Laboratory Research Of Heterogenous Reaction Of Nitrate-containing Aerosols With SO₂

Atmospheric aerosols contribute a lot to the urban pollution and affect the atmospheric environment and global climate as well.Aerosols provide large reservior for trace gases(SO2,NOx, NH3,H2S,CHx) and also act as highly reactive platform for them,thus enhancing the probability of forming secondary aerosols through solar radiation or complicated chemical reaction.With the development of economy and transportation, the amount of NOx, particles and VOCs emitted by automobile is increasing rapidly in mainland of China in recent years, especially in metropolis, such as Beijing, Shanghai and Guangzhou, the atmospheric pollution of which are much more serious.Research results show that atmospheric HN03(g), NO2(g), N2O5(g), ClONO2(g) can form nitrate on the sea-salt and mineral dust through a series of heterogeneous reaction.Field measurements showed that both sea-salt and mineral dust particles collected in different regions of the world are often found to be associated with nitrate.Since nitrate is always associated with mineral dust and sea-salt in the real atmospheric particles, and sea salt is believed to mix with mineral dust transported from arid and semiarid regions, the existence of nitrate salt particles may make the surface properties of nitrate-containing mineral particles much differently than those of previous particles and may impact its reactivity, it is reasonable to anticipate that the nitrate should influence the heterogeneous oxidation of atmospheric SO2, and it is very significant to investigate the reactivity of particles containing nitrate.However, most studies of heterogeneous reaction of SO2 focus on the surface of typical mineral oxides, China Loess, synthetic sea salt and Saharan mineral.In fact, little attention was paid to the influence of nitrate on the heterogeneous reaction of SO2 with atmospheric aerosols, and no one investigated the heterogeneous reactivity of SO2 on mineral particles containing nitrate up to now. A better understanding of the heterogeneous reaction of trace gases in the mixture of mineral dust aerosols and nitrate is therefore highly desirable.Heterogeneous oxidation of the gaseous SO2 on nitrate and Hematite mixtures compared to other metal oxides-nitrate mixtures along with the real particles (China Loess and atmospheric dust)were first studied using an in suit diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).Main results are as followed.Firstly, heterogeneous oxidation of gaseous SO2 on ammonium nitrate containing Hematite particles compared to other ammonium nitrate-containing oxides (MgO, Al2O3,CaO, SiO2)were studied.Obvious sulfate(SO42-)formation can be seen from the infrared spectrum of nitrate-containing oxides, and the catalytic activity of Hematite is much higher compared to other oxides.Using BET area as the reactive surface area, the Hematite sample containing 6% of NH4NO3(W/W) presents the highestγBET value (2.42×10-9),which increases by a factor of 1.8 compared to that of Hematite.No formation of sulfate is observed on the pure NH4NO3.Secondly, Heterogeneous uptake of SO2 on Hematite in the presence of sodium nitrate at ambient temperature was studied.It is found that SO2 can undergo heterogeneous reaction to form adsorbed sulfate on the surface of Hematite-nitrate mixtures and evolve some other species in the gas phase, such as a small amount of N2O and HNO3.No uptake of SO2 and formation of sulfate were observed on the pure NaN03.The initial uptake coefficient calculated using the BET surface areas of the mixture samples ranged from 3.68×10-10 to 3.22×10-9.The reactivity of Hematite-nitrate mixtures was higher than that of Hematite solely. The 40% w/w Hematite+60% w/w NaNO3 sample presented the highest reactivity, its correspondingγBET was about 8.7 times larger than that of Hematite.These results clearly indicate that the presence of nitrate promoted the reactivity of samples and leaded to an increase in the uptake and oxidation of SO2 to sulfate in Hematite particles.Thirdly, heterogeneous oxidation of gaseous SO2 on other nitrate-containing (KNO3,Ca(NO3)2) Hematite particles compared to NH4NO3 and NaNO3 were studied. KNO3 enjoys the highest reactivity while Ca(NO3)2 completely impedes it. Furthermore, we also discussed some other factors that would affect the above reaction, including that organic compounds (HCHO and CH3CHO) impede the above reaction.Finally, Real particles (China Loess and dust) reacting with gaseous SO2 were discussed.Small amount of sulfate formed on the particles, which manifested the real meaning of this research.