The ozonesonde observations in Hanoi, Vietnam, over fourteen years since
2004 have confirmed the enhancement in lower tropospheric ozone
concentration at about 3 km altitude in the spring season. We
investigated the evolution of the ozone enhancement from analysis of
meteorological data, backward trajectories, and model sensitivity
experiments. In spring, air masses over Hanoi exhibit strong height
dependence. At 3km, the high-ozone air masses originate from the land
area to the west of Hanoi, while low-ozone air masses below about 1.5 km
are from the oceanic area to the east. Above 4 km, the air masses are
mostly traced back to the farther west area. The chemical transport
model simulations revealed that precursor emissions from biomass burning
in the inland Indochina Peninsula have the largest contribution to the
lower tropospheric ozone enhancement, which is transported upward and
eastward and overhangs the clean air intrusion from the ocean to the
east of Hanoi. At this height level, the polluted air has the horizontal
extent of about 20 degrees in longitude and latitude. The polluted air
observed in Hanoi is transported further east and widely spread over the
northern Pacific Ocean.