Atmospheric Composite Sounding Analysis as a Forecasting Tool for Ozone Accumulation in a Semiarid Metropolitan Area during the North America Monsoon: Phoenix, Arizona

Abstract

Abstract Phoenix, Arizona, observes a high summertime frequency for daily maximum 8-h ozone averages (DMO8) exceeding 70 parts per billion, resulting in nonattainment status by the U.S. Environmental Protection Agency. This study discusses the use of composite atmospheric sounding analysis (constructing average sounding conditions for specific recurring events) to forecast different air quality index daily ozone classifications, including “good,” “moderate,” and collectively categories exceeding the 2015 ozone standard. Composite sounding analysis, using the Phoenix 1200 UTC (0500 LST) rawinsonde data (2006–17), identifies “pollutant dispersion windows” for ozone accumulation or dispersal for Phoenix during the North American monsoon (July and August period). A favorable ozone exceedance atmosphere is associated with a “Four Corners high” synoptic pattern bringing relatively light winds at and below 700 hPa (≤4.5 ms−1) and higher easterly winds above (≤12.3 ms−1). Healthy ozone days (good category) are common when Pacific low pressure troughing over the western United States creates deep and faster westerly flow above the surface reaching speeds of 6.7 (19.5) m s−1 by 700 (200) hPa. Surprisingly, large standard deviations over 10°C for dewpoint temperature at midlevels (500–400 hPa) were determined for all three daily ozone classification ranges used in this study. Additionally, modest temperature deviations and mean differences are noted at significant pressure levels. Consequently, wind speed and direction are better indicators when forecasting ozone accumulation potential. These results are pertinent to air quality meteorologists responsible for disseminating ozone forecasts for heavily urbanized areas of the U.S. Southwest.