China
Africa
Explore chapters and articles related to this topic
Radiative transfer in the atmosphere
Published in Lucien Wald, Fundamentals of Solar Radiation, 2021
Some authors use the terms optical depth and optical thickness interchangeably. This is not the general case. For example, the World Meteorological Organization limits the use of optical thickness to oblique paths.4 If τdepth and τthickness denote respectively the optical depth and thickness, the following relationship holds: τthickness=τdepth/cos(θS)
Solar Spectral Measurements
Published in Frank Vignola, Joseph Michalsky, Thomas Stoffel, Solar and Infrared Radiation Measurements, 2019
Frank Vignola, Joseph Michalsky, Thomas Stoffel
Molecular scattering is the elastic scattering of solar radiation by the gaseous molecules in the atmosphere first explained by Lord Rayleigh (1871). Many historical papers have been written describing the scattering of solar radiation by atmospheric gases and it continues to be the subject of ongoing research (Eberhard 2010). Bodhaine et al. (1999) thoroughly examined the problem of calculating the molecular scattering optical depth as a function of wavelength. Optical depth, in general, is a measure of the wavelength dependent extinction (by scattering or absorption) that occurs as a beam of radiation propagates through a medium. It can be defined using the Beer-Lambert law: () I(λ)/I0(λ)=e−τ(λ)m
Greenhouse gases and climate change
Published in Abhishek Tiwary, Jeremy Colls, Air Pollution, 2017
where V0(λ) is the solar flux at zero airmass at wavelength λ, Vm(λ) is the measured solar flux, τatm is the total optical depth of the atmosphere, and m is the airmass factor, expressed as 1/cos (zenith angle). Typically, the measured optical depth is the sum of several components, including those due to Rayleigh scattering and gaseous absorption; these are measured or calculated and the aerosol component obtained by difference. Measurements at many wavelengths enable the spectrum to be inverted and the particle size distribution to be calculated. The size distributions of aerosol from volcanoes such as Pinatubo have been modelled by formulas such as:
Retrieval and validation of long-term aerosol optical depth from AVHRR data over China
Published in International Journal of Digital Earth, 2022
Chunlin Jin, Yong Xue, Xingxing Jiang, Shuhui Wu, Yuxin Sun
The capacity to quantify aerosol loading in the atmosphere makes aerosol optical depth one of the most important indicators for comprehending atmospheric physics and local air quality (Wei et al. 2019b). With the development of multi-spectral sensor, multi-angle sensor, high spectrum sensor, polarization sensor, radar, etc. and different platforms such as polar orbit satellite and earth synchronous satellite, researchers put forward many inversion algorithms which makes the retrieval of AOD from spaceborne sensor observations a major technique for monitoring aerosol loads on a large scale. As the first sensor that has been applied to retrieve AOD, AVHRR has the advantages of a wide swath, global observation and long-time observations and AOD from AVHRR over the ocean has been produced operationally (Zhao et al. 2002; Zhao 2004; Zhao et al. 2008). However, several widely used algorithms such as ‘Dark Target’ (DT) and ‘Deep Blue’ (DB) cannot be employed directly in AVHRR AOD retrieval over land since it only has a red channel in the visible spectrum and lacks the 2.1μm channel (Kaufman et al. 1997; Hsu et al. 2004, 2013; Levy, Remer, Mattoo et al. 2007).
Laboratory and field evaluation of real-time and near real-time PM2.5 smoke monitors
Published in Journal of the Air & Waste Management Association, 2020
Ahmed Mehadi, Hans Moosmüller, David E. Campbell, Walter Ham, Donald Schweizer, Leland Tarnay, Julie Hunter
At very high PM concentrations, light scattering instruments may be influenced by extinction and multiple scattering artifacts. This will be the case if the optical depth (OD; i.e., the product of extinction (mostly scattering) coefficient and optical path length in the instrument sample volume) becomes comparable to one (1). For the compact candidate instruments, the optical path is limited to less than 0.1 m and therefore the extinction coefficient would have to be 1 m−1 to achieve an OD of 0.1 where extinction and multiple scattering artifacts become significant. For a PM mass extinction efficiency of 1 m2/g, this would correspond to a PM mass concentration of 1 g/m3 = 106 μg m-3, about three orders of magnitude larger than any PM mass concentration encountered during our lab and field studies. This analysis failed to explain the concentration effect. However, there could be concentrations in which these systems would underestimate the mass.
A statistical model for predicting PM2.5 for the western United States
Published in Journal of the Air & Waste Management Association, 2019
Amy Marsha, Narasimhan K. Larkin
Aerosol optical depth is a unitless measure of light absorption and scattering as a result of aerosol particles in an atmospheric column. We retrieved gridded AOD data from the MODIS (MODerate Resolution Imaging Spectroradiometer) instruments aboard the National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) satellites Aqua and Terra (Levy and Hsu 2015). Data were available at a 10-km resolution. We used the Level 2 Aerosol products from both satellites. The MODIS instruments aboard the two satellites overpass the Earth up to four times daily. All files covering our study area were downloaded for each day in the training period. For use in our model, each monitor location was assigned the daily average AOD within 50 km for each training day. The AOD measurement for a column of the atmosphere can be nullified by the presence of clouds; if no values were measured within 50 km of a monitor for a particular day, that day’s value was treated as missing data. The previous day’s average AOD (AODi-1) was used as a predictor in each site-specific regression model.