title: Removing negative values in pandas column keeping NaN
body: <p>I was wondering how I can remove rows which have a negative value but keep the NaNs. At the moment I am using: </p>
  
  <pre><code>DF = DF.ix[DF['RAF01Time'] &gt;= 0]
  </code></pre>
  
  <p>But this removes the NaNs.</p>
  
  <p>Thanks in advance.</p>