Nitrous Acid and Nitrite in the Atmosphere Gerhard Lammel Max-Planck-lnstitut fur Meteorologie, Bundesstr. 55, 20 146 Hamburg, Germany J. Neil Cape Institute of Terrestrial Ecology, Bush Estate, Penicuik, UK EH26 OQB 1 Introduction Despite this importance, nitrous acid is one of the least Why is nitrous acid important as an atmospheric trace gas? It is researched species in inorganic tropospheric chemistry. Why is this present in very small quantities, typically up to a few parts in lo9 by so, and why do we know so little of its major sources and sinks? The volume (ppbV), and usually makes up only a small fraction of the aim of this review is to unravel the puzzle of HNO, occurrence in total amount of gaseous nitrogen oxides present in the atmosphere. the atmospheric boundary layer close to the earth’s surface, as far One reason for an increasing number of recent studies into the as present knowledge allows, by investigating evidence from both chemistry of HNO, is the increasing importance of nitrogen oxides laboratory and field studies. as atmospheric pollutants. Unlike the emissions of sulfur dioxide, which have been decreasing markedly in recent years in Europe and North America, if not in the developing world, the emissions of 2 Why do we know so little? nitrogen oxides are far from decreasing. In developed countries the The apparent lack of information about HNO, in the troposphere is major source of nitrogen oxides is vehicle traffic, and the steadily primarily because it is very difficult to make reliable measurements increasing number of cars and car journeys more than compensate, of HNO, at ambient concentrations. Conventional chemilumines- in many cases, for emission reduction measures. On a global scale cent gas analysers, which are used to measure NO (in the chemilu- the importance of NO, pollutants (= NO + NO, ) is on the rise too, minescence reaction with 0,) and NO, (as NO after reduction), because people produce these trace gases directly from a range of include a signal from HNO, as part of the NO, signal. The catalytic activities (e.g. fossil fuel combustion in the energy sector, biomass converters used in these instruments are designed to reduce NO, burning in the context of agriculture), or enhance natural sources selectively to NO, but are likely also to reduce HNO,. In many sam- (e.g. soil NO emission following fertilization with nitrogen). pling systems HNO, may be lost to the walls of sampling lines or The global significanceof the nitrogen oxides in the atmosphere is inlet filters before it even reaches the catalytic converter, because it in determining the production and consumption of the highly reac- appears to be a ‘stickier’ molecule than NO or NO,. The potential tive hydroxyl and hydroperoxyl radicals, HO, (= OH + HO,). The presence of HNO, as part of the NO, signal from such analysers is OH radical is the most important trace species in tropospheric chem- usually ignored. istry, and its abundance governs the oxidation and eventual removal of most trace gases from the atmosphere. It is in this respect that HNO, shows its importance as a trace gas; on photolysis (A< 390 2.1 Gas-phase Measurement nm) HNO, directly produces OH. This OH source is of greatest Nitrous acid can be determined in ambient air using differential importance during the early morning, when HNO, concentrations optical absorption spectroscopy (DOAS) or denuder systems (DS). may be high after night-time accumulation,and when OH production Spectroscopy, as an in situ method, is clearly advantageous when rates from other sources (photolysis of ozone and formaldehyde)are dealing with molecules which are reactive on surfaces. Gaseous slow. Concentrations of OH radicals later in the day are also influ- HNO, absorbs light in the near UV with maxima at 354 and 368 nm; enced by HNO,. Apart from its role in tropospheric chemistry, its absorption coefficient at these wavelengths is 5.0 X and nitrous acid is of toxicological relevance. It is suspected to be a pre- 4.5 X cm2 per molecule, respectively.’ Most HNO, concen- cursor of carcinogens (nitrosamines) under atmospheric conditions. tration measurements have been based on earlier cross-section data, ~~ Gerhard Lammel was born in 1960 and studied chemistry in John Neil Cape studied chemistry at the University of St Andrews, Regensburg and Freiburg, Germany, where he received a diploma Scotland, before lecturing in chemistry at the National University in 1985, then moved to Mainz, Germany, where he worked for his of Lesotho (1974-1976). He returned to the Department of doctorate (Dr rer. nat.) with the Air Chemistry Department of the Physical Chemistry in Cambridge to study for a PhD, using molec- Max Planck Institute for Chemistry. From 1988 to 1993 he was afJ- ular dynamics to investigate phase changes in simple systems. He iated to the Karlsriihe Nuclear Research Centre, a time period joined the NERC institute of Terrestrial Ecology as an environmen- which included a stay as a research fellow with the Lawrence tal chemist in 1979, and is now a Principal Scientijic Oflcer in the Berkeley Laboratory, Berkeley, California. Since I993 he has been trace gases research section at ITE’S Edinburgh Research Station. a research Jcientist at the Max He is also an Honorat-y Planck institute for Lecturer in the Department of Meteorology at Hamburg. He is Chemistry at the University of a lecturer in Environmental Edinburgh. Current interests Analytical Chemistry at the include a continuing involve- University of Hohenheim, ment with chemical processes at Stuttgart, and works for the interfaces, in particular German Advisory Council on between leaf surfaces and the Global Change, WBGU. His atmosphere, and with the chem- main scientijc interests are istry of the troposphere, both in atmospheric chemistry in the field and in the laboratory. clouds and aerosols, and inter- disciplinary aspects of global environmental change. 36 1 362 CHEMICAL SOCIETY REVIEWS, 1996 which led to an overestimation by ca 5% Much larger discrepan- 2.2 Aqueous-phase Measurement cies in the absorption cross-section have now been resolved I The Dissolved HNO, and nitrite, Nil1,associated with particles, or fog first unequivocal detection of HNO, in the ambient atmosphere and cloud droplets, is determined by wet chemical or ion chro- became possible through the development of an optical absorption matographic methods A detection limit of a few ng m can be technique with a rapid-scanning device A slotted disc rotating in achieved for particles by integrating over many hours ( 12-24 h) and the focal plane allows for ca 100 scans of the spectrum per second ignoring the size distribution An additional risk with long Sam Rapid scanning of the spectrum is necessary, because atmospheric pling periods is the possibility that aqueous samples might be fluctuations with frequencies peaking in the range 1-10 Hz would, subject to rapid oxidation Therefore, on-line analysis or stabiliza- if resolved, dominate spectral features Diode array detectors are tion through a derivatization reaction is strongly recommended On- coming into use now for HNO, measurements Interfering trace line measurement of nitrite in cloud water has been achieved? with species can be identified and corrected for with DOAS using simple detection limits around 10 nmol dm spectral deconvolution methods The sensitivity of the method increases with the path length, which can be up to ca 10 km The method’s performance is such that under favourable atmospheric conditions a detection limit of ca 0 2 ppbV could be achieved with 3 Observations of Nitrous Acid in the a time resolution of 15-30 min with an uncertainty in the HNO, Troposphere determination of ca ? 10% The uncertainty of the absorption coef- ficient applied to calculate the concentrations introduces another ca 5 15% uncertainty 3.1 Field Measurements of Gaseous Nitrous Acid In a denuder tube (usually made from glass), trace gases are sep- Since the first unequivocal detection of HNO, in the atmosphere? arated from the aerosol in sampled air by diffusion Gases diffuse this species has been observed together with NOx in the night time from a laminar flow regime through the denuder to the walls, where atmosphere at both urban and rural sites, and also (at distinctly they are absorbed by an appropriate coating, while aerosols, having lower concentrations) during day-time Most of the measurements very much smaller diffusion rates, pass through the denuder without have been made during winter and spring, although there are now a being trapped The geometry of the denuder tube, and flow rates, few sets of year-round data Night-time HNO, concentrations up to can be adjusted such that a high (usually close to 100%)and known several ppbV have been observed Concentrations in rural areas are collection efficiency is achieved Alkaline surfaces, mainly sodium not necessarily lower than in regions with strong NO, sources carbonate coatings, make effective traps for HNO, After collec- (Figure 1) Rural concentrations over a two year period in Lower tion, the material is then washed from the surface and the extract is Saxonyioaveraged 0 6 ppbV, but with daily maxima in winter up analysed off-line by ion chromatography DS techniques are time- to 8 ppbV HNO, has also been measured inside buildings and consuming, but developments have been made recently in the use vehicles,S I where very much larger concentrations may be found, of automated denuders up to 40 ppbV No denuder system is specific for HNO, A correction for the After sunset,