Recovering long-term aerosol optical depth series (1976–2012) from an astronomical potassium-based resonance scattering spectrometer

Abstract

Abstract. A 37-year long-term series of monochromatic aerosol optical depth (AOD) has been recovered from solar irradiance measurements performed with the solar spectrometer Mark-I, deployed at Izaña mountain since 1976. The instrument operation is based on the method of resonant scattering, which affords wavelength absolute reference and stability (long-term stability and high precision) in comparison to other instruments based purely on interference filters. However, it has been specifically designed as a reference instrument for helioseismology, and its ability to determine AOD from transmitted and scattered monochromatic radiation at 769.9 nm inside a potassium vapour cell in the presence of a permanent magnetic field is evaluated in this paper. Particularly, the use of an exposed mirror arrangement to collect sunlight as well as the Sun–laboratory velocity dependence of the scattered component introduces some important inconveniences to overcome when we perform the instrument's calibration. We have solved this problem using a quasi-continuous Langley calibration technique and a refinement procedure to correct for calibration errors as well as for the fictitious diurnal cycle on AOD data. Our results showed similar calibration errors retrieved by means of this quasi-continuous Langley technique applied in different aerosol load events (from 0.04 to 0.3), provided aerosol concentration remains constant throughout the calibration interval. It assures the validity of this technique when it is applied in those periods with relatively high aerosol content. The comparative analysis between the recovered AOD data set from the Mark-I and collocated quasi-simultaneous data from the Cimel-AErosol RObotic NETwork (AERONET) and Precision Filter Radiometer (PFR) instruments showed an absolute mean bias ≤ 0.01 in the 10- and 12-year comparison, respectively. High correlation coefficients between AERONET and Mark-I and PFR/Mark-I pairs confirmed a very good linear relationship between instruments, proving that recovered AOD data series from Mark-I can be used together with PFR and AERONET AOD data to build a long-term AOD data series at the Izaña site (1976–now), suitable for future analysis of aerosols trends and inter-annual variability. Finally, the AOD preliminary trend analysis in the 29-year period from 1984 to 2012 with Mark-I AOD revealed no significant trends.

Aerosol optical depth retrievals at the Izaña Atmospheric Observatory from 1941 to 2013 by using artificial neural networks

“…However, the overall ANN AOD/ Mark-I AOD scatter (0.06) duplicates that observed for the ANN AOD/AERONET AOD comparison (0.03). This agreement is within the AOD uncertainty of Mark-I (Barreto et al, 2014) and within our error estimation ( Table 2). Notice that the experimental scatter is significantly smaller than the theoretical one, suggesting that our assumed uncertainties could be very conservative.…”

“…The ANN AOD estimates have been validated with coincident AERONET CIMEL photometers of Level 2.0 AOD (cloud screened and quality ensured) from 2004 to 2009, and with a long-term AOD at 769.9 nm data series retrieved by Barreto et al (2014) from a solar spectrometer Mark-I for the period 1975-2012. Table 2.…”

“…The typical background free troposphere conditions at IZO are only significantly modified in summer, mainly in July and August, when the most intense and relatively frequent Saharan air mass outbreaks in the subtropical free troposphere reach the observatory (Cuevas, 1996;Rodríguez et al, 2011;Cuevas et al, 2013Cuevas et al, , 2015bBarreto et al, 2014;Rodríguez et al, 2015). During these months, Saharan dust long-range transport over the Atlantic that can reach the Caribbean is driven by incursions of the so-called Saharan air layer (SAL) over the North Atlantic (Prospero et al, 2002, and references therein).…”

“…CIMEL photometers retrieve AOD measurements at different wavelengths between 340-1640 nm from direct Sun observations under cloud-free conditions, with an expected uncertainty of 0.01 at 500 nm for field instruments (Eck et al, 1999). The validation procedure of Mark-I AOD time series was performed by Barreto et al (2014), showing a rootmean-square error of 0.022 (R = 0.94) and 0.034 (R = 0.92) in comparison with the PFR reference and AERONET master instruments, respectively. In order to compare the Mark-I AOD values at 769.9 nm and the ANN AOD estimates at 500 nm, we have extrapolated the ANN AOD values from 500 to 769.9 nm by using the Ångström law (Ångström, 1929) and the α data retrieved from PFR observations.…”

“…Kudo et al (2011) estimated aerosol optical depth (AOD) combining broadband direct and diffuse irradiance measurements performed at Tsukuba, Japan, from 1975. Shaw (1979 and Holben et al (2001) studied long-term series of AOD from sun photometry at Mauna Loa since 1976, and Barreto et al (2014) derived AOD from solar irradiance measurements at Izaña Atmospheric Observatory (IZO) since 1976. All of these studies are based on solar spectrometry, but a different approach is needed to obtain longer AOD time series.…”