Aircraft measurement of HONO vertical profiles over a forested region

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 36,, doi: /2009gl038999, 2009 Aircraft measurement of HONO vertical profiles over a forested region Ning Zhang, 1 Xianliang Zhou, 1,2 Paul B. Shepson, 3 Honglian Gao, 1 Marjan Alaghmand, 3 and Brian Stirm 4 Received 4 May 2009; revised 8 June 2009; accepted 1 July 2009; published 11 August [1] Here we present the first HONO vertical profiles in the atmospheric boundary layer (BL) and the lower free troposphere (FT) over a forested region in northern Michigan and the neighboring Great Lakes, measured from a small aircraft in summer of The HONO mixing ratios ranged from 4 to 17 pptv in the FT and from 8 to 74 pptv in the BL. The HONO distribution pattern was strongly influenced by the air column stability, i.e., strong negative HONO gradients existed in stable BL in the morning hours, whereas the HONO distribution was relatively uniform in the unstable and well-mixed BL in the afternoons. The ground surface was a major source of HONO in the lower BL. The presence of substantial daytime HONO in the FT (8 pptv) and in the upper BL (25 pptv) suggests that a significant in situ source of HONO exists in the air column. Citation: Zhang, N., X. Zhou, P. B. Shepson, H. Gao, M. Alaghmand, and B. Stirm (2009), Aircraft measurement of HONO vertical profiles over a forested region, Geophys. Res. Lett., 36,, doi: / 2009GL Introduction [2] Recent studies have demonstrated that photolysis of nitrous acid (HONO) is a significant or even a major source of the hydroxyl radical (OH) in both urban [Acker et al., 2006a; Kleffmann et al., 2005; Ren et al., 2003] and rural atmospheres [Acker et al., 2006b; Ren et al., 2006; Zhou et al., 2002, 2007]. It is now generally accepted that in urban atmospheres, HONO is produced mainly by heterogeneous reactions involving NO x on the ground or aerosol surfaces [Kleffmann et al., 2003; Lammel and Cape, 1996; Reisinger, 2000]; this production can be greatly enhanced by the presence of organics on surfaces and by exposure to sunlight [Bejan et al., 2006; George et al., 2005; Stemmler et al., 2006]. In rural low-no x environments, HNO 3 photolysis on surfaces including vegetation surfaces has been suggested as a major daytime HONO source [Zhou et al., 2003, 2007]. [3] However, almost all of the HONO measurements to date have been made on ground-based platforms [e.g., Acker et al., 2006b; Harrison and Kitto, 1994; He et al., 2006; Kleffmann et al., 2003, 2005; Zhou et al., 2002, 2007] and the reported HONO concentration profiles have all been near the 1 Department of Environmental Health Sciences, State University of New York, Albany, New York, USA. 2 Wadsworth Center, New York State Department of Health, Albany, New York, USA. 3 Department of Chemistry, Purdue University, West Lafayette, Indiana, USA. 4 Department of Aviation Technology, Purdue University, West Lafayette, Indiana, USA. Copyright 2009 by the American Geophysical Union /09/2009GL bottom of the boundary layer [He et al., 2006; Kleffmann et al., 2005]. To our knowledge, no vertical HONO concentration profiles through and above the boundary layer are available in the literature. To understand the importance of HONO in tropospheric photochemistry and to estimate the relative importance of ground surfaces and aerosol surfaces in heterogeneous production of HONO, vertical HONO profile measurements are necessary, over the altitudes from the surface mixed layer to the free troposphere. Here we report the first aircraft-based measurements of HONO vertical distribution profiles over a forested region and the Great Lakes. 2. Experiment [4] Aircraft HONO measurements were carried out over the northern tip of the Lower Peninsula of Michigan and the neighboring Great Lakes region (Figure 1) from July 30 to August 6, 2007, on board the Purdue University Airborne Laboratory for Atmospheric Research (ALAR; a Beechcraft Duchess twin-engine airplane. The cruising speed is 180 km hr 1. Each flight took off from Pellston (KPLN) airport, Michigan (45.6 N, 84.9 W) and lasted 2 3hr, spending min at each altitude to establish HONO concentration profiles. The typical altitudes include m, m, m, m, m and m above ground level (AGL). During HONO profiling over the land, the aircraft circled over the same area centered on the PROPHET site of the University of Michigan Biological Station, 5.5 km to the southeast of the Pellston airport (Figure 1). This area is a mixed deciduous/ coniferous forest, in a rural environment, with a canopy height of 20 m [Carroll et al., 2001]. In addition to aircraft measurement, HONO concentrations were measured at the PROPHET site about 32 m above ground. For HONO profiles over the lake, either Lake Huron or Lake Michigan was chosen for sampling depending on the wind direction during the sampling period to ensure that air mass traveled over the lake for the last 20 km prior to its arrival. The HONO contribution from the land surface source was thus negligible due to the fast photolysis of HONO during the day. To assess the changes in HONO vertical distributions throughout the day, missions were conducted at different times of the day. [5] The HONO measurement technique has been described in detail elsewhere [He et al., 2006; Huang et al., 2002]. In brief, air was sampled using a coil sampler at a flow rate of 3 L min 1 on the aircraft and 2 L min 1 at the PROPHET tower. HONO was scrubbed from the air with a phosphate buffer (ph = 7), at a flow rate of 0.2 ml min 1. The scrubbed nitrite was derivatized with 2 mm sulfanilamide (SA) and 0.2 mm n-(1-naphthyl)-ethylenediamine (NED) in 40 mm HCl, to form an azo dye within 1 min. The azo dye was detected 1of5

2 ZHANG ET AL.: AIRCRAFT HONO MEASUREMENT Figure 1. Locations of sampling sites and major urban and industrial centers in the Great Lakes region. Sampling locations over the land centered on the UMBS forest (star). A, B and C represent the sampling locations over the lakes on August 1, 4 and 6, respectively. The dotted and dashed lines are 24-hr back trajectories of air masses arriving at the PROPHET site and the sampling location C, respectively, in the August 6 afternoon. The asterisk indicates the location of 2 power plants in the Upper Michigan Peninsula. by light absorbance at 540 nm using a miniature fiber optic spectrometer (USB2000, Ocean Optics) with a 1-m liquidwaveguide capillary flow cell (WPI). All solutions were freshly prepared, with a one-point calibration before each flight. Two sets of full range calibrations were done at the beginning and the end of the campaign. The detection limit (3s of noise at a low HONO concentration) was 2 pptv. The data collection interval was 1 min for the aircraft system and 5 min for the ground system. Prior to this aircraft study, potential interference from atmospheric species such as NOx, particulate nitrite and other neutral NOy species were studied during a 1-week field test at a remote mountain site in Whiteface Mountain, NY, by removing HONO with a Na2CO3-coated denuder from air before sampling. We found that the interference signal was below our detection limit of 2 pptv in ambient air containing HONO concentrations 20 pptv, and accounted for 10% of the overall signal for in ambient air containing [HONO] 20 pptv (r2 = 0.72, N = 542). This finding agrees with an earlier report showing the interference is 8% of the overall signal using a similar technique in the urban environment [Huang et al., 2002], but disagrees with a recent report [Kleffmann and Wiesen, 2008] suggesting a significant increase in the relative artifact signal with decreasing ambient HONO concentration under lowhono conditions. The discrepancy may be due to the different solutions used for gaseous HONO scrubbing: 1-mM phosphate buffer solution (ph 7) was used in this study and by Huang et al. [2002], and acidified SA/NED reagent solution by Kleffmann and Wiesen [2008]. All the HONO concentrations reported here have been corrected for the observed interference in the signal in the channel with a Na2CO3-coated denuder to remove HONO, by multiplying a factor 0.9. [6] HONO photolysis rate constants were calculated using NCAR TUV model ( Interactive_TUV/). Air mass source regions were identified by back trajectory calculation using the HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) Model [Draxler and Rolph, 2003]. 3. Results and Discussion [7] Eight flights were made over the forested region, three of which were also conducted over one of the lakes. Detailed flight information and measurement results are summarized in Table 1. In the BL over the forested region, HONO mixing ratios ranged from 8 to 70 pptv, with a mean of 28 pptv and a median of 29 pptv. The HONO concentrations at the altitudes closest to the ground surfaces were in good agreement with those measured at PROPHET tower during the same periods (Figures 2b 2e), and within the HONO concentration range previously reported in the rural atmosphere [Acker et al., 2006b; He et al., 2006; Zhou et al., 2002, 2007]. Over the Great Lakes, HONO mixing ratios were in the range Figure 2. HONO mixing ratios over the forested region (dots), over the Great Lakes (triangles), and at the PROPHET tower (crosses), and the potential temperature profiles over the land (solid lines). 2 of 5

3 ZHANG ET AL.: AIRCRAFT HONO MEASUREMENT Table 1. Summary of the Aircraft Measurements Over the Northern Tip of the Lower Peninsula of Michigan and the Neighboring Great Lakes Region During Summer of 2007 Flight a Date Time (EDT) Altitude AGL (m) Air Mass Origin Boundary Layer HONO Range/Mean (pptv) Free Troposphere HONO Range/Mean (pptv) 1 7/30/07 12:02 14: SW 25 66/ /14 3 8/01/07 8:18 10: SW 21 74/ /10 4 8/01/07 13:54 16: SW 15 42/ /15 4 b 8/01/07 16:25 17: SW 21 32/29 N/M 5 8/03/07 16:13 17: NW 8 22/15 4 8/6 6 8/04/07 8:13 10: NW 10 52/ /9 7 8/04/07 15:02 17: NW 22 46/ /8 7 b 8/04/07 17:15 17: NW 22 28/25 N/M 8 8/06/07 13:29 15: NW 12 23/20 6 9/7 8 b 8/06/07 15:23 16: NW 12 39/28 N/M a The data are not available for the second flight. b Observation over the lakes. N/M, not measured pptv, with a mean and a median of 26 pptv. HONO mixing ratios in the FT, which started between 1000 m and 1900 m, were low and relatively constant, in the range of 4 17 pptv, with a mean of 9 pptv and a median of 8 pptv. The variations of the ranges of HONO concentration for different days have a strong dependence on air mass origin, based on backward trajectory calculations. The BL HONO concentrations were high, up to 70 pptv, during polluted southwesterly flow (July 30 August 1) originating from Chicago and Milwaukee (Figure 2a 2c), but were less than 30 pptv during relatively clean northwesterly flow (August 3 August 6), which originated in Canada (Figures 2d 2f). The observations are consistent with the fact that higher concentrations of HONO precursors exist in southerly flow (NO y 2.9 ppbv and NO x 1.1 ppbv) than in the northerly flow (NO y 0.91 ppbv and NO x 0.46 ppbv) [Thornberry et al., 2001]. [8] All the HONO concentration profiles show the highest values near the ground surface, and a general decreasing trend with increasing altitude. In the BL, the HONO vertical profiles were found to be strongly influenced by the air column stabilities, as indicated by the potential temperature profiles (Figures 2a 2e). In the morning hours when the air column was relatively stable, significant HONO concentration gradients were observed in the lower BL (Figures 2b and 2e), suggesting the ground/canopy surface to be a HONO source. These morning HONO profiles are in agreement with previous observations by Kleffmann et al. [2003] showing significant HONO gradients in 190-m nighttime vertical profiles over a forested area in Germany. Somewhat lower but relatively constant concentrations of HONO above the surface mixed layer probably represent background values from in situ production, i.e., not influenced by the surface, due to the stable surface inversion inhibiting vertical mixing. In the morning of August 4 (Figure 2e), a significant change in HONO concentration was observed at approximately 70 m above the ground, from 17 pptv measured at 08:30 EDT to 48 pptv at 10:00 EDT. At 08:30 EDT, the ground surface temperature was 284 K, 8 K lower than at 70 m (Figure 2e). The steep temperature gradient in the stable surface mixing layer prevented vertical mixing of the surface-emitted HONO to the measurement altitude. By 10:00 EDT, surface heating raised the ground surface temperature to about 293 K, reducing air column stability and allowing surface HONO to be transported to the measurement altitude. [9] Several vertical HONO profile/gradient measurements have been reported, made on high towers [He et al., 2006; Kleffmann et al., 2003] or by DOAS with multiple light-path heights [Stutz et al., 2002; Veitel et al., 2002]. Although it is difficult to directly compare our results with those from previous studies, due to the difference in measurement environments, heights and the times of a day, all studies showed significant vertical HONO gradients within the lower portion of the boundary layer, indicating the ground surface to be a significant HONO source. [10] We estimated HONO vertical transport by turbulent diffusion using equation (1) [Jacob, 1999]. s ¼ ð2k z t HONO Þ 1 = 2 : ð1þ In equation (1), s is the vertical transport distance, K z is the turbulent diffusion coefficient, and t HONO is the HONO lifetime. During the morning hours when the BL was stable, as suggested by the temperature profiles (Figures 2b and 2e), K z is in the range of cm 2 s 1. Assuming a t HONO value of 28 min, most of the surface-derived HONO would be trapped within the lowest m above the canopy surface, resulting in a steep near-surface HONO concentration gradient (Figures 2b and 2e). Under the near-neutral conditions that we encountered during the afternoons (Figures 2c and 2d), the K z value is on the order of 10 6 cm 2 s 1.In this case, the vertical transport distance would be 350 m above the canopy surface, assuming a t HONO of 10 min. The increase in the HONO vertical transport distance would result in significant dilution and thus relatively uniform vertical HONO profiles throughout the boundary layer, as observed (Figures 2c, 2d, 2f, and 2g). The July 30 case (Figure 2a) represents a unique case in which a stable surface layer persisted until solar noon. [11] The HONO concentrations were significantly lower in the lower BL over the Great Lakes than over the forest, i.e., 29 ± 4 pptv (N = 8) versus 37 ± 3 pptv (N = 14) on August 1, and 25 ± 2 pptv (N = 11) versus 36 ± 3 pptv (N = 11) on August 4 (Figures 2c and 2f), except on August 6, which will be discussed below. Lake surfaces should behave more like a sink of HONO than a source (as in the case of ground surfaces), due to the high solubility of HONO in water (e.g., H Matm 1 at ph 6). However, a shallow and stable layer of air usually exists over colder water surface and is decoupled from the air aloft, reducing the depositional loss of HONO from the air column to the lake surface. Therefore, the measured HONO concentrations over the lake likely rep- 3of5

4 ZHANG ET AL.: AIRCRAFT HONO MEASUREMENT resents the background values supported by in situ HONO production alone. [12] It was surprising to observe higher HONO concentrations over the lake than over the land on August 6 (Figure 2g). Comparison of back trajectories of the air masses at the two sampling locations during the times of measurement indicates that while both air masses arrived from the northwest, the air mass arriving at the Lake Michigan location C had passed over a significant NO x source region about 20 hr earlier, where 2 power plants were located, in the Michigan Upper Peninsula (Figure 1; EPA 2002 facility emissions map available at The land surface HONO source should be negligible, since photolysis should have effectively removed all the surfaceemitted HONO during the 9-hr travel of the air mass over Lake Michigan. The elevated HONO concentrations over Lake Michigan probably resulted from higher in situ HONO production in this anthropogenically impacted air mass. [13] One of the major objectives of this research was to characterize in situ production of HONO in the air column. The observed median HONO concentrations of 8 pptv in the FT (i.e., typically 1500 m AGL) and 25 pptv and 15 pptv in the upper BL ( m AGL) in the southwesterly and northwesterly flows, respectively, probably represent background HONO concentrations sustained by in situ production, since the ground/canopy surface influence is negligible for these altitudes. Despite the large difference in HONO photolytic lifetime, from 8.3 min at midday to 30 min in the morning, the observed HONO concentrations did not vary much with time of day (Figure 2), suggesting that in situ HONO production was photochemical in nature and that ambient HONO was in a near photo-steady state. To maintain the observed HONO concentrations against photolysis at noontime, an in situ HONO production source of 57 pptv hr 1 would be required in the FT, and 180 pptv hr 1 and 110 pptv hr 1 in the upper BL in the southwesterly and northwesterly flows, respectively, assuming a HONO photolysis rate constant of J HONO s 1.Inthe continental FT, the NO x level is relatively low, 40 pptv [Huebert et al., 1990], and the gas-phase NO-OH reaction is only a minor source, i.e., 2 pptv hr 1 or 4% of the needed in situ production rate, assuming [OH] = molecules cm 3, [NO] = molecules cm 3, and k 1 = cm 3 molecule 1 s 1 [Atkinson et al., 2004]. In the upper BL, the contribution of the gas-phase NO-OH reaction is estimated to be about 13 pptv hr 1 in southwesterly flow and 5 pptv hr 1 in northwesterly flow, or 5 7% of the needed in situ production rate, assuming [OH] = molecules cm 3, and [NO] = and molecules cm 3 in southwesterly and northwesterly flows, respectively [Thornberry et al., 2001]. The remaining >90% of the needed HONO in situ production could derive from sources involving a range of HONO precursors, such as photolysis of nitrophenols in the gas phase and on surfaces [Bejan et al., 2006; Rohrer et al., 2005], photo-enhanced heterogeneous reactions involving NO 2 on organic surfaces [George et al., 2005; Stemmler et al., 2006], and photolysis of HNO 3 /nitrate on and in aerosol particles [He et al., 2006; Zhou et al., 2003, 2007]. Since the air at UMBS is generally relatively clean with respect to anthropogenic VOCs, the first mechanism is unlikely to be important. It is difficult to quantitatively evaluate the relative source strengths of the latter two HONO formation mechanisms, due to the lack of specific information, such as precursor concentrations and characteristics of aerosol organics. In particular, if photolysis of HNO 3 /nitrate on and in aerosol particles had been the major in situ HONO source, there would be significant implications with respect to the cycling of reactive nitrogen in the troposphere [Zhou et al., 2003]. [14] We estimated the contribution of surface processes on the ground/canopy to BL HONO (P surface ) as expressed as a flux by equation (2), by subtracting the in situ HONO production from the overall HONO source strength needed to sustain the observed HONO concentration in the air column throughout the BL, assuming a photo-steady state. P surface ¼ J HONO S CHONO i DZ i C o HONO Z =RT: ð2þ In equation (2), J HONO is the HONO photolysis rate constant, Z is the BL height, C i HONO is the average HONO concentration in the height interval DZ i, and C o HONO is the background HONO concentration, i.e., the HONO concentration observed in the upper BL or over the lake where ground surface contribution can be neglected (Figure 2). The calculated P surface ranges from moles m 2 hr 1,in the northwesterly flow during the morning of August 4, to moles m 2 hr 1, in the southwesterly flow during the early afternoon of August 1, with a mean of moles m 2 hr 1 and a median of moles m 2 hr 1. The HONO emitted from the ground/canopy surfaces accounted for 16 27% of the overall HONO in the boundary layer, most of which was distributed in the lower portion of the BL. The calculated HONO contribution from the ground/canopy surface appears to be in good agreement with the HONO flux measured at the PROPHET site in summer of 2008, which shows a noontime mean of moles m 2 hr 1 (X. Zhou, unpublished results, 2008). The mean P surface value of moles m 2 hr 1 represents a significant fraction of the summer noontime NO y downward flux mean of moles m 2 hr 1 at the PROPHET site (A. J. Hogg, NO x and NO y fluxes, manuscript in preparation, 2009). Thus a significant fraction of the deposited N is recycled back into the atmosphere as, effectively, NO x [Zhou et al., 2003]. That is consistent with the results of Hill et al. [2005], who found that much of the nitrate that dry deposits to the forest canopy at this site appears to be recycled to the atmosphere. [15] Photolysis of HONO is a significant source of the OH radical in the lower BL, i.e., up to 390 pptv hr 1 at midday, similar to reports for rural ground measurement sites [Acker et al., 2006b; Ren et al., 2006; Zhou et al., 2002, 2007], and on the order of 20% of the total photolytic HO x production observed on the canopy level at the PROPHETsite [Tan et al., 2001]. However, HONO photolysis is only a minor OH source, 55 pptv hr 1, in the FT and can be neglected when compared to the source from ozone photolysis. [16] Acknowledgments. We thank the University of Michigan Biological Station for outstanding logistical support during the field deployments. This research was supported by National Science Foundation grants ATM and ATM We thank two anonymous reviewers for their helpful comments on this manuscript. 4of5

5 ZHANG ET AL.: AIRCRAFT HONO MEASUREMENT References Acker, K., et al. (2006a), Nitrous acid in the urban area of Rome, Atmos. Environ., 40, , doi: /j.atmosenv Acker, K., D. Möller, W. Wieprecht, F. X. Meixner, B. Bohn, S. Gilge, C. Plass-Dülmer, and H. Berresheim (2006b), Strong daytime production of OH from HNO 2 at a rural mountain site, Geophys. Res. Lett., 33, L02809, doi: /2005gl Atkinson, R., et al. (2004), Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I Gas phase reactions of O x,ho x,no x, and SO x species, Atmos. Chem. Phys., 4, Bejan, I., Y. Abd El Aal, I. Barnes, T. Benter, B. Bohn, P. Wiesen, and J. Kleffmann (2006), The photolysis of ortho-nitrophenols: A new gas phase source of HONO, Phys. Chem. Chem. Phys., 8, , doi: /b516590c. Carroll, M. A., S. B. Bertman, and P. B. Shepson (2001), Overview of the program for research on oxidants: PHotochemistry, Emissions, and Transport (PROPHET) summer 1998 measurements intensive, J. Geophys. Res., 106, 24,275 24,288, doi: /2001jd Draxler, R. R., and G. D. Rolph (2003), HYSPLIT-Hybrid Single-Particle Lagrangian Integrated Trajectory Model, hysplit4.html, NOAA Air Resour. Lab., Silver Spring, Md. George, C., R. S. Strekowski, J. Kleffmann, K. Stemmler, and M. Ammann (2005), Photoenhanced uptake of gaseous NO 2 on solid-organic compounds: A photochemical source of HONO?, Faraday Discuss., 130, , doi: /b417888m. Harrison, R. M., and A.-M. N. Kitto (1994), Evidence for a surface source of atmospheric nitrous acid, Atmos. Environ., 28, , doi: / (94) He, Y., X. Zhou, J. Hou, H. Gao, and S. B. Bertman (2006), Importance of dew in controlling the air-surface exchange of HONO in rural forested environments, Geophys. Res. Lett., 33, L02813, doi: / 2005GL Hill, K. A., P. B. Shepson, E. S. Galbavy, and C. Anastasio (2005), Measurement of wet deposition of inorganic and organic nitrogen in a forest environment, J. Geophys. Res., 110, G02010, doi: / 2005JG Huang, G., X. Zhou, G. Deng, H. Qiao, and K. Civerolo (2002), Measurements of atmospheric nitrous acid and nitric acid, Atmos. Environ., 36, , doi: /s (02)00170-x. Huebert, B. J., et al. (1990), Measurements of the nitric acid to NO x ratio in the troposphere, J. Geophys. Res., 95, 10,193 10,198, doi: / JD095iD07p Jacob, D. J. (1999), Introduction to Atmospheric Chemistry, Princeton Univ. Press, Princeton, N. J. Kleffmann, J., and P. Wiesen (2008), Quantification of interferences of wet chemical HONO measurements under simulated polar conditions, Atmos. Chem. Phys. Discuss., 8, Kleffmann, J., R. Kurtenbach, J. Lorzer, P. Wiesen, N. Kalthoff, B. Vogel, and H. Vogel (2003), Measured and simulated vertical profiles of nitrous acid Part I: Field measurements, Atmos. Environ., 37, , doi: /s (03) Kleffmann, J., T. Gavriloaiei, A. Hofzumahaus, F. Holland, R. Koppmann, L. Rupp, E. Schlosser, M. Siese, and A. Wahner (2005), Daytime formation of nitrous acid: a major source of OH radicals in a forest, Geophys. Res. Lett., 32, L05818, doi: /2005gl Lammel, G., and J. N. Cape (1996), Nitrous acid and nitrite in the atmosphere, Chem. Soc. Rev., 25, , doi: /cs Reisinger, A. R. (2000), Observations of HNO 2 in the polluted winter atmosphere: possible heterogeneous production on aerosols, Atmos. Environ., 34, , doi: /s (00) Ren, X., et al. (2003), OH and HO 2 chemistry in the urban atmosphere of New York City, Atmos. Environ., 37, , doi: /s (03)00459-x. Ren, X., et al. (2006), OH, HO 2, and OH reactivity during the PMTACS-NY Whiteface Mountain 2002 campaign: observations and model comparison, J. Geophys. Res., 111, D10S03, doi: /2005jd Rohrer, F., B. Bohn, T. Brauers, D. Bruning, F. J. Johnen, A. Wahner, and J. Kleffmann (2005), Characterisation of the photolytic HONO-source in the atmosphere simulation chamber SAPHIR, Atmos. Chem. Phys., 5, Stemmler, K., M. Ammann, C. Donders, J. Kleffmann, and C. George (2006), Photosensitized reduction of nitrogen dioxide on humic acid as a source of nitrous acid, Nature, 440, , doi: / nature Stutz, J., B. Alicke, and A. Neftel (2002), Nitrous acid formation in the urban atmosphere: Gradient measurements of NO 2 and HONO over grass in Milan, Italy, J. Geophys. Res., 107(D22), 8192, doi: / 2001JD Tan, D., et al. (2001), HO x budgets in a deciduous forest: Results from the PROPHET summer 1998 campaign, J. Geophys. Res., 106, 24,407 24,427, doi: /2001jd Thornberry, T., et al. (2001), Observations of reactive oxidized nitrogen and speciation of NO y during the PROPHET summer 1998 intensive, J. Geophys. Res., 106, 24,359 24,386, doi: /2000jd Veitel, H., B. Kromer, M. Mossner, and U. Platt (2002), New techniques for measurements of atmospheric vertical trace gas profiles using DOAS, Environ. Sci. Pollut. Res., 9, Zhou, X., K. Civerolo, H. Dai, G. Huang, J. Schwab, and K. Demerjian (2002), Summertime nitrous acid chemistry in the atmospheric boundary layer at a rural site in New York State, J. Geophys. Res., 107(D21), 4590, doi: /2001jd Zhou, X., H. Gao, Y. He, G. Huang, S. B. Bertman, K. Civerolo, and J. Schwab (2003), Nitric acid photolysis on surfaces in low-no x environments: Significant atmospheric implications, Geophys. Res. Lett., 30(23), 2217, doi: /2003gl Zhou, X., G. Huang, K. Civerolo, U. Roychowdhury, and K. L. Demerjian (2007), Summertime observations of HONO, HCHO, and O 3 at the summit of Whiteface Mountain, New York, J. Geophys. Res., 112, D08311, doi: /2006jd M. Alaghmand and P. B. Shepson, Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA. H. Gao and N. Zhang, Department of Environmental Health Sciences, State University of New York, Albany, NY 12201, USA. B. Stirm, Department of Aviation Technology, Purdue University, West Lafayette, IN 47907, USA. X. Zhou, Wadsworth Center, New York State Department of Health, Albany, NY 12201, USA. (zhoux@wadsworth.org) 5of5