Special Session on Optical Properties of Atmospheric Aerosol ― Observation, Measurement Techniques and Model Analysis for Improving the Accuracy of Aerosol Light Absorption Determinations in Polluted Sites (II)

Gang Zhao 1,3, Yingli Yu1, Ping Tian2, Jing Li1, Song Guo3, Chunsheng Zhao This email address is being protected from spambots. You need JavaScript enabled to view it.1

Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing 100871, China
Beijing Key Laboratory of Cloud, Precipitation and Atmospheric Water Resources, Beijing 100089, China
College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China


 

Received: November 22, 2019
Revised: February 4, 2020
Accepted: April 2, 2020

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.


Download Citation: ||https://doi.org/10.4209/aaqr.2019.10.0500  


Cite this article:

Zhao, G., Yu, Y., Tian, P., Li, J., Guo, S. and Zhao, C. (2020). Evaluation and Correction of the Ambient Pparticle Sectral Light Absorption Measured Using a Filter-based Aethalometer. Aerosol Air Qual. Res. 20: 1833–1841. https://doi.org/10.4209/aaqr.2019.10.0500


HIGHLIGHTS

  • A multiple-scattering compensation factor of 2.90 is recommended in Eastern China.
  • The absorption coefficient with tape 8060 is more consistent than that with tape 8050.
  • A novel method is proposed to correct the data measured using AE33 with tape 8050.
 

ABSTRACT


Spectral light-absorption properties measured with an Aethalometer (AE; Model AE33; Magee Scientific) are widely used in radiative forcing studies and source appointment in China. However, considerable uncertainty regarding the measured absorption coefficient (σabs) exists because of the multiple-scattering effects, loading effects, and differences in filter tape. This study evaluated σabs by comparing the values measured with an AE33 using Tape 8050, an AE33 using Tape 8060 (which differs from Tape 8050 in material), and a three-wavelength photoacoustic soot spectrometer (PASS-3) during two field campaigns in eastern China. The results indicated that the AE33-measured σabs using either tape exceeded the PASS-3-measured value by approximately three times, mainly owing to the multiple-scattering effect. A wavelength-independent multiple-scattering compensation factor (2.90), which varies slightly (± 0.04) for eastern China, is recommended for these regions. When σabs was measured with the AE33 using Tape 8050, the value highly depended on the loading on the tape, which led to significant uncertainty and discontinuity in the absorption Ångström exponent compared to using Tape 8060. A method was proposed to effectively correct the historical datasets of σabs and the absorption Ångström exponent by using the AE33 with Tape 8050. This work provides insight into the quality of measured absorption data when filter-based measurement technology is applied.


Keywords: Black carbon; Absorption; Multiple-scattering compensation; Loading effects.


INTRODUCTION


Light-absorbing particles exert considerable effects on aerosol radiative force by absorbing radiation, changing the aerosol single-scattering albedo and interacting with clouds (Koch and Del Genio, 2010; Bond et al., 2013). Black carbon (BC) dominates the light absorption of atmospheric particles (Saturno et al., 2017). Ice and snow albedos can be markedly changed when BC particles are deposited on them (Bond et al., 2013), which leads to great uncertainty when estimating aerosol climate effects. Absorbing aerosols can also influence the environment by suppressing turbulence in the atmospheric boundary layer (Wilcox et al., 2016).

Considerable uncertainty may also arise regarding radiative effects when the light-absorbing ability is measured at one wavelength or over a limited range because ambient aerosol light absorption is dependent on wavelength. The absorption Ångström exponent (αabs) (Ångström, 1929) is used to describe the ae rosol light-absorption property as follows: 

where σabs(λ1) and σabs(λ2) are the absorption coefficient at wavelength λ1 and λ2 respectively. Different characteristics of wavelength dependence are observed for different aerosol types (Schmid et al., 2006). The theoretical αabs values of BC from the fuel is 1.0 (Martinsson et al., 2017) whereas some organic absorption classes have αabs values spanning a wide range between 3.5 and 7.0 (Schmid et al., 2006). Thus, spectral light-absorption information can provide knowledge regarding the source and aging process of ambient aerosols (Zotter et al., 2017).

Many methods have been developed to measure spectral light-absorbing properties, and these methods have been well summarized and described by Bond et al. (2013) and Petzold et al. (2013). Filter-based instruments, such as Aethalometers (AEs) (Hansen et al., 1984), Particle Soot Absorption Photometers (Bond et al., 1999) and Multi-Angle Absorption Photometers (Petzold and Schönlinner, 2004), are widely used for both ground measurements and airborne studies because they are easy to operate (Coen et al., 2010). The σabs at different wavelengths is obtained by measuring the attenuation of light when it passes through an aerosol-laden filter. AEs are the most frequently used commercial instruments to measure ambient aerosol σabs. However, it is necessary to post-process AE-measured data to compensate for the well-known multiple-scattering effects and loading effects (Bond et al., 1999; Coen et al., 2010). Ample effort has been devoted to the methods of correcting measured σabs (Coen et al., 2010; Virkkula et al., 2015; Saturno et al., 2017). Recently, a new dual-spot Aethalometer (AE33), which measures σabs at seven different wavelengths from 370 nm to 950 nm with real-time loading effects compensation, was developed (Drinovec et al., 2015).

The official filter tape of AE33 has been changed from Model 8050 to 8060 because the nonlinear filter-loading effect from Tape 8050 causes dysfunction in the compensation process. Although AE33 with Tape 8050 has long been used for measurement worldwide, the performance of measured σabs is not well characterized. It is widely accepted that the uncorrected σabs data from AE33 is too high and that correction of the multiple-scattering effect is necessary (Moosmüller et al., 1998; Saturno et al., 2017; Sharma et al., 2017). Moreover, the σabs measured with Tape 8050 is unstable, and the loading effects of the measured σabs are not satisfactorily corrected, which was confirmed in this study. Furthermore, there have been more than 30 AE33 instruments employed to measure ambient σabs in China since 2015 (personal communication), and these data are used to infer information regarding BC emissions in China. However, these data require reconstruction to accurately reveal the light-absorption properties of ambient aerosol.

In this study, we evaluated measured σabs by using AE33 with Tape 8050 and Tape 8060 from two field measurements in eastern China. Based on the inter-comparison of the measured σabs from AE33 and a three-wavelength photoacoustic soot spectrometer (PASS-3), a wavelength-independent multiple-scattering correction factor (Cf) value was recommended for eastern China. Using the same correction factor of 2.9 for eastern China makes it easier to reconstruct the historical observation data measured by AE33.


MEASUREMENTS AND INSTRUMENTS



Measurement Sites

The field measurements were conducted at two sites: Peking University (PKU) and Taizhou. The suburban measurement at site Taizhou (119°57ʹE, 32°35ʹN), as shown in Fig. 1(a), lies at the south end of the Jianghuai Plain in eastern China. It is located approximately 118 km southeast of the megacity Nanjing and approximately 200 km northeast of another megacity, Shanghai. The industrial area between Nanjing and Shanghai has experienced severe pollution over the past 30 years. The average aerosol optical depth at the wavelength of 550 nm during 2017, as measured by the Moderate Resolution Imaging Spectroradiometer, indicates that the measurement site is more polluted than the surrounding area (Fig. 1(b)). Taizhou is a representative location polluted mainly by secondary organic aerosol and biomass burning in eastern China (Zhao et al., 2019a). The PKU site is located northwest of Beijing, between the Fourth and Fifth Ring Road. It is 11 km away from the center of the Beijing, which is surrounded by Hebei Province and the megacity Tianjin. The Beijing-Tianjin-Hebei region suffers from heavy air pollution due to various industrial activities. As revealed in Fig. 1(b), the PKU site is on the edge of the heavily polluted area of the North China Plain. Datasets for this location are representative of the urban aerosol pollution in the North China Plain (Guo et al., 2012; Guo et al., 2013; Tang et al., 2018; Zhao et al., 2018; Zhao et al., 2019b).


Fig. 1. Measurement sites of Taizhou and PKU (marked with a hexagonal star). Filled colors represent (a) the topography of eastern China and (b) the average aerosol optical depth at 550 nm during 2017 from the Moderate Resolution Imaging Spectroradiometer on board the Aqua satellite.Fig. 1.
 Measurement sites of Taizhou and PKU (marked with a hexagonal star). Filled colors represent (a) the topography of eastern China and (b) the average aerosol optical depth at 550 nm during 2017 from the Moderate Resolution Imaging Spectroradiometer on board the Aqua satellite.

Measurements at the PKU site were recorded in the spring (March 20–April 30) and in the autumn (October 10–19) of 2018, whereas observations at the Taizhou site took place in the summer (May 22–June 17) in 2018. For both sites, all of the instruments were placed in a container, in which the temperature was well controlled within 24 ± 2°C. The sample air was collected using a PM10 impactor (Model SSI2.5; Mesa Labs) mounted on top of the container. Next, the sample air was passed through a Nafion dryer tube. The relative humidity of the sample particles easured with each instrument was controlled below 30%.

 
Instruments

The instruments used for the two field measurements are listed in Table S1. The corresponding time periods when data were available are marked in blue. The σabs was measured by the PASS-3 at three wavelengths (405, 532, and 781 nm) and by two AE33s with different types of filter tape at seven wavelengths (370, 470, 520, 590, 660, 880, and 950 nm). The two AE33s had the serial numbers 0345 and 0582 and were labeled as AE33-345 and AE33-582, respectively. The aerosol scattering coefficient (σsca) was measured using an Aurora 3000 nephelometer (Müller et al., 2011) at wavelengths of 450, 525, and 635 nm. Data correction was performed following Müller et al. (2011). The aerosol particle number size distribution (PNSD), ranging from 20 nm to 10 μm, was measured by using the Scanning Mobility Particle Size spectrometer (SMPS; Model 3936; TSI Inc.) and Aerodynamic Particle Sizer (APS; Model 3321; TSI Inc.). Because the SMPS and APS provided the aerosol PNSD data every 5 minutes, all σabs and σsca data were averaged with the same time resolution of 5 minutes for further study.


PASS-3

The principle of PASS-3 has been described elsewhere (Terhune and Anderson, 1977; Utry et al., 2014; Nakayama et al., 2015; Cremer et al., 2017; Davies et al., 2018), and thus only a brief introduction is presented here. First, the PASS-3 continuously pumps the sample aerosol into an acoustic resonator, and these absorbing aerosols are irradiated by laser lights at 405, 532 and 781 nm. The laser is an amplitude-modulated square wave with a frequency of 1500 Hz. The absorbing aerosols absorb light energy and then generate periodic pressure waves in the resonator. Finally, σabs is retrieved from the amplitude of the pressure wave by using a microphone. The PASS-3 installs a scattering integrating sphere to measure the aerosol σsca concurrently with the σabs.

The PASS-3 is capable of measuring the σscaσabs, and the extinction coefficient (σext) independently for high aerosol concentrations, which makes it easy to calibrate (Terhune and Anderson, 1977; Davies et al., 2018). In our study, σsca of the integrating sphere was calibrated using pure scattering aerosol ammonium sulfate, and σabs was calibrated using Aquadag soot particles by subtracting σext from σsca. The PASS-3 can provide σabsdata with high accuracy if it is properly calibrated (Davies et al., 2018). The uncertainty of the absorption measurements was estimated to be 4%, 8%, and 11% from 405, 532, and 781 nm, respectively (Nakayama et al., 2015). Several studies have used σabs data measured by a PASS as reference to evaluate the σabs measured by a filter-based instrument (Arnott et al., 2005; Schmid et al., 2006; Lack et al., 2014; Utry et al., 2014).

Here, calibration of the PASS-3 was conducted at the beginning of field observations at the PKU site, and at the beginning and near the end of the observations at the Taizhou site. The status of the instrument did not exhibit considerable change, and the variations of the calibrated parameters were within 3%. Thus, the calibration results at the beginning of the measurement were used to derive the observations for each site.

 
AE

The AE is a well-designed instrument that is used to measure the light intensity transmitted through a fiber filter on which the aerosols are deposited (Masey et al., 2020). The filter attenuation (ATN) is calculated as: 

where I and I0 are the concurrently measured light intensities that pass through the fiber with and without aerosol loading, respectively. The aerosol attenuation coefficient σATN,0 can be calculated as follows: 

where ΔATN is the change in ATN over time period ∆t; A is the sample spot area, and F is the aerosol flow rate. The σATN,0 values are corrected as σATN after filter-loading correction. The AE33 model succeeds in correcting the loading effects by performing real-time measurements of two parallel spots at different flow rates. More details on correcting the loading effects by using AE33 are provided by Drinovec et al. (2015). In this study, the corrected attenuation coefficient σATN from the AE33 was compared with the σabs measured using the PASS-3.

 
RESULTS AND DISCUSSION



Measurement and Model of the Aerosol Scattering Coefficient

The PASS-3 is capable of measuring aerosol σsca and σabs concurrently. We first proved the reliability of the measured σsca by using the PASS-3, after which the reliability of the measured σabs by the PASS-3 was guaranteed. The σsca values measured by the PASS-3, measured by a nephelometerand calculated using the scattering model with measured aerosol PNSD were compared first. The data used were obtained from the measurements at the PKU site. Comparison within the Taizhou site yielded results that were almost the same.

The Mie scattering model (Bohren and Huffman, 2007) was employed to calculate the σsca at different wavelengths. When running the Mie scattering model, the aerosol PNSD and BC mass concentration were required. The BC mass concentrations were calculated using the corrected σabs from the AE33 at a wavelength of 880 nm, with the assumption that the mass absorption coefficient was 7.77 m2 g−1. All of the BC particles were also assumed to have mixed internally with other aerosol components. The refractive indices of BC and non-light-absorbing aerosols are 1.8 + 0.54i (Wex et al., 2002; Kuang et al., 2015) and 1.53 + 10−7i (Wex et al., 2002) respectively. With this information, σsca at different wavelengths can be calculated. More details regarding this calculation process can be found in Zhao et al. (2018).

The results of inter-comparisons between the σsca values measured by the nephelometer and those calculated using the Mie scattering model are shown in Fig. S1. We observed strong consistency between the measured and calculated σsca values (R2 = 0.99 for different wavelengths), which indicated that the aerosol PNSD and σsca measurements by the nephelometer were reliable. Because the σsca values measured using the nephelometer in the present study were raw measured data, and truncation error corrections were not conducted, the calculated σsca values were always slightly larger than the measured values, with regressed larger regression slope than unity for each wavelength.

The measured σsca values using the PASS-3 and the nephelometer at similar wavelengths were compared. The measured σsca were adjusted to the same wavelength using the scattering Ångström exponent. Our findings are presented in Fig. S2 and show that great consistency was achieved between σsca values at 525 nm from the nephelometer and σsca from the PASS-3. The slope (1.06) was slightly higher than 1.0, which was mainly caused by the different truncation errors between nephelometer and PASS-3. The σsca values measured at other wavelengths had almost the same trends (R2 > 0.99). Our inter-comparison of the σsca values measured by the PASS-3 and nephelometer demonstrated that the measurements of both instruments were reliable.

Finally, σsca values were measured by PASS-3 and calculated using the Mie scattering model at wavelengths of 405, 532, and 781 nm; the results were compared and presented in Fig. 2. Notably, the measured and calculated σsca values exhibited strong consistency. The relative difference between them was primarily less than 20%.


Fig. 2. Comparison of the σsca measured using the PASS-3 and calculated using the Mie scattering model at wavelength of (a) 405 nm, (b) 532 nm and (c) 781 nm. The dashed lines in each panel represent 1:1 and a relative difference of 20%.Fig. 2.
 Comparison of the σsca measured using the PASS-3 and calculated using the Mie scattering model at wavelength of (a) 405 nm, (b) 532 nm and (c) 781 nm. The dashed lines in each panel represent 1:1 and a relative difference of 20%.

Overall, the closures achieved between the measured σsca values by the PASS-3, the measured σsca values by the nephelometer and the calculated σsca values by the Mie scattering model demonstrated that the PASS-3 provided σsca values with high accuracy. Therefore, the measured σabs by the PASS-3 was reliable. We thus used the measured σabs by the PASS-3 as a reference for the other instruments.


σATN Properties of Different Filter Tapes in AE33 Instruments

Two AE33 instruments (AE33-345 and AE33-582) were used to measure σATN using Filter Tapes 8050 and 8060, respectively. Notably, the σATNvalues obtained from the AE33 used in the present study were different from the σabs values by a factor of Cf. As shown in Table S1, four inter-comparisons were conducted to evaluate the performance of Filter Tapes 8050 and 8060. The σATN values measured from the AE33-345 and AE33-582 were compared using (1) Tape 8060 and Tape 8060 from March 20 to April 24; (2) Tape 8050 and Tape 8060 from March 24 to April 30; (3) Tape 8060 and Tape 8050 from October 10 to October 15; and (4) Tape 8050 and Tape 8050 from October 15 to October 19.

Figs. 3(a)3(b), 3(c) and 3(d) present a comparison of the measured σATN values in Table S1 at 880 nm for Groups (1), (2), (3), and (4), respectively. According to Fig. 3(a), the σATN values from the two instruments with Tape 8060 exhibited strong agreement (R2 = 0.99, slope = 1.08). Moreover, almost all of the σATN values from the two instruments had a relative difference of less than 20%. However, the consistency between σATNvalues from the two instruments decreased when one of the instruments switched to Tape 8050 (R2 = 0.95 and 0.97 for Test Groups [2] and [3], respectively). The slopes of 1.13 and 1.12 demonstrate that the σATN values measured by the AE33-582 is about 1.1 times that of measured value by the AE33-345. However, when both instruments switched to Tape 8050, the consistency between σATN values worsened (R2 = 0.87); in many cases, the relative difference between the measured σATN values was greater than 20%. Overall, the relative difference in σATN values between the two instruments did not indicate a clear relationship with the ATN values. Thus, the stability of the measured σATN values were not improved by changing the tape more frequently.


Fig. 3. Comparison of the σabs at 880 nm measured using the AE33. The σabs values on the x and y axes were measured using the AE33-345 and AE33-582 respectively. Each panel shows comparisons of using (a) Tape 8060 versus Tape 8060, (b) Tape 8050 versus Tape 8060, (c) Tape 8060 versus Tape 8050 and (d) Tape 8050 versus Tape 8050.Fig. 3.
 Comparison of the σabs at 880 nm measured using the AE33. The σabs values on the x and y axes were measured using the AE33-345 and AE33-582 respectively. Each panel shows comparisons of using (a) Tape 8060 versus Tape 8060, (b) Tape 8050 versus Tape 8060, (c) Tape 8060 versus Tape 8050 and (d) Tape 8050 versus Tape 8050.

Our inter-comparison results suggested that the σATN values measured by Tape 8060 was more stable than that measured by Tape 8050. Nonetheless, a relative difference of 10% still existed among different AE instruments.

The αabs value was calculated using Eq. (1) with the σATNvalues at 470 nm and 950 nm measured using the AE33, which can be used as the source attribution for traffic and wood burning (Martinsson et al., 2017; Zotter et al., 2017). 

The discussion in Section 3.3 reveals that the calculated αabs values from σATN and σabs were the same. The measured αabs values at different accumulated ATNs and on different days using Tape 8050 and Tape 8060 are shown in Figs. 4(a) and 4(b), respectively. The calculated αabs values from Tape 8060 (Fig. 4(b)) did not exhibit a clear relationship with accumulated ATN, and the variations in αabs were mainly a result of the diversity in ambient aerosol absorption properties. However, the αabs from Tape 8050 was highly correlated with the ATN values shown in Fig. 4(a). When the ATN changed from 22–60 to 120, the mean αabs values changed from 1.6–1.8 to 2.0, respectively. However, the variation in αabs was approximately 0.6, which is almost the same degree of αabs variation caused by different ATNs when using Tape 8050. Therefore, the αabs value measured using the AE33 using Tape 8050 was associated with great uncertainty.


Fig. 4. Measured αabs values at different ATNs for (a) Tape 8050 and (b) Tape 8060 at the PKU site. The dots with almost the same color and small difference in ATN share the same tape spot. The number in the color bar represents the Julian days in 2018.Fig. 4.
 Measured αabs values at different ATNs for (a) Tape 8050 and (b) Tape 8060 at the PKU site. The dots with almost the same color and small difference in ATN share the same tape spot. The number in the color bar represents the Julian days in 2018.

The αabs at the Taizhou site had almost the same properties as that at the PKU site, as shown in Fig. S3. The results indicated that αabs changed significantly with the ATN for Tape 8050, but that there were no clear variations with the increased accumulation of ATN for Tape 8060.


Multiple-scattering Correction Factor

The PASS-3 estimated the instrument noise and recorded the value in its reported data. The statistical results for noise during the measurement are shown in Fig. S4. The mean and standard deviation of σabs for instrument noise were 0.34 and 0.066, respectively, with a 95.5% possibility (sum of the mean and double the standard deviation) that the noise was less than 0.473. To confirm that the measured σabs noise comprised a small partition of the measured σabs, we compared these data when the surrounding σabs was larger than 10.50 Mm−1 [0.473/(1 − 5.5%)], under which condition the noise would account for less than 4.5% of the measured σabsvalue.

Figs. 5(a)5(b), and 5(c) present a comparison of the σabs values measured using the PASS-3 and the σATN values by the AE33 at wavelengths of 405, 532, and 781 nm, respectively, at the PKU site. Section 3.2 reveals that the measured results of the AE33 with Tape 8060 were more reliable and stable than those from the AE33 with Tape 8050. Thus, the σATN value from the AE33 with Tape 8060 was used for comparison. The σATN value at 405 nm was calculated using the αATN value that had been previously calculated using the σATN values at 370 nm and 470 nm. The corresponding σATN values of AE33 at 532 and 781 nm were calculated using the σATN values at the wavelength pairs of 520 and 590 nm, and 660 and 880 nm, respectively. σabs can be derived by dividing σATN by Cf. Here, Cf was determined by calculating the ratio of the σATN values measured using the AE33 and the σabs values measured using the PASS-3. As revealed in Fig. 5, Cf was determined to be 2.91, 2.94 and 2.94 at 405, 532, and 781 nm, respectively, by using the linear regression forcing the intercept to zero. Cf did not exhibit obvious variations with changes in wavelength. The comparison was conducted for the Taizhou site and the corresponding Cf values were 2.88, 2.88 and 2.86 at 405, 532, and 781 nm, respectively, as shown in Fig. S5. The Cf value is smaller than that of 4.35 in central Oregon, USA (Laing et al., 2020).


Fig. 5. Comparison between the σabs values measured using the PASS-3 and σATN values measured using the AE33 at (a) 405 nm, (b) 532 nm and (c) 781 nm at the PKU site.Fig. 5.
 Comparison between the σabs values measured using the PASS-3 and σATN values measured using the AE33 at (a) 405 nm, (b) 532 nm and (c) 781 nm at the PKU site.

Therefore, the Cf was recommended to be 2.90 (± 0.04) for both sites, which is an average of all the ratios. Notably, this value was the same for different wavelengths.


Correction Method of σATN

According to the findings presented in Section 3.3, it is clear that the AE33 with Tape 8060 provided a reliable σabs measurement with a Cf value of 2.90. By contrast, the σATN and αabs values measured with Tape 8050 were related to the accumulation of ATN (see Section 3.2). Moreover, the σATN values measured with Tape 8060 was relatively stable and unrelated to the accumulation of ATN. However, because the AE33 with Tape 8050 has long been used worldwide for measuring ambient BC mass loadings, historical observation data from the AE33 with Tape 8050 should be corrected. We propose a straightforward method to correct σATN values measured with Tape 8050 on the basis of σATN values measured with Tape 8060.

First, the σATN values measured with Tape 8050 were grouped according to their ATN values from 20 to 140 with a step of 5. We also recorded the concurrently measured σATN values from Tape 8060. For each group, the σATN values measured with 8050 and 8060 were compared and the regression slope value was used as the correction factor for the σATN values measured with Tape 8050.

Fig. 6 shows the correction factors at different wavelengths and ATN values, revealing that the correction factors varied significantly with the amount of ATN. When the ATN was less than 80, the correction factors did not exhibit clear differences among different wavelengths. They did, however, become increasingly diverse across different wavelengths as ATN increased. The σATN values measured with Tape 8050 were slightly lower than those measured with Tape 8060 when the ATN was less than 90 across the seven wavelengths.


Fig. 6. Correction factors for the σATN measured using Tape 8050 for different wavelengths and the corresponding variations in αabs of 470 nm and 950 nm at different ATNs.Fig. 6.
 Correction factors for the σATN measured using Tape 8050 for different wavelengths and the corresponding variations in αabs of 470 nm and 950 nm at different ATNs.

Because the correction factors were different for different wavelengths, we calculated the difference in αabs values at the wavelengths of 470 nm and 950 nm. The results, presented in Fig. 6, indicated that corrections of σATN can lead to variations in αabs from 0.02–0.15 to 0.23 when ATN increased from 30–60 to 130. The variations in αabs were consistent with the results presented in Section 3.2, namely that the measured αabs values first decreased and then increased with the increase in ATN. This suggests that the correction factors can provide a more reliable σATN and αabsvalues for Tape 8050.

Fig. 7 provides one example of the effects of correction on the measured σATN and αabs values. Specifically, Figs. 7(a) and 7(b) show the raw and corrected σATN values at wavelengths 470, 590, and 950 nm. Figs. 7(c) and 7(d) display the corresponding αabs values calculated from Figs. 7(a) and 7(b) using the respective wavelengths of 470 and 950 nm. The location at which the tape was switched can be recognized directly when the ATN decreased from higher than 135 to less than 40, as shown in Figs. 7(c) and 7(d)Figs. 7(a) and 7(c) reveal that both σATN and αabs exhibited large discrepancies when the tape was switched for the raw measured σATN. However, the situation improved when σATN was corrected using the factors shown in Fig. 6, resulting in more continuous σATN and αabs values than before.


Fig. 7. (a–b) The σATN and (c–d) αabs time series: (a) and (c) show the raw data, (b) and (d) show the corrected data, (a) and (c) share the same color bar, and (b) and (d) share the same wavelengths.Fig. 7.
 (a–b) The σATN and (c–d) αabs time series: (a) and (c) show the raw data, (b) and (d) show the corrected data, (a) and (c) share the same color bar, and (b) and (d) share the same wavelengths.

In conclusion, two steps of correction should be applied for σATN values measured with Tape 8050. The first step is to correct the measured σATN values by the factor shown in Fig. 6 for different ATNs and wavelengths. The second step is the same as that used for correction for Tape 8060: correcting the multiple-scattering effects by using a recommended factor of 2.9 for each wavelength.


CONCLUSIONS


In this study, we evaluated the reliability of σabs values measured with an AE33 during two field campaigns in Taizhou and at PKU. Comparing the values measured with two different AE33s, one using Tape 8050 and the other using Tape 8060, we found that using Tape 8060 produced more stable σATN values.

As no clear relationship between the αabs and the ATN was observed, the loading effects were apparently sufficiently corrected for Tape 8060; thus, only the multiple-scattering correction should be applied when measuring σATN values with this tape. However, since the loading effects were not adequately corrected for Tape 8050, both the loading effect and multiple-scattering corrections for σATN should be considered when using this tape.

Based on the comparison between the σabs values measured with the PASS-3 and the σATN values measured with the AE33 using Tape 8060, we recommend a mean Cf value of 2.9 ± 0.4 to correct the measured σATN independent of wavelength. We also obtained a correction factor for the σATN measured using Tape 8050 by regressing values measured using Tape 8060 against those measured using Tape 8050 for different ATNs and wavelengths. This factor corrected the discrepancy in the measured σATN between the tape sampling values and the raw data, thereby resolving αabs’s variation with the ATN.

This study can serve as a reference for σabs measurements and for source appointments. Using one correction factor for the North and East China Plain offered the advantage of increasing the inter-comparability of the measurements from different sites and facilitating the reconstruction of the AE33-measured historical observation data.


DATA AVAILABILITY


The measurement data involved in this study are available upon request.


ACKNOWLEDGMENTS


This work was supported by the National Key R&D Program of China (2016YFC020000: Task 5, Task 3) and the National Natural Science Foundation of China (41875044, 41675038, 21677002).


COMPETING INTERESTS


The authors declare that they have no conflict of interest. 


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Aerosol Air Qual. Res. 20 :1833 -1841 . https://doi.org/10.4209/aaqr.2019.10.0500  


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