Jörg Kleffmann

Photosensitized reduction of nitrogen dioxide on humic acid as a source of nitrous acid

Nitrous acid is a significant photochemical precursor of the hydroxyl radical, the key oxidant in the degradation of most air pollutants in the troposphere. The sources of nitrous acid in the troposphere, however, are still poorly understood. Recent atmospheric measurements revealed a strongly enhanced formation of nitrous acid during daytime via unknown mechanisms. Here we expose humic acid films to nitrogen dioxide in an irradiated tubular gas flow reactor and find that reduction of nitrogen dioxide on light-activated humic acids is an important source of gaseous nitrous acid. Our findings indicate that soil and other surfaces containing humic acid exhibit an organic surface photochemistry that produces reductive surface species, which react selectively with nitrogen dioxide. The observed rate of nitrous acid formation could explain the recently observed high daytime concentrations of nitrous acid in the boundary layer, the photolysis of which accounts for up to 60 per cent of the integrated hydroxyl radical source strengths. We suggest that this photo-induced nitrous acid production on humic acid could have a potentially significant impact on the chemistry of the lowermost troposphere.

Photoenhanced uptake of gaseous NO2 on solid organic compounds: a photochemical source of HONO?

In several recent field campaigns the existence of a strong daytime source of nitrous acid was demonstrated. The mechanism of this source remains unclear. Accordingly, in the present laboratory study, the effect of light (in the range 300-500 nm) on the uptake kinetics of NO2 on various surfaces taken as proxies for organic surfaces encountered in the troposphere (as organic aerosol but also ground surfaces) was investigated. In this collaborative study, the uptake kinetics and product formation rate were measured by different flow tube reactors in combination with a sensitive HONO instrument. Uptake on light absorbing aromatic compounds was significantly enhanced when irradiated with light of 300-420 nm, and HONO was formed with high yield when the gas was humidified. Especially organic substrates containing a combination of electron donors, such as phenols, and of compounds yielding excited triplet states, such as aromatic ketones, showed a high reactivity towards NO2. Based on the results reported a mechanism is suggested, in which photosensitised electron transfer is occurring. The results show that HONO can be efficiently formed during the day in the atmosphere at much longer wavelengths compared to the recently proposed nitrate photolysis.

Light induced conversion of nitrogen dioxide into nitrous acid on submicron humic acid aerosol

The interactions of aerosols consisting of humic acids with gaseous nitrogen dioxide (NO 2) were investigated under different light conditions in aerosol flow tube experiments at ambient pressure and temperature. The results show that NO2 is converted on the humic acid aerosol into nitrous acid (HONO), which is released from the aerosol and can be detected in the gas phase at the reactor exit. The formation of HONO on the humic acid aerosol is strongly activated by light: In the dark, the HONO-formation was below the detection limit, but it was increasing with the intensity of the irradiation with visible light. Under simulated atmospheric conditions with respect to the actinic flux, relative humidity and NO2-concentration, reactive uptake coefficients γrxn for the NO2→HONO conversion on the aerosol between γrxn <10−7 (in the dark) andγrxn=6×10 were observed. The observed uptake coefficients decreased with increasing NO 2concentration in the range from 2.7 to 280 ppb and were dependent on the relative humidity (RH) with slightly reduced values at low humidity ( <20% RH) and high humidity (>60% RH). The measured uptake coefficients for the NO2→HONO conversion are too low to explain the HONOformation rates observed near the ground in rural and urban environments by the conversion of NO 2→HONO on organic aerosol surfaces, even if one would assume that all aerosols consist of humic acid only. It is concluded that the processes leading to HONO formation on the Earth surface will have a much larger impact on the HONO-formation in the lowermost layer of the troposphere than humic materials potentially occurring in airborne particles. Correspondence to: M. Ammann (markus.ammann@psi.ch)

Heterogeneous conversion of NO2 on carbonaceous surfaces

In the present study the heterogeneous conversion of NO2 on commercial soot, freshly prepared flame soot and commercial soot treated with sulfuric acid has been investigated. The experimental results indicate a complex reaction mechanism in which the soot has to be treated as a reactant leading to deactivation of active sites on the surface with increasing NO2 consumption. Mean uptake coefficients γ for the first few minutes of the reaction of ∽10-6 have been determined for a consumption of ∽1013 NO2 cm-2. The γ-values decrease to <10-8 for a consumption of ∽1015 NO2 cm-2, indicating that the influence of soot on the partitioning of NOx in the atmosphere is negligible. Nitrous acid (HONO) and nitric oxide (NO) were found to be the major gas phase products. HONO yields up to 80% are observed, clearly showing that the nature of the reaction is non-catalytic. The humidity dependence of the reaction system was studied and indicates that water is necessary for HONO formation. In the reaction of pure HONO on soot surfaces, high yields of NO were observed, indicating a consecutive process: NO2→HONO→NO. When soot was coated with sulfuric acid the yield of HONO decreased while that of NO increased. This behaviour is caused by an increasing reactivity of the intermediate HONO on the modified surface. The experimental results indicate that without any recycling mechanism HONO formation on soot surfaces is not of major importance in the atmosphere.

Mechanistic study of the heterogeneous conversion of NO2 into HONO and N2O on acid surfaces

The heterogeneous conversion of NO2 into HONO and N2O on acid surfaces has been investigated in an 11 l Pyrex glass reactor under simulated atmospheric conditions and in a 64 l quartz glass reactor at reduced pressure. NO2, N2O and HONO were monitored with time by long-path tunable IR diode laser absorption spectroscopy using single vibrational rotational lines at 1641.0, 1254.47 and 1254.46 cm–1, respectively.The experimental results show that NO2 is heterogeneously converted on acid surfaces into HONO and then into N2O. HONO formation in the reaction system has been found to occur by the heterogeneous reaction of NO2 with adsorbed water on the surface and does not require NO. The experimental findings indicate that the nitrosyl ion probably plays an important role in converting HONO into N2O and HNO3. A mechanism is proposed which can explain the experimental findings.

Primary NO2 emissions and their impact on air quality in traffic environments in Germany

BackgroundThe decreasing NOX concentrations at urban measurement stations in Germany are in agreement with the reduction of NOX emissions from vehicular traffic. However, the measured NO2 concentrations are stagnating nationwide. In 2010, at more than the half of the urban measurement stations in Germany, annual mean values for NO2 exceeded the new Europe-wide limit value of 40 μg/m3 (20 ppbv) NO2. Similar findings are reported from many other member states of the European Union.ResultsThe observed trend of the airborne NO2 concentrations has different reasons. Firstly, the NO2/NOx emission ratio has increased significantly during the last two decades. Furthermore, secondary NO2, caused by the titration reactions of NO with ozone (O3) and peroxy radicals (RO2), is responsible for the major fraction (approximately 70%) of the measured NO2. However, secondary NO2 shows a highly nonlinear dependency on NOx and thus, is decreasing much more slowly than expected from the decreasing NOx levels. Based on the results from the present study, the increased NO2/NOX emission ratio can only explain a minor fraction of the observed high airborne NO2 concentration in the city center.ConclusionsA further reduction of primary NO2 emissions, due to improved exhaust gas treatment, will not have a strong influence on urban NO2 levels, and a further significant reduction of the NOX emissions, in particular from vehicular traffic, is necessary in order to meet the annual mean limit value for NO2 of about 20 ppb in the future.

Nitrous oxide and methane emissions from aero engines

The emissions of nitrous oxide and methane from a Pratt & Whitney Canada PW 305 and a Rolls Royce RB 211 jet engine were measured under various flight conditions either on a ground level stationary test stand or in altitude test cells by using an off-line sampling technique. The concentrations of the gases were determined by long path infrared diode laser absorption spectroscopy in the laboratory. The calculated emission indices indicate that, at present, air traffic does not contribute significantly to the global budgets of methane and nitrous oxide.

Smog chamber studies on the influence of diesel exhaust on photosmog formation

Abstract In an outdoor smog chamber, volatile organic compounds (VOC)/NO x /air mixtures were irradiated by natural sunlight in the presence and the absence of diesel exhaust. The VOC mixture contained n -butane, ethene and toluene with a fixed mixing ratio. Diesel exhaust was generated by a diesel engine mounted on a motor test bed directly at the chamber facility. Five different diesel fuel formulations were used. Each experiment was carried out under similar initial conditions for VOC and NO x . In the presence of diesel exhaust, the formation of ozone was significantly increased. Simulation of the experiments performed using a chemical box model yielded good agreement between measured and calculated concentrations for all chamber runs. The increase in ozone formation observed on addition of diesel exhaust was mainly caused by the exhaust concentrations of nitrous acid and formaldehyde, which serve as strong radical sources in the initial phase of each exhaust experiment. A sensitivity analysis showed that the photooxidant formation was not dependent on the formulation of the diesel fuel used. The different ozone formation rates observed for the single exhaust experiments were clearly caused by deviations in initial reactant concentrations as well as photolysis conditions.

Sources and Cycling of Tropospheric Hydroxyl Radicals – An Overview

Abstract The hydroxyl radical is the principal oxidizing agent in the troposphere. It controls and determines the oxidizing power of the atmosphere and thus governs the atmospheric lifetime of many species, and their potential to contribute to climate change, air pollution and ozone formation. Owing to the development of new measurement techniques and the discovery of new reaction mechanisms, the understanding of the OH chemistry has witnessed significant advances during the last decades. This article presents a comprehensive review of the current understanding of OH radical chemistry with an outlook to future work in this active research area of atmospheric chemistry.Among the different OH radical initiation sources, e.g. O3 photolysis, HCHO photolysis, ozonolysis of alkenes and the photolysis of nitrous acid (HONO), the latter has been shown in recent field and model studies to make a major contribution to tropospheric OH levels. New photochemical sources of HONO have been discovered in the laboratory which can explain unexpectedly high concentrations of HONO observed in the atmosphere during daytime. A detailed analysis of HONO sources in the atmosphere is presented. OH radicals react with volatile organic compounds (VOCs) in a complex cycle regenerating OH radicals in the propagation cycle. Recent field and model studies show that the OH radical propagation cycle is balanced for high NOx conditions. Thus, radical losses by reactions with VOCs are balanced by OH formation through the reaction HO2+NO. However, studies at low NO and high VOC levels imply that unknown radical sources of OH exist in rural and forested areas. Possible sources of OH radicals under these conditions are discussed.

Investigation of the heterogeneous NO2 conversion on perchloric acid surfaces

The heterogeneous conversion of NO2 on perchloric acid surfaces into HONO, HNO3 and N2O was investigated in a bubbler system under simulated atmospheric conditions. NO2, HONO and N2O were monitored time resolved by long-path tunable infrared diode laser absorption spectrometry using single vibrational rotational lines. The experimental results show that NO2 is heterogeneously converted on acid surfaces into HONO and consecutively into N2O. HONO formation in the reaction system occurs by the heterogeneous reaction of NO2 with water. The solubility of HONO in perchloric acid was determined at 298 K as a function of the acid concentration. The effective Henrys law constant (KHeff) is well described by: The experimental findings indicate that the nitrosyl ion (NO+) plays an important role in converting HONO into N2O and HNO3. A reaction scheme is proposed which is in excellent agreement with the experimental observations. For the global source strength of N2O by heterogeneous conversion processes on acidic aerosols an upper limit of ca. 300 kt N2O year-1 was estimated which indicates that these reactions are probably of minor importance for the global N2O budget.