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Three observation sites were centrally located in the urban area of Nanjing City, which is the second largest city in eastern China and features a high population density and large energy consumption. Eastern China, including the middle and lower reaches of the Yangtze River and Yellow River, is a major agricultural region that comprises about one-third of China's total cultivated land and almost half of the country's agricultural yield. The three observation sites were located at Nanjing University of Information Science and Technology (NUIST) (32.21°N, 118.72°E), the Xianlin campus of Nanjing Normal University (NNU) (32.11°N, 118.92°E), and Nanjing University Xianlin campus (NJU) (32.12°N, 118.95°E), as shown in Fig. 1. NNU was approximately 21 km southeast of NUIST and less than 5 km from NJU. The main radiation data were measured at NUIST. Other data related to this work [e.g., visibility, air quality index (AQI), and surface PM2.5 and PM10 concentrations] were observed at NNU, and the total-sky images were obtained at the Atmospheric Parameters Vertical Detection Site (APVDS) at NJU (Qu et al., 2017; Yang et al., 2017).
Figure 1. Observation sites in Nanjing—NUIST (32.21°N, 118.72°E), NNU (32.11°N, 118.92°E), and NJU (32.12°N, 118.95°E).
SSR data were measured during March 2016–June 2017 at a solar radiation monitoring station (instrument model: TP-SMS-22G) located at NUIST. Due to some technical issues, there was a lack of radiation observation data between November and December 2016. A secondary standard pyranometer (instrument model: SKO MS-802F) was used for the measurement of GHI and DHI, and a pyrheliometer (instrument model: SKO MS-56), for measuring DNI, was mounted on the sun tracker. All the observational data were collected every minute. The MS-802F is sensitive to solar irradiance in the spectral range of 280 to 3000 nm, and the wavelength range of MS-56 is from 200 nm to 4000 nm. The working temperature range of these instruments is between −40°C and 80°C. Routine maintenance and calibration of the instruments was carried out during the study period, including cleaning the sensor lens weekly, checking the wiring condition monthly, and checking the level of the bubble monthly. The observation data covered all four seasons in Nanjing, including spring (March–May 2016), summer (June–August 2016), autumn (September–November 2016), and winter (December 2016–February 2017).
In addition, a visibility sensor (model GSN-1) was deployed to monitor visibility based on the Koschmider principle (Horvath and Noll, 1969), facilitating the retrieval of aerosol extinction coefficients at 500 nm. Surface PM2.5 and PM10 concentrations (Thermo TEOM-1405) were continuously monitored and hourly averages recorded. PM2.5 is considered as fine particles, while particles with diameters of 2.5–10 μm (PM2.5−10) are referred to as coarse particles. The visibility, PM10, PM2.5, and AQI data were observed at Xianlin Ambient Air Quality Monitoring Site, located at NNU. The value of AQI was calculated by the concentrations of six atmospheric pollutants—namely, sulfur dioxide, nitrogen dioxide, PM10, PM2.5, carbon monoxide, and ozone—measured at the monitoring site (https://en.wikipedia.org/wiki/Airqualityindex). The main pollutants might vary throughout the year, and AQI represented the overall pollution level. The total-sky imager was set up at the APVDS site at NJU. Specific parameters of the observation instruments are shown in Table 1.
Observation instrument Technical parameter SKO MS-802F
SKO MS-56Operating
temperature (°C)Spectral
range (nm)Spectral sensitivity Response time Long-term stability
Sensitivity
(μV W−1 m−2)−40 to 80 280−3000 <1% <5 s <0.5% Approx. 7 −40 to 80 200−4000 <1% <1 s <0.5% Approx. 10 Thermo
TEOM-1405Operating
temperature (°C)Measurement
range (g m−3)Resolution
(μg m−3)Accuracy Main flow rate
(min−1)Bypass flow
rate (min−1)−40 to 60 0−1 0.1 ±0.75% 3 13.67 GSN-1 Operating
temperature (°C)Measurement
range (km)Resolution
(m)Accuracy − − −40 to 50 0.01−35 100 ±10% Table 1. Detailed discriptions of the observation instruments. The response time is taken as the time required for the measured signal to reach 95% of its final value. The long-term stability is specified as the maximum change in the zero signal and output span signal of the instrument under reference conditions within one year.
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DNI is the maximum available beam radiation that can be measured, which is defined as the amount of solar radiation received in a plane perpendicular to the incoming solar rays. DHI is the solar radiation scattered by dust, aerosols, and particles, received on a horizontal surface. GHI is the total amount of direct and diffuse solar radiation as calculated by the following equation (Sengupta et al., 2015):
where
$ \theta $ represents the zenith angle. Thus, it is possible that DNI is greater than GHI during certain periods when the zenith angle is large (e.g., morning, afternoon, or winter).In the absorption and diffusion (scattering) of solar radiation by atmospheric components, cloud cover plays an important role in the change in SSR. Hence, it is necessary to show a distinction between clear-sky and cloudy-sky conditions when we discuss the relationship between SSR and air pollution. Clear days are usually defined as days with an average cloud cover of less than 20% (Wu et al., 2012), or days with a sunshine time of more than 75% per day (Zheng et al., 2012).
The extraterrestrial solar radiation can be expressed by the following equation (Sheng et al., 2003):
where
$ S $ represents the extraterrestrial solar radiation;$ S_0 $ represents the solar constant (1367 W m−2);$ \bar{d} $ represents the mean Earth–sun distance;$ d $ represents the actual Earth–sun distance;$ \varphi $ represents the latitude (+ for Northern Hemisphere, − for Southern Hemisphere); and$ \delta $ and$ \omega $ are the solar declination and solar hour angles, respectively.To characterize clear sky conditions, a clearness index (
$ K_{\rm T} $ ) is defined by the following equation (Liu and Jordan, 1960):Previous studies have shown that there is no standard value of
$ K_{\rm T} $ to describe clear days, and a larger clearness index corresponds to less cloud cover. Different studies have adopted different methods to characterize these conditions. Cao et al. (2016) took$ K_{\rm T} $ to be greater than or equal to 0.5 as clear-sky conditions in China. In Nigeria, Okogbue et al. (2009) regarded$ K_{\rm T} $ to be greater than or equal to 0.6 to define clear skies, but Kuye and Jagtap (1992) defined clear skies as$ K_{\rm T} $ greater than or equal to 0.65, and Li et al. (2004) proposed a$ K_{\rm T} $ greater than or equal to 0.7 for clear days in Hong Kong. Hence, because of geographical factors, there is no consensus on the standard to describe clear-sky conditions by the value of$ K_{\rm T} $ . In this study, clear days are defined as days with a GHI at least half that of the corresponding extraterrestrial solar radiation [i.e.,$ K_{\rm T} $ is equal to or greater than 0.5, following Cao et al. (2016)].Scattered light is extremely vital to the process of photosynthesis. Therefore, the diffuse fraction is important in the research of both atmospheric physics and biological systems (Che et al., 2005). The diffuse fraction (
$ K_{\rm D} $ ) is an evaluating indicator of the proportion of DHI and is defined by the following equation (Liu and Jordan, 1960):In this study, the diffuse fraction is used to study the radiation scattered by aerosols, dust, and other particulate matter in clear skies.
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The Santa Barbara DISORT atmospheric radiative transfer (SBDART, version 2.4) model is a practical tool for plane-parallel radiative transfer in the Earth's atmosphere, which is applied in studies of atmospheric energy budget and satellite remote sensing (http://www.crseo.ucsb.edu/esrg/paulsdir/). The code is based on a marriage among reliable and developed physical models, which have been consistently developed over the past few decades (Kundu et al., 2018). The model contains all important physical processes, such as infrared, visible, and ultraviolet radiation (Ricchiazzi et al., 1998). In this study, the SBDART model is used to simulate GHI, DNI, and DHI with different visibilities under clear-sky conditions, and the performance of the model under different air pollution levels is analyzed. SBDART is a software tool that computes plane-parallel radiative transfer in clear and cloudy conditions within the Earth's atmosphere and at the surface.
The software package Optical Properties of Aerosols and Clouds (OPAC) was originally created for the purpose of an easy availability of spectral optical properties of aerosol particles (Hess et al., 1998). OPAC provides a comprehensive collection of refractive indexes of basic aerosol components over a wide wavelength from 0.25 μm to 40 μm, and allows the construction of aerosol types from several basic aerosol components. In this work, the optical characteristics of PM2.5 and PM10 are analyzed and compared by using the OPAC model, and the reasons why these two pollutants have different effects on SSR are further explained. OPAC is a software package that contains the optical properties in the solar and terrestrial spectral range of atmospheric particles, i.e., water droplets, aerosol, and ice crystals.
Observation instrument | Technical parameter | |||||
SKO MS-802F SKO MS-56 | Operating temperature (°C) | Spectral range (nm) | Spectral sensitivity | Response time | Long-term stability | Sensitivity (μV W−1 m−2) |
−40 to 80 | 280−3000 | <1% | <5 s | <0.5% | Approx. 7 | |
−40 to 80 | 200−4000 | <1% | <1 s | <0.5% | Approx. 10 | |
Thermo TEOM-1405 | Operating temperature (°C) | Measurement range (g m−3) | Resolution (μg m−3) | Accuracy | Main flow rate (min−1) | Bypass flow rate (min−1) |
−40 to 60 | 0−1 | 0.1 | ±0.75% | 3 | 13.67 | |
GSN-1 | Operating temperature (°C) | Measurement range (km) | Resolution (m) | Accuracy | − | − |
−40 to 50 | 0.01−35 | 100 | ±10% |