Lerp an Alpha Gradient in Particle System Color over lifetime In script

I'm using a snow particle effect, on a 2d map and transition to rain and others depending on where your at on the map. problem I'm facing is the snow stays too long and looks sloppy when in areas it shouldn't be. I cant just set the color over lifetime alpha because the lifetime itself is too long. It needs to be so the particles don't disappear in the middle of the screen too early. I've tried changing the alpha in script but the sudden change in alpha does not look good. I've been playing with gradient keys, canvas groups and lerping methods to no avail. does anyone have any idea if you can lerp from one gradient alpha to another, I've found plenty of resources on color.lerp but not for the alpha. oddly enough I got mathf.pingpong to give me the effect i wanted but it would constantly swap back and forth between alpha 0 and 255 which I assume is correct hence "pingpong" although when replaced with the lerp method it didn't do anything except change alpha from 255 to 250..not sure why..

Heres what I have, this is called in the start method I need to add the alpha change in the //Show snow section

 private IEnumerator ShowEnvironmentParticleCo()
     {
         //Rain Particle System
         ParticleSystem rainParticle = rain.GetComponent<ParticleSystem> ();
         var rainEm = rainParticle.emission;
 
         //Mist Particle System
         ParticleSystem mistParticle = mist.GetComponent<ParticleSystem> ();
         var mistEm = mistParticle.emission;
         var mainMist = mistParticle.main;
         var mistColor = mistParticle.colorOverLifetime;
         Gradient ourGradient;
         ourGradient = new Gradient ();
 
         //Snow Particle System
         ParticleSystem snowParticle = snow.GetComponent<ParticleSystem> ();
         var snowEm = snowParticle.emission;
 
         //Snow Particle System 2
         ParticleSystem snowParticle2 = snow2.GetComponent<ParticleSystem> ();
         var snowEm2 = snowParticle2.emission;
         var snowColor2 = snowParticle2.colorOverLifetime;
 
         //Flash Particle System
         ParticleSystem flashParticle = flash.GetComponent<ParticleSystem> ();
         var flashEm = flashParticle.emission;
 
         //Show Rain based on scroll rect position
         if (scrollBar.value > 0.13 && scrollBar.value < 0.3) 
         {

             rainEm.enabled = true;
             GameObject.Find ("RainPrefab2D").GetComponent<AudioSource> ().enabled = true;
         } 
         else 
         {
             rainEm.enabled = false;
             GameObject.Find ("RainPrefab2D").GetComponent<AudioSource> ().enabled = false;
         }
 
         //Show Mist based on scroll rect position
         if (scrollBar.value > 0.59 && scrollBar.value < 0.86) 
         {
             mistEm.enabled = true;
             mainMist.simulationSpeed = 0.25f;
 
             //Change Mist Alpha
             ourGradient.SetKeys (
                 new GradientColorKey[] { new GradientColorKey (Color.white, 1.0f), new GradientColorKey (Color.white, 1.0f) },
                 new GradientAlphaKey[] { new GradientAlphaKey (0.3f, 0.0f), new GradientAlphaKey (0.0f, 1.0f) });
             
             mistColor.color = ourGradient;
         } 
         else if (scrollBar.value > 0.13 && scrollBar.value < 0.3) 
         {
             mistEm.enabled = true;
             mainMist.simulationSpeed = 1f;
 
             //Change Mist Alpha Back
             ourGradient.SetKeys (
                 new GradientColorKey[] { new GradientColorKey (Color.white, 1.0f), new GradientColorKey (Color.white, 1.0f) },
                 new GradientAlphaKey[] { new GradientAlphaKey (0.15f, 0.0f), new GradientAlphaKey (0.0f, 1.0f) });
             
             mistColor.color = ourGradient;
         } 
         else 
         {
             mistEm.enabled = false;
         }
 
         //Show Snow based on scroll rect position
         if (scrollBar.value > 0.75 && scrollBar.value < 0.94) 
         {
             snowEm.enabled = true;
             snowEm2.enabled = true;
         }
         else 
         {
             snowEm.enabled = false;
             snowEm2.enabled = false;
 
             //after disabling snow particle emission I would like to lerp from alpha 255 to 0 to quickly fade out the remaining particles here
         }
 
         //Show Flash based on scroll rect position
         if (scrollBar.value > 0.96 && scrollBar.value <= 1) 
         {
             flashEm.enabled = true;
         }
         else 
         {
             flashEm.enabled = false;
         }
 
         yield return new WaitForSeconds (2);
     }