Intercontinental transport of pollution manifested in the variability and seasonal trend of springtime O₃ at northern middle and high latitudes

Observations (0-8 km) from the Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment are analyzed to examine air masses contributing to the observed variability of springtime O₃ and its seasonal increase at 40°-85°N over North America. Factor analysis using the positive matrix factorization and principal component analysis methods is applied to the data set with 14 chemical tracers (O₃, NO_y, PAN, CO, CH₄, C₂H₂, C₃H₈, CH₃Cl, CH₃Br, C₂Cl₄, CFC-11, HCFC-141B, Halon-1211, and ⁷Be) and one dynamic tracer (potential temperature). Our analysis results are biased by the measurements at 5-8 km (70% of the data) due to the availability of ⁷Be measurements. The identified tracer characteristics for seven factors are generally consistent with the geographical origins derived from their 10 day back trajectories. Stratospherically influenced air accounts for 14 ppbv (35-40%) of the observed O₃ variability for data with O₃ concentrations <100 ppbv at middle and high latitudes. It accounts for about 2.5 ppbv/month (40%) of the seasonal O₃ trend at midlatitudes but for only 0.8 ppbv/month (<20%) at high latitudes, likely reflecting more vigorous midlatitude dynamical systems in spring. At midlatitudes, reactive nitrogen-rich air masses transported through Asia are much more significant (11 ppbv in variability and 3.5 ppbv/month in trend) than other tropospheric contributors. At high latitudes the O₃ variability is significantly influenced by air masses transported from lower latitudes (11 ppbv), which are poor in reactive nitrogen. The O₃ trend, in contrast, is largely defined by air masses rich in reactive nitrogen transported through Asia and Europe across the Pacific or the Arctic (3 ppbv/month). The influence from the stratospheric source is more apparent at 6-8 km, while the effect Of O₃ production and transport within the troposphere is more apparent at lower altitudes. The overall effect of tropospheric photochemical production, through long-range transport, on the observed O₃ variability and its seasonal trend is more important at high latitudes relative to more photochemically active midlatitudes.