Abstract

The formation and loss of NO₃ radicals and N₂O₅ during the nighttime plays important roles in many aspects of tropospheric chemistry. It influences the NOx budget directly and thus impacts the photochemical formation of ozone, contributes significantly to the degradation of volatile organic carbons and the formation of organic nitrate and secondary organic aerosols, and leads to the formation of particulate nitrate and ClNO2 which is an important precursor of Cl atoms in the troposphere. Field measurement and laboratory studies have been combined to better understand the loss mechanism of NO₃ and N₂O₅ in the troposphere.

The two-channel Cavity Ring-Down instrument (CRD), with a detection limit of 1-3 pptv in 3 s, was developed to simultaneously detect NO₃ radicals and N₂O₅, and has been successfully deployed in three large field campaigns between 2008 and 2011 at sites with large geographical differences (a coastal site in south Spain, a boreal forest in Finland, and a rural mountain site in Germany). Mixing ratios of NO₃ (from below detection limit to >200 pptv) and N₂O₅ (from below detection limit to >2 ppbv), and NO₃ lifetimes (from seconds to >1 h) were found to vary a lot and highly depend on the concentrations of precursors (i.e. NO₂ and O₃) and also the strengths of different sinks. The contribution of different NO₃/N₂O₅ loss processes to the nighttime NOx removal will be presented for in a few interesting scenarios, and the first measurement of ClNO₂ over Europe will be introduced and its potential significance will be discussed.

The heterogeneous uptake N₂O₅ to Saharan dust particles was investigated using aerosol flow tubes with detection of N₂O₅ by cavity ring-down spectroscopy. The uptake coefficient was determined to be 0.02 ± 0.01 on airborne Saharan dust particles, independent of relative humidity (RH, 0-67%) and initial N₂O₅ concentration (~ 20-1000 ppbv). The particles could be deactivated with respect to N₂O₅ uptake only when pre-treated with very high levels of HNO ₃. Analysis of gas- and particulate phase products suggests that N₂O₅ undergoes heterogeneous hydrolysis forming particulate nitrate. The heterogeneous reaction of NO₃ radicals with Saharan dust particles was studied using a novel relative rate method. The uptake coefficient ratio, γ(NO₃)/γ(N₂O₅), was determined to be 0.9±0.3 for Saharan dust, independent of relative humidity and exposure time, though surface deactivation was observed for both species even when mixing ratios were typically a few hundred pptv.

A brief summary will be followed by discussing a few challenges in nocturnal chemistry and potential solutions.