Validation of VIIRS AOD through a Comparison with a Sun Photometer and MODIS AODs over Wuhan

Abstract

Visible Infrared Imaging Radiometer Suite (VIIRS) is a next-generation polar-orbiting operational environmental sensor with a capability for global aerosol observations. A comprehensive validation of VIIRS products is significant for improving product quality, assessing environment quality for human life, and studying regional climate change. In this study, three-year (from 1 January 2014 to 31 December 2016) records of VIIRS Intermediate Product (IP) data and Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals on aerosol optical depth (AOD) at 550 nm were evaluated by comparing them to ground sun photometer measurements over Wuhan. Results indicated that VIIRS IP retrievals were underestimated by 5% for the city. A comparison of VIIRS IP retrievals and ground sun photometer measurements showed a lower R 2 of 0.55 (0.79 for Terra-MODIS and 0.76 for Aqua-MODIS), with only 52% of retrievals falling within the expected error range established by MODIS over land (i.e., ±(0.05 + 0.15AOD)). Bias analyses with different Ångström exponents (AE) demonstrated that land aerosol model selection of the VIIRS retrieval over Wuhan was appropriate. However, the larger standard deviations (i.e., uncertainty) of VIIRS AODs than MODIS AODs could be attributed to the less robust retrieval algorithm. Monthly variations displayed largely underestimated AODs of VIIRS in winter, which could be caused by a large positive bias in surface reflectance estimation due to the sparse vegetation and greater surface brightness of Wuhan in this season. The spatial distribution of VIIRS and MODIS AOD observations revealed that the VIIRS IP AODs over high-pollution areas (AOD > 0.8) with sparse vegetation were underestimated by more than 20% in Wuhan, and 40% in several regions. Analysis of several clear rural areas (AOD < 0.2) with native vegetation indicated an overestimation of about 20% in the northeastern region of the city. These findings showed that the VIIRS IP AOD at 550 nm can provide a solid dataset with a high resolution (750 m) for quantitative scientific investigations and environmental monitoring over Wuhan. However, the performance of dark target algorithms in VIIRS was associated with aerosol types and ground vegetation conditions.

Performance of the NPP-VIIRS and aqua-MODIS Aerosol Optical Depth Products over the Yangtze River Basin

“…Besides, obviously in Table 3, the best performance of DT3K AOD retrievals appears at WH with all statistical metrics (R = 0.84-0.86, RMB = 1.06-1.11, MAE = 0.05-0.07 and 59-63% of retrievals within the EE). Similar comparison results at WH were also reported by [28]. Approximately 72% of the DT3K-CE318 AOD matchups fell into the EE with a high R value (0.87) and nearly 7% of the DT3K AOD retrievals (RMB = 1.07) were overestimated [28].…”

“…Similar comparison results at WH were also reported by [28]. Approximately 72% of the DT3K-CE318 AOD matchups fell into the EE with a high R value (0.87) and nearly 7% of the DT3K AOD retrievals (RMB = 1.07) were overestimated [28]. Figure 3 also reports the comparison results of the DT10K and CE318 AOD retrievals.…”

“…In terms of the site-by-site comparison over YRB (Table 3), filtering by quality flags (QF > 1), the improvement can be observed at TH, HF and WH. However, for the KM site, the matchup statistics are poor, and increasing QF does not help, probably due to the very low number of samples (67-77) [28]. In Figure 2, the correlation coefficient (R) is in the range of 0.78-0.81, along with 43-46% of DT3K AOD retrievals falling into the expected error (EE) envelope.…”

Feasibility Study on Measuring Atmospheric CO2 in Urban Areas Using Spaceborne CO2-IPDA LIDAR

“…Moreover, our group established a Cimel sun photometer CE-318, which is the same equipment used at AERONET sites since 2010. Relationships between sun photometer-derived AODs and MODIS-derived AODs have been established in Beijing [61], Taihu Lake [62], Hong Kong [63], and Wuhan [64], respectively. Through these studies we found that the slopes of the regression equations ranged from 0.8-1.1.…”

“…The difference between the annual AOD of the two products varied from 0.01-0.34 in terms of the cities. The key factors dominating such a difference were identified as the surface reflectance, AOD, and cloud optical depth [64]. It is very difficult to draw a universal conclusion on whether MODISderived AODs overestimate or underestimate the AOD because such differences are highly seasonand location-dependent.…”