Tropospheric chemistry over the lower Great Plains of the United States 2. Trace gas profiles and distributions

Abstract

Convective clouds and thunderstorms redistribute air pollutants vertically, and by altering the chemistry and radiative balance of the upper troposphere, these local actions can have global consequences. To study these effects, measurements of trace gases ozone, Os, carbon monoxide, CO, and odd nitrogen were made aboard the NCAR Sabreliner on 18 flights over the southern Great Plains during June 1985. To demonstrate chemical changes induced by vertical motions in the atmosphere and to facilitate comparison with computer model calculations, these data were categorized according to synoptic flow patterns. Part 1 of this two-part paper details the alternating pulses of polar and maritime air masses that dominate the vertical mixing in this region. In this paper, trace gas measurements are presented as altitude profiles (0-12 km) with statistical distributions of mixing ratios for each species in each flow pattern. The polar flow regime is characterized by northwesterly winds, subsiding air, and convective stability. Concentrations of CO and total odd nitrogen (NOy) are relatively high in the shallow planetary boundary layer (PBL) but decrease rapidly with altitude. Ozone, on the other hand, is uniformly distributed, suggesting limited photochemical production; in fact, nitric oxide, NO, mixing ratios fell below 10 ppt (parts per 10 •2 by volume) in the midtroposphere. The maritime regime is characterized by southerly surface winds, convective instability, and a deep PBL; uniformly high concentrations of trace gases were found up to 4 km on one flight. Severe storms occur in maritime flow, especially when capped by a dry layer, and they transport large amounts of CO, Os, and NOy into the upper troposphere. Median NO levels at high altitude exceeded 300 ppt. Lightning produces spikes of NO (but not CO) with mixing ratios sometimes exceeding 1000 ppt. This flow pattern tends to leave the midtroposphere relatively clean with concentrations of trace gases similar to those observed in the polar category. During frontal passage both stratiform and convective clouds mix pollutants more uniformly into the middle and upper levels; high mixing ratios of CO are found at all altitudes, and Os levels are highest of any category, implicating photochemical production. These results illustrate the importance of convection in tropospheric chemistry. Use of average trace gas profiles or eddy diffusion parameterized vertical mixing can lead to errors of 30 to 50% in 03 and CO concentrations and an order of magnitude for odd nitrogen.

The global lightning-induced nitrogen oxides source

“…The measurements identified enhanced NO mixing ratios of 0.1-0.8 nmol mol −1 in the sometimes 10-50 km wide anvil plumes, with local concentration "spikes" exceeding 4 nmol mol −1 (Luke et al, 1992). The large-scale enhancements within and outside thunderstorm anvils are more important in assessing NO x contributions from lightning than such short but high peak values.…”

“…no lightning contributions to trace gases, in particular no enhancements of O 3 and CO (Dickerson et al, 1987;Luke et al, 1992;Ridley et al, 1994Ridley et al, , 2006Hauf et al, 1995;Huntrieser et al, 1998;Jeker et al, 2000). Nevertheless, a few studies reported partially large O 3 increases exceeding several 100 nmol mol −1 Zahn et al, 2002;Güsten et al, 2003) near thunderstorms using various observation methods (Dobson et al, 1946;Orville, 1967;Shantla and Moore, 1972;Clarke and Griffing, 1985;Jadhav et al, 1996;Borra et al, 1997;Suhre et al, 1997;Randriambelo et al, 1999;Winterrath et al, 1999).…”

“…It reaches an order of 0.05-1 nmol mol −1 near the tropopause, and about 20 nmol mol −1 near 3 hPa pressure altitude in the tropical stratosphere (Grooß and Russell III, 2005). The tropospheric vertical profile often shows a C-shape with low values in the mid-troposphere and high values in the polluted boundary layer and near the tropopause (Kley et al, 1981;Drummond et al, 1988;Warneck, 1988;Luke et al, 1992;Rohrer et al, 1997;Huntrieser et al, 2002), see for example Fig. 3a.…”

“…Since then a series of experiments performed measurements close to or within thunderstorms over the USA (Dickerson et al, 1987;Luke et al, 1992;Ridley et al, 1994;Poulida et al, 1996;Ridley et al, 1996). The measurements identified enhanced NO mixing ratios of 0.1-0.8 nmol mol −1 in the sometimes 10-50 km wide anvil plumes, with local concentration "spikes" exceeding 4 nmol mol −1 (Luke et al, 1992).…”

Lightning-produced NO<sub>x</sub> over Brazil during TROCCINOX: airborne measurements in tropical and subtropical thunderstorms and the importance of mesoscale convective systems

“…LINOX, EULINOX and STERAO) (Luke et al, 1992;Ridley et al, 1994;Poulida et al, 1996;Ridley et al, 1996;Huntrieser et al, 1998;Höller et al, 1999;Stith et al, 1999;Dye et al, 2000;Huntrieser et al, 2002) and partly in the subtropics (e.g. CRYSTAL-FACE) (Ridley et al, 2004), see also overview in Schumann and Huntrieser (2007).…”