Abstract
Bayesian inference has been used to determine rigorous estimates of hydroxyl radical concentrations ([OH]) and air mass dilution rates (K) averaged following air masses between linked observations of nonmethane hydrocarbons (NMHCs) spanning the North Atlantic during the Intercontinental Transport and Chemical Transformation (ITCT)-Lagrangian-2K4 experiment. The Bayesian technique obtains a refined (posterior) distribution of a parameter given data related to the parameter through a model and prior beliefs about the parameter distribution. Here, the model describes hydrocarbon loss through OH reaction and mixing with a background concentration at rate K. The Lagrangian experiment provides direct observations of hydrocarbons at two time points, removing assumptions regarding composition or sources upstream of a single observation. The estimates are sharpened by using many hydrocarbons with different reactivities and accounting for their variability and measurement uncertainty. A novel technique is used to construct prior background distributions of many species, described by variation of a single parameter α. This exploits the high correlation of species, related by the first principal component of many NMHC samples. The Bayesian method obtains posterior estimates of [OH], K and α following each air mass. Median [OH] values are typically between 0.5 and 2.0 × 106 molecules cm-3, but are elevated to between 2.5 and 3.5 × 106 molecules cm-3, in low-level pollution. A comparison of [OH] estimates from absolute NMHC concentrations and NMHC ratios assuming zero background (the "photochemical clock" method) shows similar distributions but reveals systematic high bias in the estimates from ratios. Estimates of K are ∼0.1 day-1 but show more sensitivity to the prior distribution assumed.
Original language | English (US) |
---|---|
Article number | D10S40 |
Journal | Journal of Geophysical Research Atmospheres |
Volume | 112 |
Issue number | 10 |
DOIs | |
State | Published - May 27 2007 |
ASJC Scopus subject areas
- Geophysics
- Forestry
- Oceanography
- Aquatic Science
- Ecology
- Water Science and Technology
- Soil Science
- Geochemistry and Petrology
- Earth-Surface Processes
- Atmospheric Science
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science
- Palaeontology