Le Chengfeng, Hu Chuanmin
College of Marine Science, University of South Florida, St. Petersburg, FL 33701, USA.
Opt Express. 2013 Aug 12;21(16):18849-71. doi: 10.1364/OE.21.018849.
Remote sensing of chromophoric dissolved organic matter (CDOM) from satellite measurements for estuaries has been problematic due to optical complexity of estuarine waters and uncertainties in satellite-derived remote sensing reflectance (Rrs, sr(-1)). Here we demonstrate a hybrid approach to combine empirical and semi-analytical algorithms to derive CDOM absorption coefficient at 443 nm (a(g)(443), m(-1)) in a turbid estuary (Tampa Bay) from MODIS Aqua (MODISA) and SeaWiFS measurements. The approach first used a validated empirical algorithm and a modified quasi-analytical algorithm (QAA) to derive chlorophyll-a concentration (Chla, mg m(-3)) and particulate backscattering coefficient at 443 nm (b(bp)(443), m(-1)), respectively, from which phytoplankton pigment and non-algal particulate absorption coefficient at 443 nm (a(ph)(443) and a(d)(443), m(-1)) were derived with pre-determined bio-optical relationships. Then, the modified QAA was used to estimate the total absorption coefficient at 443 nm (a(t)(443), m(-1)). Finally, a(g)(443) was estimated as (a(t)(443) - a(ph)(443) - a(d)(443) - a(w)(443)) where a(w)(443) is the absorption coefficient of pure water (a constant). Using data collected from 71 field stations and 33 near-concurrent satellite-field matchup data pairs covering a large dynamic range (0.3 - 8 m(-1)), the approach showed 23% RMS uncertainties in retrieving a(g)(443) when in situ Rrs data (N = 71) were used. The same approach applied to satellite Rrs yielded much higher uncertainties of a(g)(443) (85%) due to large errors in the satellite-retrieved Rrs(443). When the Rrs(443) was derived from the satellite-retrieved Rrs(550) and then used in the hybrid approach, uncertainties in the retrieved a(g)(443) reduced to 30% (N = 33). Application of the approach to MODISA and SeaWiFS data led to a 15-year time series of monthly mean a(g)(443) distributions in Tampa Bay between 1998 and 2012. This time series showed significant seasonal and annual variations regulated mainly by river discharge. Testing of the approach over another turbid estuary (Chesapeake Bay, the largest estuary in the U.S.) demonstrated the potential (25% uncertainties for a limited a(g)(443) range) of using this approach to establish long-term environmental data records (EDRs) of CDOM distributions in other estuaries with similar optical complexity.
由于河口水体的光学复杂性以及卫星遥感反射率（Rrs，sr⁻¹）的不确定性，通过卫星测量对河口的发色溶解有机物（CDOM）进行遥感一直存在问题。在此，我们展示了一种混合方法，该方法结合经验算法和半解析算法，利用中分辨率成像光谱仪Aqua（MODISA）和宽视场海洋观测卫星（SeaWiFS）的测量数据，推导浑浊河口（坦帕湾）443 nm处的CDOM吸收系数（a(g)(443)，m⁻¹）。该方法首先使用经过验证的经验算法和改进的准解析算法（QAA）分别推导叶绿素a浓度（Chla，mg m⁻³）和443 nm处的颗粒后向散射系数（b(bp)(443)，m⁻¹），然后利用预先确定的生物光学关系从中推导443 nm处的浮游植物色素和非藻类颗粒吸收系数（a(ph)(443)和a(d)(443)，m⁻¹）。接着，使用改进的QAA估算443 nm处的总吸收系数（a(t)(443)，m⁻¹）。最后，a(g)(443)估算为（a(t)(443) - a(ph)(443) - a(d)(443) - a(w)(443)），其中a(w)(443)是纯水的吸收系数（一个常数）。利用从71个现场站点收集的数据以及33对近乎同步的卫星 - 现场匹配数据对（涵盖较大动态范围，0.3 - 8 m⁻¹），当使用现场Rrs数据（N = 71）时，该方法在反演a(g)(443)时显示出约23%的均方根误差不确定性。由于卫星反演的Rrs(443)存在较大误差，将相同方法应用于卫星Rrs时，a(g)(443)的不确定性要高得多（约85%）。当Rrs(443)从卫星反演的Rrs(550)推导得出并用于混合方法时，反演的a(g)(443)的不确定性降至约30%（N = 33）。将该方法应用于MODISA和SeaWiFS数据，得到了1998年至2012年坦帕湾月平均a(g)(443)分布的15年时间序列。该时间序列显示出显著的季节和年际变化，主要受河流流量调节。在另一个浑浊河口（切萨皮克湾，美国最大的河口）对该方法进行测试表明，对于具有类似光学复杂性的其他河口，使用此方法建立CDOM分布的长期环境数据记录（EDR）具有潜力（在有限的a(g)(443)范围内不确定性约为25%）。
