compare two numpy arrays by first column in sorted order and create new array
<p>I have two numpy arrays a and b. I want to compare two numpy array by column 0 which contains time series data.I want to create a new numpy array with sorted time series in column 0 and their associated values with time. and if no values is found insert null to the data. example</p>

<pre><code>     a = np.array([[0.002, 0.998],
                   [0.004, 0.997],
                   [0.006, 0.996],
                   [0.008, 0.995],
                   [0.010, 0.993]])

     b = np.array([[0.002, 0.666],
                   [0.004, 0.665],
                   [0.0041, 0.664],
                   [0.0041,0.663],
                   [0.0042, 0.664],
                   [0.0043, 0.664],
                   [0.0044, 0.663],
                   [0.0045, 0.663],
                   [0.005, 0.663],
                   [0.006, 0.663],
                   [0.0061, 0.662],
                   [0.008, 0.661]])
    Expected Ouput
       c=  [[0.002, 0.998, 0.666],
            [0.004, 0.997, 0.665],
            [0.0041, null, 0.664],
            [0.0041, null ,0.663],
            [0.0042, null, 0.664],
            [0.0043, null, 0.664],
            [0.0044, null, 0.663],
            [0.0045, null, 0.663],
            [0.005, null, 0.663],
            [0.006, 0.996, 0.663],
            [0.0061, null, 0.662],
            [0.008, 0.995, 0.661],
            [0.010, 0.993,  null]] 
</code></pre>

<p>Any tricks to do like this...</p>