Vertical sampling and analysis of nonmethane hydrocarbons for ozone control in urban North Carolina

Abstract

As part of an effort by the state of North Carolina to develop a State Implementation Plan for ozone control in the Raleigh Metropolitan Statistical Area (MSA), vertical measurements of C2‐C10 hydrocarbons were made, in and above the surface inversion layer (SIL), as inputs to the urban airshed model (UAM). Three‐hour integrated ambient air samples were collected during August 1993 from 0500–0800 eastern daylight time (EDT). Additional samples were collected from 1200–1500 and 1700–2000 EDT on selected days. Vertical sampling was achieved from a 610‐m television tower located approximately 15 km southeast of the downtown area. Boundary layer wind and temperature profiles were determined by balloon soundings. For some compounds, e.g., propane, the average concentration was lower above the inversion layer (9.59 ppbC in the SIL and 2.08 ppbC above the SIL); however, other species such as 2‐methylpentane had higher concentrations above the inversion layer (1.28 ppbC in and 1.40 ppbC above the SIL). In addition, the vertical distributions of hydrocarbons within the convective boundary layer were compared to the vertical distribution estimated from calculations based on surface concentration, species reactivity, and eddy diffusivity. Compounds such as isoprene and N‐butane decreased as predicted by the equation, while others such as propane and benzene showed unexpected profiles. Calculations of propylene‐equivalent concentrations were used to estimate the effect of reactivity on the relative importance of individual hydrocarbons. The contribution of isoprene to the local hydrocarbon budget was analyzed and surface measurements were compared with data collected in Atlanta, Georgia, during the 1992 Southern Oxidants Study. Isoprene comprised more than 70% of the total propylene‐equivalent concentration in the afternoon in Raleigh but only 40% of the total in Atlanta.