Additional commenting in background_particles.gd, removal of duplicate background particle process materials due to the usage of duplicate().

Materials/Particles/down_particles.tres

@@ -6,5 +6,5 @@ emission_shape_offset = Vector3(0, -8, 0)
 emission_shape = 1
 emission_sphere_radius = 7.35
 gravity = Vector3(0, -98, 0)
-scale_max = 5.0
+scale_max = 2.5
 color = Color(0, 0, 0, 1)

Materials/Particles/down_particles_bg.tres

@@ -1,11 +0,0 @@
-[gd_resource type="ParticleProcessMaterial" format=3 uid="uid://dhirstigqbwlc"]
-
-[resource]
-particle_flag_disable_z = true
-emission_shape_offset = Vector3(0, -8, 0)
-emission_shape = 1
-emission_sphere_radius = 7.35
-gravity = Vector3(0, -98, 0)
-scale_max = 2.5
-color = Color(0, 0, 0, 0.5137255)
-collision_mode = 2

Materials/Particles/up_particles.tres

@@ -6,5 +6,5 @@ emission_shape_offset = Vector3(0, -8, 0)
 emission_shape = 1
 emission_sphere_radius = 7.35
 gravity = Vector3(0, 98, 0)
-scale_max = 2.0
+scale_max = 2.5
 color = Color(0, 0, 0, 1)

Materials/Particles/up_particles_bg.tres

@@ -1,11 +0,0 @@
-[gd_resource type="ParticleProcessMaterial" format=3 uid="uid://ch1r4nhrcpg3s"]
-
-[resource]
-particle_flag_disable_z = true
-emission_shape_offset = Vector3(0, -8, 0)
-emission_shape = 1
-emission_sphere_radius = 7.35
-gravity = Vector3(0, 98, 0)
-scale_max = 2.5
-color = Color(0, 0, 0, 0.5058824)
-collision_mode = 2

Scripts/background_particles.gd

@@ -1,36 +1,51 @@
 extends Node2D
 
-@export var number_of_particles : int
+@onready var up_material : ParticleProcessMaterial = load("res://Materials/Particles/up_particles.tres") #The process material for when gravity is upwards.
+@onready var down_material : ParticleProcessMaterial = load("res://Materials/Particles/down_particles.tres") #The process material for when gravity is downwards.
 
-var particle_child : GPUParticles2D
-var up_material : ParticleProcessMaterial = load("res://Materials/Particles/up_particles_bg.tres")
-var down_material : ParticleProcessMaterial = load("res://Materials/Particles/down_particles_bg.tres")
+@export var number_of_particles : int #The total number of particles we want emitting.
+
+var particle_child : GPUParticles2D #The child node that's holding the emitting particles.
 
 # Called when the node enters the scene tree for the first time.
 func _ready() -> void:
 	
+	#Duplicate the process materials so we have a local copy.
+	#Done so we don't modify the process material attached to the player particle emitter.
 	up_material = up_material.duplicate()
 	down_material = down_material.duplicate()
 	reloadParticles()
 	
 func _physics_process(delta: float) -> void:
+	
+	#Run reloadParticles() when gravity is changed.
 	if Input.is_action_just_pressed("flip"):
 		print("input detected")
 		reloadParticles()		
 		
+#Detect gravity, and set the appropriate process material to the GPUParticles2D child node.
+#If that child node does not exist, then create it, and then assign the process material.
 func reloadParticles() -> void:
 	
 	#If the particle node does not already exist, do the initial setup, and add it as a child node.
 	if !has_node("BGParticles"):
+		
+		#Create a new GPUParticles2D object.
 		var bgparticle := GPUParticles2D.new()
+		
+		#Set number of particles, make them emit by default, make them top level to not follow camera, and set name.
 		bgparticle.amount = number_of_particles
 		bgparticle.emitting = true
 		bgparticle.top_level = true
 		bgparticle.name = "BGParticles"
+		
+		#Add our created GPUParticles2D object as a child to this Node.
 		add_child(bgparticle)
+		
+		#Set our particle_child variable equal to our newly created child Node so we can modify it later.
 		particle_child = get_node_or_null("BGParticles")
 	
-	#Set up the material we want to set the particle node to, and set up a screen size variable.
+	#Set up a termpoary material variable, and set up a screen size variable.
 	var material : ParticleProcessMaterial
 	var size : Vector2 = get_viewport_rect().size
 	
@@ -40,8 +55,10 @@ func reloadParticles() -> void:
 	elif al_globals.y_gravity < 0:
 		material = down_material
 	
-	#Set the emission shape and the size, relative to size of screen.
+	#Set the emission shape to a box, with an assignable size.
 	material.emission_shape = ParticleProcessMaterial.EMISSION_SHAPE_BOX
+	
+	#Set the bounds of the emission box equivalent to the current screen size.
 	material.emission_box_extents = Vector3(size.x, size.y, 0)
 	
 	#Set the material of the particle child object to the created material.