N2O5 heterogeneous uptake on secondary organic aerosol: the effect of organic coating thickness, relative humidity and phase state

The heterogeneous uptake reaction of dinitrogen pentoxide (N₂O₅) has a great effect on generation of soluble nitrate, nocturnal atmospheric chemistry and regulating NO_x. Organic matter is one of the important components of atmospheric particles, and has been proved that can significantly affect the N₂O₅ heterogeneous uptake coefficients (γ(N₂O₅)). In this study, an aerosol flow tube system was used for γ(N₂O₅) measurements on ammonium sulphate aerosols ((NH₄)₂SO₄, AS) and three kinds of core-shell structured secondary organic aerosols (SOA), and explored the effects of relative humidity (RH), phase state and organic coating thickness on γ(N₂O₅). Three kinds of SOA with core-shell structures were generated successfully by seed particle AS, organic gas (α-pinene, isoprene and toluene) and O₃, and characterized by a scanning mobility particle sizer (SMPS) and TEM. The SOA yields, i.e., organic coating thicknesses, of the three organics were ranked, given the same organic gas concentration was α-pinene (17.1±2.2, 4.8 ppm) ＞ isoprene (4.7±0.04, 4.8 ppm) ＞ toluene (1.3±0.7, 4.8 ppm). The experimental results showed that the γ(N₂O₅) on AS were in the range of 0.002~0.017 (RH=1~50%), whereas that of SOA were between 3.7×10^-5~2.6×10^-3 (RH=30~50%), and the order of γ(N₂O₅) at the same RH was isoprene SOA＞ toluene SOA＞ α-pinene SOA. γ(N₂O₅) of the SOA was reduced by 1-3 orders of magnitude when compared with that of AS, which suggests that organic coating significantly inhibits the diffusion of N₂O₅ on the surface and in the bulk phase of particles. However, the γ(N₂O₅) of these three SOA were not completely correlated with the organic coating thickness. The organic coating thickness of two BVOCs showed a nonlinear negative correlation with γ(N₂O₅) (R²＞0.9), while toluene SOA with the thinnest coating thickness greatly inhibited γ(N₂O₅) due to its greater influence on particle hygroscopicity. Therefore, due to the large number of organic species, it is not possible to generalize the degree of inhibition of γ(N₂O₅) only by coating thickness. It is necessary to further consider the influence of properties and structures of organics in the future, and widely applicable parameters for evaluating the effect of organic effect need to be proposed, so as to improve the accuracy of γ(N₂O₅) estimation.