Thursday, 17 January 2002: 8:45 AM
Measurement of trace gases and PM2.5 mass and composition near the ground and at 254 m agl during TexAQS2000
Continuous (1 min) measurements of various meteorological parameters, trace gases (NO, NOx, NOy, CO, SO2, and O3), and discrete, 6 to 24 h integrated filter-based measurements of PM2.5 mass and composition, including aerosol gases (NH3, HONO, HNO3, HCl, SO2, and light organic acids), were made at LaPorte municipal airport, near the Houston ship channel, during TexAQS2000. Similar filter-based PM2.5 measurements supplemented by semi-continuous (30 min) O3 and TEOM mass measurements were made on the 62nd floor of the Williams Tower, 254 m agl, and ~12 km west of downtown Houston. At both sites, the TEOM mass concentrations were systematically low relative to the gravimetric filter mass by 5 and 11 %, respectively, which is attributed to the active humidity control employed with both TEOM. LaPorte was influenced predominantly by a strong land-sea breeze circulation with veering wind directions, causing periodic short-term impacts of plumes from nearby sources with significantly reduced (titrated) nighttime ozone levels. A signature of this circulation could even be detected in the discrete PM2.5 measurements showing occasional elevated sodium levels. The combination of this air flow pattern and the relative vicinity of various emission sources led to vertically confined ozone plumes, which caused the highest ozone readings of the study at LaPorte on August 30 and 31, with maximum hourly averages of 219 and 196 ppbv, respectively, while the elevated site at Williams Tower, only saw ~50% lower ozone maxima. With exception of this episode, the PM2.5 mass and sulfate concentrations generally follow the trends in daily ozone maxima, which points to very rapid ozone production in these plumes as suggested by other investigators. The occasional deviation from this agreement between [PM2.5] and [O3]max also indicates that ozone is formed much faster than PM2.5 under these specific conditions, which stands in contrast to the conditions typically leading to O3 and PM2.5 pollution in the southeastern U.S., where regional stagnation leads to simultaneous buildup of these pollutants as in a ‘rising tide’. A charge balance based on the sulfate-nitrate-ammonium system indicates slightly acidic conditions with 16 ±17 ne m-3 at LaPorte and 25 ±47 ne m-3 at Williams Tower, and [NH3] measured during the first half of the study only, averaged 3.2 ±1.5 ppbv at LaPorte, and 2.9 ±1.9 ppbv at Williams Tower.