Background Variability of NO₂ in a Remote North Atlantic Island and Its Association with Atmospheric Transport Regimes

1. Introduction

Reactive nitrogen (Nr) plays a central role in atmospheric chemistry and in the broader biogeochemical nitrogen cycle. Since the industrial revolution, anthropogenic activities have substantially altered the global nitrogen cycle through fossil fuel combustion, agriculture, and industrial processes, increasing the abundance and reactivity of nitrogen compounds in the atmosphere [1,2]. Among these species, nitrogen dioxide (NO₂) is of particular importance as a key component of nitrogen oxides (NOₓ = NO + NO₂), influencing ozone formation, oxidative capacity, secondary aerosol production, and nitrogen deposition [3,4,5].

While numerous studies have characterized NO₂ in urban and industrialized regions, comparatively less attention has been given to its variability under remote, background conditions. Most long-term analyses of NO₂ focus on source-dominated environments where local emissions strongly control observed concentrations [6,7]. In contrast, remote marine and insular environments offer a unique opportunity to investigate the behavior of reactive nitrogen decoupled from strong local sources and dominated instead by large-scale atmospheric transport and background chemistry [8,9,10].

Transport processes exert a fundamental influence on the distribution of reactive nitrogen in the troposphere. Advection of continental air masses, vertical mixing, and synoptic-scale circulation patterns can modulate NO₂ concentrations even in regions far removed from primary emission sources [11,12,13]. In marine boundary layer environments, background NO₂ levels typically remain low but can exhibit episodic enhancements associated with long-range transport from continental regions [14,15]. These transport-driven variations provide insight into hemispheric-scale redistribution of reactive nitrogen and its role in regional nitrogen budgets.

The North Atlantic region represents a particularly valuable natural laboratory for studying background atmospheric composition. Air masses arriving in this region are influenced by a combination of mid-latitude westerlies, subtropical circulation, and occasional incursions of continental or dust-laden air [16,17]. Remote island stations in the North Atlantic have historically contributed to advancing understanding of background atmospheric composition, including greenhouse gases and trace species [18,19]. However, sustained observational analyses focusing specifically on background NO₂ variability in such environments remain limited.

Previous work has highlighted the importance of long-term observations in detecting variability and transport signatures in reactive nitrogen species [2,20]. Non-parametric statistical approaches and percentile-based classification methods are increasingly used to distinguish between background and transport-influenced regimes in atmospheric time series without imposing strong distributional assumptions [21]. Such approaches are particularly appropriate for low-concentration environments where variability is modest and episodic.

In this study, we investigate the background variability of in situ NO₂ concentrations measured at a remote North Atlantic island (Azores) over the period 2015–2024. Using an objective percentile-based criterion applied to PM₁₀ as a transport indicator, monthly conditions are classified into background Atlantic and continental-influenced regimes. Differences between regimes are evaluated using non-parametric statistics, and wind speed data are used to support the physical interpretation of transport intensity over the overlapping period 2018–2024. By focusing on a remote insular environment, this work aims to contribute observational evidence to the understanding of how atmospheric transport regimes modulate background reactive nitrogen levels in marine settings.

The following section describes the study area, datasets, and methodological approach used in this work.

2. Materials and Methods

2.1. Study Area

The study was conducted on Faial Island, located in the Azores archipelago in the central North Atlantic Ocean. Owing to its remote oceanic location between Europe and North America, the region is predominantly influenced by marine boundary layer conditions and mid-latitude westerlies (Figure 1).

The island environment is characterized by the absence of major industrial emission sources, making it suitable for investigating background atmospheric composition and transport-driven variability of atmospheric pollutants.

Air quality observations were obtained from the Espalhafatos monitoring station, located in the parish of Ribeirinha (municipality of Horta) at approximately 38.60° N, 28.63° W, at an elevation of about 120 m above sea level (Figure 2).

The station is part of the regional air quality monitoring network of the Azores, and its measurements are integrated into the Portuguese national air quality information system QualAr, operated by the Portuguese Environment Agency. According to the classification criteria established under Directive 2008/50/EC [22], the site is categorized as a rural background monitoring station, meaning that pollutant concentrations measured at this location are minimally influenced by nearby emission sources and are representative of regional atmospheric conditions. Continuous measurements include nitrogen dioxide (NO₂), sulfur dioxide (SO₂), ozone (O₃), and particulate matter (PM₁₀ and PM₂.₅). In the present study, NO₂ and PM₁₀ observations were used to characterize background atmospheric conditions and to identify periods influenced by enhanced atmospheric transport.

Meteorological conditions were characterized using observations from the Alto do Cabouco meteorological station, located on Faial Island at approximately 38.59° N, 28.69° W, at an elevation of about 330 m above sea level (Figure 3).

The station belongs to the regional hydrometeorological monitoring network of the Azores and records standard meteorological variables including wind speed, wind direction, air temperature, relative humidity, atmospheric pressure, and precipitation. In this study, wind speed and wind direction were used to support the interpretation of atmospheric transport conditions influencing the observed variability of NO₂ concentrations.

Due to its position in the mid-latitude North Atlantic, the Azores region is frequently influenced by synoptic-scale atmospheric circulation patterns, including westerly flow and episodic continental air-mass transport, which can modulate background trace gas variability. Remote oceanic islands such as the Azores provide valuable observational platforms for studying background atmospheric composition, as they are largely decoupled from strong local emission sources while remaining sensitive to long-range transport processes.

2.2. NO₂ and PM₁₀ Observations

Hourly NO₂ and PM₁₀ data were obtained from the QualAr air quality monitoring network (Agência Portuguesa do Ambiente) for the period January 2015 to December 2024. All analyses were performed using monthly aggregates derived from hourly observations.

2.2.1. Data Completeness Criterion

To ensure representativeness of monthly statistics, a completeness threshold was applied. For each month m, data completeness was defined as shown in Equation (1):

C_m = (N_v,m / N_t,m) × 100 [1]

where ${\mathit{N}}_{\mathit{v},\mathit{m}}$ is the number of valid hourly observations in month m and ${\mathit{N}}_{\mathit{t},\mathit{m}}$ the total expected number of hourly observations in that month. Only months satisfying Equation (2):

C_m ≥ 80% [2]

were retained in subsequent analyses.

The data coverage and completeness of the NO₂ observations for the period 2015–2024 are summarized in Table 1.

2.2.2. Monthly Aggregation

Monthly mean NO₂ concentrations were computed as presented in Equation (3):

$\overline{{\mathit{N}\mathit{O}}_{2_{\mathit{m}}}}$ = ${\displaystyle \frac{1}{{\mathit{n}}_{\mathit{m}}}}$  $\sum _{\mathit{i}=1}^{{\mathit{n}}_{\mathit{m}}}{\mathit{N}\mathit{O}}_{2,\mathit{i}}$ [3]

where ${\mathit{n}}_{\mathit{m}}$ denotes the number of valid hourly observations in month $\mathit{m}$ and $\mathit{N}{\mathit{O}}_{2,\mathit{i}}$ the corresponding hourly concentration.

Monthly mean PM₁₀ concentrations were calculated analogously.

2.2.3. Transport Regime Classification

Transport regimes were defined using a percentile-based classification applied to monthly mean PM₁₀ concentrations. The 90th percentile of the monthly PM₁₀ distribution over the study period was computed as shown in Equation (4):

${\mathit{P}}_{90}$ = ${\mathit{P}\mathit{e}\mathit{r}\mathit{c}\mathit{e}\mathit{n}\mathit{t}\mathit{i}\mathit{l}\mathit{e}}_{90}$ ($\overline{{\mathit{P}\mathit{M}}_{10\mathit{m}}}$ ) [4]

where distribution of monthly mean is PM_10m.

Months were classified as:

$$\mathrm{Continental}\mathrm{regime}\mathrm{if}\overline{{\mathit{P}\mathit{M}}_{10\mathit{m}}}\ge {\mathit{P}}_{90},$$

$$\mathrm{Background}\mathrm{regime}\mathrm{i}\mathit{f}\overline{{\mathit{P}\mathit{M}}_{10\mathit{m}}}<{\mathit{P}}_{90}$$

This objective threshold was used to distinguish months influenced by enhanced continental transport from prevailing background Atlantic conditions.