The heterogeneous interaction of trace gases on mineral dust and soot:kinetics and mechanism

The present thesis work deals with the investigation of the heterogeneous reactions involving nitrate radical (NO3), dinitrogen pentoxide (N2O5) and ozone (O3) on surrogates of atmospheric mineral dust particles characteristic of the troposphere. An additional investigation of heterogeneous reaction of NO3 on flame soot was carried out. The goal is to characterize the kinetics (the uptake coefficient γ) as well as the reaction products. The obtained results are intended to provide reliable data for numerical modelling studies. The experiments were performed in a very low pressure flow reactor (Knudsen cell reactor), coupled to mass spectrometry (MS) and optical probing (Resonance Enhanced Multiphoton Ionization (REMPI)). The used mineral dust powder samples were: Kaolinite, CaCO3, natural limestone, Saharan Dust and Arizona Test Dust. Two different types of soot were produced: soot originating from a rich decane flame at a high fuel/oxygen ratio ("grey" soot) and soot generated from a lean flame at a low fuel/oxygen ratio ("black" soot). Uptake experiments of NO3 on mineral dust were carried out under continuous molecular flow conditions (steady state) at 298 ± 2 K using the thermal decomposition of N2O5 as a NO3 source. In situ laser detection (REMPI) was employed in addition to beam-sampling electron-impact mass spectrometry in order to specifically detect NO2 and NO in the presence of N2O5, NO3 and HNO3. We found a steady state uptake coefficient γss ranging from (3.4 ± 1.6) × 10-2 for natural limestone to 0.12 ± 0.08 for Saharan Dust with γss decreasing as [NO3] increased. NO3 adsorbed on mineral dust led to uptake of NO2 in an Eley-Rideal mechanism where usually no uptake is observed in the absence of NO3. The disappearance of NO3 was in part accompanied by the formation of N2O5 and HNO3 in the presence of NO2. NO3 uptake performed on small amounts of Kaolinite and CaCO3 led to formation of some N2O5 according to NO3(ads) + NO2(g) –> N2O5(ads) –> N2O5(g). Slow formation of gas phase HNO3 on Kaolinite, CaCO3, Arizona Test Dust and natural limestone has also been observed and is clearly related to the presence of adsorbed water involved in the heterogeneous hydrolysis of N2O5(ads). Uptake of N2O5 on mineral dust samples led to γss values ranging from (3.5 ± 1.1) × 10-2 for CaCO3 to 0.20 ± 0.05 for Saharan Dust with γss decreasing as [N2O5]0 increased. We have observed delayed production of HNO3 upon uptake of N2O5 for every investigated sample owing to hydrolysis of N2O5 with surface-adsorbed H2O. At high and low [N2O5] Arizona Test Dust and Kaolinite turned out to be the samples to produce the largest amount of gas phase HNO3 with respect to N2O5 taken up. In contrast, the yield of HNO3 for Saharan Dust and CaCO3 is lower. On CaCO3 the disappearance of N2O5 was also accompanied by the formation of CO2. For CaCO3 sample masses ranging from 0.33 to 2.0 g, the yield of CO2 was approximately 42 – 50% with respect to the total number of N2O5 molecules taken up. The reaction of N2O5 with mineral dust and the subsequent production of gas phase HNO3 leads to a decrease in [NOx] which may have a significant effect on global ozone decrease. The rate of uptake of ozone on various mineral dust substrates was very similar for all the examined substrates. Both initial and steady state uptake coefficients γ0 and γss were found to be similar for all examined substrates. Uptake experiments on cut marble samples have shown that γ0 and γss based on the geometric and total internal (BET) surface area may be over and underestimated between a factor of 50 to 100, respectively. Based on these considerations we proposed initial and steady state uptake coefficients of the order of 10-4 and 10-5, respectively. For all uptake experiments on mineral dust surrogates, the disappearance of O3 was accompanied by formation of O2. The different mineral dust surrogates may be more accurately distinguished by their time-dependent relative O2 yield rather than the magnitude of γ. The heterogeneous reaction of O3 on mineral dust has been found to be non-catalytic and of limited importance in the atmosphere. Uptake experiments of NO3 on decane flame soot led to a large steady state uptake coefficient γss of 0.2 ± 0.02 for grey and black soot with γss decreasing as [NO3] increased. Adsorbed NO3 led to an uptake of NO2 admitted from the hot NO3 source. On large quantities of grey soot we observed production of HONO (nitrous acid) corresponding to almost 100% of NO2 taken up, whereas no HONO was formed on black soot. The disappearance of NO3 was in part accompanied by the formation of N2O5 according to reaction: NO3(ads) + NO2(ads) –> N2O5(ads) –> N2O5(g) probably due to the presence of adsorbed NO3 on the substrate. Subsequently, hydrolysis of N2O5(ads) with adsorbed H2O led to production of gas phase HNO3. For both grey and black soot we observed production of NO which did not depend of the amount of soot and [NO3]. Decomposition of NO3 and HONO on the soot substrates has been proposed to be responsible of gas phase NO formation