Potential of AOD Retrieval Using Atmospheric Emitted Radiance Interferometer (AERI)

Aerosols in the atmosphere play an essential role in the radiative transfer process due to their scattering, absorption, and emission. Moreover, they interrupt the retrieval of atmospheric properties from ground-based and satellite remote sensing. Thus, accurate aerosol information needs to be obtained. Herein, we developed an optimal-estimation-based aerosol optical depth (AOD) retrieval algorithm using the hyperspectral infrared downwelling emitted radiance of the Atmospheric Emitted Radiance Interferometer (AERI). The proposed algorithm is based on the phenomena that the thermal infrared radiance measured by a ground-based remote sensor is sensitive to the thermodynamic profile and degree of the turbid aerosol in the atmosphere. To assess the performance of algorithm, AERI observations, measured throughout the day on 21 October 2010 at Anmyeon, South Korea, were used. The derived thermodynamic profiles and AODs were compared with those of the European center for a reanalysis of medium-range weather forecasts version 5 and global atmosphere watch precision-filter radiometer (GAW-PFR), respectively. The radiances simulated with aerosol information were more suitable for the AERI-observed radiance than those without aerosol (i.e., clear sky). The temporal variation trend of the retrieved AOD matched that of GAW-PFR well, although small discrepancies were present at high aerosol concentrations. This provides a potential possibility for the retrieval of nighttime AOD.

Angular responsivity of ground and space-based direct solar irradiance radiometers

The angular response setup of PMOD/WRC was modified to facilitate measurements of the narrow field of view of radiometers for direct solar irradiance. First, The pointing of the JTSIM-DARA radiometer was measured four times during its construction in the optic laboratory of PMOD/WRC. The final offset of the pointing before shipping relative to the optical axis, defined by a removable alignment cube, is 1.07° / 0.67° (β/γ-axis) for the four-quadrant sensor and 0.095°/-0.017° for the radiometer cavity A. Next to JTSIM-DARA the DARA for the occulter of the satellite Proba-3 was characterised at PMOD/WRC. First tests of the pointing have been carried out and the final pointing characterization will be carried out in summer 2021. Finally, the angular response setup was also used the angular responsivity of solar direct irradiance filter radiometers. The first test was carried out using Precision Filter Radiometer (PFR) F-064.

Stray-Light Correction Methodology for the Precision Solar Spectroradiometer

A stray-light correction methodology for the Precision Solar Spectroradiometer (PSR) is presented. The correction is based on laboratory-measured line spread functions also taking into account the radiation from the 2nd and 3rd grating orders. The efficiency of the correction is validated on solar and lamp measurement data. The results are compared to those obtained with a PSR equipped with an order-sorting filter and with a Precision Filter Radiometer.

Advancements in NRC’s Primary Spectral Irradiance Scale Realization

The National Research Council (NRC) of Canada has been working to establish new facilities and to improve measurement capabilities traceable to the International System of Units (SI units) in optical radiometry. The NRC primary spectral irradiance scale has transitioned from a detector-based approach in the range of 700 nm to 1600 nm to a detector and source-based realization from 250 nm to 2500 nm. A high temperature blackbody (HTBB) acts as the primary light source for the calibration of 1000 W FEL spectral irradiance standard lamps. The thermodynamic temperature of the HTBB is determined using an NRC-designed wide-band filter radiometer, with spectral responsivity SI-traceable to the NRC optical power scale. This new facility has significantly improved measurement uncertainties compared to the previous NRC spectral irradiance scale.

Reducing cloud contamination in aerosol optical depth (AOD) measurements

We propose a new cloud screening method for sun photometry that is designed to effectively filter thin clouds. Our method is based on a k-nearest-neighbour algorithm instead of scanning time series of aerosol optical depth. Using 10 years of data from a precision filter radiometer in Innsbruck, we compare our new method and the currently employed screening technique. We exemplify the performance of the two routines in different cloud conditions. While both algorithms agree on the classification of a data point as clear or cloudy in a majority of the cases, the new routine is found to be more effective in flagging thin clouds. We conclude that this simple method can serve as a valid alternative for cloud detection, and we discuss the generalizability to other observation sites.

Reducing cloud contamination in AOD measurements

We propose a new cloud screening method for sun photometry that is designed to effectively filter thin clouds. Our method is based on a k-nearest neighbour algorithm instead of scanning timeseries of aerosol optical depth. Using ten years of data from a precision filter radiometer in Innsbruck, we compare our new method and the currently employed screening technique. We exemplify the performance of the two routines in different cloud conditions. While both algorithms agree on the classification of a datapoint as clear or cloudy in a majority of the cases, the new routine is found to be more effective in flagging thin clouds. We conclude that this simple method can serve as a valid alternative for cloud detection, and discuss the generalizability to other observation sites.

Use of filter radiometer measurements to derive local photolysis rates and for future monitoring network application

Production of hydroxyl (OH) radicals is frequently dominated by the photolysis of tropospheric ozone (O3). However, photolysis of nocturnal radical reservoirs, such as nitrous acid (HONO) and nitryl chloride (ClNO2), also produces radicals (OH and Cl atoms) that contribute to the oxidising capacity of the local atmosphere, and initiate many radical-chain reactions that lead to the formation of harmful secondary pollutants. Photolysis of nitric acid (HNO3) is also a minor radical production mechanism. In this paper, locally representative photolysis rate constants (j-values) for these molecules are shown to be critical for quantifying and understanding the rate of radical production in a local atmosphere. The first long-term 4-π filter radiometer dataset in the UK (21 November 2018–20 November 2019) available for direct atmospheric model validation is reported. Measurements were made at Auchencorth Moss, a Scottish rural background site, and j(NO2) is used to generate a measurement-driven adjustment factor (MDAF) for calculated j-values that accounts for local changes in meteorological variables without significantly increasing computational cost. Modelled clear-sky j-values and actinic flux for Auchencorth Moss were generated using the Tropospheric Ultraviolet and Visible radiation model (TUV; v.5.3.1). Applying the MDAF metric resulted in the calculated photolytic production rate of OH radicals, from all sources considered, being ~40 % lower over the year. Photolysis of HONO resulted in an increased rate of OH production compared to that from O3 in low-light conditions, such as sunrise and sunset (Solar Zenith Angle > 80°). Hydroxyl radical production from HONO photolysis exceeded that from O3 consistently throughout the day during the winter and autumn (by a factor of 5 and 2.1, respectively). Radical production rates from HONO and ClNO2 reached maximum values during the early morning hours of summer (06:00–09:00 UTC), with OH produced at a rate of 1.06 × 106 OH radicals cm−3 s−1, and Cl radicals at 3.20 × 104 Cl radicals cm−3 s−1, with the MDAF metric applied. This first application of the MDAF j-values demonstrates an efficient measurement and computational approach to improve modelling of the local atmospheric photochemistry that drives NO2, O3 and PM pollution levels. The incorporation of local radiation measurements in measurement networks, and the consequent greater spatial resolution of locally-relevant photolysis coefficients in model photolysis parameterisations, will improve the accuracy of assessment of air pollution and policy-intervention impacts.

Aerosol Optical Depth Measurements at high altitude and polar WMO Global Atmospheric Watch - PFR Network Stations

<p>Multiwavelength aerosol optical depth (AOD) has been defined as an essential climate variable for the Global Climate Observing System (GCOS) and the Global Atmosphere Watch (GAW) Program of the World Meteorological Organization. It is the most important parameter related to aerosol radiative forcing studies. PMOD/WRC have developed the Precision Filter Radiometer (PFR) that has been used for long term AOD measurements under a GAW-PFR Network of sun-photometers started in 1995 at Davos Switzerland and from 1999 at other locations, worldwide.</p><p>Here we present:</p><p>An overview of the results of the long term GAW-PFR AOD series for four high altitude stations (Izana/Spain, Mauna Loa/USA, Mt. Walliguan/China and Jungfraujoch/Switzerland). Mean AODs at 500nm were from 0.015 up to 0.096 with small negative changes per year for all stations.</p><p>An overview of the results for polar stations (Ny Ålesund/Norway, Summit/Denmark, Marambio/Finland). Ny Ålesund mean AODs at 500nm were almost double compared with the other stations.</p>

Measurements of spectral irradiance during the solar eclipse of 21 August 2017: reassessment of the effect of solar limb darkening and of changes in total ozone

Measurements of spectral irradiance between 306 and 1020 nm were performed with a GUVis-3511 multi-channel filter radiometer at Smith Rock State Park, Oregon, during the total solar eclipse of 21 August 2017. The radiometer was equipped with a shadowband, allowing the separation of the global (sun and sky) and direct components of solar radiation. Data were used to study the wavelength-dependent changes in solar irradiance at Earth's surface. Results were compared with theoretical predictions using three different parameterizations of the solar limb darkening (LD) effect, which describes the change in the solar spectrum from the Sun's center to its limb. Results indicate that the LD parameterization that has been most widely used during the last 15 years underestimates the LD effect, in particular at UV wavelengths. The two alternative parameterizations are based on two independent sets of observations from the McMath–Pierce solar telescope. When these parameterizations are used, the observed and theoretical LD effects agree to within 4 % for wavelengths larger than 400 nm and occultation of the solar disk of up to 97.8 %. Maximum deviations for wavelengths between 315 and 340 nm are 7 %. These somewhat larger differences compared to the visible range may be explained with varying aerosol conditions during the period of observations. The aerosol optical depth (AOD) and its wavelength dependence was calculated from measurements of direct irradiance. When corrected for the LD effect, the AOD decreases over the period of the eclipse: from 0.41 to 0.34 at 319 nm and from 0.05 to 0.04 at 1018 nm. These results show that AODs can be accurately calculated during an eclipse if the LD effect is corrected. The total ozone column (TOC) was derived from measurements of global irradiance at 306 and 340 nm. Without correction for the LD effect, the retrieved TOC increases by 20 DU between the first and second contact of the eclipse. With LD correction, the TOC remains constant to within natural variability (±2.6 DU or ±0.9 % between first and second contact and ±1.0 DU or ±0.3 % between third and fourth contact). In contrast to results of observations from earlier solar eclipses, no fluctuations in TOC were observed that could be unambiguously attributed to gravity waves, which can be triggered by the supersonic speed of the Moon's shadow across the atmosphere. Furthermore, systematic changes in the ratio of direct and global irradiance that could be attributed to the solar eclipse were not observed, in agreement with results of three-dimensional (3-D) radiative transfer (RT) models. Our results advance the understanding of the effects of solar LD on the spectral irradiance at Earth's surface, the variations in ozone during an eclipse, and the partitioning of solar radiation in direct and diffuse components.

Aerosol Optical Depth comparison between GAW-PFR and AERONET-Cimel radiometers from long term (2005–2015) 1-minute synchronous measurements

A comprehensive comparison of more than 70000 synchronous 1-minute aerosol optical depth (AOD) data from three Global Atmosphere Watch-Precision Filter Radiometer (GAW-PFR) and 15 Aerosol Robotic Network-Cimel (AERONET-Cimel) radiometers was performed for the four nearby wavelengths (380, 440, 500 and 870 nm) in the period 2005–2015. The goal of this study is to assess whether, despite the marked differences between both networks and the number of instruments used, their long term AOD data are comparable and consistent. AOD traceability established by the World Meteorological Organization (WMO) consists in determining the percentage of synchronous data within specific limits. If, at least, 95 % of the AOD differences of an instrument compared to the WMO standards lie within these limits, both data populations are considered equivalent. The percentage of traceable data is 92.7 % (380 nm), 95.7 % (440 nm), 95.8 % (500 nm) and 98.0 % (870 nm). When small misalignments in GAW-PFR sun-pointing were fixed (period 2010–2015), the percentage of traceable data increased. The contribution of calibration related aspects to comparison outside the 95 % traceability limits is insignificant in all channels, except in 380 nm. The simultaneous failure of both cloud screening algorithms might occur only under the presence of cirrus, or altostratus clouds on the top of a dust-laden Saharan air layer. Differences in the calculation of the optical depth contribution due to Rayleigh scattering, and O3 and NO2 absorption have a negligible impact. For AOD > 0.1, a non-negligible percentage (≈ 1.9 %) of the AOD data outside the 95 % traceability limits at 380 nm can be partly assigned to the different field of view of the instruments. The comparison of the Angström exponent (AE) shows that under non-pristine conditions (AOD > 0.03 and AE