Exploring the inconsistent variations in atmospheric primary and secondary pollutants during the 2016 G20 summit in Hangzhou, China: implications from observations and models
Authors: Zhang Gen , Xu Honghui , Wang Hongli , Xue Likun , He Jianjun , Xu Wanyun , Qi Bing , Du Rongguang , Liu Chang , Li Zeyuan , Gui Ke , Jiang Wanting , Liang Linlin , Yan Yan , Meng Xiaoyan 
Published: May 2020

Abstract. Complex aerosol and photochemical pollution (ozone and
peroxyacetyl nitrate, PAN) frequently occur in eastern China, and mitigation
strategies to effectively alleviate both kinds of pollution are urgently
needed. Although the effectiveness of powerful control measures implemented
by the Chinese State Council has been comprehensively evaluated in terms of reducing
atmospheric primary pollutants, the effectiveness in mitigating
photochemical pollution is less assessed and therefore the underlying
mechanisms are still poorly understood. The stringent emission controls
implemented from 24 August to 6 September 2016 during the summit for the Group
of Twenty (G20) provide us a
unique opportunity to address this issue. Surface concentrations of
atmospheric O3, PAN, and their precursors including volatile organic
compounds (VOCs) and nitrogen dioxides (NOx), in addition to the other
trace gases and particulate matter, were measured at the National Reference
Climatological Station (NRCS) (30.22∘ N, 120.17∘ E, 41.7 m a.s.l)
in urban Hangzhou. We found significant decreases in atmospheric PAN,
NOx, total VOCs, PM2.5, and sulfur dioxide (SO2) under
the unfavorable meteorological conditions during G20 (DG20) relative to the
adjacent period before and after G20 (BG20 and AG20), indicating that the
powerful control measures were effective in reducing the pollutant
emissions in Hangzhou. Unlike with the other pollutants, daily maximum
8 h average (DMA8) O3 exhibited a slight increase and then decrease
from BG20 to AG20, which was mainly attributed to the variation in the solar
irradiation intensity and regional transport in addition to the contribution from
the implementation of stringent control measures. Results from an observation-based
chemical model (OBM) indicated that acetaldehyde and methylglyoxal (MGLY)
were the most important second-generation precursors of PAN, accounting for
37.3 %–51.6 % and 22.8 %–29.5 % of the total production rates including
the reactions of OVOCs, propagation of other radicals, and other minor
sources. Moreover, we confirmed the production of PAN and O3 was
sensitive to VOCs throughout the whole period, specifically dominated by
aromatics in BG20 and DG20 but by alkenes in AG20. These findings suggested
that reducing emissions of aromatics, alkenes, and alkanes would mitigate
photochemical pollution including PAN and O3. Source appointment
results attributed the reductions of VOC source and ozone formation
potentials (OFPs) during G20 to the effective emission controls on traffic
(vehicle exhaust) and industrial processes (solvent utilization and
industrial manufacturing). However, fuel combustion and biogenic emissions
both weakened such an effect with a sizable contribution to the VOC mixing
ratios (18.8 % and 20.9 %) and OFPs (25.6 % and 17.8 %), especially
during the latter part of G20 (G20 II) when anthropogenic VOCs were
substantially reduced. This study highlights the effectiveness of stringent
emission controls in relation to traffic and industrial sources, but a
coordinated program related to controlling fuel combustion and biogenic
emissions is also required to address secondary pollution.