In case of rainwater it is seen that authors mention about the volume weighted mean concentration. What interpretation could be drawn from that value and is it necessary to express the concentration in terms of VWM.
Lets consider a rain event of one hour. In first 10 minutes rain should contain high amount of calcium / magnesium species but then gradually it become acidic due to fast in cloud scavenging . now consider a event of 6 hours. the time of in cloud scavenging is more here. So to normalized data one have to include the rain-rate or rain fall amount in precipitation chemistry.
I am very amateur in this field. May be I am wrong. But I also eager to know the answer from expert.
I think this person was trying to express an average concentration of a particular volume recorded over a period of time. although i would love to read the content of the author.