Optical Air Mass Research Articles

Development of the method of remote sensing of low-concentration gases of the atmosphere taking into account the humidity fluctuations of the air

The emergence of strong desert vortices covering large areas, including the areas of large cities with dense population, the need to quickly obtain sufficient information on the state of desert ecosystems in large areas makes the application of remote sensing methods an urgent issue. At present, the method of measuring the desert aerosol using scattered solar radiation has been proposed, and the method of measuring the optical thickness of the dust aerosol has been substantiated. A new method of optimization of remote sensing systems and information-measurement systems in general is proposed in the article. A method of remote determination of aerosol optical thickness increase, which involves measuring the scattered radiation of the sun, has been proposed. The main goal of the study is to develop a remote sensing method that allows determining the dynamics and cause of the increase in aerosol optical thickness. The scientific novelty of the work is a new method that is invariant to the variations of the aerosol optical thickness under the influence of hydration and dehydration processes. By using the proposed three-wave biparametric Sun photometer, the effect of aerosol hygroscopic growth on the measurement results is eliminated.The given critical assessment of the state of atmospheric pollution monitoring and the consideration of the main shortcomings of the aerosol pollution monitoring carried out in the country will allow further improvement of such monitoring in the future. The proposed remote sensing method for determining the dynamics of aerosol optical thickness growth allows to determine the origin of aerosol optical thickness growth by measuring scattered solar radiation at two different wavelengths and in two optical air masses.

Detecting local and regional air pollution from biomass burning at a suburban site

Identifying pollution plumes is challenging in cities where in situ air pollution data are scarce, monitoring stations are placed close to roadways, and the smoke from biomass burning might be of local and regional origin. This work characterizes local and regional smoke events at a suburban site in Londrina, southern Brazil, during the dry season. We measured black carbon at 370 and 800 nm (BC370 and BC880), fine particulate matter (PM2.5), particle number (PN), nitrogen oxides (NOx) and ozone (O3) concentrations with 1–2 min resolutions. Ancillary data, such as meteorological variables, aerosol optical depth (AOD) and air mass backward trajectories, aided identifying the sources. The pollution datasets were analyzed by using descriptive statistics, diurnal cycles, temporal correlations, and conditional probability functions coupled with cluster analysis of backward trajectories. Hourly mean (± standard deviation) concentrations were 1.33 ± 1.28 μg m-3 (BC370), 0.91 ± 0.84 μg m-3 (BC880), 15.90 ± 12.95 μg m-3 (PM2.5), 5691 ± 4872 cm-3 (PN), 7.23 ± 7.53 μg m-3 (NOx) and 49.77 ± 17.66 μg m-3 (O3). The prevalence of aerosols from biomass burning was indicated by Ångström exponent (Å370/880) values predominantly larger than 1 (1.51 ± 0.20). Pollutant concentrations peaked on weekend evenings and, occasionally, scattered along the day due to the short-lived open burning of residential solid waste. These events led to fast changing concentrations, increasing from 282 to 929% between consecutive 2-min measurements, reaching 69.29 μg m-3 (BC370), 456.77 μg m-3 (PM2.5) and 232,500 cm-3 (PN). Emissions from biomass-fueled boilers also contributed to deteriorate the local air quality, under favorable local winds. Three regional events advected pollution from Cerrado (tropical savanna) wildfires lasting two-three days each, and were associated with the South American Low Level Jet. During these event, the AOD reached up to 0.37 and mean PM2.5, BC370, and O3 concentrations were (2.5–3.3), (1.9–2.9) and (1.3–1.5) times higher compared to a clean period. Lowering the population exposure to biomass burning requires immediate action to control local sources, and international cooperation is needed to combat regional wildfires in a warming climate.

PAR and UVA composition of global solar radiation at a high altitude Mediterranean forest site

Photosynthetically active (PAR) and ultraviolet-A (UVA) radiation have important effects on plant processes, affecting both health and ecology of the natural ecosystems. Studies dealing with the composition of global solar radiation Rs in PAR and UVA wavelengths are thus important. The present work is aimed at the investigation of the relationships of PAR, UVA with Rs, assessing the key factors that affect the corresponding ratios PAR/Rs and UVA/Rs at a high-altitude mountainous environment. The study was conducted in a protected Natura-2000 forest in Greece at a high altitude (1896 m a.s.l.) site and covers a time period of about 5 years (2014–2019). The analysis was performed on a monthly, seasonal and annual basis using hourly data. Results show that PAR/Rs and UVA/Rs present annual values 0.438 ± 0.031 and 0.034 ± 0.007, respectively with low monthly and seasonal variability. Further, sky clearness was found to have a significant effect on the composition of incident Rs, with PAR/Rs values to increase from 0.417 ± 0.013 to 0.483 ± 0.037 as sky conditions change from clear to overcast. The respective UVA/Rs values were 0.031 ± 0.002 and 0.045 ± 0.011. The hourly ratios become minimum at noon, whereas the magnitude of Rs was found to have a negative effect. In addition, PAR/Rs and UVA/Rs were increasing with relative humidity, optical thickness and relative optical air mass and decreasing with saturation vapor pressure, vapor pressure deficit, and to a less degree with air temperature. Concluding, at the high altitude Mediterranean forest site, Rs composition to PAR and UVA appears to be affected mainly by atmospheric clearness and by the geometric characteristics of the incident radiation rays, whereas atmospheric clearness was affected mainly by air humidity.

Correction of a lunar-irradiance model for aerosol optical depth retrieval and comparison with a star photometer

Abstract. The emergence of Moon photometers is allowing measurements of lunar irradiance over the world and increasing the potential to derive aerosol optical depth (AOD) at night-time, which is very important in polar areas. Actually, new photometers implement the latest technological advances that permit lunar-irradiance measurements together with classical Sun photometry measurements. However, a proper use of these instruments for AOD retrieval requires accurate time-dependent knowledge of the extraterrestrial lunar irradiance over time due to its fast change throughout the Moon's cycle. This paper uses the RIMO (ROLO Implementation for Moon's Observation) model (an implementation of the ROLO – RObotic Lunar Observatory – model) to estimate the AOD at night-time assuming that the calibration of the solar channels can be transferred to the Moon by a vicarious method. However, the obtained AOD values using a Cimel CE318-T Sun–sky–Moon photometer for 98 pristine nights with low and stable AOD at the Izaña Observatory (Tenerife, Spain) are not in agreement with the expected (low and stable) AOD values estimated by linear interpolations from daytime values obtained during the previous evening and the following morning. Actually, AOD calculated using RIMO shows negative values and with a marked cycle dependent on the optical air mass. The differences between the AOD obtained using RIMO and the expected values are assumed to be associated with inaccuracies in the RIMO model, and these differences are used to calculate the RIMO correction factor (RCF). The RCF is a proposed correction factor that, multiplied by the RIMO value, gives an effective extraterrestrial lunar irradiance that provides AOD closer to the expected values. The RCF varies with the Moon phase angle (MPA) and with wavelength, ranging from 1.01 to 1.14, which reveals an overall underestimation of RIMO compared to the lunar irradiance. These obtained RCF values are modelled for each photometer wavelength to a second-order polynomial as a function of MPA. The AOD derived by this proposed method is compared with the independent AOD measurements obtained by a star photometer at Granada (Spain) for 2 years. The mean of the Moon–star AOD differences is between −0.015 and −0.005, and the standard deviation (SD) is between 0.03 and 0.04 (which is reduced to about 0.01 if 1 month of data affected by instrumental issues is not included in the analysis) for 440, 500, 675, and 870 nm; however, for 380 nm, the mean and standard deviation of these differences are higher. The Moon–star AOD differences are also analysed as a function of MPA, showing no significant dependence.

Transmittance index characterization at two solar measurement stations in Brazil

This paper constructs probability density distributions of direct normal, diffuse horizontal, and global horizontal irradiance for 1-min clearness indexes kb, kd, and kt, respectively. Previous studies on the clearness index, conditioned to the optical air mass, are analyzed and distributions of these three indexes, using data from two solarimetric stations in Brazil, are presented. The pattern found in probability density distributions, in general, matches the patterns found in the literature. The 50th percentile (P50) of kb values from Sao Paulo city (a location with high levels of pollution) is 0.22, and from the countryside region of the state of Sao Paulo (a location with expected low levels of pollution), it is 0.55. Regarding kt values, P50 is 0.57 and 0.72 for these two regions, respectively. We conclude that different atmospheric conditions result in a different dependence relation between probability density distribution and air mass increases. These conditions include not only cloud formation, but also pollution levels and the influence of relative humidity. These specific results endorse the need for local radiation measurements to the characterization of few-minute average of transmittance indexes.

ИЗМЕРЕНИЯ СОЛНЕЧНОЙ ПОСТОЯННОЙ НОВЫМ МЕТОДОМ ИСПОЛЬЗУЯ ДВУХВОЛНОВЫХ ФОТОМЕТРОВ

The new two-wavelengths method for measuring of solar constant using sun photometers has been suggested. It is shown, that differ from the known method the measurements at the fixed wavelength should be carried out at only one value of optical air mass which makes it possible to get the two-fold advantage in amount of number of needed measurements.

Results from the Fourth WMO Filter Radiometer Comparison for aerosol optical depth measurements

Abstract. This study presents the results of the Fourth Filter Radiometer Comparison that was held in Davos, Switzerland, between 28 September and 16 October 2015. Thirty filter radiometers and spectroradiometers from 12 countries participated including reference instruments from global aerosol networks. The absolute differences of all instruments compared to the reference have been based on the World Meteorological Organization (WMO) criterion defined as follows: 95% of the measured data has to be within 0.005 ± 0.001∕m (where m is the air mass). At least 24 out of 29 instruments achieved this goal at both 500 and 865 nm, while 12 out of 17 and 13 out of 21 achieved this at 368 and 412 nm, respectively. While searching for sources of differences among different instruments, it was found that all individual differences linked to Rayleigh, NO2, ozone, water vapor calculations and related optical depths and air mass calculations were smaller than 0.01 in aerosol optical depth (AOD) at 500 and 865 nm. Different cloud-detecting algorithms used have been compared. Ångström exponent calculations showed relatively large differences among different instruments, partly because of the high calculation uncertainty of this parameter in low AOD conditions. The overall low deviations of these AOD results and the high accuracy of reference aerosol network instruments demonstrated a promising framework to achieve homogeneity, compatibility and harmonization among the different spectral AOD networks in the near future.

Worst-Case Photovoltaic Generation and Power Change Distribution Under Dense Cloud Cover

Photovoltaic (PV) power generation units located in tropical or equatorial sites frequently experience low-lying, dense cloud cover; and are thereby exposed to rapid real time changes in irradiance. Commonly referred to as cloud transients, the changes lead to short duration power variations and substantial reduction of output from individual units. The paper proposes pessimistic (or worst-case) analytical assessment of power variation statistics at a PV generation unit, as well as the consequent drop in output power. Statistical estimates are introduced for worst-case short duration mean output power, its temporal variability, and the associated power change distribution. The metrics are verified against ensemble estimates from field studies conducted in Brazil and India. Statistical estimates have the advantage of exclusive parametric dependence on optical air mass, which makes them applicable across diverse geography and climate. They can therefore be used for quick evaluation of short duration performance as well as comparative studies, both at existing as well as prospective PV generating stations.

Statistical analysis and inter-comparison of erythemal solar radiation for Athalassa and Larnaca, Cyprus

A statistical analysis and inter-comparison of the erythemal ultraviolet radiation (UVER) at two sites in Cyprus representing two different climate regimes of the island (Athalassa-inland plain vs Larnaca-coastal location) covering the period January 2013–December 2015 is presented. Mean annual and mean monthly daily totals of the UVER irradiation and their frequency distribution at both sites are computed and discussed. UVER irradiance fluctuates between 0.054 W m−2 in December to 0.227 W m−2 in July at solar noon at Athalassa. The values at Larnaca are lower than in Athalassa and they fluctuate between 0.043 W m−2 in December and 0.172 W m−2 in July at solar noon. The total accumulated UVER irradiation along a mean year reaches 1142 kJ m−2 at Athalassa and 909 kJ m−2 at Larnaca. Large fluctuations of the daily UVER irradiation are observed in the spring months and November, which are mainly due to unstable meteorological conditions during the transition from cold to warm weather and vice versa. During summer the daily UVER radiation exceeds the value of 6 kJ m−2 at Athalassa and 4.8 kJ m−2 at Larnaca, while during the winter season the lowest is about 0.2 kJ m−2 at both sites. Statistical relationships between UVER and other radiation components (UVA, global radiation, clearness index and relative optical air mass) are established. The UV Index reaches high (6–7) or very high (8–10) values in 58.1% of the cases in Athalassa, whereas in Larnaca these values are reached in 38.8%. It is observed that the cumulative doses of UVER during an average year range from 9087 Standard Erythemal Dose (SEDs) in Larnaca to 11418 SEDs in Athalassa. The most common skin type in Cyprus, phototype III (about 70% of the population), could receive an annual cumulative dose between 2596 Minimum Erythemal Doses (MEDs) in Larnaca to 3262 MEDs in Athalassa.

Performance analysis of high-concentrated multi-junction solar cells in hot climate

ABSTRACTMulti-junction concentrator solar cells are a promising technology as they can fulfill the increasing energy demand with renewable sources. Focusing sunlight upon the aperture of multi-junction photovoltaic (PV) cells can generate much greater power densities than conventional PV cells. So, concentrated PV multi-junction solar cells offer a promising way towards achieving minimum cost per kilowatt-hour. However, these cells have many aspects that must be fixed to be feasible for large-scale energy generation. In this work, a model is developed to analyze the impact of various atmospheric factors on concentrator PV performance. A single-diode equivalent circuit model is developed to examine multi-junction cells performance in hot weather conditions, considering the impacts of both temperature and concentration ratio. The impacts of spectral variations of irradiance on annual performance of various high-concentrated photovoltaic (HCPV) panels are examined, adapting spectra simulations using the SMARTS model. Also, the diode shunt resistance neglected in the existing models is considered in the present model. The present results are efficiently validated against measurements from published data to within 2% accuracy. Present predictions show that the single-diode model considering the shunt resistance gives accurate and reliable results. Also, aerosol optical depth (AOD) and air mass are most important atmospheric parameters having a significant impact on HCPV cell performance. In addition, the electrical efficiency (η) is noticed to increase with concentration to a certain concentration degree after which it decreases. Finally, based on the model predictions, let us conclude that the present model could be adapted properly to examine HCPV cells' performance over a broad range of operating conditions.

Spectral study and classification of worldwide locations considering several multijunction solar cell technologies

AbstractMulti‐junction solar cells are widely used in high‐concentration photovoltaic systems (HCPV) attaining the highest efficiencies in photovoltaic energy generation. This technology is more dependent on the spectral variations of the impinging Direct Normal Irradiance (DNI) than conventional photovoltaics based on silicon solar cells and consequently demands a deeper knowledge of the solar resource characteristics. This article explores the capabilities of spectral indexes, namely, spectral matching ratios (SMR), to spectrally characterize the annual irradiation reaching a particular location on the Earth and to provide the necessary information for the spectral optimization of a MJ solar cell in that location as a starting point for CPV module spectral tuning. Additionally, the relationship between such indexes and the atmosphere parameters, such as the aerosol optical depth (AOD), precipitable water (PW), and air mass (AM), is discussed using radiative transfer models such as SMARTS to generate the spectrally resolved DNI. The network of ground‐based sun and sky‐scanning radiometers AErosol RObotic NETwork (AERONET) is exploited to obtain the atmosphere parameters for a selected bunch of 34 sites worldwide. Finally, the SMR indexes are obtained for every location, and a comparative analysis is carried out for four architectures of triple junction solar cells, covering both lattice match and metamorphic technologies. The differences found among cell technologies are much less significant than among locations. Copyright © 2016 John Wiley & Sons, Ltd.

Calibration of the 936 nm water-vapor channel for the China aerosol remote sensing NETwork (CARSNET) and the effect of the retrieval water-vapor on aerosol optical property over Beijing, China

Based on measurements of the 936 nm water-vapor channel and the 870 nm, and 1020 nm atmospheric window band for the CE-318 sunphotometer of CARSNET, clear-sky direct solar radiation observational data and relative optical air masses of between 2 and 5, and considering sensitivity parameters such as pressure, altitude, wavelength, and AOD936nm, which may affect the calibration result, a method to calibrate the 936 nm water-vapor channel using a modified Langley method is presented. The daily average columnar water-vapor (CWV) over urban Beijing was retrieved using three methods (Methods A, B and C), and compared with the AERONET water-vapor product. Finally, the seasonal relationships of CWV with AOD500nm and α440–870 were analyzed. The calibration results showed the calibration value to differ from the original value by about 4%. The CWV retrieval result showed that AERONET and the three methods of retrieval for CWV were highly correlated. The monthly average variation trends of AOD500nm and CWV were similar, with the maximum occurring in July, and the minimum in December. The seasonal average variation trends of AOD500nm and CWV were also similar, but could be placed in the following order: summer > autumn > spring > winter. The seasonal variation of α440–870 versus AOD500nm, α440–870 versus CWV, and AOD500nm versus CWV, showed increasing AOD500nm with increasing CWV, which reflected the hygroscopicity of aerosol fine particle growth characteristics. Summer was mainly characterized by fine particles; while spring, autumn and winter were a mix of coarse and fine particles, showing typical urban aerosol properties.

Broadband and luminous extinction coefficients in a clean and dry atmosphere

The extinction coefficient as a function of optical air mass has been persistently used for many years in order to model both direct normal irradiance and illuminance (DNI). Nevertheless, such a dependence is only approximate, simply a convenient, but inaccurate application in monochromatic radiative transfer theories. The relationship between these characteristics originates from the point source exponential law incorrectly used for broadband radiation and, the set of empirical formulae used for broadband and luminous extinction coefficients have no preferred model. Therefore, in this paper we explain the fundamentals of broadband (ae) and luminous (av) extinction coefficients for a clean and dry atmosphere and introduce general formulae which originate from heuristic analytical derivation that produce exact formulae, free of any random or systematic errors which can arise from numerical integration or processing of experimental data. Another advantage is that each parameter used in new formulae for ae or av has a theoretical base, therefore optimum by delineation. The analytical formulae could replace those routinely used in modelling DNI for broadband spectra (including daylight).

Statistical estimates of short duration power generated by a photovoltaic unit in environment of scattered cloud cover

The statistics of low level scattered cloud cover (cumulus or strato-cumulus types), is evolved to formulate analytical expressions for short duration mean output power and its variability, as well as power change distribution, at the output of photovoltaic generating stations. The proposed expressions are used to obtain statistical estimates of the three, which are substantiated by comparison to empirical ensemble estimates reported in available literature.The strength of concepts and formulations presented in the paper is their dependence on a single aggregated meteorological parameter, namely the optical air mass. Their applicability therefore spans over a range of PV generation plants regardless of geographical and weather conditions.

Calibration of Photometric Systems for Atmospheric Measurements with the Use of Fuzzy Sets

An improved method of calibration of sun photometers in the case of different optical air masses is proposed. An analytic condition for assurance of stable performance of periodic calibration of such photometers, taking into account random temporal variations of atmospheric aerosol, is obtained. An example of a calibration performed on the basis of fuzzy sets is demonstrated.

Seasonal Variations of the Relative Optical Air Mass Function for Background Aerosol and Thin Cirrus Clouds at Arctic and Antarctic Sites

New calculations of the relative optical air mass function are made over the 0°–87° range of apparent solar zenith angle θ, for various vertical profiles of background aerosol, diamond dust and thin cirrus cloud particle extinction coefficient in the Arctic and Antarctic atmospheres. The calculations were carried out by following the Tomasi and Petkov (2014) procedure, in which the above-mentioned vertical profiles derived from lidar observations were used as weighting functions. Different sets of lidar measurements were examined, recorded using: (i) the Koldewey-Aerosol-Raman Lidar (KARL) system (AWI, Germany) at Ny-Ålesund (Spitsbergen, Svalbard) in January, April, July and October 2013; (ii) the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite-based sensor over Barrow (Alaska), Eureka (Nunavut, Canada) and Sodankylä (northern Finland), and Neumayer III, Mario Zucchelli and Mirny coastal stations in Antarctica in the local summer months of the last two years; (iii) the National Institute of Optics (INO), National Council of Research (CNR) Antarctic lidar at Dome C on the Antarctic Plateau for a typical “diamond dust” case; and (iv) the KARL lidar at Ny-Ålesund and the University of Rome/National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) lidar at Thule (northwestern Greenland) for some cirrus cloud layers in the middle and upper troposphere. The relative optical air mass calculations are compared with those obtained by Tomasi and Petkov (2014) to define the seasonal changes produced by aerosol particles, diamond dust and cirrus clouds. The results indicate that the corresponding air mass functions generally decrease as angle θ increases with rates that are proportional to the increase in the pure aerosol, diamond dust and cirrus cloud particle optical thickness.

Clear-sky Irradiance Model for Real-time Sky Imager Application

Abstract In a context of sustainable development, enthusiasm for concentrated solar power technologies is developing. So, the CSPIMP (Concentrated Solar Power efficiencyIMProvement) project started in 2013 in order toachieve a better competitiveness for the CSP plants. Its main target is to develop new procedures to improve steam turbine start up cycles, maintenance activities and plant control. One challenge in the project is to better forecast the solar resource in order to optimize plantsoperation. The development of a clear sky model is therefore an essential step to forecast Direct Normal Irradiance (DNI) becauseclear sky represents the nominal operating conditions of aconcentrating solar power plant.This paper focuses on estimating DNI under clear-sky conditionsusinga knowledge model based on the coefficient of relative optical air mass, DNI and atmospheric turbidity fluctuations. The satisfactory results obtainedvalidate the proposed methodology.

A comparison of one-minute probability density distributions of global horizontal solar irradiance conditioned to the optical air mass and hourly averages in different climate zones

In this study, one-minute global horizontal solar irradiance distributions conditioned to the optical air mass, m, and hourly average of global horizontal solar irradiance were studied at sites in five different climate regions. For this purpose, the clearness index, kt, which accounts for the atmospheric transmittance, has been used. These distributions are fitted by functions based on the Boltzmann statistic.The one-minute distributions of kt conditioned to m found are either unimodal or bimodal, depending on the location and the value of m. These distributions are different for each of the locations analyzed. The one-minute distributions of kt conditioned to their hourly value (kth) are unimodal, and are in turn different at each of the locations analyzed. The one-minute kt distributions conditioned to both m and kth analyzed are also unimodal. These distributions were found to be the same (Kolmogorov–Smirnov test, p>0.05) at different sites in 5% of the cases compared, the majority of which show very cloudy sky conditions and decrease monotonically at clearer-sky conditions. These results point to the importance of local distribution and type of clouds in one-minute solar irradiance distributions, and highlight the role of local atmospheric clear sky transparency in differentiating these distributions.

Comparison of different UV models for cloud effect study

Observations of UV radiation (290–400 nm) at Yanting, China (31°16′ N, 105°27′ E and 420 m above sea level) from 2005 to 2012 were used for investigating UV variability and its relationship with global solar radiation in Southwest China. Daily UV/G ratios generally increased from 3.7% in January to 4.5% in July with annual mean value of 4.2%. Clearness index was used for characterizing sky conditions and model development. Linear regression between UV and global solar radiation has been carried out and the results showed that the slope changed from 0.05 under overcast skies to 0.04 in clear skies. Nine UV models have been introduced by studying the dependence of hourly UV irradiation on the relative optical air mass, cloud modification factor and clearness index, respectively, which was assessed through the statistical indices: mean bias error, mean-absolute error and root-mean-square error. By comparing the statistical indices from different models, the most suitable UV model was chosen for analyzing the cloud effects. Finally, it was found that the attenuation of UV radiation caused by clouds increased from 35.6% in July to 64.4% in January.

Estimation of hourly and daily photosynthetically active radiation in Inner Mongolia, China, from 1990 to 2012

ABSTRACTMeasurements of photosynthetically active radiation (PAR) in Inner Mongolia, China, from 2005 to 2012 were used to investigate its relationship with global solar radiation (G) under various sky conditions. Monthly mean daily PAR generally increased from 13.98 mol m−2 d−1 in December to 40.48 mol m−2 d−1 in June with annual mean value being 29.02 mol m−2 d−1. Monthly daily Fp (ratio of PAR to G) increased from 1.68 mol MJ‐1 in December to 1.88 mol MJ‐1 in August with annual average of 1.80 mol MJ‐1. Clearness index (Kt) was used for characterizing sky conditions and it was discovered that clear skies (49.4%) were the dominated sky conditions. By studying the dependence of PAR on optical air mass, Kt and cosine of solar zenith angle, two different models have been introduced and tested at four other sites in China. The model performances have been analysed and the daily PAR values during 1990–2012 at Inner Mongolia, China, have been reconstructed. It was found that PAR increased at the rate of 0.54 mol m−2 d−1 decade‐1 since 1990 and the most evident increases were observed in summer (1.96 mol m−2 d−1 decade‐1). However, PAR decreased at the rate of ‐0.37 mol m−2 d−1 decade‐1 in spring during 1990–2012.