Nocturnal surface ozone enhancement over Portugal: Influence of different atmospheric conditions

Abstract:

Five distinct events of nocturnal surface ozone (NSO) enhancement were observed, with NSO concentration exceeding 80µg/m³, at multiple ozone (O₃) monitoring sites (32 sites) in Portugal, during the winter months (November – February) of the 2000-2010 period. Since O₃ production ceases at nighttime, a plausible reasoning for observed bimodal pattern of O₃ with enhanced NSO concentration during nighttime (2300-0500 local time) is hypothesized to be related with different atmospheric transport processes. Simultaneous measurements of O₃ at 32 spatially distributed sites in Portugal suggest that horizontal advection alone cannot explain the enhancement of NSO. Thus, detailed analysis of the atmospheric conditions with the regional Weather Research and Forecasting (WRF) model were performed. Results revealed that each of the five NSO enhancement events occurred over the same geographical location under entirely different atmospheric conditions. The first event happened due to the presence of a synoptic scale circulation and downward vertical motion from the free troposphere over Portugal. The second event was a result of a combination of vertical and horizontal transport over a large area. The third event showed that under favorable conditions the high NSO events can last for several days. The fourth event showed that the NSO enhancement can drastically exceed the daytime O₃ concentrations over a large area. Finally, the fifth event showed that the enhancement of NSO occurred due to low level jet formation from southwest corner to northeast corner, passing through the mainland of Portugal at around 1.5 km height, associated with the vertical mixing below the jet stream. This study revealed that any single physical phenomenon is not sufficient to explain the NSO enhancement at a given location and large-scale flow patterns with downward vertical motion plays a key role in the enhancement of NSO.