Effects Of Altitude And Rotational Speed On Persistent Free Radicals In Pollutants Emitted By Diesel Engines

In recent years,the development of urbanization has exacerbated the deterioration of air quality.With dramatical increasing in the number of vehicles on the road,the transportation industry become one of the important pollution sources affecting air quality.Among them,diesel engines have attracted the attention of researchers for their high utilization rate and serious harm to the environment and human health.The incomplete combustion of diesel fuel leads to the emission of many exhaust pollutants and causes various hazards.Exhaust pollutants from diesel engines are one of the important sources of air pollution.Particulate matters in the exhaust is an important cause of haze and is harmful to the respiratory system;carbon oxides could affect the global climate and the carbon cycle;nitrogen oxides could cause photochemical smog;sulfur oxides could cause acid rain.In addition,the polycyclic aromatic hydrocarbon in exhaust is an aromatic compound containing at least two benzene rings.As a toxic organic compound produced during the incomplete combustion of fossil fuels,it has potential carcinogenic and mutagenic effects,endangering the environment and human health.The pollutants in exhaust could also migrate for long distances with atmosphere,or fall into water,surface and soil through dry and wet deposition,which have an impact on a larger environmental range and a wider range of environmental mediators.Exhaust pollutants from diesel engines are mainly discharged into the atmosphere in gaseous and particulate form.At present,most of the researches focus on the qualitative and quantitative analysis of conventional pollutants in the exhaust,while ignoring the environmental persistent pollutants that have a great environmental health risk.Generally,the environmentally persistent free radicals formed by exhaust pollutants have long lifetime.They will be discharged into the atmospheric environment along with exhaust particulate matters,and migrate over long distances.After entering the human body,they could induce cells to produce OH,causing oxidative DNA damage.In this study,the atmospheric simulation integrated control system was used to simulate the emission process from diesel engines at altitudes of 0 m,1000 m,and2000 m,and diesel engine speeds of 1200 rpm and 1800 rpm.They were connected to the modified atmospheric sampler.The exhaust particles and gas phase pollutants were collected through glass fiber filter membrane and polyurethane foam,respectively.It was found that at low engine speed conditions,the concentration of exhaust particulate matters was much higher than that at high speed;at the same engine speed,with the increasing of altitude,the carbon particles emitted increased,and the degree of graphitization increased.The particles concentration also increased significantly,whose maximum value could reach 136.95μg/L.Diesel fuel produces carbon-containing compounds at different altitudes and engine speeds,some of which are adsorbed or condensed on the surface of exhaust particulate matters.At the conditions of low engine speed and high altitude,due to the lack of oxygen in the cylinder of the diesel engine,more fuel is not completely burned,thus generating more carbon-containing pollutants.All of these have led to the more emission of exhaust particulate matters from diesel engine at high altitude and low engine speed,increasing the exposure risk of exhaust particulate pollutants.Due to high fuel consumption and high engine wear,the metal concentration at 1800 rpm is higher than 1200 rpm.A variety of high-risk polycyclic aromatic hydrocarbons have been detected in diesel exhaust particulates and gas phase materials.It mainly contains six PAHs with low molecular weight,including naphthalene,acenaphthylene,phenanthrene,anthracene,fluoranthene and pyrene.PAHs in exhaust are mainly composed of two to three rings,and are mainly distributed in the gas phase,while the concentration in solid phase particles is low.The speed of diesel engine has little effect on the emission of PAHs.Compared with low altitudes,the concentration of PAHs produced at high altitudes is significantly higher.On the basis of analyzing the composition of exhaust pollutants,the emission characteristics of environmentally persistant free radicals in diesel engine exhaust pollutant at different altitudes(0 m,1000 m,2000 m)and rotation speed(1200 rpm,1800 rpm)are studied in this paper.Long-term observation experiments of particulate matters under continuous ultraviolet light and dark conditions were designed to investigate the changes of signal intensity,concentration,g factor,and half-width of environmentally persistant free radicals with operating conditions and time.Compared with the high diesel engine speed and low altitude working conditions,the signal of free radicals in solid particulates generated at low engine speed and high altitude is significantly stronger,and more carbon-centered free radicals are generated.This is due to the incomplete combustion of fuel at low engine speed and high altitude conditions,which produces more carbon-containing pollutants.In addition,at low rotation speed and high altitude,the g value of particulate free radicals is significantly lower,which further indicates that at low rotation speed and high altitude conditions more carbon-containing organic matters can be emitted.The half-width of free radicals in exhaust particulate matters discharged at low engine speed is significantly larger than that about 2.5 times at high engine speed,indicating that the exhaust particulates discharged at low engine speed contain more types of free radicals,resulting in more complex intermediate products and complicated exhaust pollutant composition.Compared with ultraviolet light conditions,the free radicals signal under dark conditions is relatively more stable,and ultraviolet light conditions may quench some free radicals.Due to the higher particles concentration,the free radical concentration normalized per unit time and volume at high altitude and low engine speed conditions is larger,whose maximum values were 6.83E+17 spins/h and1.14E+18 spins/10~3 m~3,respectively.While the low concentration of free radicals per unit mass at high altitude and low engine speed is due to the high graphitization degree of the carbonaceous structure in particulate matters.The correlation analysis of free radicals with metals and PAHs in the exhaust particles of diesel engines showed that the carbon-containing structure formed by the pyrolysis of fuel in the cylinder was the main source of persistent free radical signals in exhaust.