Research On Construction And Key Technology Of Biological Aerosol Monitoring System Based On Laser Radar Technology

Purpose:Releasing bioaerosol is the major approach for biological terrorist attacks. How to distinguish the bioaerosol and implement real-time monitoring is of great significance in the field of bio-terrorism prevention, which is one of the hot spots of researches in various countries in recent years. Many developed countries have been devoted to studying the use of lidar technology to quickly achieve the effective monitoring of bioaerosol, improve its alertand reduce response time and attendant losses. Up to now, these projects are continuously invested and deeply studies, so as to strive to promote the practical and unmannedapplication of this technology. Our nationalresearch on the detection equipment of biological terrorist attacks started late. So farreal-time monitoring technological means of bioaerosol mainly depend on the air-exhaust-type bioaerosol point monitor, there are obvious shortcomings.It has small monitoringrange, vulnerably affected by wind speed, and deployed hard. Research on bioaerosol sensing device conducted has great practical significance on biological rapid reaction treatment under the condition of war in our country and security protection of bioterrorismin civilian areas.Method and Contents:Through the literature researches on bioaerosol telemetering equipment reported abroad, this paper combined with the actual detection needs and the development status of bioaerosol monitoring equipment in our country, putting forward key technology for portable short-range bioaerosol telemetering equipment design. Accurately identifing biological particles depend on the accurate and systematic understanding of physical and chemical properties and optical properties of biological particles. In this paper, the author systematically analyzes telemetering principles of bioaerosol and key points of engineering design respectively from the three aspects, the scattering and fluorescence properties of bioaerosol particles, hardware and software simulation of telemeteringsystem and experimental results of actual detection. These include:Research on the back scattering properties of biological particles:the size of biological particles aredistributed between about 0.1～10um, its interaction with near-ultraviolet laser light has specific rules. In this paper, the author uses the Mie scattering theory and T-matrix theory to analyze and computethe simulation of the scattering properties of bioaerosol, thus recognize the relationship between particle size, refractive index and outline and back scattering signals to guide the design of detection equipment.Analysis of endogenousfluorescence properties of biological particles:The biological particles can be distinguished by fluorescence spectroscopy if the biological particles contain endogenous fluorescent molecules. Meanwhile,organism metabolism and external environment conditions will cause changes in content and activity of fluorescence molecules, resulting in change in its fluorescence properties and affecting the accuracy of the fluorescence measurement. This paper describes the fluorescence properties from the fluorescence spectra of cells and their main endogenous fluorescent molecules, and contrasts the fluorescence properties ofsome commoninterferent. Calculate the situation of fluorescence properties of biological particleswith changes in metabolism by combining the fluorescent mathematical models of biological particles withmaterial metabolism network in particles.Light source parameter optimization settings based on eye-safe consideration: Lidarhasawide range ofmonitoring, which detection performance is positively correlated to the risk degree of eye-safe, and the performance of both aspects depend on the parameter settings of light source. This paperuses detection signal-noise ratio as indicator to evaluate the detection performance, uses ANSI Z136.1-2000 American Laser Standard:Safe Use of Lasers indicator to evaluate the security ofthe system, comprehensively comparesthese three setting modes of light source, that is, traditional pulsed lidar, micro pulse lidar and pseudo-random modulation lidar, thus calculates the light source setting parameters with optimal signal-noise ratio under the same safe level.Transient analysis of dead time effect in photoncounting based on the non-homogeneous Markov chain:Micro pulse lidar and pseudo-random modulation lidaruse photon countersto improve detection sensitivity, and the dead time in the output signal of photon counter leads toa difference from signal properties by using photomultiplier, that dead-time effect will cause distortion of output signal occurred by using photon counter. In this paper, the author conducts theoretical analysis, computer simulation and experimental verification about the dead time effect of photon counter through the non-homogeneous Markov chain process, and then proposes the correction method for dead time effect.Biological particle identification technology combined fluorescence detection and polarization detection based on 405nm wavelength laser:different excitation wavelengths will lead to different fluorescence spectrums of biological particles. This paperchooses 405nm semiconductor laser as the light source of remote sensing system. Two characteristic fluorescence bands was selected by contrast 405nm excitation fluorescence spectra of several biological particles. Distinguish the bacillus subtilis, staphylococcus aureus and yeast by simultaneously conducting polarizationdetection and 2 fluorescence spectral detections.Design of portable short-range bioaerosol lidar based on pseudorandom modulation technology:the pseudo-random modulation lidar developed by using continuous semiconductor laser is with characteristics of small size, low power and convenience to carry, which is particularly suitable for rapid deployment and mobile monitoring. This paper meticulously designs the optics receiving system and collection system, and compiles data processing program to meet the needs of small short-range bioaerosol monitoring lidar system.Results:According to the far-field scattering theory, the scattering properties of randomly oriented particle swarm are the same as the scattering properties of single particle. Therefore, the scattering theoryof single particle could be usedto calculate the scattering properties of biological particle clouds. Based on the back scattering theory of biological particles, this paper defines the depolarization ratio as the indicator for polarization detection and uses it to measure the ellipsoidal particles with different axialratio, and the depolarization ratio monotonically decreases with theincrease in the axialratioof particles.The endogenous fluorescence of biological particles is mainly from endogenous molecules, such as tryptophan, tyrosine, NADH and riboflavin, etc. By combining fluorescent mathematical models of biological particles with material metabolism network in particles, this paper describes the relationship between metabolism network particles and particle fluorescence effect, and calculates the intrinsic fluorescence changes of E. coli metabolic according to metabolism under 355nm laser excitation. The simulation assumes to set ± 5mmol/gDW.h as limit threshold for material exchange between inside and outside the cells, and to set103mmol/gDW.h as flux upper limit within each reaction pathway. The calculation result shows the fluorescent scattering cross section of E.coli with the initial equivalent grain size of lum increases to 0.028umz from 0.012umz under unchanged adequate-nutrition environment.By comparing the security level and detection signal-noise ratio of traditional pulse lidar, micro-pulse radar and pseudo-random modulation lidar, the author thinks that the laser launching way of micro-pulse radar is theoretically optimal. For 405nm wavelength, when the repetition frequency and energy of the light source should be set to 55179Hz and 0.58uJ SNR is the highest under the condition of Class 1 safety; when the repetition frequency and energy of the light source should be set to 4Hzand 0.98J SNR is the highest under the condition of Class 3 safety. But within the permissible range, the pseudo-random modulated lidar can emit more laser pulses at the same time, its pulse duty cycle of 1:1 meaning the number of pulses emitted by the pseudo-random modulated lidar can reach even a hundred times of micro-pulse lidar, to improve the SNR and reduce peak power.Photon counters in micro-pulse lidar or pseudo-random modulationlidar technology will make distortion in countingdue to the dead timeeffect, which is mainly manifested that the dead-time effect in strong signal light irradiation can cause jitter up and down of signals in detection target area,reduce the measured valueof photon counting and form overshoot phenomenon when the signals fall back. The experimental results show that the scattered light channel may be inhibited by 50% for strong bioaerosol.By the comparing design of all the details, this paper presents the design plan of small eye-safe lidar prototype, whose main parameters are:405nm 100mW continuous laser the modulation frequency is 100MHz; a Cassegrain telescope with the caliber of 200mm as the receiver, the optical aperture is set with the diameter of 12mm; four channel of photon countingin which two are polarization detection channels andthe other two are fluorescence detection channels (450 ± 20nm,520 ± 20nm), the detectors have the same optical path, which simultaneously focus on 200m in front of the telescope.Field testsof three bioaerosols (bacillus subtilis, staphylococcus aureus and microzyme) and three common interferents (leaf humus, dry soil and clothing dye) based on the above prototypehave been done and the experiential results show that,the sensitivity of polarization detection and fluorescence detection is about 105ppl, which can detect 90m to the farthestand respectively work normally underthe ambient light illumination of 5Lux and 20Lux. Compared polarization detection with fluorescence detection, the prototypes can distinguish three kinds of bioaerosol clouds, the back substitution false alarm rate of whose signal respectively is 2.5%,5% and 15%. For three common interferents, there is almost no signal in 450nm channel on clothing dye (polycyclic aromatic hydrocarbons) compared to the biological particles; 520nm channel signal of mud is about twice than 450nm; fluorescence propertiesof leaf humus is similar to biological particles.Conclusion:Lidar has unique advantages in wide detection range, high spatial resolution and long-term continuous real-time unmanned monitoring. This paper uses polarization detection and fluorescence detection methods to design a four-channel simultaneously collecting small low-power portable short-distance bioaerosol monitoring lidar based on the scattered light and fluorescence properties of bioaerosol particles. The experimental results show that the prototypes can effectively distinguish between biological particles and non-biological particles.