A low-cost NO2 trace concentration measuring device based on a cavity ring-down spectroscopy (CRDS) technique is presented. We theoretically and experimentally demonstrate that, if the nonresonant direct reflection is effectively isolated, an inexpensive broadband diode laser (LD) can excite multiple cavity modes and stabilize the cavity output signal with an optical feedback (OF) method. To improve the measuring accuracy for low concentrations of NO2 gases, we also design and machine a cavity with several mixing rooms. Several standard concentrations NO2 gases are used to test the apparatus. The results prove a great agreement between measured and theoretical values over the full range.
Ideally, lasers follow Beer-Lambert's law when travel through the atmosphere. Beer-Lambert’s law is premised on single scattering. However, in the actual atmospheric environment a variety of circumstances will cause the deviation of Beer-Lambert’s law. In this study, a simulation model of laser atmospheric transmission is established based on the Monte Carlo method. The established simulation model is from the perspective of each photon. The random motion of photons is simulated, that is to say, the radiation transport equation is not involved. In simulation model, migration direction and migration distance of photons are determined by the sampling theory of Monte Carlo on the probability distribution of photons scattering angle and free path. Migration direction is determined by the scattering phase function. Migration distance is related with the extinction coefficient. The energy change caused by scattering is determined by the single scattering albedo(abs). For a photon, if its energy is absorbed to zero before it reaches at the detector, calculation of this photon transmission process would be end up. Whether a photon is received by the detector can be judged by observing whether the motion track of the photon intersects with the detection region. Thus, by tracking motion track of each photon of the laser beam, the transmission of the laser can be obtained. In this study, several key factors of laser transmittance in the atmosphere such as extinction coefficient, ratio of scattered light, and absorption are carefully studied by the established simulation model.
With the outbreak of COVID-19,masks,as the most important personal protective equipment, its necessity and importance becomes evident. Particle protective performance, as the key index of masks, the accuracy of its test result is very important. In this study, based on the high-precision photometer, the calibration method of particle protective performance testers for mask is studied. The protective performance is evaluated by the percentage of particle concentration reduction of before and after the mask. Photometric method is a relatively mature technology of particle concentration measurement, with advantages of portability and quick response. In our study, two photometers are used in the calibration. In order to ensure the accuracy, it is necessary to calibrate the two photometers first. Aerosol with concentrations about 1, 10, 20 and 30 mg/m3 is generated in the test chamber, respectively. The filter weight method is used to measure the concentration in the test chamber as the standard values. Within the weighing time, the concentration test results measured by the two photometers are recorded and calibrated with the concentration results measured by the weight method. For the two calibrated photometers, one is used to measure the particle concentration in the test chamber, the other is used to measure the particle concentration in the mask which is attached to the head mold. In this way, the particle protective performance value measured by the instrument can be calibrated. In our experiment, the extended uncertainty of the calibration results are lower than 3%.
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