Atmospheric Environment 39 (2005) 3039–3054 Plume dispersion from the MVP field experiment. Analysis of surface concentration and its fluctuations Yimin Ma à , Zafer Boybeyi, Steven Hanna, Kittisak Chayantrakom School of Computational Sciences, George Mason University, Fairfax, VA 22030 4444, USA Received 30 April 2004; received in revised form 3 December 2004; accepted 7 January 2005 Abstract Surface concentration and its fluctuations from plume dispersion under unstable conditions in a coastal environment are investigated using the model validation program field experimental data. The goal of this study is to better understand plume dispersion under such conditions. Procedures are described to derive the plume surface concentration from moving vehicle measurements. Convective boundary layer scalings are applied and cumulative density functions (CDF) are studied. The results indicate that the relative concentration fluctuation intensity (s c =CðyÞ) decreases with the normalized downwind distance (X ) and that it is relatively small at the plume central line and largely increased at the plume edges, consistent with other field and laboratory results. The relation between s c =CðyÞ at the plume centerline (s c =C) and X for elevated sources can be described by s c =C ¼ a þ b=X : The crosswind plume spread (s y ) is found to satisfy Deardorff and Willis’s (J. Appl. Meteorol., 14 (1975) 1451) form of s y =h ¼ a 1 X =ð1 þ a 2 X Þ 1=2 scaled with convective layer depth h: For elevated sources, the normalized crosswind integrated concentration (C y ) is found to satisfy a relation of C y ¼ 16X 3=2 ; with Yaglom’s (Izr. Atmos. Oceanic Phys., 8 (1972) 333) scaling rule on the free convective layer being applied. Empirical CDFs based on the gamma and the clipped probability density functions show agreements with the experimental CDFs, with the former being better than the latter when ðc  CÞ=s c 40:5: A new clipped-gamma CDF form is proposed based on the analysis of the present data, showing a better agreement. We suggest that a parameter u  0 ð12  0:5 h=LÞ 1=3 ; with combined efforts of surface friction velocity (u  0 ), Monin–Obukhov stability length (L) and unstable boundary layer height (h), replace the convective velocity scale (w  ) under weak convective conditions in a coastal environment. r 2005 Elsevier Ltd. All rights reserved. Keywords: Concentration fluctuation; CDF; Intermittency; Crosswind plume spread 1. Introduction Studies of air pollution concentration near the surface are important because human activities mainly involve the near surface layer. The concentration can be described as the mean value and its turbulent fluctua- tions. The latter is especially important when the pollutant is a highly toxic or flammable material, which has been observed to exhibit the same order of the mean value (Hanna and Insley, 1989). For dispersions from continuous source emissions, namely plume dispersions, there are two distinct types of concentration fluctua- tions: fluctuations within the plume and variations of the ARTICLE IN PRESS www.elsevier.com/locate/atmosenv 1352-2310/$-see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.01.008 à Corresponding author. Tel.: +17039933576. E-mail address: yma@scs.gmu.edu (Y. Ma).