Effects of cloud, atmospheric water vapor, and dust on photosynthetically active radiation and total solar ra-diation in a Mongolian grassland

Abstract

Photosynthetically active radiation (PAR) is an important input parameter for estimating plant productivity due to its key role in the growth and development of plants. However, a worldwide routine network for systematic PAR measurements is not yet established, and PAR is often calculated as a constant fraction of total solar radiation (SR). Although the ratio of PAR to SR (PAR/SR) has been reported from many places, few studies have been performed for dry regions. The present study was therefore carried out in an arid region of Mongolia to obtain PAR/SR and examine its dependency on sky clearness (the clearness index), water vapor in the atmosphere and aeolian dust. Continuous measurements of PAR and SR were taken every one second using quantum and pyranometer sensors, respectively, and the readings were averaged and recorded at intervals of 30 minutes for a period of 12 months. The lowest monthly mean daily PAR/SR occurred in April (0.420), while the highest ratio was observed in July (0.459). Mean daily PAR/SR during plant growing season (May−August) was estimated to be 0.442, which could be useful for modeling plant productivity in the study area. The annual mean daily PAR/SR (0.435) was lower than the values reported in many previous studies. This difference could be explained with the regional variation in climate: i.e. drier climatic condition in the study area. PAR/SR was negatively correlated with the clearness index (r=-0.36, P<0.001), but positively with atmospheric water vapor pressure (r=0.47, P<0.001). The average PAR/SR was significantly lower (P=0.02) on the dusty days compared to the non-dust days. Water vapor in the atmosphere was shown to be the strongest factor in the variation of PAR/SR. This is the first study examining PAR/SR under a semi-arid condition in Mongolia.

A Global Review of Empirical Models for Estimating Photosynthetically Active Radiation

“…That is, in cloudy and humid conditions, the absorption of solar radiation in the near infrared (NIR) portion of the solar spectrum is enhanced, whereas absorption in the PAR waveband does not vary significantly. Moreover, Bat-Oyun et al [59] equally observed that increases in PAR e /H were found under cloudy and humid conditions. The authors equally recorded a significant correlation between PAR e /H and water vapour pressure (r = 0.49, P < 0.001) for day time (08:00 -17:00, local time).…”

“…The influence of water vapour pressure on PAR/H has been roughly reported in literature in season variations such as the higher in summer (wet season) and lower in winter (dry season) [51,59,107]. This report is in agreement with recent report of dependence of PAR/H on water vapour pressure by Akitsu et al [49] who observed that the monthly mean PAR/H recorded higher values (0.465) in summer and lower value (0.420) in winter as shown in Fig.…”

“…However, numerous researchers have observed that PAR ratio varied according to location [48]; seasons [43,50]; sky conditions [51][52][53]; sky clearness, sky brightness and atmospheric depth for the solar beam [54][55]; relative sunshine duration and water vapour pressure [50,55]; altitude [56]; irradiance intensity [57]; day length [57][58]; Ozone and other atmospheric gases [53,[58][59]; relative humidity [60], minimum and maximum temperature [61]; optical air mass [62][63][64]; cloud amount and turbidity [55]; global solar radiation [41,[65][66][67]; clearness index [48,[68][69][70][71][72][73]; site, season, local time and weather conditions [49]. Thus, each ratio remains incompletely understood as to how it varies with 239 climatic factors such as water vapour pressure (WVP); solar zenith angle (ϴ) and sky cloudiness.…”

Photosynthetically active radiation and comparison of methods for its estimation in equatorial Singapore

“…The diurnal variation in PAR/H is variable between sites, e.g. with different degrees of midday depression (Alados et al 1996;Jacovides et al 2004;Udo and Aro 1999), midday increase (Ge et al 2011), slight increasing trend throughout the day (Aguiar et al 2012), as well as inconsistent diurnal changes from month to month (Bat-oyun et al 2012). The most commonly cited factor that explains the diurnal variation is water vapour.…”

Dependence of the spectral diffuse-direct irradiance ratio on aerosol spectral distribution and single scattering albedo

“…With the increasing use of spectrally selective devices, such as photovoltaic cells and coated glazings (Kanters et al, 2014;Wang et al, 2015), the scientific interest is not limited to the determining of the total or -broadband‖ solar irradiance reaching at the ground level, but now also encompasses its spectral distribution. This spectral distribution is also needed for industrial, biological, and medical applications such as evaluation of electrical energy, study of the photosynthetic activity of plants and evaluation of ultraviolet (UV) radiation (Bat-Oyun et al, 2012;Petkov et al, 2012;Bilbao et al, 2015). The scattering by air molecules and aerosols attenuates the solar radiation over its whole spectrum, while the absorption modifies it in selective spectral bands due to the action of various atmospheric constituents, mainly including ozone, water vapor, oxygen, carbon dioxide, and nitrogen dioxide (Leckner, 1978;Jacovides et al, 2005;Kanniah et al, 2010).…”