Estimation of Daytime NO<sub><b>3</b></sub> Radical Levels in the UK Urban Atmosphere Using the Steady State Approximation Method

Khan, M. A. H.; Morris, W. C.; Watson, L. A.; Galloway, M.; Hamer, P. D.; Shallcross, B. M. A.; Percival, C. J.; Shallcross, D. E.

doi:https://doi.org/10.1155/2015/294069

Advances in Meteorology

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Research Article | Open Access

Volume 2015 | Article ID 294069 | https://doi.org/10.1155/2015/294069

Estimation of Daytime NO₃ Radical Levels in the UK Urban Atmosphere Using the Steady State Approximation Method

M. A. H. Khan,¹W. C. Morris,¹L. A. Watson,¹M. Galloway,¹P. D. Hamer,¹B. M. A. Shallcross,¹C. J. Percival,²and D. E. Shallcross¹

Academic Editor: Pedro Jiménez-Guerrero

Received03 May 2015

Accepted28 Jun 2015

Published29 Jul 2015

Abstract

The steady state approximation has been applied to the UK National Environment Technology Centre (NETCEN) data at three urban sites in the UK (Marylebone Road London, London Eltham, and Harwell) over the period of 1997 to 2012 to estimate the concentrations of daytime NO₃. Despite the common assertion that NO₃ levels are negligible in the day as a consequence of photolysis, there are occasions where NO₃ reaches a few pptv. A seasonal pattern in NO₃ concentration was observed with higher levels in the spring with consistent peaks in April and May. A combination of temperature effects (the formation of NO₃ from the reaction of NO₂ with O₃ has a high activation energy barrier), a distinct pattern in O₃ concentration (peaking in spring), and loss via reaction with NO peaking in winter is responsible for this trend. Although reaction with OH is still the dominant loss process for VOCs during the day, there are VOCs (unsaturated) that will have an appreciable loss due to reaction with NO₃ in the daytime. Since the addition reaction of NO₃ with alkenes can lead directly to organic nitrate formation, there are implications for O₃ formation and secondary organic aerosol formation during daytime and these are discussed.

1. Introduction

The role of the nitrate radical (NO₃) in the atmospheric cycle has long been recognized and it is believed to be one of the most important oxidizing species in the nighttime troposphere [1–4]. NO₃ is known to photolyze rapidly in the day [5] but in addition under high NO levels, NO₃ is titrated rapidly during the day and until recently it has been assumed that it is restricted to be at night-time hours where it can be an important sink of organic species [6]. NO₃ was observed in the day in previous studies [7, 8] and through its oxidation of VOCs it may be contributing a nonnegligible fraction to the total RO₂ budget [9].

NO₃ radicals are produced by the relatively slow oxidation of NO₂ by O₃, which then react with NO₂ to form N₂O₅. N₂O₅ can thermally decompose giving back NO₂ and NO₃: In the daytime, NO₃ is readily photolysed through the reactions (3)-(4), with a life time of ~5 s with the sun overhead [10] and hence its importance as an oxidant is greatly reduced [11]:Urban daytime NO₃ can be removed efficiently via the reaction with NO [12]:Another loss process of NO₃ involves reaction with alkenes leading to the production of peroxy radicals [10]:NO₃ radicals can react with peroxy radicals via reaction (7) to produce alkoxy radicals and these can, depending on R, form HO₂ radicals and be a source of HO_x [13–22]. Recent studies have indicated that the levels of NO₃ through the day are higher than previously thought [7, 8, 23, 24]. Geyer et al. [8] found levels of daytime NO₃ to range from 5 to 31 ppt with the maximum levels measured in the hours preceding sunset, when photolysis rates were lower. Brown et al. [7] and Horowitz et al. [23] have recognised that NO₃ at even the tenths of pptv levels can begin to play a role in the oxidation of some VOCs (unsaturated). Horowitz et al. [23] have identified NO₃ reactions with isoprene around dusk and into the early evening as being a small flux for isoprene loss (6%) but a disproportionately high flux for the formation of isoprene nitrates (49%), impacting on aerosol formation. Therefore, daytime NO₃ may be a key component in controlling summertime O₃ in the Eastern USA and in semipolluted areas in general.

Measurements of NO₃ concentration in the atmosphere can be directly obtained, but typically sensitivities for various techniques (e.g., cavity ring down spectroscopy (CRDS), differential optical absorption spectroscopy (DOAS)) are in the pptv range [25]. It is known for [NO₃] that it exceeds these levels in polluted and semipolluted regions during night [26–30], but daytime measurements have rarely been made suggesting that levels are well below the instrumental detection limit [8]. The published daytime NO₃ concentrations and the understanding of their role in the daytime oxidizing cycle are limited. However, there are other trace gases (e.g., NO, NO₂, O₃, and unsaturated VOCs) that have been measured which can be used to estimate long-term daytime NO₃ concentration by the steady state approximation.

Therefore, we estimate the urban daytime levels of NO₃ over the time period of 1997 to 2012 at 3 urban sites in the UK (e.g., Marylebone Road London, London Eltham, and Harwell) using the steady state approximation as described by Brown et al. [7]. Data for NO, NO₂, O₃, and some VOCs are taken from hourly measurements made as part of the NETCEN (National Environmental Technology Centre, Culham, Oxfordshire) data archive [31]. The conceivable impacts on the seasonal and daily variations of the level of NO₃ for all of the selected sites are discussed in the study. Limitations of the method and interpretation of these results are also presented.

2. Methodology

2.1. The Steady State Approximation

Assuming the steady state approximation, the rate of total production of NO₃ and the rate of loss of NO₃ are equal; that is,Brown et al. [7] have shown that the production and loss processes of NO₃ in the day are The loss due to photolysis only exists in daylight hours. In order to determine which hours of the day photolysis would occur, the solar zenith angle () was calculated through where is equal to the declination of the sun, is the hour angle, and is the latitude. If is greater than 90°, the region is in darkness and if less than 90°, it is daylight and photolysis is present as a loss process.

A photolysis constant () was calculated as where is a specific actinic flux, is the absorption cross-section, is the quantum yield, and is the wavelength. More details about the calculation can be found in Landgraf and Crutzen [35]. It should be noted that in these calculations fixed layers of clouds are used to determine the photon flux and hence photolysis rate but in conventional parlance these are clear sky photolysis rates. The steady state concentration of NO₃ during the day becomesThe importance of alkenes to the budget of NO₃ is negligible when compared with NO and photolysis. For example, 1.5 × 10¹² molecules cm⁻³ isoprene (i.e., approximately 60 ppb) is required to achieve the loss rate of 1 s⁻¹. From London Eltham site in 2005, the median value for isoprene = 7.0 × 10⁸ molecules cm⁻³, which is 10⁴ lower than the required value, confirming the negligibility of the VOCs to the NO₃ budget. Of course a sum of alkenes is required but a calculation using available alkene data and scaling up to include other possible alkenes all suggest that the direct loss via reaction with alkene is not significant for NO₃, although we show later that the loss via reaction with NO₃ for some alkenes is very significant.

The resultant steady state equation used is thereforeThrough knowledge of the concentrations of the relevant species, rate constants, and photolysis rates, the concentration of NO₃ is calculated. The statistical language R and the associated “openair package” have been used for performing statistical analysis and making figures [36].

2.2. Site Selection

The monitoring sites in the NETCEN archive have been chosen in areas which give a range of urban environments (kerbside, suburban, and rural). Sites are different, ranging from being impacted heavily by vehicles (e.g., Marylebone Road) through to being heavily impacted by VOCs from vegetation (e.g., Harwell). All sites are at ground level and have continuous hourly data of all the species required for the calculation. The site description is summarised in Table 1.

3. Results and Discussion

The average daytime NO₃ concentrations over the time period of 1997 to 2012 for all three urban sites of UK (e.g., London Eltham, London Marylebone Road, and Harwell) vary significantly throughout the daytime (see Figure 1). NO₂ and O₃, both involved in the major NO₃ production reaction, are of great importance for the variation of daytime NO₃ levels. As a result of the photolysis, a rapid reduction of NO₃ occurs throughout the daylight hours, and the reduction is more pronounced at the middle of the day. The levels of NO₃ in the early morning and early evening time are found to be comparatively higher than the midday NO₃ level because of the reduced effect of photolysis (see Figure 1). The loss of NO₃ by reaction with NO is high throughout the daylight hours (this peaks in the morning) (see Figure 2) and is also responsible for the regional variation in the magnitude of NO₃ across the three sites. Combined, these two major loss processes contribute together to the suppression of NO₃ concentrations in the day.

(a)

(b)

(c)

Table 2 provides the statistics of the daytime NO₃ level calculated by the steady state approximation for the three monitoring sites of UK over the time period of 1997 to 2012. The daytime NO₃ values calculated in this study are comparable with the previous observed results [7, 8]. Geyer et al. [8] measured NO₃ by LP-DOAS in August, 2000, at a site near Houston and found NO₃ levels of 5 pptv during 3 hours prior to sunset with a maximum of 31 pptv at sunset. Brown et al. [7] measured daytime NO₃ by CRDP from aircraft during the New England Air Quality Study in the summer of 2004 and found approximately 0.5 pptv near midday. In this latter campaign, the measurement of NO₃ was recorded only from 2 to 3 days of campaign results, which subsequently limits the scope of the comparison with our estimated NO₃ values. However, there are many occasions in our study where NO₃ reaches up to a few pptv in summer daytimes (e.g., 4.7 ppt on 5 June 2002 8:00 pm, 3.9 ppt on 13 July 2003 8:00 pm at Harwell site, 2.9 ppt on 10 August 2001 8:00 pm, and 2.8 ppt on 16 July 2010 8:00 pm).

The daytime NO₃ levels for Harwell and London Eltham locations are found to be very similar in magnitude (average of 0.06 ppt). However, the levels at Marylebone Road are found to be the lowest (average of 0.01 ppt) because of higher NO levels with a day time average value of 135 ppb for the period of 1998 to 2012 arising from heavy traffic congestion (50,000 vehicles per day). This results in the decreased ozone levels (average of 8.8 ppb) which leads to a decrease in the production of NO₃ and hence an overall decrease of NO₃ levels in Marylebone Road. Another reaction that produces NO is the conversion via photolysis of NO₂ whose concentration is also higher at Marylebone Road (average of 57.8 ppb) and this too contributes to a reduction of NO₃ concentrations. The relatively low levels of NO in Harwell (average of 2.9 ppb) and London Eltham (average of 10.6 ppb) lead to a reduction in the loss process of NO₃ that results in higher daytime NO₃ concentrations compared with Marylebone Road.

A clear seasonal variation of the levels of daytime NO₃ is observed at the three monitoring sites in the UK over the period 1997–2012 (Figure 1). Photolysis frequencies are mainly responsible for the diurnal variation as well as contributing to the seasonal variability of NO₃. Through the daytime and the summer months, the solar zenith angle is at its lowest; hence levels of photolysis increase promoting NO₃ loss. The increase in photolysis is counteracted by increasing O₃, decreasing NO concentrations (see Figure 2), and increasing temperature which increases the production of NO₃ via reaction (1), resulting in the overall higher summer levels. However, in the summer months, the increase in OH levels leads to enhanced production of HNO₃ through the reaction of OH and NO₂, which indirectly removes NO from the troposphere.

Overall, the daytime NO₃ levels peak in spring with lower concentrations throughout the winter months for all sites. Urban O₃ levels at the monitoring sites of UK tend to peak in late spring, whereas the changes in NO₂ throughout the year are less pronounced with slightly lower levels in the summer months (see Figure 2), and the seasonal variation of O₃ leads to the highest NO₃ concentration in spring. The lower concentrations of NO₃ in winter months were observed due to the suppression of NO₃ production from the temperature dependent reaction between NO₂ and O₃. This reaction has an activation energy of 20.54 kJ mol⁻¹ and a resultant rate constant of 3.52 × 10⁻¹⁷ cm³ molecules⁻¹ s⁻¹ at 298 K; this rate constant experiences a large reduction of 50% (i.e., 1.76 × 10⁻¹⁷ cm³ molecules⁻¹ s⁻¹ at 275 K), when the temperature is reduced by 23 K [37]. It has been found that NO₃ formation rates at Marylebone Road, Eltham, and Harwell sites vary by about 20% for the average summer winter temperature difference of 11 K.

Coupled with temperature, the NO and NO₂ concentrations can affect the seasonal variations of NO₃ concentration. The concentration of NO is the highest in winter months for all of the sites (see Figure 2). The reaction of NO with NO₃ is the major daytime loss process for NO₃ in the urban environment with a rate constant at 298 K = 2.7 × 10⁻¹¹ cm³ molecules⁻¹ s⁻¹ [38]. At this rate constant, ~0.3 ppb of NO is required to produce a loss rate comparable with photolysis. On the Marylebone Road, the average winter NO concentration is found to be 165 ppb, which has a significant impact on the concentration of daytime NO₃ in winter time.

The concentrations of NO₃ for all sites vary throughout the week, as shown in Figure 1. The highest concentrations are observed at the weekends and are at their lowest during the weekdays in London Eltham and Marylebone Road. This trend follows the level of traffic congestion, which is reduced at the weekends. This causes the loss process by NO to be increased during weekdays (see Figure 2) due to high levels of congestion on the Marylebone Road as well as London Eltham and the subsequent anthropogenic contribution to the NO concentration, resulting in lower NO₃. In Harwell, an opposite trend is observed with higher concentrations of NO₃ during weekdays in comparison with the weekend. Like other sites, the NO_x concentrations at the Harwell site during weekdays ([NO] = 3.1 ppb and [NO₂] = 6.1 ppb) are higher than that during weekend ([NO] = 2.1 ppb and [NO₂] = 4.2 ppb). However, the relatively low NO at the Harwell site greatly reduces the impact of the major loss process for NO₃ and enhances the impact of the major production term, [O₃][NO₂], at higher concentration of O₃ during weekend (weekday’s [O₃] = 28.5 ppb and weekend’s [O₃] = 30.7 ppb).

In the study, extensive datasets of daytime NO₃ were obtained through the application of the steady state approximation. Although the steady state provided an excellent way of calculating the concentrations given the availability of trace gases, there is a degree of error associated with the calculations. The rates of photolysis were calculated considering clear sky; in reality there are a number of different variables (e.g., cloud cover) that can affect photolysis rates, so the NO₃ estimated in this study would be a lower limit on balance. Given that at the sites analysed the NO₃ concentration calculation is dominated by the concentration of NO and not photolysis, typically photolysis is responsible for 22% of the loss for the typical NO concentration of 1 ppb, even though the photolysis rate could be reduced by cloud cover by 50% and the overall impact of the photolysis on the calculated NO₃ concentration is around 11%. The largest uncertainty in the calculation arises from the rate coefficients (typically ± 10%) and and in the concentrations of O₃, NO₂, and NO (typically ± 10%), which combined gives an estimate of the overall uncertainty of ±40%. Nevertheless, such an analysis provides insight into long-term measurements of daytime NO₃ concentrations that are currently absent.

3.1. Impact of Daytime NO₃ on VOC Burdens

The daytime oxidation of VOCs is dominated by their reactions with OH and O₃. However, certain biogenic VOCs (e.g., terpenes) have rate coefficients for their reaction with NO₃, which are similar or greater than with OH (see Table 3) and thus, provided sufficient levels of NO₃ exist, will provide competition with or dominate the daytime reactivity of these VOCs [39]. Using urban daytime concentrations of NO₃ and OH of 0.06 ppt and 1 × 10⁶ molecules cm⁻³, respectively, for calculating the comparison of VOC loss by NO₃ and OH, the ratio of loss rates of VOC () by NO₃ and OH (Table 3) shows that monoterpenes are removed more effectively by NO₃ during the daytime than by OH. The monoterpenes can be impacted to a greater extent by daytime levels of NO₃ in the urban sites of UK, particularly in the case of limonene; the relative rate of removal by NO₃ compared with OH is 33 for Harwell and London Eltham sites and 5.5 even for the Marylebone Road London. The pinene oxidation by OH experiences competition from NO₃ in these urban sites when the concentrations of NO₃ and OH are similar. However, if phenomena such as a higher NO₃ production occurs due to higher NO₂ and O₃ levels, oxidation of pinene via NO₃ can be of great significance. Geyer et al. [8] observed that 32% of all daytime α-pinene oxidation was through its reaction with NO₃. VOCs produced in areas around the Marylebone Road such as Regents Park, the wooded areas in London Eltham, and the vegetated area surrounding Harwell are likely to be affected by the recorded daytime levels of NO₃ via oxidation of these biogenic sources.

The products of biogenic VOC oxidation by NO₃ differ from those produced via reaction by OH and are of interest due to the relative abundance of isoprene and monoterpenes in the troposphere [40]. The direct impact of the daytime NO₃ levels on VOCs has not been analysed in our study, but considering the previous studies [7, 8, 23], the nonnegligible daytime NO₃ levels estimated using the steady state approximation from UK urban sites will have significant implications on the burdens of biogenic VOCs (e.g., monoterpenes). The oxidation of alkenes by NO₃ can produce peroxy radicals [41] which can contribute to the total peroxy radical concentrations in the urban sites. The reactions of conjugated double bond alkenes (e.g., isoprene, monoterpenes) with NO₃ can produce organic nitrate compounds directly, which are significant NO_x-reservoir compounds affecting regional ozone formation [23, 42, 43]. Such nitrates have low enough vapour pressure to condense at the pressure and temperature experienced in UK urban sites and could have a large impact on SOA formation in the urban area [44–46].

4. Conclusion

National data in the UK has been assessed to calculate the daytime NO₃ levels at three sites in the UK (London Eltham, Marylebone Road, and Harwell) over the period 1997–2012. At each site, hourly measurements of NO, NO₂, and O₃, with estimates of the photolysis frequency, were used to determine daytime NO₃ radical levels. The maximum values of daytime NO₃ for all sites are found to be in the hours proceeding sunset, when photolysis rates are at a minimum. The average daytime NO₃ concentration ranges from 0.01 to 0.06 ppt, with the lowest value returned by the most polluted site (Marylebone Road). All sites experience the same overall trend in daytime NO₃ throughout the calendar year. Peaks appear in spring, where O₃ levels pass through a maximum. Decreased levels of NO vital to the loss process of NO₃ see a slight decrease during the summer and provide further explanation for the seasonal trends observed. Higher levels of OH in summer are accountable for the decreased levels of NO as it reacts with NO₂ to form HNO₃, which indirectly removes NO from the troposphere. Although the levels of NO₃ are generally sub-pptv, they are high enough to see nonnegligible effects, particularly on the burdens of biogenic terpenes making a significant contribution to organic nitrate formation which could impact upon the formation of secondary organic aerosol during the day.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors thank NERC and the Dorothy Hodgkin Foundation under whose auspices this work was carried out. The authors also thank R. K. Farmer and C. Walker for their supports in the data processing.

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Copyright © 2015 M. A. H. Khan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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