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Luo et al. Carbon Footprints 2025, 4, 14 https://dx.doi.org/10.20517/cf.2024.53 Page 17 of 24

NO concentrations in US cities on weekends, largely due to reduced heavy-duty diesel vehicle activity.
2
These findings align with the weekend effect observed in this study. Together, the studies suggest that traffic
emissions, particularly from freight transport and commuting, are the main contributors to elevated
weekday NO levels.
2

Concerning gender differences, the findings align with Mo et al. (2023), who found that men face higher
NO exposure risks due to occupational factors (e.g., transportation and outdoor work) and longer
2
durations of outdoor activity, based on a Chinese cohort study . Hu et al. (2020) reported similar results in
[45]
a multi-province survey across China . However, this contrasts with the findings of Kim et al. (2019) in
[46]
South Korea, which indicated that women experienced significantly higher average NO exposure than men
2
3
(30.11 vs. 25.83 µg/m ), potentially due to indoor environmental exposures such as cooking fuel
pollution . Furthermore, Chau et al. (2002) in Hong Kong noted that long-distance commuters (typically
[47]
male) are overexposed, but also suggested that women might encounter different exposure pathways related
to residential environments or domestic responsibilities . In addition, Setton et al. (2010), in a simulation
[48]
study, found no gender differences at all, highlighting how exposure assessment methods (e.g., variations in
spatial-temporal resolution between satellite data and individual monitoring) and differences in pollution
source structures (traffic-dominant versus mixed-source cities) may influence observed outcomes .
[49]

The finding that elderly individuals experience lower NO exposure is consistent with most literature.
2
Mo et al. (2023), in a national cohort study, found that older individuals are less exposed to high NO
2
[45]
concentrations along transportation corridors due to limited mobility . Similarly, Chau et al. (2002)
observed that reduced commuting needs among the elderly in Hong Kong contribute to lower traffic-
related exposure . However, such conclusions should be interpreted with caution, as they may overlook
[48]
the impact of residential pollution distribution. If elderly populations are concentrated in older urban areas
or near industrial zones, their static exposure risk could be underestimated.

In summary, the weekend effect and the exposure characteristics of the elderly observed in this study align
with findings from various global regions. However, the complexity of gender-based exposure differences
underscores the need to account for regional cultural contexts (e.g., occupational roles and family
responsibilities) and exposure pathways (i.e., the interplay between occupational and indoor
microenvironment exposures). Future research could benefit from integrating high-resolution pollution
data with individual behavioral trajectories (e.g., GPS-based activity patterns) to further reveal the
mechanisms driving exposure disparities among population subgroups.

DISCUSSION
Spatiotemporal heterogeneity of conversion processes
The derived NO /NO ratio range (0.3-0.7), based on Luo Minghan's proportional simulation method,
2
x
reflects the dynamic interplay between localized emission characteristics and atmospheric chemical
processes in megacities such as Shanghai. This complexity is not adequately captured by national standard
rates. Specifically:

Enhanced Photochemical Conversion: During summer in Shanghai, elevated temperatures, high relative
humidity, and high solar radiation increase O and VOC concentrations, which in turn promote the
3
conversion of NO to NO 2 [50,51] . This process pushes the NO /NO ratio toward the upper limit (0.7).
2
x
Nonlinear Effects of Particulate Matter: During heavy pollution episodes, heterogeneous reactions on PM
2.5