Heterogeneous Reaction of NO₂on Diesel Soot Particles

Abstract

Soot particles were collected from a diesel engine using a procedure that realistically mimics exhaust gas conditions in tailpipes and during dilution at room temperature. After being sampled, the particles were exposed to NO2 concentrations and relative humidity in ranges relevant for the troposphere using 13N as tracer. Gas-phase nitrous acid(HONO) and irreversibly bound (i.e., chemisorbed) species were the main reaction products with initial yields of 80-90% and about 10%, respectively. Neither NO nor HNO3 were detectable. The HONO formation increased with increasing engine load (i.e., with a decreasing air to fuel ratio, lambda). The reaction rates of HONO and chemisorbed NO2 increased with increasing NO2 concentration and did not depend on relative humidity. At the beginning of reaction, the uptake coefficient averaged over 3 min ranged from 5 x 10(-6) to 10(-5) for NO2 concentrations between 2 and 40 ppb. The HONO formation rates decreased with time, indicating consumption of reactive surface species, while the chemisorption rates remained almost constant. The total HONO formation potential of the particles was estimated to about 1.3 x 10(17) molecules/mg of diesel soot or to about 4.7 mg/kg of diesel fuel, indicating that the reaction between NO2 and diesel soot particles does not provide a significant secondary HONO source in the atmosphere. A Langmuir-Hinshelwood type reaction mechanism was proposed that adequately describes the observed results and also allows discussing important general features of reactions on soot.

Source Identification and Trends in Concentrations of Gaseous and Fine Particulate Principal Species in Seoul, South Korea

“…33 Several heterogeneous reactions have been proposed for atmospheric HONO formation, and motor vehicle exhausts can also be considered as another source in urban areas. 26,[33][34][35] SO 2 concentrations were the highest (10.5 g/m Ϫ3 ) in winter, followed by spring (9.14 g/m Ϫ3 ), fall (6.57 g/m Ϫ3 ), and summer (the lowest, 3.10 g/m Ϫ3 ). Seasonal SO 2 pattern was consistent with the amount of fuel used in Seoul; the general trend of fuel consumption is in order (winter Ͼ spring Ͼ fall Ͼ summer).…”

Influence of relative humidity on heterogeneous kinetics of NO2 on kaolin and hematite

“…The reaction kinetics, k obs , was reported in terms of the uptake coefficient, which was proportional to the measured first-order rate constant derived from the uptake curves when the Na 2 CO 3 denuder was used, according to the following equation, 18,33 k obs ¼ g obs c h i 2r tube (1) where r tube , g obs and hci are the flow tube radius, the geometric uptake coefficient and the average molecular velocity of NO 2 , respectively. The k obs was determined by pulling the injector back to the end of the sample tube.…”

Measurements of nitrous acid (HONO) in urban area of Shanghai, China

“…This includes different surfaces (ground and aerosols) such as the photocatalytic conversion of NO 2 on mineral dust (Ndour et al 2008), the dark heterogeneous conversion of NO 2 on suspended soot particles (Ammann et al 1998;Arens et al 2001), the heterogeneous hydrolysis of NO 2 (Finlayson-Pitts et al 2003), the photosensitized reduction of NO 2 on organic surfaces (George et al 2005;Stemmler et al 2006), the photolysis of adsorbed nitric acid (HNO 3 ) (Zhou et al 2003) and nitrate (NO 3 − ) (Zhou et al 2001), the HNO 3 conversion on primary organic aerosols (Ziemba et al 2010) and from soilemitted nitrite (Su et al 2011). Direct emissions from combustion processes (i.e.…”