Effects of NO x control and plume mixing on nighttime chemical processing of plumes from coal-fired power plants

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117,, doi: /2011jd016954, 2012 Effects of NO x control and plume mixing on nighttime chemical processing of plumes from coal-fired power plants Steven S. Brown, 1 William P. Dubé, 1,2 Prakash Karamchandani, 3 Greg Yarwood, 3 Jeff Peischl, 1,2 Thomas B. Ryerson, 1 J. Andrew Neuman, 1,2 John B. Nowak, 1,2 John S. Holloway, 1,2 Rebecca A. Washenfelder, 1,2 Charles A. Brock, 1 Gregory J. Frost, 1,2 Michael Trainer, 1 David D. Parrish, 1 Frederick C. Fehsenfeld, 1,2 and A. R. Ravishankara 1 Received 30 September 2011; revised 9 February 2012; accepted 12 February 2012; published 5 April [1] Coal-fired electric power plants produce a large fraction of total U.S. NO x emissions, but NO x from this sector has been declining in the last decade owing to installation of control technology. Nighttime aircraft intercepts of plumes from two different Texas power plants (Oklaunion near Wichita Falls and W. A. Parish near Houston) with different control technologies demonstrate the effect of these reductions on nighttime NO x oxidation rates. The analysis shows that the spatial extent of nighttime-emitted plumes to be quite limited and that mixing of highly concentrated plume NO x with ambient ozone is a determining factor for its nighttime oxidation. The plume from the uncontrolled plant had full titration of ozone through 74 km/2.4 h of downwind transport that suppressed nighttime oxidation of NO 2 to higher oxides of nitrogen across the majority of the plume. The plume from the controlled plant did not have sufficient NO x to titrate background ozone, which led to rapid nighttime oxidation of NO 2 during downwind transport. A plume model that includes horizontal mixing and nighttime chemistry reproduces the observed structures of the nitrogen species in the plumes from the two plants. The model shows that NO x controls not only reduce the emissions directly but also lead to an additional overnight NO x loss of 36 44% on average. The maximum reduction for 12 h of transport in darkness was 73%. The results imply that power plant NO x emissions controls may produce a larger than linear reduction in next-day, downwind ozone production following nighttime transport. Citation: Brown, S. S., et al. (2012), Effects of NO x control and plume mixing on nighttime chemical processing of plumes from coal-fired power plants, J. Geophys. Res., 117,, doi: /2011jd Introduction [2] Nitrogen oxides (NO x =NO+NO 2 ) are one of the key precursors to tropospheric photochemical ozone production [Crutzen, 1970]. Electric power generation from fossil fuel burning is the second largest source of NO x emissions in the United States, after on and off road vehicles, representing 26% of total emissions in 1998 and 18% in 2008 ( Among mobile NO x sources, on-road emission from Diesel engines is currently the largest single contribution [Dallmann and Harley, 2010]. Coal-fired power plants are responsible for 45% of the total U.S. electric power, and Texas has the largest coal generating capacity of any state. The declining trend in NO x 1 Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA. 2 Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Boulder, Colorado, USA. 3 Environ, Inc., Novato, California, USA. Copyright 2012 by the American Geophysical Union /12/2011JD emissions from the U.S. electric power generation sector is largely due to the installation of control technologies on these plants after 1999, which resulted in, for example, a 50% reduction of NO x point source emissions from 1990 baseline levels by 2003, and further reductions thereafter, in the northeastern United States [Frost et al., 2006]. These reductions have been confirmed by satellite NO 2 measurements and have played a significant role in air quality improvements in the eastern United States in the last decade [Frost et al., 2006; Godowitch et al., 2008; Hudman et al., 2009; Kim et al., 2006]. [3] Aircraft studies of power plant emissions during daytime have provided a detailed quantification of their ozone production efficiency, which can range from 1 to 7 ppbv O 3 /ppbv NO x emitted depending on the NO x source strength and background hydrocarbon reactivity [Ryerson et al., 1998, 2001]. Nighttime power plant emissions do not lead directly to ozone formation during transport in darkness but they do influence next-day ozone production in downwind areas to a varying extent depending on the overnight fate of NO x [Brown et al., 2006b]. Nocturnal oxidation of NO x in power plant plumes occurs by the following set of reactions 1of14

2 following the emission of NO, which is the primary component of directly emitted NO x. ðr1þ ðr2þ ðr3þ ðr4þ ðr5þ ðr6þ NO þ O 3 NO 2 þ O 2 NO 2 þ O 3 NO 3 þ O 2 NO 2 þ NO 3 N 2 O 5 NO þ NO 3 2NO 2 NO 3 þ VOC Products N 2 O 5 þ H 2 OðhetÞ 2HNO 3 The key reactive intermediates are NO 3 and N 2 O 5, which are generally not present during daytime because NO 3 photolyzes rapidly in sunlight and reacts with NO (R4), present in photostationary state with NO 2 during daytime [Leighton, 1961]. The reaction of NO 3 with NO can also be important at night in highly concentrated NO x plumes from point sources if emitted NO exceeds background ozone. Reaction (R5) represents the sum of all reactions of NO 3 with volatile organic compounds (VOCs). Budgets for NO 3 -VOC reactions from aircraft measurements have been described in a recent analysis of the TexAQS 2006 night flights [Brown et al., 2011]. This analysis showed that 30 54% of NO 3 and N 2 O 5 loss outside of power plant plumes was due to reactions of NO 3 with anthropogenic VOCs, mainly alkenes, but also aromatic VOC, oxygenates (e.g., aldehydes) and alkanes. Power plant plumes are more concentrated NO x sources than urban plumes, in general, and the larger NO 2 concentration increases the ratio of N 2 O 5 to NO 3 via the equilibrium in (R3), increasing the importance of N 2 O 5 reactions over those of NO 3. Our prior analysis specifically excluded power plant plumes. Reaction (R6) occurs heterogeneously via N 2 O 5 uptake to aerosol and is an important determining factor to the overall fate of nighttime NO x. For simplicity, this paper considers only the HNO 3 producing branch of N 2 O 5 heterogeneous uptake. Production of ClNO 2 by uptake to chloride-containing aerosol (which converts at least 50% of NO x to HNO 3 ) can also be significant in some air masses and may be important in power plant plumes, particularly in coastal areas [Osthoff et al., 2008; Thornton et al., 2010]. [4] The study of chemical evolution in power plant plumes at night is difficult because of their limited vertical depth (<100 m in some cases) and their transport above the nocturnal boundary layer [Brown et al., 2007]. As a result, there have been few nighttime studies of these plumes, and the influence of overnight transport and chemistry of power plant NO x is poorly characterized. Zaveri et al. [2010] have analyzed aircraft intercepts of power plant plumes in New England at night with the assistance of a balloon to track the air mass forward from the point source. They found evidence for overnight production of organic nitrate aerosol and concluded that the heterogeneous hydrolysis of N 2 O 5,a key reaction that determines the overnight conversion of NO x to soluble nitrate, was negligibly slow. However, they presented little information on the spatial scales relevant to plume mixing during transport. Luria et al. [2008] presented data from two nighttime aircraft transects of power plant plumes in east Texas in 2005 and suggested that the plumes showed evidence for more rapid oxidation of NO x at night than during the day. Brown et al. [2006a] analyzed power plant intercepts from a study in the northeast United States in 2004 and demonstrated the variability in the conversion of NO x to HNO 3 using a mass balance approach for nocturnal odd oxygen (O x =O 3 +NO 2 + 2NO 3 +3N 2 O 5 ) in plumes that had NO x levels that did not exceed background O 3. [5] Here we present multiple nighttime aircraft transects of plumes from two NO x power plant sources. The data include fast time response measurements of relevant nitrogen, sulfur and aerosol species, including the nighttime nitrogen oxides, NO 3 and N 2 O 5, at 100 m horizontal resolution. The analysis demonstrates that the spatial scale of the plumes is limited during transport, producing plumes that are at most a few kilometers in width and 100 m in depth. Because these scales are limited, plume NO x is highly concentrated, and mixing of plume NO x with ambient O 3 is a determining factor for the rate of NO x oxidation during transport. The lack of observational data on nighttime power plant plumes, along with the fine scale required to accurately model these plumes, means that the interaction between mixing and chemistry is poorly constrained. Effects arising from this interaction, such as the acceleration of nighttime oxidation rates with decreasing NO x emissions owing to implementation of control technology, are unlikely to have been recognized to this point. We are unaware of any such prior analysis. [6] The analysis examines two separate power plant plumes that differed by an order of magnitude in their NO x emissions relative to coal burned. Concentrations of NO x in plumes from the plant with NO x control were less than that of background ozone at plume center, leading to more rapid nighttime NO x oxidation. The results indicate that NO x control may lead to larger than expected reductions of NO x owing to the increase in its nighttime oxidation rate, potentially influencing ozone formation in downwind areas at sunrise. 2. Field Measurements and Models 2.1. Selected Texas Power Plants [7] Figure 1 shows a map of Texas with the locations of NO x point sources, sized according to their emissions [Washenfelder et al., 2010]. In this study, we examine the nighttime chemistry in the plumes of two of these power plants: Oklaunion and W. A. Parish. These power plants differ in their total NO x emissions owing to selective catalytic reduction (SCR), an operational technique that uses ammonia or urea to reduce NO to N 2 in the flue gas. At the time of the 2006 study, the Oklaunion plant, which consists of a single 670 megawatt boiler with an exhaust stack at 140 m above ground level, operated with low-no x burner technology but without SCR. The W. A. Parish plant has both natural gas and coal fired units; the coal units consist of four boilers with a total generating capacity of 2470 megawatts that exhaust through a set of four separate stacks at two altitudes (150 and 180 m). These operated with both 2of14

3 Figure 1. Outline of the state of Texas showing the locations of NO x point sources sized according to their reported emissions (scale is shown with emission proportional to the circle diameter rather than area). Emissions are from a 2004 Texas Commission on Environmental Quality database, except for power plants, which are the average daily values from September and October 2006 from the CEMS (see text) database. Red squares mark the locations of the Oklaunion and W. A. Parish coal-fired power plants. Blue lines show major urban areas in Texas. Black lines with stars show 14 h forward trajectories at 2 h intervals starting at sunset from Oklaunion on 10 October and W. A. Parish on 11 October 2006, as described in section 4. low-no x burners and SCR in Thus, although the Oklaunion plant is a smaller facility, its total NO x emissions were 31.3 tons d 1 compared to 13.1 tons d 1 for the coal units at W. A. Parish. The reported hourly molar ratio of NO x to CO 2 from the continuous emissions monitoring systems (CEMS) for the two plants differed by approximately one order of magnitude (Oklaunion = , Parish = ) during the periods when the P-3 sampled the two sources ( epa.gov/gdm/index.cfm?fuseaction = emissions.wizard) Aircraft Measurements and Plume Intercepts [8] The 2006 Texas Air Quality Study (TexAQS 2006) [Parrish et al., 2009] was a field campaign to understand the emissions, chemical transformations and transport that influence regulated air pollutants such as ozone and particulate matter in eastern Texas. As part of that study, the NOAA P-3 aircraft undertook a series of three night flights to investigate the importance of nighttime chemical processes. Prior analyses from these night flights have included a determination of the uptake coefficients for N 2 O 5 hydrolysis to aerosol [Brown et al., 2009] and budgets for NO 3 oxidation of highly reactive VOCs [Brown et al., 2011]. Here, we examine emissions from coal fired electric utility power plants, their nighttime chemical transformations, and the implications for next-day ozone formation. [9] Relevant measurements include nitrogen oxides (NO, NO 2, NO y = total reactive nitrogen) and ozone (O 3 ) by chemiluminescence (CL) [Ryerson et al., 1999, 2000], NO 2, NO 3 and N 2 O 5 by cavity ring-down spectroscopy (CRDS) [Dubé et al., 2006; Osthoff et al., 2006], HNO 3 by chemical ionization mass spectrometry (CIMS) [Neuman et al., 2002], SO 2 by pulsed UV fluorescence [Ryerson et al., 1998], CO 2 by nondispersive infrared absorption [Peischl et al., 2010] and particulate surface area by a series of optical particle counters [Brock et al., 2003]. Instrument details can be found in the relevant references and are summarized in the work of Parrish et al. [2009, Table A1]. Additional meteorological data measured from the aircraft and used in this analysis include wind speed, wind direction, and temperature. [10] Nighttime plume intercepts of the Oklaunion plant in north Texas, near the Oklahoma border and the city of Wichita Falls, Texas, were acquired during the October flight. Nighttime plume intercepts of the W. A. Parish plant were acquired on the October flight. Both nights were clear and free of precipitation, although there had been a frontal passage during the day prior to the flight at Oklaunion. Transport times for plumes were calculated from measured local wind speed and distance from the plume intercept to the source. These two P-3 night flights included multiple intercepts of these plumes at varying distances downwind, but mainly in the range from 0.2 to 4 h of transport during darkness. Plumes varied in width from 0.5 to >10 km and were sampled with fast time response (i.e., 1 Hz) instruments at the nominal 100 m s 1 speed of the P-3 to provide 0.1 km spatial resolution. Plumes from coal-fired plants were identified by their SO 2 content, as well as their ratio of SO 2 /NO x, as described in section 3.1 and Plume Modeling [11] A 1-D model simulation that included horizontal mixing and nighttime chemistry as a function of time was used to analyze the observed transects. The model does not include vertical mixing, which, as discussed further below, is likely determined by plume rise in a stable nocturnal atmosphere. The horizontal mixing is represented as a process in which the time rate of change of any chemical species is proportional to the concentration gradient in that species, similar to the treatment of molecular diffusion. In practice, the numerical integration to calculate the time rate of change of the concentration, C i, of a species in the ith box uses the following equation. dc i dt ¼ k Chem Dx 2 DCL R C i i þ DC i þ t [12] Here k is an arbitrary constant (units of length 2 time 1 ), Dx is the size of the horizontal box used in the integration, DC L i and DC R i are the concentration differences between the ith box and the next box to the left or right, respectively, and C Chem / t is the concentration change owing to chemistry alone during the time step (see sections 3.1 and 3.3 for chemical scheme). The horizontal spatial resolution of the model was 0.1 km (approximately the same as the chemical measurements). The horizontal dimension was constructed of 201, 0.1 km boxes to span a range of ð1þ 3of14

4 mixing scales, including an initial, finite width from a rapid mixing process after emission, and a slower mixing during transport, is required to reproduce observations. In this respect, the model is empirical since the physical reason for a separation in mixing time scales is not clear. The simulation produces plumes that are Gaussian in shape as the plume evolves with time. Measured plumes of SO 2 or total NO x from the single stack source at Oklaunion were also Gaussian in shape. The model is constrained by the measured background O 3 level, and by the average NO x level across plume center where the O 3 is titrated to zero. For simplicity, the initial NO x is taken as a constant value over the finite, initial width to produce an initial square-wave horizontal profile. The chemical scheme includes reactions (R1) (R6) above, where the temperaturedependent rate coefficients for (R1) (R4) are taken from literature recommendations [Sander et al., 2006]. The effective first-order rate coefficient for NO 3 loss in (R5) is k = s 1, corresponding to a lifetime of 20 min, for consistency with our prior analysis [Brown et al., 2011] (see below). As noted in the introduction, the concentrated NO 2 levels in these plumes increase the importance of N 2 O 5 reactions over those of NO 3. The uptake coefficient of N 2 O 5 to aerosol, g(n 2 O 5 ), is also taken from a prior analysis (see below) [Brown et al., 2009]. The model is not particularly sensitive to the NO 3 loss rate coefficient because of the large NO x levels, which shift the equilibrium in (R3) in favor of N 2 O 5 and thus weight heterogeneous loss of N 2 O 5 much more strongly than reactions of NO 3. The model does not consider secondary effects, such as the more rapid oxidation and depletion of reactive VOCs at plume edge or plume center. Such effects would only serve to increase the importance of N 2 O 5 heterogeneous loss, however. First-order loss rate coefficients for N 2 O 5 are then calculated using measured aerosol surface area. These loss rate coefficients ranged from 0.5 to s 1, with an average of s 1 (equivalent to a lifetime of 2.5 h) for the Oklaunion intercepts, and s 1, with an average of s 1 (4.5 h lifetime) for the Parish intercepts. Figure 2. Map of the P-3 transects downwind of the Oklaunion power plant on 10 October Transects are color and size coded by the mixing ratio of NO 2 (color shown in bar at top, which saturates at 30 ppbv for clarity of display between large and small plumes). (top) Horizontal transects plotted as distance south and east of the plant (blue diamond). Numbers on the top graph are sequential in time for the 18 intercepts. Arrows indicate wind direction, with length proportional to wind speed over a range from 4 to 9.5 m s 1. (bottom) The same transects plotted as a function of altitude and distance east of the plant, indicating the location and height of the stack (blue bar). 20 km, with the initial plume emission in the center of the domain. A 20 km domain was sufficient to describe plume evolution over a 12 h simulation. The model includes an initial emission of NO with a finite width into a background level of ozone. Measurements in the Oklaunion plume showed modest (6% of total NO x )directno 2 emissions that, for simplicity, are not accounted for in this model [Peischl et al., 2010]. Background ozone is taken from measurements just outside the plume, while initial NO x is adjusted to match observations at a given transport time. As described below, this separation of 3. Results [13] This section considers the plume widths and nighttime plume chemistry for the Oklaunion and Parish intercepts separately. Plume widths and mixing are important because they determine the concentration of the NO x at plume center, which the analysis shows to have the strongest influence on the subsequent chemistry. Widths are determined observationally as the full width at half maximum (FWHM) of a Gaussian fit to SO 2 ; where multiple plumes are present, they are fit individually, as described in sections 3.1 and Oklaunion Plume Widths [14] Figure 2 shows a map of the P-3 flight pattern downwind of the Oklaunion power plant on 10 October 2006, colored and sized by the mixing ratio of NO 2. The wind direction was approximately due north, such that the plumes were encountered on a series of east-west running transects at varying distances south of the plant. Maximum 100 m averaged NO 2 mixing ratios were 50 ppbv, limited largely by the background ozone mixing ratios, which varied between 30 and 50 ppbv. Many of the plume intercepts had excess NO (furthest downwind measurements were 2.4 h or 4of14

5 Table 1. Power Plant Plume Intercepts, Including Plume Numbers Shown in Figures 2 and 6, Distance of Intercept From Plant, Altitude Above Ground Level, Transport Time, Time in Darkness, Full Width at Half Maximum (FWHM) of Gaussian Fit to SO 2 Plume, and Integrated Plume SO 2 and NO x Plume Distance (km) Alt AGL (m) Transport Time (h) Time in Darkness (h) SO 2 FWHM (km) R SO2 (ppm m) R NOx (ppm m) Oklaunion, 10 October a Parish, October b c a FWHM of NO x plume; SO 2 data unavailable. b Vertical profile shown in Figure 9. c Fits to single plume features within multiple plume structures. 74 km) and essentially zero O 3 at plume center, indicating complete titration of the background ozone by the NO x emissions from the power plant via reaction (R1) above. Maximum observed NO was as high as 300 ppbv, but varied widely depending on the particular plume intercept, as described below. All of the plume intercepts showed clear SO 2 enhancements (but no enhancements in fine particle volume or mass), with an SO 2 /NO x ratio of , within 15% of the ratio reported for the time of emission from the CEMS data. The agreement is within the 16% the combined uncertainty and variability of the measurements (9% variability, 13% combined uncertainty in NO 2 and SO 2 measurements). The consistency of this ratio across all plume transects and the lack of any other large NO x or SO 2 point sources upwind of these transects unambiguously identifies the source of the observed plumes. Table 1 gives a list of the plume intercepts shown in Figure 2, including distance from plant, height above ground level (AGL), plume transport time and time in darkness, and plume integrated SO 2 and NO x to give an indication of the intensity of the intercept. [15] Locating plumes advected downwind of a large point source from an aircraft at night is difficult because of the shallow plume depth in a vertically stratified nighttime atmosphere. The horizontal view of the plume transects in Figure 2 (top) shows that intercepts do not all appear at the same east-west location on successive transects. The stack height, shown in Figure 2 (bottom), is 140 m. Gray arrows mark the wind direction, with length proportional to wind speed. Many transects failed to locate the plume at all. Figure 2 (bottom) shows the same transects plotted as height above ground level against the east-west distance from the plant. Transects separated by as little as 150 m vertically encountered either concentrated plumes or background air, indicating a plume depth of no more than 300 m. Since the aircraft was required to fly above 300 m AGL at night over these locations, there were no vertical profiles that could directly measure the entire plume vertical structure; however, previous aircraft measurements in the northeast United States in which nighttime power plant plumes were intercepted during descent to an airfield showed them to be m in depth at an altitude of m AGL [Brown et al., 2007]. The majority of intercepts in Figure 2, especially later in the flight and well after sunset, found the plume only at the lowest operational P-3 altitude. The rise height of the plume may have been variable with time, especially if the structure of the nocturnal boundary layer became more stable and shallower with time after sunset. It is possible that the bulk of the plume was below the lowest P-3 transects. The vertical structure of these nighttime intercepts contrasts with daytime observations, in which plumes from point sources mix rapidly throughout a deeper daytime boundary layer within a short distance downwind [Zhou et al., 2012]. 5of14

6 Figure 3. Segment of the flight track from Figure 2 showing a series of four downwind plume intercepts at level altitude 345 m above ground level. Color coding on the flight track and location of the power plant are as in Figure 2. The black trace superimposed on the track (top) shows SO 2, which gives a measure of the plume width as it advects downwind. Numbers next to each intercept indicate the transport time based on the local wind direction and distance from the source, as well as the width (Gaussian full width at half maximum) of the observed SO 2 plumes. [16] Figure 3 shows a subset of the flight track with four plume intercepts (intercepts from Figure 2) at successive downwind distances at an altitude of m AGL. The intercepts occurred at h after sunset, such that the plumes were emitted and transported entirely in darkness. Transport times are indicated on Figure 3. The SO 2 mixing ratio (arbitrary units) is overlaid on the flight track (which is also color coded by NO 2, as above) to indicate the plume widths. Despite an 2 h range of transport time (as determined by local wind speed and distance to the plant for each intercept), the widths of these plumes did not vary systematically with transport distance. Gaussian fits to the transect data, corrected for the transect distance perpendicular to the wind direction, gave an average width (FWHM) of (1s) km. This invariance indicates that horizontal mixing, following more rapid horizontal dilution between emission and our closest transect, was slow on the time scale of this transport. This conclusion is only qualitative, however, since the width of any observed individual intercept is also likely a function of the intercept height relative to the actual vertical center of the plume. For example, in spite of the invariance in plume width with downwind distance in Figure 3, the magnitude of the SO 2 (and NO x, not shown) peak enhancement decreased systematically, consistent with successive intercepts further from plume center at constant altitude with downwind distance. Potential temperature increased systematically with distance, indicating that the aircraft may have risen within the nocturnal boundary layer structure. A fifth downwind transect at the same altitude (not shown) failed to find the plume at all. Interestingly, the fourth transect found two plumes; the width of the narrow feature is given on Figure 3. The wider feature, encountered at the same altitude, had the same SO 2 /NO x ratio as the narrow feature but a slower wind speed, indicating that it may have had a different transport history than the other four plumes. Its presence in close proximity to the narrow plume indicates the complexity of nighttime transport. [17] The four plumes in Figure 3 were among the narrower of the 18 plumes encountered. Plume intercepts varied in width from 0.53 to 6.5 km, with an average width of 2.1 km FWHM. Some of these plumes were emitted during daylight (on the basis of transport time and time since sunset) and may have undergone different mixing processes or even modest photochemistry. Of the plumes that were emitted after dark, the range in widths was km, with an average of 1.0 km. These widths are in general larger than the 100 m depth suggested by the presence or absence of plumes on transects at different altitudes in this study and the vertical profiles in our prior study, indicating that the plumes are elliptical in shape (wide in the crosswind horizontal, shallow in the vertical) during nighttime transport. Such fanning plumes are characteristic of emissions in the presence of a negative lapse rate, when vertical mixing is restricted [Stull, 1988]. The finite width of the plumes, but lack of a systematic dependence on distance or transport time, suggests more rapid initial horizontal mixing after emission, followed by a slower mixing process during transport. In the following analysis the slower mixing during transport is represented as a first-order process that transports mass perpendicular to the direction of flow Chemistry and Modeling of the Oklaunion Plume [18] Figure 4 shows an expanded view of reactive nitrogen and ozone species from one intercept (intercept 3 from Figure 2) that most clearly illustrates the role of mixing and 6of14

7 Figure 4. Expanded view of a single nighttime Oklaunion power plant intercept (intercept 3 from Figure 2) showing (top) NO, NO 2, and O 3 ; (middle) NO 3 and N 2 O 5 ; and (bottom) HNO 3. Two separate NO 2 measurements (chemiluminescence (CL) and cavity ring-down spectroscopy (CRDS)) are shown in Figure 4 (top). Dashed lines in each panel are the results of a model that incorporates mixing and chemistry, as described in the text. The model has been offset to the background HNO 3 level outside of the plume in Figure 4 (bottom). chemistry in creating a structured plume. The estimated transport time for this plume was 1.4 h, almost identical to the time since sunset (1.3 h). The presence of unreacted NO and essentially zero O 3 at plume center shows that the initial NO x in this plume exceeded the background ozone, and that the rate of mixing was slow enough that NO x remained in excess of background ozone at this transport time. Nighttime chemistry involving the further oxidation of NO 2 to NO 3 and N 2 O 5 was effectively suppressed at plume center since this oxidation requires O 3, and since the rapid reaction of NO (R4) with NO 3 suppresses formation of these nocturnal nitrogen oxides. The observed mixing ratios of NO 3 and N 2 O 5 in the center graph are indeed zero at plume center, and exhibit structure such that they are present only in the region where the NO falls to zero but the NO 2 and O 3 are nonzero (hereafter referred to as wings ). Thus, the width of these wing structures is an indication of how rapidly the NO x mixes out of plume center. [19] The dashed lines in Figure 4 are the output of the plume model described in section 2. The model transects in Figure 4 reproduce the shape of the observed NO 2, though not its peak value; the measured NO 2 exceeds the background ozone, indicating that some fraction of the NO x emissions was in the form of NO 2. Peischl et al. [2010] have analyzed the direct NO 2 emissions from this set of transects and found that the initial NO 2 /NO x ratio was 6% on average and 10% for this particular intercept. Model parameters required to reproduce this shape included an initial width of 1.2 km and a horizontal mixing rate coefficient k = 5 m 2 s 1 (see equation (1)). [20] This relatively slow mixing rate constant also reasonably reproduces the observed structure in NO 3 and N 2 O 5, including the wing structures seen at plume edge in Figure 4. Such wing structures have been predicted in a previous model of transport of urban plumes at night [Jones et al., 2005]. Wings in NO 3 lie slightly to the outside of those in N 2 O 5 owing to the falloff of NO 2 toward the edge of the plume; this falloff favors N 2 O 5 via the equilibrium in (R3) at high NO x, closer to plume center, and NO 3 at low NO 2, closer to plume edge. The observed structure follows this trend, except that NO 3 and N 2 O 5 do not fall to zero at plume edge owing to the presence of background NO 2 levels, which, for simplicity, were not included in the model. Omission of modest background NO 2 does not significantly alter the conclusions about loss of plume NO x. The model predicts symmetric NO 3 and N 2 O 5 plume structure, while the observed structure is slightly asymmetric. The model overpredicts the NO 3 and N 2 O 5 by up to 60%. Rate constants for loss of NO 3 and N 2 O 5 were taken as and s 1 in (R5) and (R6), respectively, as described in section 2, on the basis of a prior analysis that showed an NO 3 lifetime of 20 min for this flight and g(n 2 O 5 ) of [Brown et al., 2009]. The N 2 O 5 loss rate coefficient is derived from this uptake coefficient and a humidity-corrected, measured aerosol surface area of 340 mm 2 cm 3 for this plume. [21] The model predicts a modest production of HNO 3 through reaction (R6) in the wings of the plume, as Figure 4 (bottom) shows. The HNO 3 measurements do not show this structure, possibly because the average uptake coefficient used in the simulation is too large for this particular plume, or because the HNO 3 was present as aerosol phase nitrate (time resolution of aerosol nitrate, measured by aerosol mass spectrometer [Bahreini et al., 2008], was too low to resolve this wing structure). A decrease in the N 2 O 5 uptake coefficient would bring modeled HNO 3 into better agreement with the observation, but would increase the difference between modeled and observed N 2 O 5. If the differences between the model and measurements are due to chemistry and not plume mixing, it indicates that the previously determined parameters only approximately describe the concentrations of N 2 O 5 and HNO 3 in the plume wings. These effects do not change the overall conclusions regarding the much stronger effect of plume mixing on nighttime NO x oxidation rates. [22] Figure 5 shows the complete simulation associated with the model transects in Figure 4, which are horizontal 7of14

8 Figure 5. Plume model results for transect shown in Figure 4. (left) Horizontal structure in NO x,n 2 O 5, and HNO 3 as a function of transport time over 12 h, initialized by the NO x levels that best reproduce the observations at 1.4 h. (right) The same simulation, except with an arbitrary fourfold reduction in initial NO x. The color scale in the first panel of Figure 5 (right) is four times smaller than in the first panel of Figure 5 (left) to show the full range in NO x for the two simulations. Color scales for the other graphs in Figure 5 (right) are identical to those in Figure 5 (left). slices across the time series. The simulation time is 12 h, roughly consistent with the time during darkness for the modeled plumes, which were emitted near sunset (18:00 local standard time) and slightly after the fall equinox. Figures 5 (left) shows the evolution of the chemical composition of selected nitrogen oxides (total NO x,n 2 O 5 and HNO 3 ) for the observed NO x to O 3 mixing ratios. The mixing rate constant used to simulate the NO x plume width and the widths of the wing structures in NO 3 and N 2 O 5 at 1.4 h results in a plume that has NO x >O 3 at center during the entire course of its transport. After 12 h of transport, with nighttime chemistry occurring only in the wings, 91% of the initial NO x is still present in the simulation. [23] Figure 5 (right) shows the same simulation, but with an arbitrary, fourfold reduction in the plant NO x emissions, a reduction achievable with SCR installation [Shelef, 1995]. The difference in nighttime chemistry between the two scenarios is striking. The reduced emitter has insufficient NO x to fully titrate O 3 at plume center and undergoes full nighttime chemical conversion of NO x to soluble nitrate (via N 2 O 5 ) and other products (via NO 3 oxidation chemistry). This modeled plume lacks the wing structures associated with the observations, and gives rise to a strong, Gaussian shaped plume of HNO 3 at the end of the simulation and a much weaker plume of NO x. The NO x remaining after 12 h of transport in the reduced emission scenario is 34% of the initial. (There is also a reduction in total O x =O 3 +NO 2 + 2NO 3 +3N 2 O 5, but it is less dramatic than for NO x owing to the presence of O 3 at plume center in the reduced NO x case. The net O x reduction is 8% across the plume relative to the high-no x case) Thus, for a plume emitted near sunset (i.e., one that would undergo the maximum transport during darkness), emissions controls sufficient to reduce the NO x at plume center below the level of background O 3 subsequent to the initial mixing result in an additional 73% reduction (i.e., 1 34/91) in NO x relative to the uncontrolled case for 8of14

9 Figure 6. P-3 flight track on October 2006, color and size coded by the mixing ratio of SO 2 (color shown in bar). Numbers and arrows indicate the location of nine separate intercepts of the plume from the Parish power plant, which are sorted by distance from plant but are not sequential in time. Arrows indicate wind direction at each intercept. the simulated conditions. This reduction is a maximum since it refers to emission at sunset that undergoes the maximum amount of transport and chemistry in darkness. The average reduction for plumes emitted between sunset and sunrise, assuming 12 h of darkness, is 36% (i.e., 96% NO x remaining, at sunrise, in the high-no x case, and 61% remaining in the low-no x case; 1 61/96 = 36%.). Actual reductions for any given plume would be variable depending on vertical mixing and shear, background ozone, temperature and plume width subsequent to the initial mixing process. However, any emission reduction that yields NO x mixing ratios at plume center below that of the background ozone after the initial mixing phase will have an effect qualitatively similar to that shown here Parish Plume Widths and Depth [24] Figure 6 shows the P-3 flight track on October 2006, color and size coded by SO 2 mixing ratio. The Parish power plant, located 50 km southwest of downtown Houston, was the largest source of sulfur encountered on this flight. Due in part to installation of SCR NO x controls, it was not the largest NO x source in this highly polluted area, and the markers in Figure 6 use SO 2 rather than NO 2 for plume identification. The P-3 sampled the Parish plume and other sources of urban and industrial pollution during a series of east-west transects at varying altitudes and distances north of the Parish plant under southwesterly wind flow. Ratios of SO 2 to NO 2 are consistent with all of the marked plumes originating from the coal-fired units at Parish, as described in section 3.4. The lack of a definitively identified plume from the gas stacks indicates that coal and gas sources were well separated in the stratified, nighttime boundary layer. Like the Oklaunion plume, the Parish plume was only observed on transects at specific altitudes; all of the plume intercepts for Parish were between 550 and 800 m AGL, somewhat higher than the Oklaunion intercepts. Also like the Oklaunion night flight, the intercepts were irregularly spaced and not at uniform directions downwind of the plant on each transect. With the exception of intercept 8, transport times for all plume intercepts indicated that the time of emission for each was well after sunset, so that all mixing and chemistry occurred in darkness. A list of plume intercepts appears in Table 1. [25] Although Parish burns coal at four times the rate of Oklaunion, observed NO x downwind of Parish was less concentrated. Only one of the nine intercepts in Figure 6 showed excess NO and complete O 3 titration at plume center. In the one intercept that did have excess NO (intercept 5 in Figure 6), it was small (<1 ppbv) and extended over a small range (<0.5 km) of a relatively wide (>10 km) plume. A vertical profile taken near the plant (Figure 7) provides an estimate of the minimum vertical extent of the nighttime Parish plume. The profile shows two distinct SO 2 and NO x plumes centered at 560 and 440 m altitude and a number of other smaller structures. The concentrated plumes have Gaussian FWHM of m and edge-to-edge distances of 100 m in the vertical. The plume was encountered 5.9 km and 19 min downwind of the plant. The 50 m FWHM cannot be assigned unequivocally to the vertical extent of the plume since the descent rate of the P-3 is such that it traveled 1 km horizontally during the same interval. A second intercept, also 19 min downwind, but at a level cruising altitude of 640 m (intercept 1 in Figure 6) also encountered two separate plumes with horizontal Gaussian FWHM of 1.1 and 0.9 km each. Because the plume widths measured on a level leg are similar to the horizontal distance traveled during the plume encounter on the vertical profile, the plume vertical extent from Figure 7 is a lower limit to the actual depths. [26] Figure 8 shows data from a horizontal transect of the Parish plant (intercept 3 in Figure 6) at 640 m altitude and 1.33 h (40 km) downwind. The double plume structure, similar to several of the other intercepts, is likely due to the two sets of coal-fired stacks at 150 and 180 m (see section 2). The shorter gas-fired stacks are not likely the source of this plume as they emit very little SO 2 and are thus inconsistent with these measurements. The separation between the measured plumes, which is more than 1 km, may be due in part to the 0.35 km physical separation of the stacks, and in part to wind shear between the two sources during plume rise and transport. Wind shear was evident within the altitude range of the two plumes in Figure 7, for example. Even more complicated, multiple peaked structures were observed on several of the other transects. [27] Similar to the Oklaunion intercepts, the data suggest that mixing of this plume with background air was initially rapid. The two plumes sampled 19 min downwind (one of which is shown in Figure 7) showed nonzero ozone across the entire intercept and no excess NO. Further, the plume widths for the different intercepts, although variable owing to the multiple stacks, are finite but did not show a systematic increase with distance downwind. Gaussian fits of 9of14

10 Figure 7. Vertical profile of the Parish power plant plume (intercept 0 in Figure 6) showing the potential temperature (Q), SO 2,O 3,NO 2, and NO. individual peaks within each intercept FWHM ranging from 0.5 to 3 km, with an average value of km, similar to but slightly larger than plumes emitted from the single stack at Oklaunion. Widths for Parish intercepts were not all perpendicular to the wind direction (see Figure 6), and have been corrected to the perpendicular distance across the plume. W Actual ¼ W Observed cosð90 qþ Here, W Actual and W observed are the actual and observed plume widths, and Q is the angle between the aircraft direction and the wind direction. Qualitatively at least, the horizontal mixing associated with each plant appears to have been similar. Both are consistent with an initial, rapid mixing that established the horizontal extent of the plume, followed by slower mixing during downwind transport Chemistry and Modeling of the Parish Plume [28] Mixing ratios of NO 3 and N 2 O 5 in Figure 8 (middle) are large and consistent with rapid nocturnal oxidation of NO 2 by O 3 across the plume during the entire course of its transport. Modest production of HNO 3, attributable to N 2 O 5 hydrolysis, is also evident in Figure 8 (bottom). The dashed line in each panel is the output of the plume model described in section 2.3. [29] The model included two separate but symmetric initial NO x mixing ratios of 30 ppbv in a width of 1 km to match the observed double peak structure. The rate constant for mixing was the same as that used for the Oklaunion plant, k = 5 m 2 s 1. These parameters do not reflect the 20% correction in plume width arising from the 125, rather than 90, intercept angle (Figure 6). Although the approximate match to the observed profile in NO 2 was largely ð2þ determined by the choice of initial NO mixing ratio and initial mixing width, the relative widths of the NO 3 and N 2 O 5 profiles are more sensitive to the mixing rate constant. The NO 3 profile is somewhat wider and also somewhat more uniform than that of N 2 O 5 as a result of the NO 2 dependence of the equilibrium ratio of N 2 O 5 to NO 3, which produces more NO 3 at lower NO 2 at plume edge. This is the same mechanism that gives rise to the wing structures in Oklaunion plumes that were O 3 titrated at center. Although the model reasonably reproduces the falloff of N 2 O 5 at plume edge, it produces stronger wings and a slightly wider NO 3 profile than observed. A doubling of the mixing rate constant increases this disagreement, suggesting that the mixing rate constant is an upper limit for this plume. The model approximately reproduces the observed levels of NO 3 and N 2 O 5 as well as that of HNO 3, although the shape of the HNO 3 transect does not match that of the other species or of the model. The plume was simulated using a previously determined uptake coefficient, g(n 2 O 5 ) = (specific to this plume and lower than the average value of 0.003), and a measured aerosol surface area of 420 mm 2 cm 3 [Brown et al., 2009]. As our prior analysis describes, this uptake coefficient is 10 times smaller than the parameterizations developed for regional and global chemical models [e.g., Davis et al., 2008; Evans and Jacob, 2005]. [30] Figure 9 shows the plume mixing and chemistry model for 12 h of transport associated with the single intercept in Figure 8. Aside from use of two separate, adjacent emissions, the evolution of the plume in Figure 9 (left), which represent the observed NO x levels, is similar to the reduced NO x case simulated for the Oklaunion plume. Even with a relatively small N 2 O 5 uptake coefficient, the plume converts 74% of its NO x to HNO 3 and oxidation products of NO 3 over the 12 h simulation. Figure 9 (right) shows the 10 of 14

11 Figure 8. Example intercept of the Parish power plant (intercept 3 in Figure 6) showing (top) NO, NO 2, and O 3 ; (middle) NO 3 and N 2 O 5 ; and (bottom) HNO 3. The dashed lines are the output of a plume model that includes mixing and chemistry, as described in the text. The model has been offset to the background HNO 3 level outside of the plume in the third panel. effect of a fourfold increase in NO x emissions, intended to approximately represent NO x emissions at Parish prior to SCR installation. This plume has wing structures similar to those observed for the Oklaunion transects, and the NO titration at plume center for the adjacent plumes eventually mixes to produce a single plume with no further oxidation of NO 2 except at the outer edges. The effect of mixing in this higher-no x case leads to only a 7% conversion of the initial NO x to HNO 3 and other products. The maximum additional downwind NO x reduction due to the effects of mixing and chemistry for the controlled plume is 72% (i.e., 1 26/93). The average reduction for plumes emitted over the course of 12 h is 44%. The contrast between the large power plant with SCR represented by Parish and the smaller plant without such controls represented by Oklaunion shows that emissions controls result in greater overnight oxidation of NO x, which is unavailable to participate in photochemistry during the following day. [31] Finally, Figure 10 shows the ratios of nocturnal nitrogen oxides, NNO x =NO 3 +2N 2 O 5, and HNO 3 to total reactive nitrogen, NO y for each of the Parish intercepts against the transport time. The ratios were determined as the slopes of linear least squares fits of each species to NO y for the plume data. All of the intercepts of plumes emitted after dark had transport times in the range h, and the ratio NNO x /NO y increased systematically over this time scale. The ratio HNO 3 /NO y increased only modestly for these intercepts. The shaded areas in Figure 10 (top) represent the range of model outputs for this ratio at plume center. The model range is determined by the range of observed, average NO x within each plume and by the range in observed aerosol surface areas. Large injections of NO x into the background ozone lead to slower oxidation of NO x via reaction (R2) (i.e., the effective first-order rate coefficient for NO 2 oxidation, k 2 [O 3 ], decreases as the NO injection becomes a larger fraction of the background ozone), so that the NNO x /NO y ratio increases more slowly with time at high NO x. Larger aerosol surface area also leads to faster N 2 O 5 hydrolysis (with g(n 2 O 5 ) held fixed at 0.002) and a more rapid increase in HNO 3 /NO y. This model includes an arbitrary 0.1 h time offset to represent the initial, rapid mixing phase. The plume evolution over the first few hours shows that NO 3 and N 2 O 5 are the dominant products of NO x oxidation, largely as a result of the small uptake coefficient for N 2 O 5 hydrolysis. A separate analysis of NO 3 -VOC oxidation shows that these reactions are important to the nitrogen budget in areas downwind of the Houston Ship Channel, where there are large emissions of reactive anthropogenic VOC, but less important in the background air into which the Parish plume was emitted [Brown et al., 2011]. Over longer time scales, as shown in the lower-no x plume model results in Figure 5 and Figure 9, the NO 3 and N 2 O 5 production rate (i.e., k 2 [O 3 ] [NO 2 ]) eventually decreases as the NO 2 levels fall, leading to a decrease in the fraction of NO y present as NNO x. The HNO 3 fraction increases steadily with time owing to the absence of loss to dry deposition in a plume that is isolated from the surface at night. Plumes with smaller aerosol surface areas and lower-n 2 O 5 heterogeneous losses would preserve a larger fraction of their NO y in this reservoir, even at long time scales. [32] Figure 10 (bottom) shows the SO 2 to NO 2 ratio as a function of transport time. Nighttime oxidation of SO 2 is assumed to be negligibly slow in the presence of small or zero nighttime OH. Nighttime NO 2 oxidation then leads to a systematic increase in SO 2 /NO 2 with plume transport time. Figure 10 shows that the observed ratios are consistent with the modeled range, where the modeled NO 2 decrease is described above. The evolution of the SO 2 /NO 2 ratio provides further confirmation of the time evolution of the nitrogen chemistry described above and also identifies the origin of the selected plume intercepts as the Parish plant. The SO 2 /NO 2 ratio at short transport times was , within 15% of the CEMS data for Parish coal-fired units ( for SO 2 /NO x ) during the time period of the October flight. The model curves in Figure 10 use 11 of 14

12 Figure 9. Plume mixing model for the Parish intercepts. The model is similar to that shown in Figure 5, except that it includes two emission sources, consistent with the two heights of coal-fired stacks at the Parish plant. (left) Observed NO x. (right) A fourfold increase in emitted NO x. an initial ratio of 7.5 and an arbitrary time offset of 0.1 h prior to the onset of nighttime NO x oxidation. 4. Discussion and Implications [33] The spatial scales associated with power plant plumes emitted and transported at night are small compared to those encountered during daytime. Nighttime plumes analyzed in this paper had widths of 1 3 km for distances as much as 140 km downwind from the source. Vertical depths were more difficult to analyze systematically, but near source vertical profiling and downwind horizontal transects at different altitudes were consistent with plume depths on the order of 100 m or less. This is in contrast to daytime measurements, which show a horizontal width of 3 10 km at transport distances of km, respectively [Ryerson et al., 1998]. Daytime plumes are also likely to have vertical depths equal to that of a well-mixed, convective boundary layer (e.g., km). Nighttime plumes are thus far more likely to be highly concentrated and to have plume NO x in excess of background O 3 owing to reduced dilution. This concentration of plume NO x in turn has implications for its nighttime oxidation rate, as well as for the change in nighttime oxidation rates with the implementation of NO x control technology. [34] The comparison of nighttime plumes from two different coal-fired power plants, one with SCR control and one without, demonstrates that the uncontrolled plume had sufficiently large NO x emissions to completely titrate ozone at plume center, which in turn led to much slower NO x oxidation in the plume. Plume models constrained to the observed levels of NO x, and with horizontal mixing rates that are consistent with observations, confirm this result. The models indicate that the difference in overnight NO x consumption is large, with >90% of NO x at sunset remaining in the plume at sunrise in the uncontrolled case, but <40% remaining in the controlled case. Average reductions for plumes emitted during 12 h of darkness were 36 44%. These effects will not be evident in the immediate vicinity of the plant owing to the nighttime transport of the plume aloft, 12 of 14

13 Figure 10. (top) Ratio of the nocturnal nitrogen oxides (NNO x =NO 3 +2N 2 O 5 ) to total reactive nitrogen (NO y ) and HNO 3 to NO y, as measured from the slope of linear least squares fits to the data for each of the Parish plume intercepts. Error bars are the quadratic sum of the fit errors and the measurement inaccuracies. The shaded region represents the range of model outputs for the range in observed NO x and aerosol surface area in each plume. (bottom) Ratio of SO 2 to NO x (taken here as NO 2 ) determined from the slope of the correlation plot for each plume intercept. Shaded region is the range of model outputs. without surface contact. They are likely to affect next day ozone formation in downwind areas, however. The map in Figure 1 includes two 14 h forward trajectories (R. R. Draxler and G. D. Rolph, HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Tracker) Model, NOAA Air Resource Laboratory, Silver Spring, Maryland, 2003; available at starting at sunset on 10 October at the Oklaunion plant and at the same time of day on 11 October at the Parish plant. These forward trajectories terminate at 2 h after sunrise, during the time period when the nocturnal boundary layer would break up and the plumes would mix to surface level. The trajectories, while representative only of the particular cases studied here, illustrate the potential horizontal extent of overnight plume transport. The October transport of the Parish plume in particular was to an area with large emissions of biogenic hydrocarbons [Guenther et al., 2006]. Air masses rich in biogenic emissions, such as isoprene, are known to have greater efficiency for ozone production per unit NO x than air masses poor in such emissions. For example, NO x emissions in isoprene rich areas have been shown to produce up to 7 molecules of ozone per molecule NO x emitted, compared to less than 2 in isoprene poor areas [Ryerson et al., 2001]. Thus, injection of a large NO x source in this area during morning hours, when isoprene emissions increase, could lead to additional ozone production. The results of this study imply that the SCR controls have decreased the potential for this next-day ozone generation by an additional 40%, on average, owing to increased nighttime NO x oxidation. [35] Recent studies have shown that the daytime, photochemical ozone reductions resulting from NO x controls are less than proportional to the NO x controls themselves (i.e., smaller percentage decrease in ozone produced than in NO x emitted) [Ryerson et al., 1998, 2001]. Nighttime chemical processing, which this study suggests lead to NO x reductions that are more than directly proportional to the NO x control itself (i.e., a larger percentage decrease in downwind NO x than in NO x emitted), may serve to at least partially offset the nonlinear response of the photochemical side of the cycle. Such effects may be most important in forested regions with large hydrocarbon emissions and reactivity and may apply generally to the emissions controls instituted across the eastern United States in the last decade. This region is heavily forested, such that overnight plume transport is likely to impact downwind areas with large biogenic VOC emissions. This result should be investigated in a full three-dimensional model simulation, including changes in NO x emissions, detailed nighttime plume chemistry as well as daytime photochemistry, and emissions of reactive hydrocarbons. [36] Acknowledgments. The preparation of this report is based on work supported by the state of Texas through the Air Quality Research Program administered by the University of Texas at Austin by means of a grant from the Texas Commission on Environmental Quality. The work was also supported in part by NOAA s Atmospheric Chemistry and Climate Program. References Bahreini, R., E. J. Dunlea, B. M. Matthew, C. Simons, K. S. Docherty, P. F. DeCarlo, J. L. Jimenez, C. A. Brock, and A. M. Middlebrook (2008), Design and operation of a pressure-controlled inlet for airborne sampling with an aerodynamic aerosol lens, Aerosol Sci. Technol., 42, , doi: / Brock, C. A., et al. (2003), Particle growth in urban and industrial plumes in Texas, J. Geophys. Res., 108(D3), 4111, doi: /2002jd Brown, S. S., et al. (2006a), Nocturnal odd-oxygen budget and its implications for ozone loss in the lower troposphere, Geophys. Res. Lett., 33, L08801, doi: /2006gl Brown, S. S., et al. (2006b), Variability in nocturnal nitrogen oxide processing and its role in regional air quality, Science, 311,67 70, doi: / science Brown, S. S., et al. (2007), Vertical profiles in NO 3 and N 2 O 5 measured from an aircraft: Results from the NOAA P-3 and surface platforms during NEAQS 2004, J. Geophys. Res., 112, D22304, doi: / 2007JD Brown, S. S., et al. (2009), Reactive uptake coefficients for N 2 O 5 determined from aircraft measurements during TexAQS 2006; Comparison to current model parameterizations, J. Geophys. Res., 114, D00F10, doi: /2008jd of 14