NO3 has been measured in slightly polluted Colorado mountain air up to a few hundred ppt. There is clear evidence that it is removed by scavenging reactions in less than one hour. The identify of the scavenger is not yet established in mountain air, nor is it yet known whether the reaction removes odd nitrogen from the troposphere. In urban air the scavenging occurs in less than a few hundred seconds and may in part be due to the reaction of NO3 with NO.