Transport mechanisms of nocturnal surface ozone over Riyadh, Kingdom of Saudi Arabia

Abstract

This study investigated ozone (O3) transport mechanisms and their impact on observed nocturnal surface O3 (NSO) enhancements over Riyadh, Kingdom of Saudi Arabia (KSA). We used O3 measurements for the summer of 2012 to examine the NSO enhancements across the city. We found that out of the 88 days for which observations were available, NSO enhancements could be observed on 38 days. The average difference in the NSO concentration between NSO-enhanced days and nonenhanced days was ∼15 ppb, which was statistically significant (p < 0.05). Our analysis of the regional surface emission sources, potential vorticity (PV), and back trajectories using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model indicated that out of 38 days, the NSO enhancements on 18 days (47%), 14 days (37%), 4 days (11%), and 2 days (5%) are due to local transport, long-range transport from highly polluted regions and from stratospheric intrusion regions located at a considerable distance from Riyadh, and a combination of long-range transport from stratospheric intrusion and highly polluted regions, respectively. We then investigated the relevance of each transport mechanism for the increased NSO concentrations during five protracted episodes, each marked by three consecutive days of NSO enhancement. Our results demonstrated that the local transport, long-range transport of O3-rich air masses from highly polluted regions to the east and northeast of the KSA, and local descent over Riyadh increased the average NSO concentration by up to 43%. Moreover, we found that long-range transport from stratospheric intrusion regions north of the KSA increased the average NSO concentration by up to 46%. The combination of long-range transport and stratospheric intrusion increased the average NSO concentration by ∼180%. These increases were usually associated with strong northwest regional winds flowing over highly polluted and major stratospheric intrusion regions.