Comparison between Ron Brown and Sagar Kanya results on light scattering, absorption and aerosol optical depth

During the INDOEX –IFP campaign research vessels Ronald H Brown and Sagar Kanya cruised together in the Arabian Sea (Figure 1) for about a day from the 9^th March afternoon hours to the 10^th March afternoon hours. Coordinated measurements were made on atmospheric chemistry and aerosol optical parameters. Results of the intercomparison between the aerosol scattering, absorption and optical depth are discussed here. Table 1 shows the details of the instruments used and the summary of the results.

Table 1. Details of the data compared and the instruments used during the joint cruise between Ron Brown and Sagar Kanya on 9 to 10 March 1999, over the Arabian Sea

Parameter measured	Ron Brown		Sagar Kanya		Percentage difference
Parameter measured	Instrument used	Mean Value	Instrument used	Mean Value	Percentage difference
Scattering Coeff.	TSI three wavelength Nephelometer	29.56 (1/Mm)	Radiance Research single wavelength Nephelometer	24.11 (1/Mm)	18.4%
Absorption Coeff.	Radiance Research PSAP	1.13 (1/Mm)	Radiance Research PSAP	0.92 (1/Mm)	18.6%
Single scat. albedo	From the above two data set	0.96	From the above two data set	0.96	0.4%
Aerosol optical depth spectra	Microtops, Micro Pulse Lidar and Shadow band photometer	see Figures 4&5	Hand-held sunphotometer	see Figures 4&5	less than 0.02 in absolute value

(i) Scattering Coefficient The Ron Brown data used for the intercomprison purpose is of NOAA-PMEL measurements made for sub 10 m m aerosol size at 55% RH. Data correspond to green wavelength is used. In case of Sagar Kanya, it is bulk aerosol sample without using an inlet impactor and the measurements are made at the ambient humidity. While both the data sets are corrected by the individual experimenters for the zero and calibration drift and for angular non-idealities in the scattering volume [Anderson and Ogren, 1998], the Sagar Kanya data are further adjusted for the 55% RH using the empirical fit proposed by Ogren (Harmony Workshop, 1999) to facilitate comparison. The two data sets are plotted in Figure 2 for the period 9 to 10 Match, decimal day from 68.4605 to 69.4396. Data are averaged for every 30 minutes interval. While both the data sets reproduce the relative temporal/spatial variation uniformly, there is an absolute difference between the two data sets, the Sagar Kanya values are lower than the Ron Brown values by about 18%. Never the less the difference is not very alarming, as the nephelometer data are expected to have an uncertainty of ± 7% based on laboratory closure experiments [Anderson and Ogren, 1998].

(ii) Absorption Coefficient The absorption measurements are made using Radiance Research Particle/Soot Absorption Photometer (PSAP) both in the case of Ron Brown as well as Sagar Kanya. In case of PSAP measurement, the raw data are corrected for the spot size difference (by a factor of 0.873), for the scattering loss (2% of the scattering obtained from the nephelometer) and the calibration constant (by a factor of 0.82) [Bond et al., 1999] and Hal Maring (1999, private communication). The absorption data are plotted in Figure 3 for the same period as in the case of the Nephelometer data. In spite of the over all agreement seen between the two data sets the mean value differ by about 18%. This is within the measurement uncertainty of PSAP, which is estimated to be about 20% [Bond et al., 1999].

(iii) Single scattering albedo The single scattering albedo is a derived quantity, defined as the ratio of scattering coefficient to the extinction coefficient (scattering + absorption). The good agreement seen between the Sagar Kanya and Ron Brown values is fortuitous because both the absorption and scattering has similar difference between the measurements which got cancelled in the albedo estimation.

(iv) Aerosol optical depth Compared to scattering and absorption measurements, which depend very much on the sampling condition, the aerosol optical depth measurements are robust as it is representative of the whole air column. Though difference will be there between different set of instruments in terms of the wavelength being used, comparison of the optical depth spectrum (as shown in Figure 4) eliminates the bias in the wavelength used. In the case of Ron Brown the measurements were made using the Microtops sunphotometer, while in the case of Sagar Kanya an indigenously developed hand-held sun-photometer was employed. The same instrument was used in the earlier three INDOEX Sagar Kanya cruises conducted in 1996, 1997 and 1998. Details of the instrument calibration and data reduction are found in Jayaraman et al., 1998. While the shape of the optical depth spectra are different, particularly in the 450 to 500 nm range, the absolute difference between the two data sets is within 0.02 which is the accuracy of aerosol optical depth measurement. Another comparison attempted between Ron Brown and Sagar Kanya is using the column integrated value from the Micro Pulse Lidar (MPL) data with the sun-photometer time series data, shown in Figure 5 (E.J. Welton, private communication). Ron Brown microtops data and shadow band photometer are also shown. There is a general agreement between the data sets within the measurement accuracy of ± 0.02 in the aerosol optical depth.

Figure 1. Portions of the cruise tracks of Ron Brown and Sagar Kanya during the INDOEX-IFP showing the coordinated measurements location (shaded by thick arrow).

Figure 2. Intercomparison between the aerosol scattering coefficient measured using nephelometer (at 550 nm) onboard Ron Brown and Sagar Kanya during the INDOEX-IFP coordinated measurements.

Figure 3. Comparison between the absorption coefficient measured using PSAP onboard Ron Brown and Sagar Kanya during the INDOEX-IFP coordinated measurements

Figure 4. Comparison of aerosol optical depth spectra measured using ‘Microtops’ sunphotometer onboard Ron Brown and the hand-held sunphotometer onboard the Sagar Kanya during the INDOEX-IFP coordinated measurements

Figure 5. Comparison of aerosol optical depth measured using ‘Microtops’ sunphotometer, the Micro Pulse Lidar (MPL) and the shadow band photometer onboard Ron Brown, and the hand-held sunphotometer onboard Sagar Kanya (courtesy: J.E. Welton)