Comprehensive volatile organic compound measurements and their implications for ground-level ozone formation in the two main urban areas of Vietnam
30/12/21 05:43PM
To Thi Hien, Duong Huu Huy, Pamela A. Dominutti, et al. Atmospheric Environment: 118872, 2021.
Abstract:
Volatile organic compounds (VOCs) and oxygenated VOCs (OVOCs) were
measured in Ho Chi Minh City (HCMC) and Hanoi, the two largest and most
populous cities in Vietnam. The purpose of this study is to better
understand the VOC atmospheric composition and their role in
ground-level ozone formation. Online measurements of a wide range of
VOCs and other pollutants were conducted using numerous instruments
during different seasons (dry and rainy) in HCMC and Hanoi (spring). Our
results show that the mean mixing ratio of total measured VOCs in Hanoi
was 80.8 ± 40.7 ppb (mean ± standard deviation), and was similar to
that observed during the rainy season (75.2 ± 44.8 ppb) in HCMC. During
the dry season campaign in HCMC, which was coincident with the Hanoi
campaign, total VOC was around 50% lower (40.7 ± 19.5 ppb), largely a
result of increased planetary boundary layer (PBL) height and the
direction of the prevailing wind. VOC profiles in both cities were
dominated by alkanes (31–35%) and OVOCs (27–33%) and the proportion of
alkenes (13–17%) and aromatics (12–19%) were comparable. Similarities in
diurnal variation for most VOC species (except for isoprene) are seen
in both cities with two clear peaks during the morning (7:00–8:00 a.m.)
and evening (18:00 p.m.) rush hours, as observed for
vehicular-combustion tracers (acetylene and CO). Comparisons of the
ambient ratios of paired VOCs, namely i/n-pentane, and toluene/benzene,
with those reported in motorcycle exhaust, roadside and gasoline samples
indicate that motorcycle-related emission is likely a major contributor
to VOC pollution. According to the propylene-equivalent concentration
(PE conc.) and maximum incremental reactivity (MIR) methods, alkenes and
aromatics were determined to be the main contributors to reactivity and
ozone potential formation. Furthermore, the initial mixing ratio of VOC
species was estimated based on the photochemical age method. The
consumed VOCs (initial VOCs minus measured VOCs) has a similar variation
trend to ground-level ozone, and a good correlation is observed in
HCMC. In contrast, this result was not seen in Hanoi despite relatively
high levels of PE conc. and MIR.
