Heterogeneous Reaction of NO2 on Al2O3: The Effect of Temperature

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Heterogeneous Reaction of NO on AlO: The Effect of Temperature on the Nitrite and Nitrate Formation Lingyan Wu, Shengrui Tong, and Maofa Ge J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp402773c • Publication Date (Web): 20 May 2013 Downloaded from http://pubs.acs.org on May 20, 2013

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The Journal of Physical Chemistry

Heterogeneous Reaction of NO2 on Al2O3: The Effect of Temperature on the Nitrite and Nitrate Formation Lingyan Wu 1, 2, Shengrui Tong 1, ∗, and Maofa Ge 1, ∗

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Beijing National Laboratory for Molecular Science (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China

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Beijing National Laboratory for Molecular Science (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Peking University, Beijing, 100871, P. R. China

∗ Corresponding author. Tel.: 86-10-62554518; Fax: 86-10-62559373; E-mail: [email protected] (Maofa Ge). ∗ Corresponding author. Tel.: 86-10-62558682; Fax: 86-10-62559373; E-mail: [email protected] (Shengrui Tong).

1 ACS Paragon Plus Environment


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Abstract: Although recent evidence suggests that the heterogeneous reaction of NO2 on the surface of mineral aerosol plays an important role in the atmospheric chemistry, a fundamental understanding of how temperature influences the rate and extent of nitrate formation processes remains unclear. This work presents the first laboratory study of the effect of temperature on heterogeneous reaction of NO2 on the surface of γ-Al2O3 in the temperature range of 250-318 K at ambient pressure. From the analysis of IR spectra, nitrite was found to be an intermediate product at temperatures between 250 and 318 K. It is proved by our experiments that nitrite would convert to the bidentate nitrate as the reaction proceeded. In addition, it is interesting to find that the rate of conversion increased with decreasing temperature. Along with nitrite decrease, the initial rate of nitrate formation increased while the rate of nitrate formation in the steady region decreased with decreasing temperature. The uptake coefficients at seasonal temperatures were determined for the first time and were found to be sensitive to temperature. Finally, atmospheric implications of the role of temperature on the heterogeneous reaction of NO2 with mineral aerosol are discussed.

Keywords: temperature effect, DRIFTS, mineral aerosol, uptake coefficient


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1. Introduction Nitrogen oxides are a major component of air pollution and their chemical reactivity is of great importance in the atmospheric chemistry.

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It can contribute to both stratospheric ozone depletion

and the formation of acid rain. 3-6 Typical concentration of NO2 in the air photochemical smog may be as high as 70 part per billion (ppb), but in the vicinity of emission sources, such as fumes of coal power stations or waste gases of motor engines, the NO2 concentration may increase by several orders of magnitude reaching ~400 part per million (ppm). 7-8 At present, the major chemical sink of nitrogen oxides is the reaction of NO2 with OH radicals, followed by formation and precipitation of nitric acid.

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Another pathway to remove nitrogen oxides from the gas phase is heterogeneous

processes, for example, the uptake of NO2 on mineral aerosol with forming adsorbed products such as surface nitrate.

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A number of reactions of nitrogen oxides that are slow in the gas phase do

occur at significant rates on the surfaces of laboratory systems.

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Modeling studies suggested that

approximately 40% of nitrate formation was associated with mineral aerosols. 14 It was reported that the heterogeneous reaction of NO2 with dust particles could account for the accumulation of nitrate during high dust events and long-term transport. 15-16 Mineral aerosol, constituting 36% of total primary aerosol emissions, is the most widespread and concentrated tropospheric aerosol.

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Recent field studies estimated that 1000 to 3000 Tg of

mineral aerosol is uplifted into the atmosphere annually. 14 Particles exceeding a radius of 10 µm are redeposited within the source region, leaving approximately 350 Tg/yr available for long-range transport thousands of miles downwind at altitudes between 1 and ∼4.5 km to affect other regions on



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intercontinental or hemispheric scales.

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Under such altitude scope, environmentally relevant

conditions such as temperature and relative humidity are greatly different from the earth’s surface. However, to the best of our knowledge, most of previous laboratory studies on heterogeneous reactions were performed at room temperature. Less is known about the heterogeneous reaction of NO2 on mineral aerosol at temperatures less than 298 K. The experimental determination of rate constants for important atmospheric reactions and how these rate constants vary with temperature remain a crucially important part of atmospheric science.

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Furthermore, field investigation found

that particulate nitrate ion concentration in the northern and central Great Plains had increased at the rate of over 5% yr-1 in wintertime.

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Thus it is important to explore the heterogeneous reaction of

NO2 on mineral aerosol surfaces as a function of temperature. Based on the above reasons, we investigated the effect of temperature on the heterogeneous reaction of NO2 on the surface of γ-Al2O3 particles in the present study. Alumina is a significant fraction of the Earth’s crust and a major component of mineral aerosol in the Earth’s atmosphere. It also emitted into the stratosphere from solid-propellant rocket motor exhaust.

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γ-Al2O3 was

widely used as a model for mineral aerosol because of its better quality of spectra information to obtain useful information about the mechanism of atmospheric heterogeneous reaction.

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Using

diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), a series of reactive uptake coefficients for the heterogeneous reaction of NO2 on the surface of γ-Al2O3 particles at different temperatures were obtained. The reactive uptake coefficients at different temperatures will supply basic data for atmospheric chemistry modeling studies. Moreover, the mechanism of the temperature dependence of nitrite and nitrate formation rate was also discussed. Understanding of how the


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temperature affects secondary nitrate and nitrite aerosols will offer insight into the mechanism of the heterogeneous reaction of NO2 on mineral aerosol particles in the troposphere. 2. Experimental Section

2.1 Chemicals Commercially available γ–Al2O3 particles (with a stated minimum purity of 99+%) purchased from Alfa Aesar were used for the spectroscopic measurements. The Brunauer-Emmett-Teller (BET) surface area was determined to be 220.59 m2 g-1 from a multipoint BET analysis (Autosorb-IQ automatic equipment (Quanta Chrome Instrument Co.)). NO2 (99.9%, Beijing Huayuan Gas Chemical Industry Co., Ltd.) was diluted and mixed with N2 (> 99.999%, Beijing Tailong Electronics Co., Ltd.) before used. The NO2 was diluted by N2 in a glass bottle and the partial pressure was monitored by absolute pressure transducer (DL-4 range of 0.1-100 KPa). The NO2 concentration was measured by a NOx chemiluminescence analyzer (Thermo 42i). O2 (> 99.998%, Orient Center Gas Science & Technology Co., Ltd.) was used to simulate the ambient air. N2 and O2 were dehumidified by silica gel and molecular sieve before flowing into the system, and the relative humidity (RH) was less than 1%. All gases were mixed together before entering the reactor chamber, resulting in a total flow of 400 sccm synthetic air (21% O2 and 79% N2). Ultrapure water with resistivity of 18.2 MΩ·cm was purified by the Thermo Scientific Barnstead Easypure II systems (Model UF).

2.2 Experimental methods In this work the heterogeneous reaction of nitrogen dioxide with γ–Al2O3 has been studied using



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diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) technique. The experimental setup has been described previously,

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therefore only relevant features will be described here. The

sample compartment of the FTIR spectrometer (Nicolet 6700) with a liquid-nitrogen-cooled narrow band mercury-cadmium-telluride (MCT) detector houses a reaction chamber (Model CHC-CHA-3, Harrick Sci. Corp.). The reaction chamber, which is designed for operation from -150oC up to 600oC, was part of the gas flow system. The volume of the reaction chamber is about 15 mL. To obtain a reproducible packing of the DRIFTS sampling cup, the γ-Al203 powder (about 16 mg) was placed in the stainless steel cup (10mm diameter, 0.5mm depth) and compressed in order to form a solid pellet. NO2 with specific concentration in the dry synthetic air were then introduced into the reaction chamber and passed through the powder. Average residence time of gases inside the DRIFTS cell was approximately 2.5 s. The experimental conditions were controlled at ambient pressure under dry synthetic air at temperatures from 250 to 318 K. During the whole experimental procedure, the temperature uncertainty was ±1 K. Vibrational spectra were recorded in the spectral range from 4000 to 650 cm-1 with a resolution of 4 cm-1 during the exposure of the γ-Al203 sample to NO2. A typical experiment at each temperature lasted 200 min. To improve the signal to noise ratio 100 scans were co-added for each spectrum resulting in a time resolution of 40 s. The integrated absorption bands of the products were calibrated absolutely by analysing the sample by ion chromatography after reaction. The reacted alumina samples were sonicated in 1.5 mL of ultrapure water for 20 min. The filtered solution was analyzed using a Dionex ICS 900 system, which was equipped with a Dionex AS 14A analytical column and a conductivity detector (DS5).


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3. Results and Discussion

3.1 Observed products Prior to initiation of the experiments, an ultra-pure sample was in situ pretreated by heating in synthetic air at 573 K for 3 h before an experiment. This treatment can remove surface-adsorbed substances such as adsorbed water from the surface.

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Then the pretreated sample was cooled

to the desired experimental temperature. In all uptake experiments, the DRIFTS-spectrum of the unreacted alumina sample has been used as a background spectrum. Therefore, reaction products formed during the uptake can be observed as positive absorption bands whereas negative bands indicate the loss of the corresponding species. When alumina particles were exposed to NO2 (1.21×1015 molecules cm-3) under dry conditions (RH

Heterogeneous Reaction of NO2 on Al2O3: The Effect of Temperature

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