HCFCs have very low atmospheric concentrations measured in parts per what?

Hydrochlorofluorocarbons (HCFCs) are a class of ozone-depleting substances that, while present in the atmosphere, generally exist in very low concentrations. Their concentrations are indeed measured in parts per trillion (ppt). This measurement reflects the relatively small amounts of HCFCs compared to other substances in the atmosphere.

In atmospheric science, parts per trillion is commonly used to quantify trace gases like HCFCs because they are present in such minuscule quantities. For comparison, other measurement units like parts per million (ppm) or parts per billion (ppb) are typically reserved for more abundant substances. Thus, recognizing that HCFCs are characterized by their very low atmospheric presence underscores their classification as trace gases, possibly due to their regulation and phased-out usage in many applications to mitigate their impact on ozone depletion.

In this context, selecting parts per trillion accurately reflects the nature of HCFC concentrations in the environment.