Vertical distribution of ozone and VOCs in the low boundary layer of Mexico City

Abstract

Abstract. The evolution of ozone and 13 volatile organic compounds (VOCs) in the boundary layer of Mexico City was investigated during 2000–2004 to improve our understanding of the complex interactions between those trace gases and meteorological variables, and their influence on the air quality of a polluted megacity. A tethered balloon, fitted with electrochemical and meteorological sondes, was used to obtain detailed vertical profiles of ozone and meteorological parameters up to 1000 m above ground during part of the diurnal cycle (02:00–18:00 h). VOCs samples were collected up to 200 m by pumping air to canisters with a Teflon tube attached to the tether line. Overall, features of these profiles were found to be consistent with a simple picture of nighttime trapping of ozone in an upper residual layer and of VOCs in a shallow unstable layer above the ground. After sunrise an ozone balance is determined by photochemical production, entrainment from the upper residual layer and destruction by titration with NO, delaying the ground-level ozone rise by 2 h. The subsequent evolution of the conductive boundary layer and vertical distribution of pollutants are discussed in terms of the energy balance, the presence of turbulence and the atmospheric stability.

Tethered balloon-based soundings of ozone, aerosols, and solar radiation near Mexico City during MIRAGE-MEX

“…This is expected, resulting from the vigorous mixing of the boundary layer and the low productivity of the arid landscape (which precluded deposition to vegetation through stomatal conductance). The observations are also consistent in pattern with those reported previously using the electrochemical sonde from late morning through mid-afternoon (Wö hrnschimmel et al, 2006;Velasco et al, 2008).…”

“…A sudden cooling of the boundary layer to an altitude of 2250 m occurs between 16:30 and 19:30 LST, after the surface energy budget reverses (Velasco et al, 2008). Since most tethered balloonbased observations were made between 9:30 and 16:00 LST, when boundary layer growth was rapid, deep and uniform mixing of air through this layer is expected.…”

“…On the tethered balloon platform, it is also an economical alternative to electrochemical sondes (Komhyr, 1969). (The electrochemical ozone sonde measures the change in conductivity resulting from the reaction of potassium iodide in a solution through which ozone-containing air is bubbled; this technique requires separate and repeated calibration of each sonde, both for chemical reaction and for sample flow rates and is subject to chemical interferences; Parrish and Fehsenfeld, 2000;Velasco et al, 2008).…”

Satellite observations of Mexico City pollution outflow from the Tropospheric Emissions Spectrometer (TES)

“…There are large temporal and spatial variabilities in both O 3 and CO during the course of the aircraft flights. The peaks in the in situ observations occurred after 13:00 local time due to the development of strong nocturnal vertical inversions over the MCMA (Velasco et al, 2007). TES data provide a snap shot at around 01:45 and 13:45 local time only, thus missing the intra-day temporal variability.…”

Measurement of Ozone Concentration on the Elevation Gradient of a Low Hill by a Semiconductor-Based Portable Monitor

“…Monitoring on towers [12] and measurement using tethered balloons [13,14] are standard methods to obtain vertical profiles. Transect measurements along mountain slopes is also an efficient way to grasp the vertical structure at high temporal resolution for a long time [15].…”