophysical Research 108, 4144, DOI: 10.1029/2002JD002436. (pdf)