the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Dec 2025
Novel insights on causes of disproportionate trends between particulate NO3− and NOx emissions in Canadian urban atmospheres
Abstract. Particulate nitrate (NO3−) is a key target for controlling air pollution, yet its response to NOx abatement remains uncertain in cold climates. This study assesses trends of fine- and coarse-mode NO3− (f-NO3− and c-NO3−) during 1990–2019 in seven Canadian cities, making use of the long-term data collected by the National Air Pollution Surveillance (NAPS) network, and revealed disproportionate trends between NO3− and NOx emissions across Canada. In Edmonton, annual mean f-NO3− decreased by ~60 % from 2007–2019 while provincial NOx emissions declined by only 10–20 %; comparable patterns were also observed in five out of the six other cities in the most recent decade. Such disproportionate trends were diagnosed to be caused by reduced primary f-NO3− emissions, localized dispersion, and Arctic Oscillation–modulated wind anomalies. Conversely, all cities exhibited a transient f-NO3− increase during 1998–2007, coincident with early NOx controls and consistent with unintended enhancement of primary emissions of f-NO3− formed within stationary-combustion plumes. c-NO3− was largely insensitive to NOx reduction in most cities (except Edmonton), with its trends governed by neutralization reactions with alkaline aerosols rather than HNO3 availability. These findings can help interpret the weak or absent f-NO3− response to NOx reductions worldwide, especially in cold-climate regions.
Competing interests: One of the coauthors is a member of the editorial board of ACP
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 26 Jan 2026)
- CC1: 'Comment on egusphere-2025-6080', Nima Zafarmomen, 02 Jan 2026 reply
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RC1: 'Comment on egusphere-2025-6080', Anonymous Referee #1, 05 Jan 2026
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This manuscript utilizes long-term observational data to investigate the characteristics of fine particulate nitrate (f-NO3-) variations in Canada under the context of NOx emission reductions. It proposes that primary fine particulate nitrate may play a significant role in annual average concentrations and their trends. The research topic is of practical relevance and attempts to explain the observed nonlinear responses from perspectives of meteorological modulation and chemical mechanisms. However, the current version suffers from several weaknesses and the following concerns are addressed:
- The Introduction provides a general motivation related to NOx reductions and nitrate responses but offers only limited discussion of previous studies that have examined the effects of emission changes, climate or meteorological variability on air pollution in Canada. A more comprehensive and regionally focused literature review is needed to better justify the scientific scope and originality of the work.
- The manuscript’s key conclusion is based primarily on indirect evidence, including trend mismatches between NOx and f-NO3- and seasonal behavior. While these analyses are suggestive, they largely rely on exclusion and correlation rather than direct constraints. Without additional lines of evidence (e.g., source apportionment or model-based sensitivity tests), it remains difficult to clearly separate primary nitrate formation from complex secondary or heterogeneous processes. The uncertainty associated with this inference should be more explicitly acknowledged.
- The manuscript argues that wintertime stagnant conditions enhance local accumulation of f-NO3-, thereby supporting a primary formation pathway. However, stagnant meteorology would also be expected to suppress dispersion and increase coarse nitrate (c-NO3-) concentrations. The manuscript does not sufficiently explain why c-NO3- responds much more weakly than f-NO3- under similar conditions, nor does it quantitatively compare their sensitivities to stagnation. A clearer discussion of the differing source regions, formation rates, and transport characteristics of fine versus coarse nitrate is necessary to strengthen this argument.
- The manuscript introduces the Arctic Oscillation (AO) as a key climate factor modulating wintertime pollution levels, but the rationale for focusing exclusively on AO is not sufficiently developed. Other climate drivers, such as ENSO, Arctic sea ice variability, or long-term warming trends, can also influence regional meteorology and air quality in Canada. The authors should better justify why AO was selected over other factors, or at least briefly discuss the potential roles of these climate influences and why they were not considered.
- To investigate controls on annual mean f-NO3-, the analysis focuses on a single site (S-90132) and two representative years (2010 and 2015). The manuscript does not sufficiently justify the representativeness of these years, nor does it demonstrate that the inferred mechanisms are robust across the full observational record.
- In Section 3.5, the analysis is based on a total of only 58 samples divided into three groups. However, the manuscript does not clearly define what constitutes a “sample,” nor does it specify the associated site(s), temporal resolution, observation period, or selection criteria. Given the small sample size and subsequent grouping, the statistical representativeness and robustness of the results are questionable.
- The trend analyses presented in the manuscript appear to be based on annual mean concentrations. Given the pronounced seasonal variability of air pollutants, it remains unclear whether the reported trends remain significant when the data are analyzed on a seasonal basis. Seasonal trend analysis would help determine whether the inferred long-term changes are robust or dominated by specific seasons. In addition, the time period used for pollutant trend analysis does not appear to be fully aligned with the period of major NOx emission reductions. This temporal mismatch complicates causal interpretation and weakens the linkage between observed concentration trends and emission control measures. Clarification and additional analyses addressing these issues would strengthen the trend attribution.
- The random forest (RF) model identifies temperature, PM2.5, and NO2 as key drivers of daily f-NO3- variability. However, the use of approximately 3,000 trees raises potential concerns about overfitting, which should be discussed. In addition, the inclusion of interaction analyses or partial dependence plots for major predictors (e.g., temperature and NO2) would substantially enhance the interpretability and physical relevance of the RF results.
- Figures 1-4 share very similar structures and differ mainly by site, resulting in a degree of redundancy that reduces information density and visual clarity. The authors are encouraged to consider alternative visualization strategies, such as multi-panel figures, combined plots, or summary representations, to improve readability and overall presentation quality.
- The meanings of the open circles and filled circles are inconsistent between Figures 1a and 1c, which may cause confusion for readers. The slanted lines shown in the figures appear to represent regression lines; however, this is not specified in the figure captions. The authors should explicitly clarify this in the captions to avoid ambiguity.
Citation: https://doi.org/10.5194/egusphere-2025-6080-RC1
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This study presents a comprehensive long-term (1990–2019) analysis of fine- and coarse-mode particulate nitrate (f-NO₃⁻ and c-NO₃⁻) in seven Canadian urban atmospheres using NAPS observations. The authors identify systematic, disproportionate trends between particulate nitrate concentrations and NOₓ emission reductions, particularly in cold-climate cities. Despite modest declines in provincial NOₓ emissions (typically 10–30%), f-NO₃⁻ concentrations decreased by up to ~60–70% in recent decades, while c-NO₃⁻ remained largely insensitive to NOₓ controls.
Primary f-NO₃⁻ emissions hypothesis: The explanation is physically plausible and well-argued, but remains indirect. The manuscript would benefit from clearer discussion of how future studies (e.g., near-source plume measurements or isotopic constraints) could directly validate this mechanism.
HNO₃* measurements: The clarification that denuder-based HNO₃ represents an upper bound (HNO₃ + N₂O₅) is important and appropriately handled. Consider briefly discussing how this uncertainty might bias wintertime interpretations (even qualitatively).
Given the study’s emphasis on spatial inhomogeneity and the impact of localized urban sources (as discussed in Category ii uncertainties, Section 3.6), it is essential to contextualize these findings within the broader framework of high-resolution urban monitoring.
I strongly suggest citing the following paper to bolster the discussion on how localized traffic and industrial emissions create complex urban aerosol patterns that traditional stationary sites might struggle to represent: Comprehensive spatiotemporal analysis of long-term mobile monitoring for traffic-related particles in a complex urban environment. > DOI: 10.1016/j.apr.2025.102870