Nocturnal Ozone Depletion Events at the Amphitrite Point Observatory on West Vancouver Island

Abstract:

Routine monitoring stations on the West coast of North America serve to monitor baseline levels of criteria pollutants such as ozone (O₃) arriving from the Pacific Ocean. In Canada, the Amphitrite Point Observatory (APO) in Ucluelet on the West coast of Vancouver Island has been added to this network to provide regional baseline measurements. Recently, McKendry and coworkers have reported frequent episodes of nocturnal O₃ depletion events (ODEs) at APO (range: 5-20 ppbv) that generally correlate with alongshore winds, elevated levels of carbon dioxide (CO₂), and low vertical entrainment but whose cause(s) has (have) remained unclear.

In this work, results from the Ozone-depleting reactions in a coastal atmosphere (ORCA) campaign, which took place at APO from July 6 - 31, 2015, are presented. In addition to the long-term measurements that include aerosol size distribution and composition measurements, mixing ratios of speciated monoterpenes (e.g., α- and β-pinene, limonene), molecular halogens (i.e., Cl₂, I₂), halogen oxides (i.e., OIO), plus a full suite of nitrogen oxides (including N₂O₅, PAN, PPN, ΣPN, ΣAN, HNO₃, HONO, and ClNO₂) were quantified. Synoptic conditions at the site varied greatly between nights. During westerly flow of relatively clean marine air, O₃ was generally conserved at night, indicating that deposition of O₃ to the ocean surface is a minor loss pathway. When the air mass originated from other sectors, episodes of nocturnal ODEs were observed on several occasions, in which mixing ratios of biogenic VOCs were enhanced. These included air masses that originated from densely forested areas to the East, air masses polluted by marine traffic emissions from the southeast, and air masses from the NW that have traveled parallel to the coastline. In this sector, the air was likely in contact with terrestrial vegetation via land-sea breeze circulations. The results suggest that nocturnal ODEs at APO are mainly driven by local or regional processes involving near-shore vegetation.