Sources of atmospheric nitrous acid: State of the science, current research needs, and future prospects

Abstract

Nitrous acid (HONO) plays a key role in tropospheric photochemistry, primarily due to its role as a source of hydroxyl (OH) radicals via its rapid photolysis. OH radicals are involved in photooxidation processes, such as the formation of tropospheric 03 and other secondary atmospheric pollutants (peroxyacetyl nitrate/PAN] and secondary particles). Recent field and modeling studies have postulated the occurrence of a strong and unknown daytime HONO source, but there are still many significant uncertainties concerning the identification and formation mechanisms of these unknown sources. Up to now, five HONO formation pathways are known: direct emission, homogeneous gas-phase reactions, heterogeneous reactions, surface photolysis; and biological processes. In this review paper the HONO sources proposed to explain the observed HONO budget, especially during daytime, are discussed, highlighting the knowledge gaps that need further investigation. In this framework it is crucial to have available accurate and reliable measurements of atmospheric HONO concentrations; thus, a short description ofHONO measurement techniques currently available is also reported. The techniquesare divided into three basic categories: spectroscopic techniques, wet chemical techniques, and off-line methods.

Impacts of Additional HONO Sources on Concentrations and Deposition of NO_y in the Beijing-Tianjin-Hebei Region of China

“…In summer, interferences on the order of 2−9% from nitrite-forming species were observed and corrected using sodium carbonate denuders (Spataro et al 2013). The detection limit was 0.88 ppt (Spataro et al 2013;Spataro and Ianniello 2014). The uncertainty for HONO measurements was ~10% (Liu et al 2010).…”

Comparisons of measured nitrous acid (HONO) concentrations in a pollution period at urban and suburban Beijing, in autumn of 2014

“…The higher ratios of HONO/ NO 2 in suburbans indicated higher conversion efficiency of NO 2 heterogeneous reactions [11,33]. The heterogeneous formation of HONO could have occurred on various surfaces with different conversion rates, and the preferences to aerosol or ground surface were still in discussions [34]. However, compared to the urban surfaces of concrete, glass and foliage, the suburban surroundings provided much more surfaces for the heterogeneous reactions such as soils, lake, mineral dusts as well as numerous particles [34].…”

Measurements of PM10 ions and trace gases with the online system MARGA at the research station Melpitz in Germany – A five-year study

“…Beside possible positive artifacts caused by wet chemical sampling, HONO daytime sources also exist and explain low HONO concentrations despite of a strong photolysis. Several HONO daytime sources were discussed in literature (Li et al 2014;Michoud et al 2014;Spataro and Ianniello 2014;VandenBoer et al 2014).…”