scholarly journals Analysis of relationships between ultraviolet radiation (295–385 nm) and aerosols as well as shortwave radiation in North China Plain

2008 ◽  
Vol 26 (7) ◽  
pp. 2043-2052 ◽  
Author(s):  
X. Xia ◽  
Z. Li ◽  
P. Wang ◽  
M. Cribb ◽  
H. Chen ◽  
...  
Keyword(s):  
Uv Radiation ◽  
Zenith Angle ◽  
Solar Zenith Angle ◽  
Precipitable Water ◽  
Ground Level ◽  
Weather Conditions ◽  
Northern China ◽  
Shortwave Radiation ◽  
Relevant Variables

Abstract. The fraction of ultraviolet (UV) radiation to broadband shortwave (SW) radiation (FUV=UV/SW) and the influences of aerosol, precipitable water vapor content (PWV) and snow on FUV were examined using two year's worth of ground-based measurements of relevant variables in northern China. The annual mean FUV was 3.85%. Larger monthly values occurred in summer and minimum appeared in winter. Under cloudless condition, FUV decreased linearly with τ500 nm and the resulting regression indicated a reduction of about 26% in daily FUV per unit τ500 nm, implying that aerosol is an efficient agent in lowering the ground-level UV radiation, especially when the sun is high. Given that the annual mean τ500 nm is 0.82, aerosol induced reduction in surface UV radiation was from 24% to 74% when the solar zenith angle (θ) changed from 20° to 80°. One cm of PWV led to an increase of about 17% in daily FUV. One case study of snow influence on surface irradiance showed that UV and SW radiation increased simultaneously when the ground was covered with snow, but SW radiation increased much less than UV radiation. Accordingly, FUV increased by 20% for this case. Models were developed to describe the dependence of instantaneous UV radiation on aerosol optical depth, the cosine of the solar zenith angle (μ), and clearness index (Kt) under both clear and all-weather conditions.

scholarly journals Relationship between the NO<sub>2</sub> photolysis frequency and the solar global irradiance

2009 ◽  
Vol 2 (4) ◽  
pp. 1537-1573 ◽  
Author(s):  
I. Trebs ◽  
B. Bohn ◽  
C. Ammann ◽  
U. Rummel ◽  
M. Blumthaler ◽  
...  
Keyword(s):  
Zenith Angle ◽  
Solar Zenith Angle ◽  
Field Experiments ◽  
Global Radiation ◽  
Ground Level ◽  
Field Studies ◽  
Atmospheric Conditions ◽  
Global Irradiance ◽  
Tropical Environments ◽  
Photolysis Frequency

Abstract. Representative values of the atmospheric NO2 photolysis frequency, (j(NO2)), are required for the adequate calculation and interpretation of NO and NO2 concentrations and exchange fluxes near the surface. Direct measurements of j(NO2) at ground level are often not available in field studies. In most cases, modeling approaches involving complex radiative transfer calculations are used to estimate j(NO2) and other photolysis frequencies for air chemistry studies. However, important input parameters for accurate modeling are often missing, most importantly with regard to the radiative effects of clouds. On the other hand, solar global irradiance ("global radiation", G) is nowadays measured as a standard parameter in most field experiments and in many meteorological observation networks around the world. A linear relationship between j(NO2) and G was reported in previous studies and has been used to estimate j(NO2) from G in the past 30 years. We have measured j(NO2) using spectro- or filter radiometers and G using pyranometers side-by-side at several field sites. Our results cover a solar zenith angle range of 0–90°, and are based on nine field campaigns in temperate, subtropical and tropical environments during the period 1994–2008. We show that a second-order polynomial function (intercept=0): j(NO2)=(1+α)×(B1×G+B2×G2), with α defined as the site-dependent UV-A surface albedo and the polynomial coefficients (including uncertainty ranges): B1=(1.47±0.03)×10−5 W−1 m2 s−1 and B2=(−4.84±0.31)×10−9 W−2 m4 s−1 can be used to estimate ground-level j(NO2) directly from G, independent of solar zenith angle under all atmospheric conditions. The absolute j(NO2)↓ residual of the empirical function is ±6×10−4 s−1 (95.45% confidence level). The relationship is valid for sites below 800 m a.s.l. and under low background albedo conditions. It is not valid in alpine regions, above snow or ice and sandy or dry soil surfaces. Our function can be applied to estimate chemical life times of the NO2 molecule with respect to photolysis, and is useful for surface-atmosphere exchange and photochemistry studies close to the ground, e.g., above fields with short vegetation and above forest canopies.

scholarly journals Rethinking the correction for absorbing aerosols in the satellite-based surface UV products

2021 ◽  
Author(s):  
Antti Arola ◽  
William Wandji Nyamsi ◽  
Antti Lipponen ◽  
Stelios Kazadzis ◽  
Nickolay A. Krotkov ◽  
...  
Keyword(s):  
Optical Depth ◽  
Uv Radiation ◽  
Correction Factor ◽  
Zenith Angle ◽  
Solar Zenith Angle ◽  
Retrieval Algorithm ◽  
Angle Dependence ◽  
Aerosol Absorption ◽  
Uv Irradiance ◽  
Absorbing Aerosols

Abstract. Satellite estimates of surface UV irradiance have been available since 1978 from TOMS UV spectrometer and continued with significantly improved ground resolution using Ozone Monitoring Instrument (OMI 2004-current) and Sentinel 5 Precursor (S5P 2017-current). The surface UV retrieval algorithm remains essentially the same: it first estimates the clear-sky UV irradiance based on measured ozone and then accounts for the attenuation by clouds and aerosols applying two consecutive correction factors. When estimating the total aerosol effect in surface UV irradiance, there are two major classes of aerosols to be considered: 1) aerosols that only scatter UV radiation and 2) aerosols that both scatter and absorb UV radiation. The former effect is implicitly included in the measured effective Lambertian Equivalent scene reflectivity (LER), so the scattering aerosol influence is estimated through cloud correction factor. Aerosols that absorb UV radiation attenuate the surface UV radiation more strongly than non-absorbing aerosols of the same extinction optical depth (AOD). Moreover, since these aerosols also attenuate the outgoing satellite-measured radiance, the cloud correction factor that treats these aerosols as purely scattering underestimates their AOD causing underestimation of LER and overestimation of surface UV irradiance. Therefore, for correction of aerosol absorption additional information is needed, such as the UV absorbing Aerosol Index (UVAI) or a model-based monthly climatology of aerosol absorption optical depth (AAOD). A correction for absorbing aerosols was proposed almost a decade ago and later implemented in the operational OMI and TROPOMI UV algorithms. In this study, however, we show that there is still room for an improvement to better account for the solar zenith angle dependence and non-linearity in the absorbing aerosol attenuation and as a result we propose an improved correction scheme. There are two main differences between the new proposed correction and the one that is currently operational in OMI and TROPOMI UV-algorithms. First, the currently operational correction for absorbing aerosols is a function of AAOD only, while the new correction takes additionally the solar zenith angle dependence into account. Second, the 2nd order polynomial of the new correction takes better into account the non-linearity in the correction as a function of AAOD, if compared to the currently operational one, and thus better describes the effect by absorbing aerosols over larger range of AAOD. To illustrate the potential impact of the new correction in the global UV estimates, we applied the current and new proposed correction for global fields of AAOD from the aerosol climatology currently used in OMI UV algorithm, showing a typical differences of ±5 %. This new correction is easy to implement operationally using information of solar zenith angle and existing AAOD climatology.

scholarly journals Rethinking the correction for absorbing aerosols in the OMI- and TROPOMI-like surface UV algorithms

2021 ◽  
Vol 14 (7) ◽  
pp. 4947-4957
Author(s):  
Antti Arola ◽  
William Wandji Nyamsi ◽  
Antti Lipponen ◽  
Stelios Kazadzis ◽  
Nickolay A. Krotkov ◽  
...  
Keyword(s):  
Optical Depth ◽  
Uv Radiation ◽  
Correction Factor ◽  
Zenith Angle ◽  
Solar Zenith Angle ◽  
Retrieval Algorithm ◽  
Ozone Monitoring Instrument ◽  
Aerosol Absorption ◽  
Uv Irradiance ◽  
Absorbing Aerosols

Abstract. Satellite estimates of surface UV irradiance have been available since 1978 from the TOMS UV spectrometer and have continued with significantly improved ground resolution using the Ozone Monitoring Instrument (OMI 2004–current) and Sentinel 5 Precursor (S5P 2017–current). The surface UV retrieval algorithm remains essentially the same: it first estimates the clear-sky UV irradiance based on measured ozone and then accounts for the attenuation by clouds and aerosols, applying two consecutive correction factors. When estimating the total aerosol effect in surface UV irradiance, there are two major classes of aerosols to be considered: (1) aerosols that only scatter UV radiation and (2) aerosols that both scatter and absorb UV radiation. The former effect is implicitly included in the measured effective Lambertian-equivalent scene reflectivity (LER), so the scattering aerosol influence is estimated through cloud correction factor. Aerosols that absorb UV radiation attenuate the surface UV radiation more strongly than non-absorbing aerosols of the same extinction optical depth. Moreover, since these aerosols also attenuate the outgoing satellite-measured radiance, the cloud correction factor that treats these aerosols as purely scattering underestimates their aerosol optical depth (AOD), causing underestimation of LER and overestimation of surface UV irradiance. Therefore, for correction of aerosol absorption, additional information is needed, such as a model-based monthly climatology of aerosol absorption optical depth (AAOD). A correction for absorbing aerosols was proposed almost a decade ago and later implemented in the operational OMI and TROPOMI UV algorithms. In this study, however, we show that there is still room for improvement to better account for the solar zenith angle (SZA) dependence and nonlinearity in the absorbing aerosol attenuation, and as a result we propose an improved correction scheme. There are two main differences between the new proposed correction and the one that is currently operational in OMI and TROPOMI UV algorithms. First, the currently operational correction for absorbing aerosols is a function of AAOD only, while the new correction additionally takes the solar zenith angle dependence into account. Second, the second-order polynomial of the new correction takes the nonlinearity in the correction as a function of AAOD better into account, if compared to the currently operational one, and thus better describes the effect by absorbing aerosols over a larger range of AAOD. To illustrate the potential impact of the new correction in the global UV estimates, we applied the current and new proposed correction for global fields of AAOD from the aerosol climatology currently used in OMI UV algorithm, showing a typical differences of ±5 %. This new correction is easy to implement operationally using information of solar zenith angle and existing AAOD climatology.

scholarly journals Relationship between the NO<sub>2</sub> photolysis frequency and the solar global irradiance

2009 ◽  
Vol 2 (2) ◽  
pp. 725-739 ◽  
Author(s):  
I. Trebs ◽  
B. Bohn ◽  
C. Ammann ◽  
U. Rummel ◽  
M. Blumthaler ◽  
...  
Keyword(s):  
Zenith Angle ◽  
Solar Zenith Angle ◽  
Surface Albedo ◽  
Field Experiments ◽  
Global Radiation ◽  
Ground Level ◽  
Field Studies ◽  
Atmospheric Conditions ◽  
Global Irradiance ◽  
Photolysis Frequency

Abstract. Representative values of the atmospheric NO2 photolysis frequency j(NO2) are required for the adequate calculation and interpretation of NO and NO2 concentrations and exchange fluxes near the surface. Direct measurements of j(NO2) at ground level are often not available in field studies. In most cases, modeling approaches involving complex radiative transfer calculations are used to estimate j(NO2) and other photolysis frequencies for air chemistry studies. However, important input parameters for accurate modeling are often missing, most importantly with regard to the radiative effects of clouds. On the other hand, solar global irradiance ("global radiation", G) is nowadays measured as a standard parameter in most field experiments and in many meteorological observation networks around the world. Previous studies mainly reported linear relationships between j(NO2) and G. We have measured j(NO2) using spectro- or filter radiometers and G using pyranometers side-by-side at several field sites. Our results cover a solar zenith angle range of 0–90°, and are based on nine field campaigns in temperate, subtropical and tropical environments during the period 1994–2008. We show that a second-order polynomial function (intercept = 0): j(NO2)=(1+α)× (B1×G+B2×G2), with α defined as the site-dependent UV-A surface albedo and the polynomial coefficients: B1=(1.47± 0.03)×10-5 W−1 m2 s−1 and B2=(-4.84±0.31)×10-9 W−2 m4 s−1 can be used to estimate ground-level j(NO2) directly from G, independent of solar zenith angle under all atmospheric conditions. The absolute j(NO2) residual of the empirical function is ±6×10-4 s−1(2σ). The relationship is valid for sites below 800 m a.s.l. and with low surface albedo (α<0.2). It is not valid in high mountains, above snow or ice and sandy or dry soil surfaces.

Sediment transport under dry weather conditions in a small sewer system

1998 ◽  
Vol 37 (1) ◽  
pp. 155-162
Author(s):  
Flemming Schlütter ◽  
Kjeld Schaarup-Jensen
Keyword(s):  
Sediment Transport ◽  
Field Data ◽  
Field Measurements ◽  
Weather Conditions ◽  
Sewer System ◽  
Transport Models ◽  
Sewer Systems ◽  
Combined Sewer ◽  
Initial Results

Increased knowledge of the processes which govern the transport of solids in sewers is necessary in order to develop more reliable and applicable sediment transport models for sewer systems. Proper validation of these are essential. For that purpose thorough field measurements are imperative. This paper renders initial results obtained in an ongoing case study of a Danish combined sewer system in Frejlev, a small town southwest of Aalborg, Denmark. Field data are presented concerning estimation of the sediment transport during dry weather. Finally, considerations on how to approach numerical modelling is made based on numerical simulations using MOUSE TRAP (DHI 1993).

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