Use of variance techniques to measure dry air-surface exchange rates

The variances of fluctuations of scalar quantities can be measured and interpreted to yield indirect estimates of their vertical fluxes in the atmospheric surface layer. Strong correlations among scalar fluctuations indicate a similarity of transfer mechanisms, which is utilized in some of the variance techniques. The ratios of the standard deviations of two scalar quantities, for example, can be used to estimate the flux of one if the flux of the other is measured, without knowledge of atmospheric stability. This is akin to a modified Bowen ratio approach. Other methods such as the normalized standard-deviation technique and the correlation-coefficient technique can be utilized effectively if atmospheric stability is evaluated and certain semi-empirical functions are known. In these cases, iterative calculations involving measured variances of fluctuations of temperature and vertical wind velocity can be used in place of direct flux measurements. For a chemical sensor whose output is contaminated by non-atmospheric noise, covariances with fluctuations of scalar quantities measured with a very good signal-to-noise ratio can be used to extract the needed standard deviation. Field measurements have shown that many of these approaches are successful for gases such as ozone and sulfur dioxide, as well as for temperature and water vapor, and could be extended to other trace substances. In humid areas, it appears that water vapor fluctuations often have a higher degree of correlation to fluctuations of other trace gases than do temperature fluctuations; this makes water vapor a more reliable companion or reference scalar. These techniques provide some reliable research approaches but, for routine or operational measurement, they are limited by the need for fast-response sensors. Also, all variance approaches require some independent means to estimate the direction of the flux.This research has been funded as part of the National Acid Precipitation Assessment Program by the U.S. Environmental Protection Agency through IAGDW89930069-01 to the U.S. Department of Energy.