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A 2004 study reported unusually high relative humidity with respect to ice (RHi) in cold natural cirrus and aircraft contrails, and proposed that nitric acid on ice particles could help explain it. The measurements are distinct from the explanation: the team observed the humidity pattern, while its proposed “Delta-ice” mechanism remains a hypothesis in the sources cited here.
What did the study measure?
Gao and colleagues reported in situ measurements of RHi and nitric acid (HNO3) in upper-tropospheric natural cirrus and aircraft contrails. RHi means relative humidity referenced to ice; it is not the same quantity as relative humidity referenced to liquid water.
In the paper’s abstract, the researchers wrote: “At temperatures lower than 202 kelvin, RHi values show a sharp increase to average values of over 130% in both cloud types.” This is the study’s reported average under those temperature conditions, not a claim that every cirrus cloud or contrail reaches that humidity.
The Science paper was published January 23, 2004. PubMed’s record and abstract for Gao et al. identify the measurements and the authors’ interpretation. Chemistry World’s Emma Davies reported on the work on March 1, 2004, describing how contrails can mix with natural cirrus; that is contextual reporting rather than a separate measurement. Chemistry World
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How could nitric acid affect the ice?
The authors attributed the elevated RHi to a proposed class of nitric-acid-containing ice particles they called “Delta-ice.” Their suggested mechanism was that HNO3 at the ice-particle surface could impede equilibration between the ice and surrounding water vapor. If the exchange were slowed, the air and ice could remain out of equilibrium, consistent with higher measured RHi.
This is a causal explanation proposed by the study, not a direct demonstration that surface nitric acid produced the observed humidity values. The measurements and the mechanism therefore should be kept separate: the former are reported observations; the latter is the authors’ interpretation of them.
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Why did the authors think it mattered for climate models?
Cirrus clouds influence the distribution of water vapor in the upper troposphere, so a particle type that changes how ice exchanges vapor could affect simulated cloud properties and humidity. Gao and colleagues said that adding Delta-ice to climate models would alter simulated cirrus properties and the distribution of upper-tropospheric water vapor. NASA’s record for the paper also describes those stated implications. NASA Technical Reports Server record
The implication was a modeling prediction, not a quantified climate effect. The cited records do not establish whether later research confirmed or revised the proposed mechanism, whether current models include Delta-ice, or how large any present-day climate impact might be.
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What remains uncertain?
This is a historical account of a 2004 result, not evidence by itself of current scientific consensus. The sources cited here establish what the study measured, the explanation it proposed, and the modeling consequence its authors anticipated; they do not establish the subsequent scientific status of that explanation.
The unresolved question is whether later observations and modeling work show that nitric-acid-containing ice materially affects present-day representations of natural cirrus or aircraft contrails.
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