Nitrous Acid (HONO)

Principal Investigators:
Jörg Kleffmann / Guillermo Villena Tapia

Affiliation:
University of Wuppertal/Germany

Measurement:
Nitrous Acid (HONO)

PI:

PD Dr. Jörg Kleffmann
Physikalische Chemie /FB C
Bergische Universität Wuppertal
42097 Wuppertal /Germany
Tel.:      +49 202 439 3534
Fax.:     +49 202 439 2505
e-mail: kleffman@uni-wuppertal.de

Objectives:

Nitrous acid (HONO) is an important precursor of the hydroxyl radical (OH) the “cleaning detergent” of the atmosphere. Recent polar studies indicate the existence of a strong surface source of HONO over snow and ice surfaces, however, the mechanism is still under discussion. The photolysis of nitrate/HNO3 and photosensitized reactions including NO2 and humic substances were proposed to explain high HONO levels over snow surfaces, which may have a significantly impact on the oxidation capacity of the polar atmosphere.

Recently, it was proposed that high measured HONO levels may be caused, at least in part, by strong interferences of chemical instruments used in polar studies. This is confirmed by modeling studies including measurement data of NOx, HOx and HONO and by the unrealistic high HONO/NOx ratios of up to 100 % measured in polar studies, which is not in accordance with the known lifetimes of both species. In addition, a recent intercomparison study between the spectroscopic LIF technique and a chemical instrument (mist chamber) showed strong positive interferences of the chemical instrument (difference by a factor of seven). These high interferences were recently confirmed by our LOPAP measurements for simulated polar conditions on the mountain “Jungfraujoch” in the Swiss Alps (3580 m altitude).

Thus, the objectives of the University of Wuppertal in the OASIS activities are (a) quantification of HONO levels under polar conditions by an instrument for which interferences are corrected for and (b) clarification of the potential formation mechanisms of HONO over irradiated snow and ice surfaces.

The LOPAP technique:

In the LOPAP instrument, HONO is sampled by a fast, selective chemical reaction in a stripping coil, is converted into a strongly absorbing dye and measured in long path absorption in a liquid core waveguide. To minimize sampling artifacts, HONO is sampled in an external sampling unit directly at the atmosphere of interest, thus excluding the use of any sampling lines. To correct for chemical interferences, two channels are used in series. In the first channel, HONO and interfering substances are sampled, while in the second channel only the interferences are measured. From the difference of both channels HONO concentrations are determined. Caused by the fast chemical reaction, leading to the shortest gas/liquid contact time of all known chemical instruments and the high acidity of the sampling solution, all interferences tested so far can be corrected for. This is confirmed by the very good agreement obtained in recent intercomparison studies against the well accepted spectroscopic DOAS technique. Optimized to remote conditions, the instrument has a time resolution of 7 min and a detection limit of 0.2 ppt.

Fig. 1:   The LOPAP instrument

Example of previous work:

HONO was recently measured for simulated polar conditions on the mountain “Jungrfraujoch” (3580 m altitude) and high correlation of HONO with the irradiance was obtained supporting a photochemical formation mechanism. Average HONO/NOx and HONO/NOy ratios of 4.6 % and 1.1 % were observed, which are much lower compared to other remote studies.

Fig. 2:   Field site at the mountain “Jungfraujoch” in the Swiss Alps (3580 m altitude)

Fig. 3:  External sampling unit of the LOPAP instrument during a supercooled cloud event.

Fig. 4:  Average diurnal variation of HONO, HONO/NOx, HONO/NOy and irradiance from the field campaign at “Jungraujoch”.