Use this class (instead of a PostProcessingBehaviour) to dynamically instantiate the PostProcessingProfile at runtime: this allows you to modify the properties of the profile during playtime, without modifying the profile asset

InstantiatePostProcessingProfile.cs

using UnityEngine;
using UnityEngine.PostProcessing;
using System.Collections.Generic;

/// <summary>
/// Use this component to dynamically create a PostProcessingBehaviour and instantiate a PostProcessingProfile on a Camera
/// This allows you to dynamically modify at runtime the PostProcessingProfile, without modifying the asset.
/// This component keeps track of the Profile and Instances. This means that if 2 different camera use the same Profile, they will use the same Instance.
/// </summary>
[RequireComponent(typeof(Camera))]
public class InstantiatePostProcessingProfile : MonoBehaviour
{
    [SerializeField] PostProcessingProfile m_Profile = null;

    static Dictionary<PostProcessingProfile, PostProcessingProfile> ms_RefToInstance = new Dictionary<PostProcessingProfile, PostProcessingProfile>();
    static PostProcessingProfile AssignProfile(PostProcessingProfile reference)
    {
        if (!reference)
            return null;

        // keep track of the profile and instances: only 1 instance is created per profile
        // (event if multiple cameras share 1 profile)
        if (!ms_RefToInstance.ContainsKey(reference))
        {
            var profileInstance = Object.Instantiate(reference);
            QualitySettingsAdjustments(profileInstance);
            ms_RefToInstance.Add(reference, profileInstance);
        }

        return ms_RefToInstance[reference];
    }

    T GetOrAddComponent<T>() where T : Component
    {
        var component = gameObject.GetComponent<T>();
        if (component == null)
            component = gameObject.AddComponent<T>();
        return component;
    }

    void Start()
    {
        if (m_Profile)
        {
            var ppb = GetOrAddComponent<PostProcessingBehaviour>();
            Debug.Assert(ppb);
            ppb.profile = AssignProfile(m_Profile);
        }

        Object.Destroy(this);
    }

    static void QualitySettingsAdjustments(PostProcessingProfile profile)
    {
        // Here you can adjust the PostProcessingProfile just after instantiation.
        // For example you can change it according to the Quality Settings.
        // Like here we disable Motion Blur for low quality.
        if (QualitySettings.GetQualityLevel() < 1)
            profile.motionBlur.enabled = false;
    }
}