Nitrate formation from heterogeneous uptake of dinitrogen pentoxide during a severe winter haze in southern China
2018
Abstract. Nitrate ( NO 3 - ) has become a major component of fine particulate matter
(PM 2.5 ) during hazy days in China. However, the role of the
heterogeneous reactions of dinitrogen pentoxide ( N 2 O 5 ) in
nitrate formation is not well constrained. In January 2017, a severe haze
event occurred in the Pearl River Delta (PRD) of southern China during which
high levels of PM 2.5 ( ∼400 µ g m −3 ) and
O 3 ( ∼160 ppbv) were observed at a semi-rural site (Heshan)
in the western PRD. Nitrate concentrations reached 108 µ g m −3
(1 h time resolution), and the contribution of nitrate to PM 2.5 was
nearly 40 %. Concurrent increases in NO 3 - and
ClNO 2 (with a maximum value of 8.3 ppbv at a 1 min time
resolution) were observed in the first several hours after sunset, indicating
an intense N 2 O 5 heterogeneous uptake by aerosols.
The formation potential of NO 3 - via N 2 O 5 heterogeneous reactions was
estimated to be between 29.0 and 77.3 µ g m −3 in the early
hours (2 to 6 h) after sunset based on the measurement data, which could
completely explain the measured increase in the NO 3 -
concentration during the same time period. Daytime production of nitric acid
from the gas-phase reaction of OH+NO 2 was calculated
with a chemical box model built using the Master Chemical Mechanism
(MCM v3.3.1) and constrained by the measurement data. The integrated
nocturnal nitrate formed via N 2 O 5 chemistry was comparable to or
even higher than the nitric acid formed during the day. This study confirms
that N 2 O 5 heterogeneous chemistry was a significant source of
aerosol nitrate during hazy days in southern China.
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