Converting .NET 4 Threading to .NET 2

I am using someone else's .NET 4 open source smoothed particle hydrodynamics code and I'm trying to convert it for a Unity project which you can only count on to be up to .NET 2 standards. Unfortunately the code uses the Parallels class (which is awesome!) but he used one of the more obscure overloads. Can anyone see a good way to achieve the same thing in .NET 2 without a huge performance hit?

    Parallel.For(
        0,
        _numActiveParticles,
        () => new Vector2[MAX_PARTICLES],
        (i, state, accumulatedDelta) => calculateForce(_activeParticles[i], accumulatedDelta),
        (accumulatedDelta) =>
        {
            lock (_calculateForcesLock)
            {
                for (int i = _numActiveParticles - 1; i >= 0; i--)
                {
                    int index = _activeParticles[i];
                    _delta[index] += accumulatedDelta[index] / MULTIPLIER;
                }
            }
        }
    );

I think this is what the code does (non threaded):

    for (int i = 0; i < _numActiveParticles; i++)
    {
        Vector2[] temp = new Vector2[MAX_PARTICLES];
        temp = calculateForce(_activeParticles[i], temp);


        for (int k = _numActiveParticles - 1; k >= 0; k--)
        {
            int index = _activeParticles[k];
            _delta[index] += temp[index] / MULTIPLIER;
        }
    }