Heterogeneous chemistry of monocarboxylic acids on α-Al<sub>2</sub>O<sub>3</sub> at different relative humidities

Abstract

A study of the atmospheric heterogeneous reactions of formic acid, acetic acid, and propionic acid on &alpha;-Al₂O₃ was performed at ambient condition by using a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reactor. From the analysis of the spectral features, observations of carboxylates formation provide strong evidence for an efficient reactive uptake process. Comparison of the calculated and experimental vibrational frequencies of adsorbed carboxylates establishes the bridging coordinated structures on the surface. The uptake coefficients of formic acid, acetic acid, and propionic acid on &alpha;-Al₂O₃ particles are (2.07&plusmn;0.26)&times;10⁻³ or (2.37&plusmn;0.30) &times;10⁻⁷, (5.00&plusmn;0.69)&times;10⁻³ or (5.99&plusmn;0.78)&times;10⁻⁷, and (3.04&plusmn;0.63)&times;10⁻³ or (3.03&plusmn;0.52)&times;10⁻⁷, respectively (using geometric or BET surface area). Furthermore, the effect of varying relative humidity (RH) on these heterogeneous reactions was studied. The uptake coefficients of monocarboxylic acids on &alpha;-Al₂O₃ particles increase initially (RH<20%) and then decrease with the increased RH (RH>20%) which was due to the effect of water on carboxylic acid solvation, particle surface hydroxylation, and competition for reactive sites. On the basis of the results of experimental simulation, the mechanism of heterogeneous reaction of &alpha;-Al₂O₃ with carboxylic acids at ambient RH was discussed. The loss of atmospheric monocarboxylic acids due to reactive uptake on available mineral dust particles may be competitive with homogeneous loss pathways, especially in dusty urban and desertified environments

Heterogeneous reactions of mineral dust aerosol: implications for tropospheric oxidation capacity

“…Mineral dust aerosol has a myriad of significant impacts on atmospheric chemistry and climate. For example, dust aerosol particles can influence the radiative balance of the Earth system directly by scattering and absorbing solar and terrestrial radiation (Balkanski et al,11728 M. Tang et al: Heterogeneous reactions of mineral dust aerosol as cloud condensation nuclei (CCN) to form cloud droplets (Koehler et al, 2009;Kumar et al, 2009;Twohy et al, 2009;Garimella et al, 2014;Tang et al, 2016a) and ice nucleation particles (INPs) to form ice particles (DeMott et al, 2003;Hoose and Moehler, 2012;Murray et al, 2012;Ladino et al, 2013;DeMott et al, 2015). Mineral dust particles are believed to be the dominant ice nucleation particles in the troposphere (Hoose et al, 2010;Creamean et al, 2013;Cziczo et al, 2013), therefore having a large impact on the radiative balance, precipitation, and the hydrological cycle (Rosenfeld et al, 2001;Lohmann and Feichter, 2005;Rosenfeld et al, 2008).…”

“…Alternatively, reaction rates can also be measured by detecting changes in particle composition (Goodman et al, 2000;Sullivan et al, 2009a;Li et al, 2010;Tong et al, 2010;Ma et al, 2012;Kong et al, 2014). A number of experimental techniques have been developed and utilized to investigate heterogeneous reactions of mineral dust particles, as summarized in Table 2.…”

“…Multiple layers of powdered dust samples are typically deposited on a substrate. Consequently, it is not uncommon that the BET surface area is several orders of magnitude larger than the projected area Liu et al, 2010;Tong et al, 2010). The surface area actually available for heterogeneous uptake falls between the two extreme cases and varies for different studies.…”

Influence of ambient conditions on the evolution of wettability properties of an IR-, ns-laser textured aluminium alloy

“…Eqn (3) details the chemical reaction that occurred on the surface when samples were exposed to ambient air. 42 Chemisorbed carboxylates present in air moisture, such as formic and acetic acid, 43 represent short chain organic molecules that are non polar and therefore lower the total surface free energy. This depolarization of the surface was responsible for the hydrophilic to hydrophobic transition, and explains the reason why samples that were not exposed to ambient air have a slower transition.…”

Effect of aluminium dust on secondary organic aerosol formation in m-xylene/NO x photo-oxidation

“…Aluminium dust including boehmite (AlOOH) and alumina (Al 2 O 3 ) were chosen as seed particles. Boehmite is a major component of aluminium ore bauxite (Mameli et al, 2007), and alumina is usually chosen as the model oxide for mineral dust in laboratory researches (Liu et al, 2012;Ma et al, 2008;Tong et al, 2010). The influences of aluminium seeds on SOA yield and SOA size were also discussed.…”