Calibration Algorithm Of Aethalometer Based On Aerosol Optical In Situ Measurement Technology

Black carbon(BC)is the main light absorbing component in atmospheric aerosols,which is produced by incomplete combustion of fossil fuels or biomass.Its strong absorption characteristics directly or indirectly affect the atmospheric environment and climate change.Accurate measurement of BC absorption characteristics is very important for understanding its environmental and climatic effects.At present,the commonly used absorption coefficient measurement methods are divided into two categories:filter-based technology and in-situ method.For the filter-based technology,aerosol particles are collected on the filter fibers,and the aerosol absorption coefficient is obtained by measuring the attenuation of transmitted light.In-situ method is to obtain aerosol absorption coefficient directly or indirectly by measuring the signal changes after the interaction between particles and light without changing the suspension state of aerosol.The measurement accuracy is high,and it has become a reference standard for measurement and comparison of absorption coefficient.As the most commonly used filter-based commercial instrument to measure BC absorption coefficient,Aethalometer has the advantages of simple operation,easy maintenance and no interference from gas absorption.It is widely used in operational observation and scientific research of atmospheric composition.However,due to the limitation of measurement method,the accuracy of absorption coefficient measurement is affected by the multiple scattering effect of the filter fibers,filter-loading effect,as well as the scattering of aerosols embedded in the filter.In this paper,based on the selfdeveloped aerosol optical properties in-situ measurement instrument(Cavity enhanced albedometer),through the biomass and diesel lamp combustion experiments,as well as the actual atmospheric observation experiments,the deviation and influence factors of the absorption coefficient measured by the Aethalometer(AE33 model)under different BC sources were studied,and the correction algorithm of the absorption coefficient measured by the Aethalometer was established.The main research work and results are as follows:(1)The filter fibers multiple scattering factor(C)and its influencing factors of Aethalometer with different BC sources and different reference instruments were analyzed comprehensively.The range of C was 1.57-5.23(the peak value of frequency distribution was 3).The source of BC had a great influence on C.Aging package maked C value increase,and C increased with single scattering albedo(SSA).That was,the increase of non BC aerosols content would increase the measurement deviation of absorption coefficient of Aethalometer.The factors affecting C maight include the mixed state of BC(morphology,size and material of BC),the size and chemical composition of aerosols,etc.(2)Researched on correction of load effect of Aethalometer.The Aethalometer uses(1-k·ATN)to correct the filter-loading effect(k is obtained by real-time calculation).After correcting the filter-loading effect,the abrupt change point of absorption coefficient disappeared.The filter-loading parameter(k)decreased with the increase of SSA.Namely,the smaller the proportion of absorbent material,the smaller the effect of particle shielding on the measurement of absorption coefficient.For atmospheric aerosols and biomass combustion aerosols,the linear relationship between(1-k·ATN)and ATN were y=1-0.00335x and y=1-0.0057x,respectively.This linear relationship could be used to correct the filter-loading effect.(3)Studied on the measurement deviation(Cf)of absorption coefficient of Aethalometer under different sources of BC.For BC produced by diesel lamp ignition experiment,it had a large absorption coefficient and the variation of absorption coefficient,which affected the correction of(1-k·ATN)to the filter-loading effect,and leaded to the decrease of Cf with the increase of particle loading.AE33 overestimated the absorption of highly scattering aerosols for biomass combustion aerosols and atmospheric aerosols at Guangzhou station.For the atmospheric aerosols at Shouxian station,AE33 overestimated the absorption of absorbing aerosols.For the diesel lamp ignition experiment,the change of Cf was more complex.(4)Researched on correction algorithm of absorption coefficient measured by Aethalometer.When there was no particle loading on the filter fibers,the absorption coefficient measured by the Aethalometer still had deviation,which was affected by the multiple scattering effect of the filter fibers.That was,it was not accurate to use the constant value C(1.39)to correct the multiple scattering effect of the filter fibers.In this paper,the multiple scattering factors(Cref)of different filter fibers were obtained by two methods.Then obtained the best Cref of BC aerosol from different sources.The Cref of biomass combustion experiment(wheat straw and corn straw combustion experiments),diesel lamp ignition experiment,Guangzhou and Shouxian observation experiments were 3.45,3.51,4.49 and 1.92,respectively.kold-new and kold were used to modify the filter-loading effect respectively,to explore the influence of the selection of parameter kold on the filter-loading effect correction.And the change percentage of absorption coefficient Δbabs(%)were smaller.That was,kold had less effect on the filter-loading effect correction.Finally,Cref and the linear relationship between(1-k·ATN)and ATN were used to correct the four groups of experimental data.It was proved that the linear relationship between(1-k·ATN)and ATN could be used to correct the filter-loading effect of Aethalometer.