Shader Material blending

Questions
1

I want to make particles trail. I think that WebGL render target would be the best method.
My idea is to use WebGL render target to save the previous frame, use it as a background with an opacity that is smaller than 1, and draw (blend) the current frame to it, repeat it again and again. I think this method can make trails. But I don’t know how to set shader material blend. Anyone can help me?
https://mapbox.github.io/webgl-wind/demo/

2

I’m sorry. I solved it by simply shader
I let it here and hope it could help someone face the same problem.

precision mediump float;

uniform sampler2D current;
uniform sampler2D background;

varying vec2 vUv;
float when_gt(float x,float y){
  return max(sign(x-y),0.);
}
void main(){
  vec4 cr=texture2D(current,vUv);
  vec4 bg=texture2D(background,vUv)*.96;
  bg.r*=when_gt(bg.r,.2);
  bg.g*=when_gt(bg.g,.2);
  bg.b*=when_gt(bg.b,.2);
  bg.a*=when_gt(bg.a,.2);
  gl_FragColor=cr+bg;
}