Articles | Volume 15, issue 2
Atmos. Chem. Phys., 15, 799–813, 2015
Atmos. Chem. Phys., 15, 799–813, 2015
Research article
23 Jan 2015
Research article | 23 Jan 2015

A comparison of HONO budgets for two measurement heights at a field station within the boreal forest in Finland

R. Oswald1,2, M. Ermel1,2, K. Hens3, A. Novelli3, H. G. Ouwersloot3,4, P. Paasonen5, T. Petäjä5, M. Sipilä5, P. Keronen5, J. Bäck6, R. Königstedt3, Z. Hosaynali Beygi3, H. Fischer3, B. Bohn7, D. Kubistin3,8, H. Harder3, M. Martinez3, J. Williams3, T. Hoffmann2, I. Trebs1,9, and M. Sörgel1 R. Oswald et al.
  • 1Biogeochemistry Department, Max Planck Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany
  • 2Institute for Inorganic and Analytical Chemistry, Johannes Gutenberg University Mainz, 55128 Mainz, Germany
  • 3Airchemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany
  • 4Meteorology and Air Quality, Wageningen University, Wageningen, the Netherlands
  • 5Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
  • 6Department of Forest Sciences, University of Helsinki, P.O. Box 27, 00014 Helsinki, Finland
  • 7Institut für Energie- und Klimaforschung IEK-8: Troposphäre, Forschungszentrum Jülich GmbH, 52428 Jülich, Germany
  • 8University of Wollongong, School of Chemistry, Wollongong, Australia
  • 9Luxembourg Institute of Science and Technology, Environmental Research and Innovation (ERIN) Department, 5 avenue des Hauts-Fourneaux, 4362 Esch/Alzette, Luxembourg

Abstract. Atmospheric concentrations of nitrous acid (HONO), one of the major precursors of the hydroxyl radical (OH) in the troposphere, significantly exceed the values predicted by the assumption of a photostationary state (PSS) during daytime. Therefore, additional sources of HONO were intensively investigated in the last decades. This study presents budget calculations of HONO based on simultaneous measurements of all relevant species, including HONO and OH at two different measurement heights, i.e. 1 m above the ground and about 2 to 3 m above the canopy (24 m above the ground), conducted in a boreal forest environment. We observed mean HONO concentrations of about 6.5 × 108 molecules cm−3 (26 ppt) during daytime, more than 20 times higher than expected from the PSS of 0.2 × 108 molecules cm−3 (1 ppt). To close the budgets at both heights, a strong additional source term during daytime is required. This unidentified source is at its maximum at noon (up to 1.1 × 106 molecules cm−3 s−1, 160 ppt h−1) and in general up to 2.3 times stronger above the canopy than close to the ground. The insignificance of known gas phase reactions and other processes like dry deposition or advection compared to the photolytic decomposition of HONO at this measurement site was an ideal prerequisite to study possible correlations of this unknown term to proposed HONO sources. But neither the proposed emissions from soils nor the proposed photolysis of adsorbed HNO3 contributed substantially to the unknown source. However, the unknown source was found to be perfectly correlated to the unbalanced photolytic loss of HONO.

Short summary
Nitrous acid (HONO) is a key species in atmospheric photochemistry since the photolysis leads to the important hydroxyl radical (OH). Although the importance of HONO as a precursor of OH is known, the formation pathways of HONO, especially during daytime, are a major challenge in atmospheric science. We present a detailed analysis of sources and sinks for HONO in the atmosphere for a field measurement campaign in the boreal forest in Finland and wonder if there is really a source term missing.
Final-revised paper