By examining the five years observational data at high altitudes at a North Carolina rural site, it is found that ozone anomaly is a more suitable indicator than concentration to identify built-up ozone aloft. On the days of surface ozone exceeding 80 ppb, upper air ozone anomaly is greater than 20 ppb albeit low absolute values. The visualization of the temporal variation in vertical ozone profiles suggests that a large number of surface ozone episodes start with the downward propagation of high aloft ozone on the previous day.

To demonstrate that the aloft ozone reservoir at a rural site is the result of long distance transport, a spectral analysis is first conducted to distill the principle frequency and the times of its occurrence. The frequency at the order of long-range transport processes turns out to coincide with the occurrence of the ozone episodes that are triggered by the ozone propagation from upper air to the surface. Second, the climatology of the flow patterns at different heights prior to the episodes are constructed using a trajectory model, HYSPLIT. The results suggest that when there is a deep system of northwesterly, southwesterly and northeasterly flows, the episodes at the site are most likely triggered by high aloft ozone transported via long distance. Over the episodes due to local mechanism, the two days trajectories are short with varying directions at different altitudes. Third, process analysis on photochemical model simulations is applied to quantify the contribution of transport to the high ozone. At the North Carolina rural site, the transport term makes the most contribution to the ozone concentration when the ozone episode starts with downward ozone propagation.