Evaluation of maritime aerosol optical depth and precipitable water vapor content from the Microtops II Sun photometer

Aerosols and precipitable water vapor over oceans represent the main components of the land-atmosphere-ocean ecosystem and play an important role in the exchange of substances and the radiative balance on the global scale. Using AERONET level 2.0 data from islands and coastal sites as reference data, this study evaluated the validity of maritime aerosol optical depth (AOD) in each band and precipitable water vapor (PWV) content retrieved from Microtops II Sun photometer measurements based on ship-borne data collections. The results showed that the bias of the AOD at each wavelength from Microtops II was in the range of −0.043 to −0.005, which could be caused by improper pointing at the sun, dirty optics or unstable elctronics. The root mean square error (RMSE) was in the range of 0.035-0.093, and the mean absolute percentage error (MAPE) varied from 15.88% to 34.42%, which were attributed to the comprehensive effect of both random and systematic errors during the measurements. The average Ångström exponent was 1.402 and the bias, RMSE, and MAPE were −0.081, 0.249, and 14.06% respectively, and the error of Ångström exponent was mainly derived from the cumulative one from the AOD at each wavelength. The average PWV content over oceans was 2.740 cm and the bias, RMSE, and MAPE were −0.071, 0.268 cm, and 8.58% respectively; this indicated that the retrieved PWV had a good accuracy.