Abstract. PM2.5, particulate matter with a diameter of 2.5 µm or less,
is one of the major components of air pollution in eastern China. In the
past few years, China's government has made strong efforts to reduce
PM2.5 pollution. However, another important pollutant (ozone) is becoming
a problem in eastern China. Ozone (O3) is produced by
photochemistry, which requires solar radiation for the formation of O3.
Under heavy PM2.5 pollution, solar radiation is often depressed,
and the photochemical production of O3 is prohibited. This study shows
that during late spring and early fall in eastern China, under heavy
PM2.5 pollution, there was often strong O3 photochemical
production, causing a co-occurrence of high PM2.5 and O3
concentrations. This co-occurrence of high PM2.5 and O3 is
unusual and is the main focus of this study. Recent measurements show that
there were often high HONO surface concentrations in major Chinese megacities, especially during daytime, with maximum concentrations ranging from
0.5 to 2 ppbv. It is also interesting to note that high HONO
concentrations occurred during high aerosol concentration periods,
suggesting that there were additional HONO surface sources in eastern China.
Under high daytime HONO concentrations, HONO can be photodissociated to
OH radicals, which enhance the photochemical production of O3. In
order to study the above scientific issues, a radiative transfer model (TUV;
tropospheric ultraviolet–visible) is used in this study, and a chemical
steady-state model is established to calculate OH radical concentrations.
The calculations show that by including the OH production of
photodissociated HONO, the calculated OH concentrations are
significantly higher than the values without including this production. For
example, by including HONO production, the maximum OH concentration under
high aerosol conditions (AOD = 2.5) is similar to the value under low
aerosol conditions (AOD = 0.25) in the no-HONO case. This result suggests
that even under high aerosol conditions, the chemical oxidizing process
for O3 production can occur, which explains the co-occurrence of high
PM2.5 and high O3 in late spring and early fall in
eastern China. However, the O3 concentrations were not significantly
affected by the appearance of HONO in winter. This study shows that the
seasonal variation of solar radiation plays important roles for controlling
the OH production in winter. Because solar radiation is at a very low
level in winter, adding the photolysis of HONO has a smaller effect in winter
than in other seasons, and OH remains at low values by including the HONO
production term. This study provides some important scientific insight to
better understand O3 pollution in eastern China.
Premature leaf discoloration of European deciduous trees is caused by drought and heat in late spring and cold spells in early fall
