Light changes the atmospheric reactivity of soot

Abstract

Soot particles produced by incomplete combustion processes are one of the major components of urban air pollution. Chemistry at their surfaces lead to the heterogeneous conversion of several key trace gases; for example NO(2) interacts with soot and is converted into HONO, which rapidly photodissociates to form OH in the troposphere. In the dark, soot surfaces are rapidly deactivated under atmospheric conditions, leading to the current understanding that soot chemistry affects tropospheric chemical composition only in a minor way. We demonstrate here that the conversion of NO(2) to HONO on soot particles is drastically enhanced in the presence of artificial solar radiation, and leads to persistent reactivity over long periods. Soot photochemistry may therefore be a key player in urban air pollution.

Fig. 1.

NO₂ uptake in the dark and under irradiation for soot (fuel-rich conditions). The HONO concentration is obtained as the difference in the detector signal without and with the carbonate denuder in the sampling line. The gray bars indicate exposure in the dark.

Fig. 2.

(A) Soot produced with a rich flame exposed to 150 ppbv of NO₂ under irradiation for almost 7 hrs. The Red Line corresponds to NO and the Green Line to NO₂. (B) Time-integrated NO, NO₂ and HONO concentrations. The Black Dotted Line indicates the upper limit for NO₂ loss in the dark.

Fig. 3.

Uptake coefficients for the heterogeneous reaction between soot (stoichiometric flame) and NO₂ under irradiation as a function of the initial NO₂ gas concentration. Error bars are derived from the uncertainties associated to the calculation of the uptake coefficients.

Fig. 4.

NO₂ loss, NO and HONO formation on propane soot samples exposed to 40 ppbv of NO₂ and generated with a lean, a stoichiometric and a rich flame. SEM images are shown in the upper panels of the figure. Error bars indicate the standard deviation from independent measurements.

Fig. 5.

Effect of light on soot particles (rich flame), which had previously been exposed to 40 ppbv of NO₂ and light. In Red is shown the NO signal and in Blue the NO₂ signal, which was proved to be HONO when the carbonate denuder was switched into the sample line. Gray bars indicate dark conditions.

Fig. 6.

Proposed reaction mechanism for HONO formation.

Fig. 7.

Effect of irradiation intensity on the uptake coefficient for the heterogeneous reaction of NO₂ and soot produced with a stoichiometric flame when exposed towards 25 ppbv of NO₂. An NO₂ uptake of (2.0 ± 0.6) × 10^-6 would be expected if these results are extrapolated to the solar irradiance (in the 300–420 nm range). The arrow indicates the point at zero irradiance which shows the result under dark conditions.