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The nitrogen oxides are important trace gas constituents of the troposphere for many reasons, but foremost because of their involvement in the HOx (=OH+HO2) and NOx (=NO+NO2) catalytic radical chain reactions that drive photochemical O3 production. Reactions of HOx or organic peroxy radicals (ROx) with NOx generally terminate these radical chains, resulting in formation of nitric acid (HONO2) from reaction of NO2 with OH in the high-[NOx] regime, of peroxides (e.g., H2O2) from self-reactions of HOx radicals at low NOx concentration, or of organic nitrates, e.g., of alkyl nitrates (AN, RONO2) formed as side products in the reactions of RO2 radicals with NO. This pathway is the major chain-termination step at intermediate NOx concentration.
The dominant organic nitrate classes are the AN and peroxycarboxylic nitric anhydrides (PAN, RC(O)O2NO2, R≠H), also called peroxyacyl nitrates. Concentrations of the most abundant PAN, peroxyacetic nitric anhydride, are generally well-predicted by photochemical models, and the impact of PAN in the Earth system is thus reasonably well understood. The impact of AN formation on tropospheric O3 and NOx budgets, on the other hand, remains uncertain as there are gaps in our knowledge of AN chemistry governing their production and losss.
The HOx radicals driving this chemistry are generated by photo-dissociation of a free radical precursor, or reservoir, species. Their chemistry needs to be understood for an accurate assessment of radical-driven processes in the troposphere. Many of these are generated "in the dark", i.e., at night, e.g., nitrous acid (HONO) or nitryl chloride (ClNO2). At night, the major oxidant is the nitrate radical (NO3), produced by reaction of NO2 with O3, which exists in a thermal equilibrium with dinitrogen pentoxide (N2O5) and NO2 [Osthoff et al., 2007]. Hydrolysis of N2O5 on surfaces is a major NOx removal pathway. It is now clear, though, through chemical ionization mass spectrometric (CIMS) measurements pioneered by me when I was a PDF [Osthoff et al., 2008], that reaction of N2O5 with aerosol chloride efficiently produces ClNO2 and not only reduces rates of nocturnal NOx loss but also generates Cl atoms [Mielke et al., 2013; Osthoff et al., 2017; Osthoff et al., 2008]. The Cl radicals efficiently feed into the HOx-driven cycles and lead to higher net O3 production in polluted areas, or catalyze O3 destruction in remote areas such as the polar boundary layer.
Our current picture of these reservoir species, in particular the abundances and formation pathways of dihalogens (Cl2, Br2, I2, and the interhalogens) and reservoir species such as chlorine nitrate (ClNO3), is incomplete, hampering our ability to make predictions, e.g., of the effects of Arctic climate change. Measurement techniques with improved limits of detection are thus needed.
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