Characterization of surface functional groups present on laboratory-generated and ambient aerosol particles by means of heterogeneous titration reactions

Abstract

A Knudsen flow reactor has been used to quantify surface functional groups on aerosols collected in the field. This technique is based on a heterogeneous titration reaction between a probe gas and a specific functional group on the particle surface. In the first part of this work, the reactivity of different probe gases on laboratory-generated aerosols (limonene SOA, Pb(NO 3 ) 2 , Cd(NO 3 ) 2 ) and diesel reference soot (SRM 2975) has been studied. Five probe gases have been selected for the quantitative determination of important functional groups: N(CH 3 ) 3 (for the titration of acidic sites), NH 2 OH (for carbonyl functions), CF 3 COOH and HCl (for basic sites of different strength), and O 3 (for oxidizable groups). The second part describes a field campaign that has been undertaken in several bus depots in Switzerland, where ambient fine and ultrafine particles were collected on suitable filters and quantitatively investigated using the Knudsen flow reactor. Results point to important differences in the surface reactivity of ambient particles, depending on the sampling site and season. The particle surface appears to be multi-functional, with the simultaneous presence of antagonistic functional groups which do not undergo internal chemical reactions, such as acid-base neutralization. Results also indicate that the surface of ambient particles was characterized by a high density of carbonyl functions (reactivity towards NH 2 OH probe in the range 0.26-6 formal molecular monolayers) and a low density of acidic sites (reactivity towards N(CH 3 ) 3 probe in the range 0.01-0.20 formal molecular monolayer). Kinetic parameters point to fast redox reactions (uptake coefficient 0 > 10 −3 for O 3 probe) and slow acid-base reactions ( 0 < 10 −4 for N(CH 3 ) 3 probe) on the particle surface.

Probing Functional Groups at the Gas–Aerosol Interface Using Heterogeneous Titration Reactions: A Tool for Predicting Aerosol Health Effects?

“…An interesting case is provided by the results for the two limonene SOA entries in Table 1 that differ in their degree of oxidation. The yield of weakly oxidized limonene SOA was only 2 % based on consumed limonene, [21] in contrast to 55 % for the strongly oxidized case. [36] Both the ozone concentration and the gas residence time were larger for the latter conditions which should lead to a higher degree of SOA oxidation.…”

“…[39] Table 1 reveals that the amorphous carbons, namely FS 101, Printex 60 and FW 2 and to a lesser extent hexane soot from a lean flame apparently contain basic oxides on their surface by virtue of the larger uptake of CF 3 COOH compared to HCl. Setyan et al [21] have investigated a SOA generated from ozone oxidation of limonene whose ratio N i CF3COOH /N i HCl amounted to a factor of ten and indicated the presence of basic oxides. In contrast, "aged" Diesel soot substrates (SRM2975, Diesel BD), as well as PM 4 collected in the field [21] take up higher amounts of HCl than CF 3 COOH.…”

“…Setyan et al [21] have investigated a SOA generated from ozone oxidation of limonene whose ratio N i CF3COOH /N i HCl amounted to a factor of ten and indicated the presence of basic oxides. In contrast, "aged" Diesel soot substrates (SRM2975, Diesel BD), as well as PM 4 collected in the field [21] take up higher amounts of HCl than CF 3 COOH. Diesel BD soot is noteworthy for its high uptake of significant amounts of HCl.…”

“…However, we cannot exclude the formation of Cd 2 + and Pb 2 + hydroxides through hydrolysis and subsequent neutralization by HCl opposed to direct halogen exchange of Cl À against nitrates. [21] The description of the sampling sites and the whole data set may be found in Setyan et al [21] The discussion is based on N i M values expressed as the number of probe molecules taken up per unit of particle surface area of the filter or as a fraction of a molecular monolayer. [21,43] The choice of a surface-based metric is motivated by previous studies which highlighted the toxicity of nanoparticles in terms of their surface area [66] and the ability of particulate matter to adsorb to a wide variety of compounds (proteins, surfactants, lipids) in the lung.…”

“…[19,20] Setyan et al have recently successfully used this technique for the investigation of aerosols sampled in the field with a much reduced mass requirement, namely one to a few mg of sample. [21] This work deals with gas-phase probe molecules that specifically interact with the interface of nanoparticles in a soft, that is, non-invasive manner without significant perturbation of the molecular composition of the interface in contrast to many analytical techniques investigating the aerosol bulk. [19] This highly surface-sensitive and "soft" chemical "interrogation" of the particle interface has the following practical advantages: a) it enables the use of MS-based techniques sensitive that are sensitive to less than 0.3 % of a molecular monolayer; b) the low sample mass requirement in the range of one mg or so directly enables the investigation of ambient or smog chamber aerosol samples collected on filters; c) the present approach enables the collection of molecular information on interfacial or subsurface physical and chemical properties of nanoparticles not easily available from spectroscopic methods.…”