How do I keep down-arrow straight and have up-arrow curved in graphviz?

I have the following example. How do I get the lines going between a2 -> a3 to be straight and the line from a3 to a2 to be curved?

digraph G {

  subgraph cluster_0 {
    style=filled;
    color=lightgrey;
    node [style=filled,color=white];
    a0 -> a1 -> a2 -> a3;
    label = "process #1";
  }

  subgraph cluster_1 {
    node [style=filled];
    b0 -> b1 -> b2 -> b3;
    label = "process #2";
    color=blue
  }
  start -> a0;
  start -> b0;
  a1 -> b3;
  b2 -> a3;
  a3 -> a2;
  a3 -> end;
  b3 -> end;

  start [shape=Mdiamond];
  end [shape=Msquare];
}

*example created from https://dreampuf.github.io/GraphvizOnline