Mineral dust aerosols promote the formation of toxic nitropolycyclic aromatic compounds

Atmospheric nitrated polycyclic aromatic hydrocarbons (NPAHs), which have been shown to have adverse health effects such as carcinogenicity, are formed in part through nitration reactions of their parent polycyclic aromatic hydrocarbons (PAHs) in the atmosphere. However, little is known about heterogeneous nitration rates of PAHs by gaseous NO2 on natural mineral substrates, such as desert dust aerosols. Herein by employing kinetic experiments using a flow reactor and surface analysis by Fourier transform infrared spectroscopy with pyridine adsorption, we demonstrate that the reaction is accelerated on acidic surfaces of mineral dust, particularly on those of clay minerals. In support of this finding, we show that levels of ambient particle-associated NPAHs in Beijing, China, significantly increased during heavy dust storms. These results suggest that mineral dust surface reactions are an unrecognized source of toxic organic chemicals in the atmosphere and that they enhance the toxicity of mineral dust aerosols in urban environments.


Heterogeneous reaction of Py with HNO3
The substrates used in this study were coated with Py by liquid-solid adsorption at a ratio of ~1 nmol mg -1 . Py was heterogeneously reacted with gaseous HNO3 in a Pyrex cylindrical reaction vessel (6 cm ID × 18 cm height, ca. 500 mL) under ambient pressure at 298 ± 1 K and < 2% relative humidity in the dark. A portion of the Py-coated substrate (ca. 10 mg) was spread on a Pyrex petri dish (1.9 cm ID × 1.2 cm depth). The Petri dish was placed in the reactor and subsequently exposed to 2 ppmv HNO3/air which was supplied by passing pure air through a permeation tube (KIN-TEK, 2022) at a constant flow rate of 0.5 L min -1 . Prior to the reaction, the inside of the reaction system was continuously exposed to 2 ppmv HNO3 for 24 h in order to minimize the loss of the reactant gas onto the walls of the system during the experiment. Reaction products and the residual Py after the prescribed reaction time were extracted with dichloromethane as described below. The extracted chemicals were identified and quantified by GC/MS by comparing their retention times, fragmentation patterns, and peak areas with those of authentic standards.

Extraction of soluble organic fractions (SOF) from airborne particulate and laboratory experiment samples
The filter samples of ambient airborne particulates, onto which deuterated 1-nitropyrene  and deuterated chrysene (1-NP-d9 and Chry-d12, respectively; internal standards) were added, were cut into fine pieces before extraction. SOF from the filter samples were extracted twice with 100 mL of dichloromethane under sonication for 20 min. The extract solution was filtered with a cellulose acetate filter to remove solid residue, followed by adding 100 L of dimethyl sulfoxide (DMSO) into the filtrate to avoid complete dryness of the solvent during the concentration steps. After concentration to ca. 5 mL and filtration with a 0.45 µm membrane filter, the samples were concentrated to ca. 100 µL under a nitrogen stream to leave DMSO, and then 400 µL of methanol was added. An aliquot of each of the sample solution was subjected to the quantification of PAHs and 1-NP by high-performance liquid chromatograph (HPLC) analysis.
The NO2 or HNO3 exposure experiment products and the remaining reactant after each reaction were extracted with 10 mL of dichloromethane under sonication for 20 min after adding 1-NP-d9 and deuterated pyrene (Py-d10) as internal standards. The extract solution was filtered with a 0.45 µm membrane filter, and then concentrated to ca. 1 mL under a nitrogen stream.

S4
An HPLC system with column-switching and chemiluminescence detection was employed for ambient particle-associated 1-NP quantification as reported previously. 1 Briefly, the system consists of four HPLC pumps, a 6-port switching valve, a clean up column (GL Sciences, Inertsil ODS-P, 3 S5 Chemicals 1-NP, Chry-d12, and Py were purchased from Wako Pure Chemical Industries. 1-NP-d9 and Py-d10 were obtained from C/D/N Isotopes and Cambridge Isotope Laboratories, respectively. PAH standard mixture (EPA 610 Polynuclear Aromatic Hydrocarbons Mix) and DNPs were purchased from Supelco and Accu Standard, respectively. All solvents and other chemicals used were HPLC or analytical grades from Wako Pure Chemical Industries.

Estimation of fractional surface coverage of Py on ambient dust
It was reported that in Beijing, the surface area density of the atmospheric mineral dust was 6.3 × 10 -6 cm 2 cm -3 when the dust concentration was 61 μg m -3 . 3 Hence, the specific surface area of the atmospheric dust is calculated to be 10 m 2 g -1 . Using the calculated specific surface area and the mean concentration of the atmospheric dust observed on 20 March, 2010 in Beijing (247 μg m -3 ), the surface area density of the dust is estimated to have been 2.5 × 10 -5 cm 2 cm -3 when the dust storm hit Beijing.
Using the mean concentration of Py during 19 -21 March, 2010 (8.6 pmol m -3 ) and the effective cross-section of a Py molecule (~0.8 nm 2 ), the surface area density of Py molecules is calculated to have been 4.1 × 10 -8 cm 2 cm -3 . Assuming that all the molecules of Py that we determined were adsorbed on the dust particles, thus, the fractional surface coverage of Py on the dust particles is estimated to have been 1.7 × 10 -3 , i.e., the surface-adsorbed Py is believed to have formed a submonolayer on the dust particles.

Heterogeneous reaction of Py with HNO3
Supplementary Fig. S6 shows time profiles of Py degradation and 1-NP formation under 2 ppmv HNO3-air in the dark on CDD (a) and on ATD particles (b). The parent Py moderately decreased on both the substrates, and then ~30% of the initial Py was converted to 1-NP after 12 h of the reaction. No DNP was formed during the exposure. The apparent rate constants, kobs, were (3.6 ± 0.1) × 10 -5 sec -1 and (2.4 ± 0.4) × 10 -5 sec -1 on CDD and ATD, respectively (errors represent one standard error). The kobs values for the NO2 reactions were an order of magnitude larger than those for the HNO3 reactions. Concentration of NO2 is typically 1 -2 orders of magnitude larger than that of HNO3 in urban air. 4 Therefore, the reaction of Py with HNO3 appears to have little influence on the formation of 1-NP on dust particles.