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Advances in Meteorology
Volume 2015 (2015), Article ID 294069, 9 pages
http://dx.doi.org/10.1155/2015/294069
Research Article

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

1Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK
2The Centre for Atmospheric Science, The School of Earth, Atmospheric and Environmental Science, The University of Manchester, Simon Building, Brunswick Street, Manchester M13 9PL, UK

Received 3 May 2015; Accepted 28 June 2015

Academic Editor: Pedro Jiménez-Guerrero

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.

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 [14]. 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 HOx [1322]. 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 [2630], 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.

Table 1: Site description.

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.

Figure 1: The hourly, monthly, and daily daytime NO3 concentration in the three urban sites of UK over the time series of 1997 to 2012.
Figure 2: The hourly, monthly, and daily daytime NO, NO2, and O3 concentration in the three sites of UK for the time series 1997–2012, (a) London Eltham, (b) Marylebone Road London, and (c) Harwell.

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).

Table 2: Average and maximum daytime NO3 at the different urban sites of UK for the time period of 1997 to 2012.

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 NOx 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.

Table 3: Relative rates of removal of hydrocarbon [] of some selected VOCs by NO3 and OH considering the mixing ratios of NO3 and OH are 0.06 ppt (this study for UK urban representative sites (e.g., London Eltham)) and 1 × 106 molecules cm−3 (tropospheric average global concentration), respectively.

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 NOx-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 [4446].

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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