Bioaerosols are extremely easy to spread in the atmosphere and cause widespread disease infections, and a large number of bioaerosols are produced by natural human activities, environmental pollution, and sewage treatment processes. In addition, in the military field, bioaerosols are also the main combat form of biological agents, which have the characteristics of low cost and large destructive range and are used in battlefield or terrorist attacks with high possibility. Therefore, there is a need to develop technical means for real-time detection and early warning of bioaerosols. Lidar technology based on physical effects such as elastic and inelastic scattering between laser and bioaerosol particles, as well as laser-induced fluorescence (LIF), can provide information about the size, shape and composition of aerosol particles and has very fast response, and more importantly, long-range non-contact detection makes the safety significantly improved. So it has gained extensive research from scholars at home and abroad. LIF lidar is a broad-spectrum system, which is affected by atmospheric visibility and background radiation. It is obviously different from narrow-spectrum systems such as Mie scattering lidar. Different atmospheric conditions can affect the detection performance of LIF lidar, and we hope to quantify the influences.
In order to evaluate the detection performance of LIF lidar under different atmospheric conditions, a LIF lidar system is designed, and the formula of the system signal-to-noise ratio (SNR) is given. Then the simulation of atmospheric radiation transmission is started, and the broad-spectrum background radiation and atmospheric transmission transmittance on the horizontal path of several typical atmospheric visibility and background radiation (or operating hours) conditions are carried out using Modtran5. Finally, the parameters of LIF lidar are set, and then the signal-to-noise ratio of LIF lidar is simulated numerically according to the broad-spectrum background radiation and atmospheric transmission transmittance of different atmospheric conditions obtained in the previous step, and the variation function of signal-to-noise ratio with distance is obtained. The system works for detecting whether the aerosol contains biological substances and then for identifying biological aerosol species at two levels of detection needs. The simulation analysis is carried out separately.
The fluorescence emission spectra of 355 nm light excitation of six biological substances are measured (Fig.1), which verify the basic principle of LIF lidar that spectral discrimination can be performed based on the characteristic fluorescence emission spectra of various biological substances. Then the real-world atmospheric background radiation is measured, and the comparison of the measured real-world atmospheric background radiation with the simulation results (Fig.3) fits well, proving that the simulation results of Modtran5 are credible. Subsequently, the simulated broad-spectrum background radiation and atmospheric transmission transmittance on the horizontal path under different atmospheric conditions (Figs.4, 5, 6, and 8) are obtained to provide sufficient atmospheric data for the following simulation of the system performance. Finally, the signal-to-noise ratio of the total fluorescence spectroscopy system (Fig.9) and the that of the 360‒370 nm channel system (Fig.10) are calculated. The results show that the atmospheric conditions largely affect the detection performance of the system: the higher the atmospheric visibility, the longer the effective detection distance of the system. Comparing the two atmospheric environments of visibility of 23 km and 3 km, the difference in the effective detection distance for different working hours is 2‒4 times. The working hours of the system also have a great impact on the detection performance. The detection performance of the system during daytime is poor and the difference between different time periods is small, while the effective detection distance of the system is generally increased (at least doubling) at night because the intensity of the atmospheric background radiation is greatly reduced, and the higher the atmospheric visibility, the larger the increase. Under the visibility of 23 km, the effective detection distance at night is 3.7 times that at the daytime. At the same time, the effective detection distance of the aerosol biological detection based on the total fluorescence intensity is higher than that of the aerosol biological component identification based on the spectrally distinguishable fluorescence intensity, and the former is more significantly affected by atmospheric conditions, i.e., good atmospheric conditions will bring higher improvement to the aerosol biological detection function.
In this paper, the detection performance of LIF lidar under different atmospheric conditions is evaluated through simulations. The LIF lidar is suitable for biofluorescence measurements at night because of the high atmospheric background radiation intensity in the fluorescence band. The decrease of atmospheric visibility leads to the decrease of background radiation intensity and also the decrease of transmittance of broad fluorescence spectrum, which in turn leads to the decrease of LIF lidar detection performance, but the influence is far less than the effect of changing operating hours between daytime and night. The most suitable working environment for LIF lidar is the night with high visibility.
