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The understanding of aerosol properties in troposphere, especially their behavior near the ground level, is indispensable for precise evaluation of their impact on the Earth’s radiation studies. Although a sunphotometer or a skyradiometer can provide the aerosol optical thickness (AOT), their application is limited to daytime under near cloud free conditions. In order to attain the multi-wavelength observation for both day- and night-time including cloudy conditions, here we propose a novel monitoring technique by means of simultaneous measurement using a nephelometer (450, 550, and 700 nm), an aethalometer (370, 470, 520, 590, 660, 880, and 950 nm), and a visibility meter (550 nm). On the basis of the multi-wavelength data of scattering and absorption coefficients from the nephelometer and aethalometer, respectively, first we calculate the real-time values of aerosol extinction coefficient in addition to the Angstrom exponent (AE). Then, correction of these values is carried out by comparing the resulting extinction coefficient with the corresponding value obtained from the optical data of visibility-meter. The major reason for this correction is the loss of relatively coarse particles due to the aerodynamic effect as well as evaporation of water content from particles during the sampling procedure. Then, with the ancillary data of vertical aerosol profile obtained with a lidar (532 nm), the temporal change of AOT is estimated. In this way, information from the sampling can be converted to the ambient properties in the atmospheric boundary layer. Furthermore, daytime data from a sunphotometer (368, 500, 675, and 778 nm) and a skyradiometer (340, 380, 400, 500, 675, 870, and 1020 nm) are used to validate the resulting AOT values. From the overall procedure, we can estimate the AE and AOT values from the sampling data, with uncertainties of approximately 5% for AE and 10% for AOT. Such a capability will be useful for studying aerosol properties throughout 24 hours regardless of the solar radiation and cloud coverage.

The importance of both direct and indirect effects of gaseous pollutants and aerosols to the atmosphere has been discussed in the context of the earth radiation budget and global climate change [

The most fundamental parameters that are used to describe the optical influence of aerosol particles are the extinction coefficient and optical thickness. For vertical measurements, aerosol extinction coefficient (AEC) is given as a function of altitude, z, and wavelength, λ, representing the light attenuation due to the combined effects of scattering and absorption [

To attain the multi-wavelength observation of aerosol optical thickness near the surface level regardless of the cloud coverage conditions, here we propose a novel monitoring technique based on measurements of scattering coefficient, absorption coefficient, and visibility for calculating AOT. To obtain such aerosol parameters, and concurrent measurement data from a multi-wavelength integrating nephelometer (scattering coefficient), a multi-wavelengthaethalometer (absorption coefficient), and a visibilitymeter are exploited. Such measurements are routinely conducted at the Center for Environmental Remote Sensing (CEReS), Chiba University, Japan. The validation of the resulting AOT, on the other hand, is carried out using the data of a sunphotometer, and also operated at CEReS. The continuous estimation of AOT near the surface is considered to be useful for studying sources and sinks of aerosol particles in relation to the monitoring of local environment [

The remaining part of this paper is organized as follows. In Section 2, the instrumentation is given, whereas in Section 3, the methodology is described. Section 4 gives the results and discussion, followed by the conclusion section.

All the sampling and optical data used in this study are obtained from instruments operated on the campus of Chiba University (35˚37'30''N and 140˚06'14''E). The university is in the mid of Chiba city, which in turn is located on the east coast of Tokyo Bay (

For the sampling measurement, a 3-m long vertical pipe made of stainless steel is used as an inlet of aerosol particles from the ambient atmosphere on the rooftop of an eight-story building of CEReS. An integrating nephelometer (TSI3563) provides the scattering coefficients measured at the three wavelengths of 450, 550, and 700 nm. An aethalometer (Magee, AE31) measures the black carbon (BC) concentration values at the seven wavelengths of 370, 470, 520, 590, 660, 880, and 950 nm. In addition to these sampling instruments, a visibility meter (Vaisala, PWD52) is operated to measure the meteorological visibility on the same rooftop, approximately 30 m above the surface level (50 m above sea level). The visibility value provided from this instrument is the value that has been converted to the wavelength of 550 nm, though its operational wavelength is 875 nm. The data from these instruments are employed to derive the AOT value

throughout day- and night time, with ancillary information from a dual wavelength (532 and 1064 nm) lidar of National Institute for Environmental Research (NIES), operated also on the Chiba University campus (NIES Chiba lidar). Besides, the supporting information on the local weather is obtained from a weather monitor (Davis, Vantage Pro). Also these instruments are routinely operated as CEReS facilities.

For the purpose of validating the resulting AOT, we exploit the data from a sun photometer (Prede, PSF-100) (368, 500, 675, and 778 nm) and a sky radiometer (Prede, POM-02) (340, 380, 400, 500, 675, 870, and 1020 nm). This latter instrument is operated as part of the SKYNET, an international network of radiation-measurement instruments used for recording and characterizing regional properties of aerosol, cloud, and solar radiation [

Here we explain the theoretical basis how the instrumental data near the surface level can be converted into the estimated values of AOT. The value of AEC, α e x t S ( λ ) is computed as [

α e x t S ( λ ) = α s c a ( λ ) + α a b s ( λ ) , (1)

where the superscript S indicates the value from the sampling measurement. The scattering coefficient, α s c a ( λ ) , can readily be obtained from the integrating nephelometer. The absorption coefficient, α a b s ( λ ) , can be calculated from the BC (black carbon) concentration data of the aethalometer as

α a b s ( λ ) = ( B C ( λ ) × 10 − 9 ) 6834 λ . (2)

Here the conversion factor of 6834 has been obtained by considering the correction due to multiple scattering effects on the fiber filter of the instrument [

q = − ln [ α e x t S ( λ 2 ) α e x t S ( λ 1 ) ] / ln ( λ 2 λ 1 ) . (3)

This equation can easily be extended for fitting the extinction values observed at more than two wavelengths. The value of q is of the order of unity, and the value becomes larger (smaller) for the dominance of fine-mode (coarse-mode) particles [

When calculating the AE from the sampling data, the following two corrections must be applied before the use of Equation (3). The first correction is usually called the truncation error, which accounts for the loss of signal intensity due to relatively coarse particles [

f ( R H , λ ) = σ ( R H , λ ) σ ( R H d r y , λ ) . (4)

Here σ ( R H , λ ) is the aerosol scattering cross-section at the wavelength λ under an ambient RH, whereas σ ( R H d r y , λ ) is that under the dry condition inside the instrument. Because of the hysteresis behavior of aerosol growth/evaporation process [

From the data of visibility meter, on the other hand, the extinction coefficient at 550 nm can be calculated by using the Koschmeider equation with the attenuation ratio of 5% [

α e x t O ( λ ) = K a V ( λ 550 ) − q − P T 0 T P 0 ( 1.095 × 10 − 5 ) ( λ 550 ) − 4.05 . (5)

Here, λ is in units of nanometer, the superscript O indicates the value from the optical measurement, and K_{a} = ln(1/0.05) = 2.996 is the Koschmieder coefficient [_{0} = 1013.25 hPa and T_{0} = 288.15 K).

The value of AOT can be calculated on the basis of α e x t O ( λ ) given by Equation (5). If we assume that the aerosol vertical distribution is given by a simple exponential profile with a scale height of h_{a}, the value of AOT ( τ a ) can readily be calculated as

τ a ( λ ) = α e x t O ( λ ) ∫ 0 ∞ exp ( − z h a ) d z = h a α e x t O ( λ ) . (6)

In the actual situation, the aerosol profile shows some deviation from this simple exponential formula. Thus, we employ the extinction profile observed with the vertical lidar (NIES lidar) to estimate the effective value of h_{a}. It is noted that the resulting value reflects the altitude dependence of AEC, but it is not sensitive to the choice of the lidar ratio (the ratio between the extinction and back-scattering coefficients) used for solving the lidar data. By combining Equations (5) and (6), we obtain

τ a ( λ ) = K a h a V ( λ 550 ) − q − P P 0 ( 9.88 × 10 − 3 ) ( λ 550 ) − 4.05 . (7)

Here the second term has been adopted from the approximation formula of Dutton et al. [

The proposed methodology is applied to two cases of observation periods of March 19-20, 2017 and May 19-21, 2017, representing relatively low and high RH, respectively. The temporal changes of ambient RH and RH inside the nephelometer are plotted for these two cases in

ambient value of the aerosol scattering coefficient has been calculated as the difference between the extinction coefficient from the visibility meter and the absorption coefficient from the aethalometer, while the dry scattering coefficient is obtained directly from the scattering coefficient of the nephelometer. This correction factor is applied only to the higher RH: no f(RH) correction is considered to the low RH case in March 19-20, 2017.

small as compared with the optically measured value. The AEC values after correcting the truncation error are shown in

case observed on May 19-21, 2017. The original values of AEC are shown in

In _{a}, from the vertical lidar data shown in _{a} is determined by the vertical profile of AEC, and the value is not critically dependent on the assumed value of the lidar ratio.

In the low RH case during March 19-20, 2017, the AOT curves resulted from the coupled analysis of the sampling and visibility-meter data show similar behavior to the skyradiometer, indicating relatively stable condition in relation to low RH. On March 19, the value of AOT at 500 nm from the optical measurement (skyradiometer and sunphotometer,

by ~0.1 than that estimated from the sampling (

The temporal variation of AOT in the case of high RH (

accuracy for AOT. Such a capability will be useful for studying aerosol properties throughout 24-hours regardless of the solar radiation and cloud coverage.

A novel methodology of estimating AOT from continuous data of ground-based sampling instruments (an integrating nephelometer and an aethalometer) and a visibility meter have been proposed and demonstrated. The vertical profile of AEC has been derived from an ancillary data observed with a Mie-scattering lidar. In order to convert the value of aerosol scattering coefficient measured with a nephelometer to ambient value in the atmospheric boundary layer, the correction of the truncation error has to be applied. The magnitude of this error has been successfully evaluated by comparing the nephelometer (sampling) data and visibility-meter (optical) data. Under high RH conditions, an additional correction that arises from the evaporation of hygroscopic particles inside the instrument (nephelometer) has to be taken into account. In the present work, we have evaluated the value off (RH) from the comparison between the visibility-meter-derived AEC and the raw data of the sampling measurement. The temporal change of the estimated AOT has been compared with the observed value from either a sunphotometer or a skyradiometer. It has been found that relatively stable estimation is feasible for the case of relatively low RH situation, though more fluctuating behavior of AOT is seen for relatively high RH case. The present approach will be generally useful to estimate the optical properties of ambient aerosols on the basis of ground-based sampling data. The capability of uninterrupted estimation of AOT will provide new insight in the source and sink investigation of aerosols as well as in monitoring local environment.

The first author (JA) would like to thank to Ministry of Research, Technology, and Higher Education Republic of Indonesia for supporting fellowship named Beasiswa Dikti.

Aminuddin, J., Okude, S., Lagrosas, N., Manago, N. and Kuze, H. (2018) Real Time Derivation of Atmospheric Aerosol Optical Properties by Concurrent Measurements of Optical and Sampling Instruments. Open Journal of Air Pollution, 7, 140-155. https://doi.org/10.4236/ojap.2018.72008