Table 2. The impact factor for high pollution days (ozone and PM2.5) over the United States associated with various extreme meteorological events (None: no event; HW: only heat waves; TI: only temperature inversions; AS: only atmospheric stagnation episodes; All: three kinds of events happened at the same time).
| Species | Season | None | HW | TI | AS | HW&TI | HW&AS | TI&AS | All |
|---|---|---|---|---|---|---|---|---|---|
| O3 | Spring | −0.5 | 0.1 | 0.0 | 0.0 | 1.2 | 1.0 | 1.1 | 3.0 |
| Summer | −0.5 | 1.2 | 0.4 | 0.2 | 3.0 | 2.1 | 1.3 | 3.3 | |
| Fall | −0.5 | −0.1 | −0.2 | 0.4 | 0.8 | 0.8 | 0.6 | 2.1 | |
| Winter | 0.1 | −0.4 | −0.1 | 0.2 | −0.1 | 0.1 | 0.1 | 0.0 | |
| PM2.5 | Spring | −0.4 | 0.0 | 0.1 | 0.2 | 0.6 | 0.7 | 0.8 | 1.9 |
| Summer | −0.3 | 0.8 | 0.2 | 0.2 | 1.9 | 1.2 | 0.7 | 2.5 | |
| Fall | −0.4 | 0.2 | −0.3 | 0.4 | 1.0 | 1.0 | 0.5 | 2.0 | |
| Winter | −0.5 | −0.7 | −0.1 | 0.2 | −0.2 | 0.3 | 1.2 | 0.8 |
High pollution days are defined as the top 10% most polluted days for each season during 2001–2010. The impact factor is defined as the enhancement in the probability of high pollution episodes due to extreme meteorological events.