Showing papers by "André S. H. Prévôt published in 1998"

OH measurements during the First Aerosol Characterization Experiment (ACE 1): Observations and model comparisons

Abstract: Airborne measurements of the hydroxyl radical, OH, performed during the First Aerosol Characterization Experiment (ACE 1), using the Selected Ion Chemical lonization Mass Spectrometry (SICIMS) technique are presented. Adaptations of the previous ground-based technique for measurement aboard an aircraft platform are discussed, including an inlet for the straightening and slowing of the airflow, calibration, and measurement considerations at changing pressure altitudes. Steady state model calculations of the concentration of OH, [OH], throughout the entire mission were generally in good agreement, with a slight bias toward an overestimate of the measured [OH]. The largest discrepancies between measured and modeled values occurred for measurements in the boundary layer, and those in and around clouds, with the model overestimating the [OH] by ∼40% in the boundary layer or inside clouds, and underestimating it by ∼30% near clouds. The low model [OH] near clouds can be attributed to underestimating the actinic flux calculated from Eppley radiometer measurements. The model overestimates in the boundary layer and inside clouds may in part be due to a lack of heterogeneous losses of HOx species in the model. Models developed at the National Oceanic and Atmospheric Administration Aeronomy Laboratory and at Georgia Institute of Technology produce similar results with differences being attributed to the methods of calculating photolysis rates. Calculations of high noon [OH] for a flight out of Hobart using the O(1D) quantum yields from Talukdar et al. [1998] produce 17% higher values than those calculated using the currently recommended Jet Propulsion Laboratory values.

Physico-chemical modeling of the First Aerosol Characterization Experiment (ACE 1) Lagrangian B: 1. A moving column approach

Abstract: During Lagrangian experiment B (LB in the following) of the First Aerosol Characterization Experiment (ACE 1), a clean maritime air mass was followed over a period of 28 hours. During that time span, the vertical distribution of aerosols and their gas phase precursors were characterized by a total of nine aircraft soundings which were performed during three research flights that followed the trajectory of a set of marked tetroons. The objective of this paper is to study the time evolution of gas phase photochemistry in this Lagrangian framework. A box model approach to the wind shear driven and vertically stratified boundary layer is questionable, since its basic assumption of instantaneous turbulent mixing of the entire air column is not satisfied here. To overcome this obstacle, a one-dimensional Lagrangian boundary layer meteorological model with coupled gas phase photochemistry is used. To our knowledge, this is the first time that such a model is applied to a Lagrangian experiment and that enough measurements are available to fully constrain the simulations. A major part of this paper is devoted to the question of to what degree our model is able to reproduce the time evolution and the vertical distribution of the observed species. Comparison with observations of O3, OH, H2O2, CH3OOH, DMS, and CH3I, made on the nine Lagrangian aircraft soundings shows that this is in general the case, although the dynamical simulation started to deviate from the observations on the last Lagrangian flight. In agreement with experimental findings reported by Q. Wang et al. (unpublished manuscript, 1998b), generation of turbulence in the model appears to be most sensitive to the imposed sea surface temperature. Concerning the different modeled and observed chemical species, a number of conclusions are drawn: (1) Ozone, having a relatively long photochemical lifetime in the clean marine boundary layer, is found to be controlled by vertical transport processes, in particular synoptic-scale subsidence or ascent. (2) Starting with initally constant vertical profiles, the model is able to “create” qualitatively the vertical structure of the observed peroxides. (3) OH concentrations are in agreement with observations, both on cloudy and noncloudy days. On the first flight, a layer of dry ozone rich air topped the boundary layer. The model predicts a minimum in OH and peroxides at that altitude consistent with observations. (4) Atmospheric DMS concentrations are modeled correctly only when using the Liss and Merlivat [1986] flux parameterization, the Wanninkhof [1992] flux parameterization giving values twice those observed. To arrive at this conclusion, OH is assumed to be the major DMS oxidant, but no assumptions about mixing heights or entrainment rates are necessary in this type of model. DMS seawater concentrations are constrained by observations.

Carbon monoxide measurements from 76° N to 59° S and over the South Tasman Sea

Abstract: In November and December of 1995, carbon monoxide (CO) measurements were made in a Pacific transect and over the South Tasman Sea as part of the First Aerosol Characterization Experiment (ACE 1) program. Airborne CO measurements were made from 76° N to 59° S. A clear latitudinal gradient in CO concentrations was measured, with the southern hemisphere average about 80 parts per billion by volume (ppbv), and increasing to 120–130 ppbv at the most northern latitudes. Plumes of CO with a 30–40 ppbv concentration increase over the general background concentrations could be seen at several latitudes. The National Oceanic and Atmospheric Administration R/V Discoverer made CO measurements over the South Tasman Sea from November 15 to December 9, 1995. A systematic decrease of 0.31 ppbv/d CO was observed. Vertical profile measurements of CO from near the ocean surface to 2500 m altitude during the Lagrangian B intensive of ACE 1 suggested the mixing of stratospheric air with reduced CO concentrations.