The first airborne comparison of N 2 O 5 measurements over the UK using a CIMS and BBCEAS during the RONOCO campaign

Dinitrogen pentoxide (N2O5) plays a central role in nighttime tropospheric chemistry as its formation and subsequent loss in sink processes limits the potential for tropospheric photochemistry to generate ozone the next day. Since accurate observational data for N2O5 are critical to examine our understanding of this chemistry, it is vital also to evaluate the capabilities of N2O5 measurement techniques through the co-deployment of the available instrumentation. This work compares measurements of N2O5 from two aircraft instruments onboard the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft during the Role of Nighttime Chemistry in Controlling the Oxidising Capacity of the 2 Atmosphere (RONOCO) measurement campaigns over the United Kingdom in 2010 and 2011. A chemical ionisation mass spectrometer (CIMS), deployed for the first time for ambient N2O5 detection during RONOCO, measured N2O5 directly using I ionisation chemistry and an aircraft-based broadband cavity enhanced absorption spectrometer (BBCEAS), developed specifically for RONOCO, measured N2O5 by thermally dissociating N2O5 and quantifying the resultant NO3 spectroscopically within a high finesse optical cavity. N2O5 mixing ratios were simultaneously measured at 1 second time resolution (1 Hz) by the two instruments for 8 flights during RONOCO. The sensitivity for the CIMS instrument was 52 ion counts pptv with a limit of detection of 7.4 pptv for 1 Hz measurements. BBCEAS, a proven technique for N2O5 measurement, had a limit of detection of 2 pptv. Comparison of the observed N2O5 mixing ratios show excellent agreement between the CIMS and BBCEAS methods for the whole dataset, as indicated by the square of the linear correlation coefficient, R = 0.89. Even stronger correlations (R values up to 0.98) were found for individual flights. Altitudinal profiles of N2O5 obtained by CIMS and BBCEAS also showed close agreement (R = 0.93). Similarly, N2O5 mixing ratios from both instruments were greatest within pollution plumes and were strongly positively correlated with the NO2 concentrations. The transition from day to night time chemistry was observed during a dusk-to-dawn flight during the summer 2011 RONOCO campaign: the CIMS and BBCEAS instruments simultaneously detected the increasing N2O5 concentrations after sunset. The performance of the CIMS and BBCEAS techniques demonstrated in the RONOCO dataset illustrate the benefits that accurate, high-frequency, aircraft-based measurements have for improving understanding the nighttime chemistry of N2O5.


Introduction
Dinitrogen pentoxide (N2O5) is an important nighttime oxide of nitrogen in both the stratosphere and troposphere. 1,2Nighttime production of N2O5 has a significant impact on the lifetime of NOx, enabling N2O5 to act as a nighttime sink of NOx or a reversible storage of NOx allowing possible transport. 3Furthermore, it also has a major impact on the formation of NO3, the main tropospheric nighttime oxidant.Formation of N2O5 in the stratosphere limits ozone (O3) production 4 and its presence in the troposphere enables halogen activation and the production of inorganic nitrate 5 by reaction with salt aerosols, forming ClNO2. 6The efficiency of daytime tropospheric O3 production and the formation of secondary aerosols are affected by nitrate radical (NO3) and N2O5 levels the previous night 7 NO3 initiates the processing of anthropogenic and biogenic emissions at nighttime and has been shown to compete effectively with the daytime hydroxyl radical processing, especially for unsaturated volatile organic compounds (VOC) due to their high reactivity with NO3. 8 To improve the understanding of the processes affecting O3 formation and air quality, it is necessary to improve the understanding of the atmospheric cycle of N2O5 and NO3 through its formation, loss, spatial variability and role in the regulation of NOx and budgets of VOCs. 9 NO3 is formed through the reaction of O3 and NO2, which can then further react with NO2 to form N2O5. N2O5 is in thermal equilibrium with NO3 which is typically established in a few minutes in the atmosphere. 10 + O 3 → NO 2 + O 2 R1 NO 2 + O 3 → NO 3 + O 2 R2 NO 2 + NO 3 + M ⇄ N 2 O 5 + M R3 NO3 and its equilibrium partner N2O5 are only abundant at night due to the rapid daytime photolysis of NO3; j (NO3) will vary during the day, season and latitude of course but a typical daytime value is (0.2 s -1 ). 11NO3 and N2O5 are also suppressed in the presence of fresh pollution sources because NO3 undergoes a fast reaction with NO.N2O5 mixing ratios can build up during the night reaching maximum concentrations of a few ppbv. 12,13O5 acts as a sink for NOx in the troposphere through its reaction with water to produce nitric acid (HNO3). 14,152 O 5 + H 2 O het → 2HNO 3 R4 Therefore, the nighttime oxidative capacity of the troposphere and NO3 availability is partially dependent upon the hydrolysis of N2O5.The removal of NO3 and N2O5 via reaction (4) directly impacts the production of daytime oxidants such as OH and O3.The relationship between NOx availability and tropospheric O3 production rates is complex, but the hydrolysis of N2O5 is thought to decrease O3 concentrations in low NOx conditions and increase O3 in high NOx regions. 16,17O5 also affects the tropospheric aerosol budget as the nitric acid produced, via its hydrolysis, partitions to the aerosol phase at low temperatures or in regions of excess ammonia. 18,19N2O5 can also be directly taken up on particles or fog droplets resulting in a production of dissolved nitrate. 203][24] Additionally, the uptake co-efficient for N2O5 is rather variable, depending on aerosol composition and meteorological conditions. 25,26Recent studies have shown heterogeneous chemistry of N2O5 on chloride containing aerosols efficiently releases chlorine radicals to the atmosphere via the formation and subsequent photolysis of ClNO2 .6The first measurements of NO3 in the troposphere using differential optical absorption spectroscopy DOAS 27 were followed by a wide range of ground-based studies investigating the role of NO3 in polluted and clean tropospheric environments. 28,29The measurement of N2O5 during these DOAS studies was not possible as it does not absorb at any convenient wavelengths.Techniques for measuring NO3 were then adapted to infer N2O5 concentrations by forcing the thermal equilibrium between NO3 and N2O5 to favour the detectable NO3 species.9][40][41][42][43] The LED broadband cavity enhanced absorption spectrometer used for RONOCO provides NO3, N2O5 and NO2 measurements using three separate channels, each with their own LED light source, cavity and grating spectrometer.The instrument has been described in detail by Kennedy et al. (2011)  13 and builds on our previous work measuring NO3 and N2O5 by BBCEAS in the marine boundary layer 44 and the polluted urban atmosphere. 45iefly, channels 1 and 2 of the BBCEAS instrument operated at red wavelengths to target N2O5 and NO3 respectively, and channel 3 operated at blue wavelengths to target NO2.The BBCEAS instrument sampled air through two rear facing inlets situated on the fuselage of the FAAM BAe-146 aircraft.The flow through the first inlet (50 litres per minute) was divided between channels 1 and 2. Prior to entering the cavity of channel 1, the air flow passed through a preheater at 120C to thermally decompose N2O5 in the sample into NO3 and NO2 with an efficiency of >99.6% for the range of inlet air temperatures and NO2 mixing ratios encountered during RONOCO flights.The NO3 produced from N2O5 decomposition, plus any ambient NO3, was quantified via the 662 nm absorption band of NO3 inside the cavity of channel 1.The cavity was held at 80C to prevent the recombination of NO3 with NO2.The same 662 nm absorption band was used to measure ambient NO3 in channel 2, the cavity of which was maintained close to the air temperature outside the aircraft in order to minimise any perturbation of the NO3/ N2O5 equilibrium.This thermal stabilisation was achieved by flowing ambient air sampled through the instrument's second inlet through a sheath surrounding the cavity.The N2O5 concentration was thus calculated from the difference between the NO3 signals recorded in the heated and ambient temperature cavities.The concentrations of N2O5 and NO3 were corrected for the measured losses of these species in the inlet and on the walls of channels 1 and 2.
The gas flow exhausted from the cavity of channel 2 was then passed into the cavity of channel 3 wherein NO2 was quantified via its highly-structured absorption features between 440 and 480 nm.Excellent agreement was obtained between the NO2 measurements from the BBCEAS instrument and from a commercial, photolytic converter chemiluminescence (CL) detector on board the FAAM BAe-146 aircraft 1 .Typical 1σ detection limits of the BBCEAS instrument were 2.4 pptv for N2O5 and 1 pptv for NO3 (1 s measurements), and 10 pptv for NO2 (10 s measurements).

CIMS instrument
The CIMS technique has been implemented for field measurements of gaseous species since the detection of H2SO4 in 1991. 46Following this work, CIMS has been further developed to successfully detect a wide range of gaseous species using a number of different ionisation schemes.This development of CIMS and the advances made in the technology have been reviewed by Huey (2006). 47Here, the development and implementation of CIMS to detect N2O5 is presented and compared to BBCEAS.The CIMS instrument deployed during RONOCO was built by the Georgia Institute of Technology as previously described by Nowak et al. (2007)  48 and Le Breton et al. (2012). 49The schematic in figure 1 shows the set up used and operating conditions of the CIMS on board the airborne platform FAAM BAe-146 research aircraft.Arrows indicate direction of gas flow.Dimensions are not to scale.

Inlet and ionisation
The CIMS is fitted with two identical inlets, one for background measurements and one for sampling.They consist of 6 cm OD diameter PFA tubing of length 580 mm and are heated to 50 ⁰ C to reduce surface loss.A 3-way valve is used to automate switching between measuring the ambient atmospheric air and the background line which passes the ambient air through and acid scrubber, removing all acids and N2O5 from the flow. 49,50An orifice of diameter 0.9 mm was positioned at the front of the inlet to restrict the flow to 5.8 SLM which was brought in using a rotary vane pump (Picolino VTE-3, Gardner Denver Thomas).This corresponded to a residence time of 0.28 s at standard temperature and pressure in the total length of the . 49These reagent ions then allow the species of interest in the air sample to be detected.

Ion filtration and detection
The ions then passed through a pinhole of a charged plate, which entered the mass spectrometer section of the instrument, i.e. the first octopole ion guide chamber which is the collisional dissociation chamber (CDC).Here, weakly-bound ion-water clusters are broken up to simplify the resultant mass spectrum.Inside the CDC, the pressure was 0.25 Torr and the local electric field divided by the gas number density (E/N) was 180 Townsend (Td = 10 −17 V cm 2 ).The pressure in the CDC of less than 1 Torr was achieved by the use of a molecular drag pump (MDP-5011, Adixen Alcatel Vacuum Technology).After the CDC, the ions passed through the second octopole ion guide, which has the effect of collimating the ions.The octopole chamber was held at a pressure of 10 -3 Torr which was maintained by a turbomolecular pump (V-81M Navigator, Varian Inc. Vacuum Technologies).Beyond the second octopole chamber, the ions were mass selected by a quadrupole with pre and post filters with entrance and exit lenses (Tri-filter Quadrupole Mass Filter, Extrel CMS).
The quadrupole section was kept at a pressure of 10 -4 Torr by a second 15 turbomolecular pump (V-81M Navigator, Varian Inc. Vacuum Technologies).The selected ions were then detected and counted by a continuous dynode electron multiplier (7550M detector, ITT Power Solutions, Inc.).

Ionisation scheme
The ion-molecule chemistry using iodide ions (I -) for trace gas detection has been described by Slusher et al. (2004)  32 and was utilised here to detect N2O5 and nitric acid (HNO3).The heated inlet and lower electric field strength (25 V cm 2 compared with 180 V cm 2 ) allows the CIMS to detect NO3 and N2O5 as NO3 -.Here, a small peak is observed for the ion N2O5 -, although at a sensitivity ratio of 200:1 and therefore is negligible.Laboratory calibrations confirm an interference at mass 62 by NO3 is not observed, deeming the system in this setup is unable to detect NO3.
A gas mixture of methyl iodide (CH3I) and H2O in N2 is used to obtain reagent ions I -and water clusters I -.H2O.The mix was produced using a manifold by evaporating the liquid deionised H2O and CH3I sequentially into the manifold to reach the following partial pressures of 10 Torr H2O and 15 Torr CH3I.Nitrogen was then added up to 5 bar to make an ionisation gas mixture of 0.39 % CH3I and 0.26 % H2O.CH3I ( 99.5 %) was purchased from Sigma Aldrich and used as provided.
N2O5 and HNO3 were ionised by through the I -ionisation scheme via reactions ( 5) and ( 6);

R6
which enabled N2O5 to be detected selectively via the NO3 -ion signal at m/z =62 and HNO3 to form an adduct with I -and be detected at m/z = 189 as shown in figure 2. Typical reagent ion count values were I -= 1.5 ×10 6 Hz, and I.H2O -= 2.5 ×10 6 Hz.As the ionisation efficiency depends on the presence of water vapour through the production of I -.H2O, 32,50 water vapour was added to the ionisation gas mixture, so as to produce an excess of I -.H2O cluster ions over the I -ions and hence allow operation in the water vapour independent regime 32 .The dependency of CIMS sensitivity to I.H2O is shown in figure 3, presenting a formic acid calibration over a range of RH and therefore I - .H2O counts.Mass 145 (I - .H2O) counts never fell below 150 000 during operation onboard the aircraft due to this addition of water vapour.Formic acid has been chosen as the reference mass due to the extensive development on this CIMS for detection and calibration of this species.The sensitivity of the CIMS for N2O5 is assumed to be independent of water vapour amounts, as laboratory tests have shown formic acid and N2O5 sensitivities are linear, as explained in detail in section 2.2.4.above 100 000 I - .H2O counts is independent of I.H2O -counts.

Calibrations
The CIMS was unable to be calibrated for N2O5 during the RONOCO campaign, therefore a single BBCEAS data point was taken to estimate sensitivity of the CIMS to N2O5.The formic acid calibration for this flight was then used to calculate the relative sensitivity ratio, allowing the campaigns formic acid calibrations to determine the CIMS sensitivity towards N2O5.
Airborne formic acid calibrations have been well developed for operation of this CIMS and are performed in-flight and post flight as described in Le Breton et al. (2012, 2013). 49,50N2O5 was calibrated in the laboratory after the campaign by producing a known concentration of N2O5 as described later and simultaneously calibrating the instrument for formic acid.A linear relationship was found for formic acid and N2O5 sensitivities. Figure 4 shows how the CIMS sensitivity to formic acid and N2O5 increase linearly. 3.0x10

FAAM BAe-146 onboard instruments
In addition to the N2O5 data from the BBCEAS and CIMS, NO2 measurements are used in this analysis.Nitric oxide (NO) and nitrogen dioxide (NO2) were measured using separate channels of a photolytic "blue light" converter chemiluminescence detector and were reported every 1 second with an uncertainty of ± 6% ppbv 51 Ozone was measured using a UV Photometric Ozone Analyser at 1 Hz with an uncertainty of 15 ±3 ppbv. 52The FAAM core GPS-aided inertial Navigation system is also used to provide altitude, longitude and latitude.

RONOCO 2010 and 2011 campaign
The two RONOCO flying campaigns were conducted in July 2010 and January 2011 based at the East Midlands Airport, in central United Kingdom.The scientific objectives of RONOCO were to determine the spatio-temporal variation of tropospheric NO3 in different meteorological conditions and seasons, and in a range of gas phase and aerosol environments, in order to quantify the key processes and pathways of oxidized nitrogen chemistry at night in the troposphere.The ultimate aim was to assess the pervasiveness and importance of nighttime chemical processes, and in particular NO3, for UK regional and Western European

Overall comparison
Figure 5 shows the flight tracks of the aircraft for the data presented within this paper.A typical time series for the BBCEAS and CIMS N2O5 data can be seen in figure 6 for flight B566 on January 16 th 2011, showing good agreement between the instruments.The concentrations measured and statistics reported here are at 1 Hz for both instruments.
The CIMS sensitivity is calculated as the average sensitivity for the 8 flights presented here.
The average sensitivity was 52 ± 2 ion counts pptv -1 s -1 , with a limit of detection of 7.8 pptv, calculated as 3 standard deviations above the background counts, and a total measurement error of 19%.The BBCEAS sensitivity was calculated in Kennedy et al. (2011) 13 to be 2.4 pptv for 1 Hz data.Good agreement was obtained between the N2O5 measurements using both CIMS and BBCEAS for the 8 flights presented here (top panel of Fig 5).The linear regression exhibits a line of best fit with a correlation coefficient R 2 = 0.89.The agreement between the CIMS and BBCEAS measurements varies from flight to flight as shown in figure 7 and Table 1.Flight B566 has the highest R 2 value of 0.98, whereas as flight B537 has the lowest R 2 of 0.74.This non linearity may be a result of the difference in the instruments method of concentration retrieval.Spectral techniques can be impeded by pressure broadening and interference by water.However the CIMS sensitivity depends on I.H2O counts, which may decrease at high N2O5 concentrations as shown in figure 3. The mean N2O5 mixing ratio over the 8 flights presented in this work was 114 pptv for CIMS and 115 pptv for BBCEAS.Maximum concentrations reported by the CIMS and BBCEAS were 890 ± 133 pptv and 1007 ± 141 pptv respectively, although these peak concentrations do not originate from the same air mass.These maxima were intercepted during measurements of the London plume travelling North East over the North Sea, but the BBCEAS maxima was reported during flight B534, whereas the CIMS report the measurement during flight B565.

Comparison as a function of altitude
Vertical profiles obtained from aircraft measurements offer the ability to derive profiles from a variety of air masses, locations and meteorological conditions.Previous profiles obtained from aircraft measurements have shown that concentrations of N2O5 are larger and longer lived aloft as compared with the surface 12 , as heterogeneous loss of N2O5 will generally decrease with altitude 53 .At the lowest altitude of 64 metres, both instruments return an average concentration of 45 pptv.A steady increase is observed up to 600 metres where the CIMS records a maximum concentration of 552 pptv and the BBCEAS records a maximum concentration of 353 pptv.

Flight
Both instruments observe a sharp decrease in N2O5 concentration to 100 pptv at 820 metres.An increase again is observed steadily to 300 metres where the CIMS and BBCEAS record concentrations of 512 and 392 pptv respectively.Both instruments observe a very similar drop above this altitude to very small N2O5 concentrations (15 pptv) above 1500 metres.
If it is assumed that nitric acid is produced by the hydrolysis of N2O5, correlations to the nitric acid profile can aid a comparison between the instruments.Both instruments profiles show a very similar structure to that of nitric acid, although rapid decreases in nitric acid at 200 metres and 1200 metres are not observed in either N2O5 measurements.Further analysis using the steady state approximation is presented in a later section.

Comparison as a function of NO2
NO2 plays a key role in nighttime chemistry as it is a primary reactant to N2O5 formation; therefore it is useful to observe NO2 concentrations at the same time as N2O5 measurements to understand the N2O5 formation and trends.Figure 9 shows the NO2 data correlated with N2O5 concentrations from plume detected during flight B566 at 20:44, which increases above background concentrations during the flight (1.5 ppbv) to 15.7 ppbv.Under these conditions the CIMS and BBCEAS detect a similar increase in N2O5 concentrations at this time with close to identical structure.The correlation of N2O5 to NO2 can be seen in figure 9 for the CIMS and BBCEAS which both show a very similar trend (41.82  0.83 pptv N2O5 (ppb NO2) -1 for CIMS, 41.10  0.65 pptv ppb -1 for BBCEAS) and the same high R 2 value; 0.93.
The R 2 for all the data obtained in flight B566 for CIMS and BBCEAS vs NO2 was 0.59 and 0.62 respectively.The daytime average N2O5 concentrations measured by the CIMS and BBCEAS were 22 ± 3 pptv and 18 ± 3 pptv respectively.An increase in concentration is then observed from the point of sunset, indicating the transition from daytime chemistry to nighttime chemistry.This transition is confirmed by the increasing NO2 concentration above the earlier background levels, which is expected due to the reduction in photolysis.The high NO2 concentrations observed from 17:25 until 17:50 correlate with a decrease in O3 concentrations.Low O3 mixing ratios are expected and observed to reduce NO3 and N2O5 concentrations.Following the recovery to higher O3 levels, N2O5 and NO3 progress to their maximum concentrations on the flight.The time series in figure 8 shows that these will originate from the same air mass, but CIMS was calibrating during the time when the BBCEAS measured the maxima and therefore no data could be obtained.This flight shows the ability of both instruments, and measurements of N2O5, to track accurately the transition from day to nighttime chemistry and the emergence of NO2, NO3 and N2O5.

Steady state analysis
Application of the steady state approximation to NO3 and N2O5 yields the expressions

II
Further manipulations using equations I and II then yield an expression for [N2O5] involving just NO, NO2 and O3.Comparison of the [N2O5] derived using equation III using the measurement data of NO, NO2 and O3, together with the rate coefficients taken from laboratory studies, with the measured data produces good agreement in general (figure 11).When one includes the combined uncertainties in rate coefficients and species measurements to derive a lower and upper limit for the steady state analysis, these limits easily bracket the measured data.The dataset splits into two regions, one where nighttime NO is large such that NO3 loss is large via reaction (7).
Here, N2O5 levels cannot build to high levels and is in keeping with the study by Zheng et   al.,(2008). 54These workers studied NO3 and N2O5 vertical profiles during the Milagro campaign over Mexico City and concluded that nighttime NO levels were large and led to a suppression of both species.Alternatively if NO2 production rate dominates, equation III can be simplified to equation IV which further simplifies to equation V (and is in keeping with the linear correlation between N2O5 and NO2.Rearranging equation V yields an expression for k9 which involves all parameters that are measured in this work.The range of values returned for flight B568 is ~ from 5 x 10 -4 s -1 to 7 x 10 -3 s -1 , leading to an estimate of the lifetime for N2O5 of ca. 3 minutes up to about 30 minutes.Assuming that all the N2O5 lost produces gas phase HNO3 (unlikely as one would imagine that a substantial fraction will be incorporated onto aerosol) leads to an upper limit production rate of HNO3 ~ 30 ppt min -1 .Although we note that this is an upper limit, it compares with a typical production rate during daytime through the reaction between OH and NO2, assuming

Summary and conclusions
The

Figure 1 .
Figure 1.Schematic of chemical ionisation mass spectrometer (CIMS) used in this study.
inlet tubing.The pressure in the ion molecule region (IMR) was maintained at 19 Torr throughout the flight and was controlled and measured using a mass flow controller (MKS 1179 and MKS M100 Mass flow controllers, MKS Instruments, UK) and Baratron (1000 Torr range, Pressure Transducer, Model No. 722A, MKS Instruments, UK) and a dry scroll pump (UL-DISL 100, ULVAC Industrial).Here, N2 and the ionisation gas mixture of CH3I/H2O/N2 at a rate of 1 standard cubic centimetres (SCCM) passed over the ion source (Polonium-210 inline ioniser, NRD inc Static Solutions Limited) producing an excess of I -and I -.H2O ions in the ion molecule region (IMR) as described in Le Breton et al. (2012)

Figure 3 .
Figure 3. Formic acid calibration at a range of RH values, indicating that the sensitivity
air quality, eutrophication, and ultimately to quantify its links to climate change.The CIMS instrument measured formic acid, propanoic acid, butanoic acid, nitric acid and N2O5 during the RONOCO campaign.The BBCEAS measured NO3, N2O5 water and NO2.35 hours of data from eight RONOCO flights are presented within this paper for comparison; 5 flights at nighttime in summer 2010, 2 in winter 2011 and 1dusk to dawn transition in winter to study the transition between daytime and nighttime chemistry.All flights sampled air over the UK, North Sea and English Channel which are impacted by pollution from the UK and occasionally continental Europe.

Figure 5 .
Figure 5. Flight tracks from the RONOCO 2010/2011 campaign for the data presented in this work.Flights B534 to B538 were taken during the summer 2010 campaign and B565 to B568 were taken in January 2011.

Figure 6 .
Figure 6.Time series of CIMS (red) and BBCEAS (blue) N2O5 concentrations on flight B566 on the 16 th January 2011.

Figure 7 :Table 1 .
Figure 7: Scatter plots for the entire RONOCO dataset accumulated and for each flight from RONOCO where CIMS and BBCEAS measured [N2O5].The black lines illustrate the linear regression.

Figure 8 :
Figure 8: Vertical profiles of N2O5 and nitric acid (HNO3) mixing ratios during a missed approach at Lydd airport, Kent, during B568.The altitude ranges from 64 to 1711 metre.Linear regression line for this data, R 2 = 0.98.HNO3 measurements are averaged to 15 seconds.The error for CIMS and BBCEAS are 19% and 14% respectively.

Figure 8
Figure8illustrates a concentration profile obtained during flight B568, increasing with altitude from 64 metres to 1711 metres.Good agreement between concentrations returned by both instruments, R 2 = 0.98 (as shown in figure6), confirms the accuracy of the instruments measurements during altitudinal profiles.

Figure 9 :
Figure 9: CIMS and BBCEAS N2O5 concentration time series during flight B566.NO2 scatter plot for both N2O5 measurements (CIMS in red, BBCEAS in blue) and linear regression line for each (CIMS in black, BBCEAS in green).

3. 4 .
Dusk to nighttime transition flight Flight B568 took off from East Midlands Airport, Leicestershire, UK at 14:53 on the 19 th January 2011 and flew south, operating in the English Channel during the dusk to nighttime transition as observed in figure 10, enabling measurements during daylight and nighttime, enabling observation of the transition from day to nighttime chemistry as sunset was at 16:30.

Figure 10 :
Figure 10: Time series plot and flight track from flight B568 for concentrations of N2O5 from the CIMS (red line) and BBCEAS (blue line), NO2 concentrations (green line) and altitude (black line).A correlation plot of the B568 CIMS and BBCEAS N2O5 data set was shown in the bottom right panel of Fig 5.

Figure 11 :
Figure 11: Time series plot from flight B568 of N2O5 concentrations from the CIMS (red line) and BBCEAS (blue line) with the results from the model (green line), model minimum (purple line) and model maximum (grey line). 10 [NO2] = 10 ppb, [OH] = 1 x 10 6 molecule cm-3  , this produces a production rate of approx.20 ppt min -1 .This result would support the modelling work of Jones et al.(2005)55 who show that the nighttime production of HNO3 from N2O5 chemistry can be an efficient sink for NOx, comparable with the daytime production of HNO3 from the hydroxyl radical.Daytime and nighttime measurements byBrown et al. (2004) 56 also confirm similar HNOO3 production in the daytime and nighttime.
scheme.The high correlation during altitudinal profiles (R 2 = 0.93) suggest that the sensitivity of each instrument remains constant throughout varying flight conditions and Brown et al. (2003)10have described in detail the conditions under which the steady state approximation is valid for the analysis of atmospheric levels of NO3 and N2O5.Weak sinks for NO3 in clean conditions can render the steady state inappropriate and under polluted (e.g large NO2 concentrations) the equilibrium between NO3 and N2O5 can slow the approach to steady state.We have analysed the dataset presented in this paper and conclude that the steady state approximation can be applied to the first airborne NO3 and N2O5 measurements over the UK at nighttime during RONOCO.Following the work ofBrown et al., (2003)10we note that five reactions exist under the conditions encountered that will control both species.