"WriteLine" does not work

I have a problem

I use this code to save to txt file, how to do not to overwrite the file, but would write line by line the next time a function call??

Command WriteLine does not work. Write overwrite the file.

private async Task WriteToFile()
{
    string ResultString = string.Join("\n", locationData.ToArray());

    byte[] fileBytes = System.Text.Encoding.UTF8.GetBytes(ResultString);

    StorageFolder local = Windows.Storage.ApplicationData.Current.LocalFolder;
    var dataFolder = await local.CreateFolderAsync("DataFolder", CreationCollisionOption.OpenIfExists);
    var file = await dataFolder.CreateFileAsync("DataFile.txt", CreationCollisionOption.OpenIfExists);

    using (var s = await file.OpenStreamForWriteAsync())
    {
        s.Write(fileBytes, 0, fileBytes.Length);
    }
}

I used Windows Phone 8.1 for my app.