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Summertime nitrous acid chemistry in the atmospheric boundary

layer at a rural site in New York State

Xianliang Zhou,1 Kevin Civerolo,2 and Hongping DaiWadsworth Center, New York State Department of Health, Albany, New York, USA

Gu HuangSchool of Public Health, State University of New York at Albany, Albany, New York, USA

James Schwab and Kenneth DemerjianAtmospheric Sciences Research Center, State University of New York at Albany, Albany, New York, USA

Received 26 November 2001; revised 7 March 2002; accepted 11 March 2002; published 13 November 2002.

[1] Ambient measurements of HONO and HNO3, using a highly sensitive coil scrubbing/HPLC/visible detection technique, were made at a rural site in southwestern NewYork Statefrom 26 June to 14 July 1998, along with concurrent measurements of NOx, NOy, O3, andvarious meteorological parameters. The mean (and median) half-hour concentrations ofHONO and HNO3 during this period were 63 (and 56) pptv and 418 (and 339) pptv,respectively. On average, there were two HONO concentration peaks, the first around0200–0300 LT and the second around 0700–0800 LT, and a minimum at about 2000 LT.The sum of NOx, HONO, and HNO3 (�NOyi) was highly correlated with the measured NOy

concentration (r2 = 0.64). The average HONO/NOx ratio was 0.07, while the average�NOyi/NOy ratio was 0.66. During the early morning hours, the photolysis of HONOappeared to be a dominant source of HOx radicals in boundary layer near the ground surface.The average daily radical production from HONO photolysis was 2.3 ppbv, accounting for24% of the total production from photolyses of HONO, O3, and HCHO at themeasurement height of 4 m above the ground. Diurnal patterns of HONO and relativehumidity suggest that the ground and vegetation surfaces were sinks for HONO in theboundary layer when dew droplets were formed at night and that the subsequent release ofthe trapped nitrous acid/nitrite from the surfaces acted as a strong HONO source in themorning as the dew droplets evaporated. Our data also suggest that, in order to maintain theobserved daytime HONO concentration of �60 pptv, there should be a strong daytimesource of 220 pptv hr�1, which was much greater than the nighttime source of 13 pptv hr�1

and the estimated production of � 40 pptv hr�1 from the gas-phase NO-OH reaction.Photolysis of HNO3, which deposits and accumulates on the ground and vegetation surfaces,may contribute significantly to the ‘‘missing’’ daytime HONO sources. INDEX TERMS: 0322

Atmospheric Composition and Structure: Constituent sources and sinks; 0365 Atmospheric Composition

and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure:

Troposphere—constituent transport and chemistry; KEYWORDS: HONO, HNO3 photolysis, dew, rural

atmosphere

Citation: Zhou, X., K. Civerolo, H. Dai, G. Huang, J. J. Schwab, and K. L. Demerjian, Summertime nitrous acid chemistry in the

atmospheric boundary layer at a rural site in New York State, J. Geophys. Res., 107(D21), 4590, doi:10.1029/2001JD001539, 2002.

1. Introduction

[2] Nitrous acid (HONO) and nitric acid (HNO3) areimportant trace gases in the ambient atmosphere. Both

species contribute to the total reactive nitrogen budget,NOy (NOy = nitric oxide (NO) + nitrogen dioxide (NO2) +peroxyacetyl nitrate (PAN) + HONO + HNO3 + particulatenitrates ( p-NO3) + dinitrogen pentoxide (N2O5) + organicnitrates +.). In urban areas, HONO can accumulate to ppbvlevels at night [Andres-Hernandes et al., 1996; Harris etal., 1982; Pitts et al., 1984; Reisinger, 2000; Vecera andDasgupta, 1991], and subsequent photolysis in the follow-ing morning can be a significant source of hydroxylradicals (OH). However, continental background concen-trations of HONO are generally in the range of a few pptv

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D21, 4590, doi:10.1029/2001JD001539, 2002

1Also at School of Public Health, State University of New York atAlbany, USA.

2Now at New York State Department of Environmental Conservation,Albany, New York, USA.

Copyright 2002 by the American Geophysical Union.0148-0227/02/2001JD001539

ACH 13 - 1

to hundreds of pptv [Li, 1994; Harrison et al., 1996; Zhouet al., 2001], and to date few measurements in the lowNOx rural areas exist.[3] The gas-phase formation of HNO3 represents an

important mechanism for the removal of nitrogen oxides(NOx = NO + NO2) and odd hydrogen (HOx = OH + HO2)from the atmosphere:

NO2 þ OHþM ! HNO3 þMðR1Þ

This loss mechanism disrupts the steady state catalyticcycling of O3 and NOx. Subsequent deposition of HNO3

and nitrates contributes to acidification of sensitiveecosystems such as the Adirondack Park region of NewYork State [e.g., Driscoll et al., 1998]. Typical backgroundcontinental concentrations of HNO3 are of the order of 1–1.5 ppbv during the summer months [Buhr et al., 1990;Aneja et al., 1998].[4] Gas-phase formation of HONO is thought to be

relatively unimportant in the urban atmosphere, whileheterogeneous reactions on surfaces, involving NOx, soot,and water vapor, are considered more important for HONOformation [Lammel and Perner, 1988; Calvert et al., 1994;Andres-Hernandez et al., 1996; Ammann et al., 1998]:

NOþ NO2 þ H2Oþ surface ! 2HONOðR2Þ

2NO2 þ H2Oþ surface ! HONOþ HNO3ðR3Þ

NO2 þ soot ! HONOðR4Þ

Hence, an understanding of the mechanisms that control thefate of HONO in the atmosphere requires knowledge ofNOx, water vapor, HONO and HNO3 levels.[5] To date, little is known about the details of these

reactions, and few ambient measurements of these trace

species in rural areas exist. The reader is referred to Lammeland Cape [1996] for a detailed overview of the atmosphericchemistry of HONO, as well as a summary of measure-ments in both urban and rural areas. In this paper, we reportmeasurements of HONO and HNO3 at a rural site insouthwestern New York State made with a high sensitivitycoil scrubbing/high performance liquid chromatography(HPLC)/visible detection technique, during the summer of1998.

2. Site Description and Measurement Techniques

[6] Measurements were made from June 26 through July14, 1998 at Pinnacle State Park (77.21�W, 42.09�N,elevation 515 m MSL), located in the village of Addison,in the Southern Tier region of New York State (see Figure1). The site is located on a clearing and is surrounded by anine-hole golf course, mixed deciduous and coniferoustrees, a 50-acre pond, and pastureland [Vertefeuille, 1997].The nearest city is Corning, about 15 km northeast of thesite, with a population of about 12,000.[7] Our instruments were housed in a trailer adjacent to

an Atmospheric Sciences Research Center (ASRC) ofSUNY Albany air monitoring site, operated since 1995with logistical support from the New York State Depart-ment of Environmental Conservation (NYSDEC). Ozone,oxidized nitrogen species (NO/NOx/NOy), carbon monox-ide (CO), sulfur dioxide (SO2), various hydrocarbons, andmeteorological parameters are monitored continuously atthis site.[8] HONO and HNO3 were measured using a two-

channel measurement system. The details of the techniqueare described elsewhere [Zhou et al., 1999b; Huang et al.,2002], and are only briefly discussed here. Ambient airwas sampled through two 10-turn coil samplers [Lee andZhou, 1993] mounted 1 m atop the trailer (about 4 m

Figure 1. The Pinnacle State Park measurement site and surrounding region: Albany (ALB), Syracuse(SYR), Rochester (ROC), Buffalo (BUF), Jamestown (JAM), Corning (COR), Elmira (ELM), and NewYork City (NYC), New York; Erie (ERI) and Pittsburgh (PIT), Pennsylvania; and Toronto (TOR),Ontario, Canada.

ACH 13 - 2 ZHOU ET AL.: SUMMERTIME NITROUS ACID CHEMISTRY

above ground), at a rate of 2 SLPM per channel. A 1-mMphosphate buffer solution (pH 7) and deionized water wereused to scrub HONO and HNO3, respectively, at a flowrate of 0.24 mL min�1. In the HONO channel, thescrubbed nitrite was converted to a highly light-absorbingazo dye by reacting with two aromatic amines, sulfanila-mide (SA) and n-(1-naphthyl)-ethylenediamine (NED).The resultant azo derivative was separated from otherlight-absorbing compounds in the solution along a C18reverse-phase column, and was monitored with a UV-visible detector at 540 nm (Model 100, Thermo-Separa-tion). HNO3 was measured with the same technique afterfirst converting it to nitrite by a hydrazine reductionmethod. The nitrate to nitrite conversion occurred in anon-line 6-mL glass coil with a reaction medium containing0.8 mM hydrazine, 8 mM CuSO4 and 42 mM NaOH. Thenitrate to nitrite conversion efficiency was (86 ± 5)% andnitrite recovery was �95%. In the HNO3 channel, themeasured signal was due to ambient HONO plus anyHNO3 converted to nitrite; hence, to obtain the HNO3

signal, we took the difference between the signal recordedduring this cycle and the signal from the next ‘‘HONO’’cycle. The HONO and HNO3 channels were calibratedusing aqueous NaNO2 and NaNO3 standards, respectively.Chromatograms were stored and processed using MacIn-tegrator (Rainin), a Macintosh-based HPLC data acquis-ition system. Sampling was automated, providing nearreal-time data. Using a 10-port injector, we alternatedbetween the HNO3 and HONO modes every five minutes,yielding 10-minute data resolution. The overall measure-ment uncertainties were estimated to be about ±(4 + 0.2 �HONO) pptv for HONO and ±(20 + 0.3 � HNO3) pptvfor HNO3, respectively, taking into account the uncertain-ties associated with HPLC analysis, gas and liquid flowrates, blanks and calibrations, and potential interferencefrom gaseous and aerosol species.[9] To examine the accuracy of measurement system

and the validity of aqueous standard calibration, a blindintercomparison experiment was carried out during thePolar Sunrise Experiment 2000 at Alert, Canada. Detailsare described by Beine et al. [2002]. Briefly, a gas-phaseHONO standard at a concentration of about 2 ppbv wasgenerated using a system developed by Febo et al. [1995].The purity of HONO was better than 99%. The HONOconcentration in the standard gas was measured simulta-neously by our HONO system and by the NOy channel ofa NOx/NOy system. Excellent agreement was achievedbetween the data from our HONO system, the NOy

measurement system, and the theoretically calculatedHONO concentration, within 5%. This good agreementsupports the validity of aqueous calibration for our meas-urement system.[10] Potential interferences from NO, NO2, PAN, nitrate

and O3 have been investigated in our previous papers[Zhou et al., 1999b; Huang et al., 2002]. It was found that,in the polluted urban environments with up to 100 ppbvNOx and several ppbv of PAN, interferences from NO,NO2 and PAN were equivalent to a few pptv of HONO,insignificant compared to the ambient HONO concentra-tion of �100 pptv under these conditions. In a ruralenvironment, interferences from sub-ppbv levels of NOx

and PAN were too low to be detected. No interference

from a high concentration of nitrate on HONO analysiswas observed. Loss of scrubbed nitrite due to its aqueousreaction with ambient O3 during sampling was also negli-gible. We did, however, observe some loss of detectionsensitivity for both HONO and HNO3 by this techniquewith time in the field, especially in the polluted environ-ment, over a period of days to weeks. This uncertaintycould be completely eliminated by flushing the samplingand derivatization lines daily with HPLC mobile phasesolution.[11] While aqueous scrubbing of gaseous HONO and

HNO3 is quantitative using our coil sampler, a fraction ofaerosols may be collected. We recently conducted a seriesof experiments in Albany, NY, to examine the significanceof the aerosol contribution to the observed HONO andHNO3 signals using a two channel system, with onechannel sampling ambient air directly and the other con-nected downstream of a Na2CO3-coated annular denuder(URG) to remove gaseous HONO and HNO3. The inter-fering signals collected in the coil sampler after thedenuder resulted from particulate species as well as fromNOx and PAN in the ambient air. We found that theHONO signal (including reagent blank) in the denuderchannel contributed only a minor fraction, (6 ± 2)%, of theobserved signal without the denuder, with a maximum ofabout 8% in the early afternoon when the gaseous HONOconcentration was low (�200 pptv) due to its effectivephotolysis, and a minimum of about 4% in the earlymorning when the HONO concentration was high (�1.3ppbv) due to overnight accumulation. This observation isconsistent with recent results by Heland et al. [2001]showing insignificant interferences from NOx, PAN andparticulate species to the measurement of HONO using asimilar aqueous scrubbing-SA/NED derivatization-lightabsorbance system. The percentage of HNO3 signal(including reagent blank) by the denuder channel was(14 ± 6)%, with a maximum of about 20% in the earlymorning when gaseous HNO3 was relatively depleted dueto dry deposition in the relatively shallow and stablesurface layer, and a minimum of about 8% in the afternoonwhen gaseous HNO3 accumulated via photochemical reac-tions. The higher denuder channel signal for HNO3,compared to HONO, may reflect the fact that particulatenitrate represents a large fraction of total nitrate during theexperiment. Past measurement results [e.g., Buhr et al.,1990; Aneja et al., 1998] have suggested that particulatenitrate may only account for a minor fraction of the totalnitrate budget, about 1 – 25%, in rural eastern U.S. areas.If this applies to the Pinnacle State Park sampling site,contributions from aerosol interference on gaseous HONOand HNO3 would be relative minor, within our estimatedmeasurement uncertainties of ±20% for HONO and ±30%for HNO3, respectively.[12] Ozone was measured with Thermo Environmental

Instruments (TEI) Model 49 analyzer, with a detectionlimit of 2 ppbv for 1-minute averaged data. NO, NO2, andNOy were measured with TEI Model 42S analyzers usingthe technique of NO chemiluminescence. NO2 was con-verted to NO by a filtered Xenon arc lamp, and NOy wasconverted to NO by a heated molybdenum oxide convertermounted outside, 5 cm from the sample inlet. For 1-minuteaveraged data, the detection limits for NO, NO2, and NOy

ZHOU ET AL.: SUMMERTIME NITROUS ACID CHEMISTRY ACH 13 - 3

were 60, 180, and 120 pptv, respectively. Relative humid-ity was measured with a Rotronics MP101A integratedtemperature and humidity probe, with a nominal accuracyof ± 1.5%. UV radiation was measured with an EppleyTUVR with a detection limit of 0.11 W m�2 for 1-minuteaveraged data.

3. Back Trajectory Analysis

[13] In order to examine the general airflow at themeasurement site, we computed 1-day back trajectoriesusing Version 4 of the HYbrid Single-Particle LagrangianIntegrated Model (HYSPLIT4, available at http://www.arl.noaa.gov/ready/hysplit4.html) for each day, using thearchived ETA model meteorological data. In order toestimate afternoon mixed-layer trajectories, each trajectorywas terminated at 1600 LT and 500 m above the ground.While individual trajectories can be in error by about 30%of the trajectory length [Stohl, 1998], and they cannot takeinto account high-resolution physical and chemical pro-cesses, they can be used to qualitatively describe the large-scale air movement.

4. Results and Discussion

4.1. Statistical Summary

[14] Basic summary statistics of the data are presented inTable 1. The HNO3 mixing ratio from late-June to mid-July ranged from 15 to 1372 pptv, with a mean of 418pptv, while the HONO mixing ratio ranged from 9 to 213pptv, with a mean of 63 pptv. For comparison, the averageHNO3 concentrations at the Pinnacle site were lower thanthose reported by Buhr et al. [1990] near State College,Pennsylvania, a site that is more affected by point sourcesin the Ohio River Valley and population centers in centraland western Pennsylvania. The average HONO concen-trations at the Pinnacle site were higher than thoseobserved after polar sunrise in the Northwest Territories,Canada [Li, 1994; Zhou et al., 2001], where it was mostlybelow 20 pptv. On the other hand, Zhou et al. [1999a]reported summertime average concentrations in downtownAlbany, NY, which were often >300 pptv, and as high asabout 2 ppbv.[15] The sum of HONO and HNO3 accounted for nearly

20% of the observed NOy, ranging from 3% when freshemissions from local sources affected the site, to 61%for more aged air masses. The sum of NOx, HONO,and HNO3 (�NOyi) was strongly correlated with NOy

(Figure 2), with a correlation coefficient (r2) of 0.64,significantly higher than that between NOy and NOx (r2 =0.37). In Figure 2, the solid line denotes the least squaresbest fit, while the dashed line indicates the 1:1 line. Onaverage, �NOyi accounted for about two-thirds of the totalNOy at this site. The remaining NOy was probablycomprised mostly of PAN and p-NO3.[16] In urban areas, reactions R2 and R3 are important

at night. It has previously been shown [Andres-Hernandezet al., 1996; Harrison et al., 1996; Reisinger, 2000] thatnighttime HONO concentrations tend to correlate wellwith NO2, and the product NO � NO2. However, at thePinnacle site the correlation coefficients between HONOand these two parameters were less than 0.05; inclusionof water vapor and exclusion of daytime HONO data didnot improve the correlations. This may suggest that someother physical and chemical factors rather than straightfor-ward NOx-H2O reactions were controlling the temporalHONO distribution; thus, simple correlations betweenHONO and NOx-H2O may not be very meaningful. Itis interesting to note that HONO and HNO3 were wellcorrelated over all conditions (r2 = 0.63). This may bedue to some common factors controlling the temporaldistribution of these two species, such as the transport ofpolluted air masses containing high concentrations ofHNO3 and other HONO precursors, and wet scavengingby rain and dry deposition when dew occurred on groundsurfaces. In addition, this good correlation is consistentwith the hypothesis that HNO3 photolysis on the groundsurface is a major source for daytime HONO in boundarylayer, as discussed later.

4.2. General Trends

[17] The observed time series of HONO and HNO3

(bottom panels) from June 26 to July 14 are presentedin Figure 3, along with wind speed and direction (toppanels), UV radiation and relative humidity (middle pan-els). Periods of rain are denoted with solid dark lines in

Table 1. Basic Statistics of Observed HONO, HNO3, NOx, NOy

Concentrations, and Various Ratiosa

Species or Ratio Range Median Mean s Number

HONO, pptv 9 – 213 56 63 33 811HNO3, pptv 15 – 1372 339 418 303 811NOx, ppbv 0.22 – 5.36 0.97 1.11 0.63 907NOy, ppbv 0.61 – 6.91 2.40 2.61 1.10 885HONO/NOx <0.01 – 0.30 0.06 0.07 0.04 748HONO/NOy <0.01 – 0.13 0.03 0.03 0.01 694

(HONO + HNO3)/NOy 0.03–0.61 0.17 0.19 0.10 693�NOyi/NOy 0.25 – 1 0.64 0.66 0.20 783

aThe data are half-hour averages. The �NOyi is the sum of HONO,HNO3, and NOx.

Figure 2. The correlation between NOy and the sum ofNOx, HONO and HNO3 (�NOyi). The dashed line denotesthe 1:1 line. The solid line denotes the least squares best fitfor the data, with r2 = 0.64.


the middle panels. In general, the HONO concentrationsincreased during the night. There were two situationsobserved at night and the following morning during therain-free periods (Figure 3). The first situation occurredduring the nights when dew droplets formed on theground and vegetation surfaces, with HONO concentra-tions usually reaching the first peak after midnight whenthe relative humidity was near 100%, then decreasing dueto scavenging by dew droplets formed on surfaces. TheHONO trapped on the surface was later released aftersunrise when the dew droplets evaporated, resulting in asecond HONO concentration peak in the early to mid-morning period. This situation occurred on June 26, July3, July 6, July 8 – 10, and July 13. The second situationwas evident on more dew-free nights, i.e., relative humid-

ity < 90% throughout the nights, when HONO tended toaccumulate throughout the night and reached a maximumin the early morning, as happened on the mornings ofJune 29, July 7, July 11–12, and July 14. The HONOconcentration generally decreased throughout the day as aresult of its photolytic removal, and reached a late-dayminimum.[18] Wind speed and direction can affect HONO levels

in different ways. In general, winds from emissions-richregions, such as the Ohio River Valley (westerly andsouthwesterly) and the nearby Corning/Elmira area (east-erly and northeasterly) tended to bring higher concentra-tions of pollutants, including NOx, to the site, resulting inhigher observed HONO concentrations. Northerly flows,on the other hand, should transport relatively cleaner air

Figure 3. Time series of concentrations of HONO and HNO3 (lower panels), along with wind speedand direction (upper panels), UV intensity and relative humidity (middle panels). The heavy solid lines inthe middle panels indicate the periods when significant precipitation was observed. The panels in 3arepresent the June 26 to July 4 period, and the panels in 3b represent the July 5–14 period.


to the site, resulting in lower observed HONO concen-trations. It should be pointed out, however, that HONO isrelatively short-lived, around 10 min under full sunconditions, and that the observed relationship betweenHONO concentration and wind direction is not due tolong-distance transport of HONO itself, but its precursorssuch as NOx and perhaps HNO3. HONO was formedmore locally from its precursors on surfaces. SinceHONO is produced heterogeneously on surfaces [Calvertet al., 1994; Lammel and Cape, 1996], and since groundsurfaces (including vegetation surfaces) provide most ofthe surface area for the HONO formation in the atmos-pheric boundary layer, the stability of the atmosphericboundary layer becomes an important factor in controllingHONO concentration near ground surface. Indeed, closeexamination of Figure 3 shows that most of the highHONO events occurred in the relatively low wind period,

while strong winds helped dilute concentrations of HONOreleased from ground surfaces. There were many factorsthat controlled HONO concentrations in atmosphericboundary layer, as will be discussed below.

4.3. Diurnal Variations

[19] Figure 4 displays the average diurnal variations ofHONO, NOx, NOy, UV radiation, and relative humidity.During the nighttime and early morning hours, the HONOconcentration was on average greater than 70 pptv, and theminimum late afternoon concentration was about 40 pptv.The morning peak in NOx and NOy was related to thebreak-up of the nocturnal boundary layer and subsequentvertical mixing near sunrise. On the other hand, thereappeared to be a time lag between the concentrations ofNOx and NOy and that of HONO during the night, due tothe net accumulation of HONO from heterogeneous reac-

Figure 3. (continued)


tions between NOx and H2O. There were two morningpeaks in the average HONO concentration plot, the firstaround 0200–0300 LT and the second after sunrise (Figure4). The intermediate ‘‘dip’’ may be a result of scavenging ofHONO by dew droplets formed on the ground and vegeta-tion surfaces, while the second peak may be caused by therelease of the trapped HONO as relative humidity began todecrease and dew began to evaporate. There appeared to bestrong sources of HONO sufficient to partially offset thephotolytic loss and sustain higher than expected concen-trations throughout the day, as will be discussed below.

4.4. HONO as a Source of Radicals

[20] To assess the importance of HONO as a source ofradicals at the Pinnacle site near the surface, we comparedthe radical production rates from the photolysis of HONO,HCHO, and O3 (Figure 5), using the half-hour averageconcentrations of HONO, HCHO, O3, solar UV, and H2Omixing ratio. All kinetic rate constants were obtained fromAtkinson et al. [1996], and photolysis rate constants werecomputed using actinic flux values, absorption cross sec-tions, and quantum yields obtained from Finlayson-Pittsand Pitts [2000] and references therein. The photolysis rateconstants were corrected for the variable cloud coverageusing the Eppley radiometer measurements following themethod of Kleinman et al. [1995].[21] From Figure 5, it appears that during the early

morning hours, the dominant near-ground source of HOx

radicals was through the photolysis of HONO. At 0600 LT,radical production by HONO photolysis was five times

greater than that by HCHO photolysis, and nearly an orderof magnitude larger than that by O3 photolysis. By about0830 LT, radical production by HONO photolysis was aboutthe same as that of O3 photolysis and about twice as large asthat of HCHO. From noontime onward, radical productionfrom HONO and HCHO was comparable, although O3

photolysis appeared to be the major HOx source duringthe late morning and afternoon hours. Radical productionfrom both O3 and HCHO photolysis at noontime was about1.1 ppbv hr�1, lower than �1.6 ppbv hr�1 reported at arural site in the southeastern U.S. [Kleinman et al., 1995],due to lower concentrations of water vapor and HCHO, andslightly lower UV solar intensity. The daily radical produc-tions were 2.3, 5.2 and 2.2 ppbv from the photolyses ofHONO, O3 and HCHO, respectively. Therefore, the con-tribution from HONO photolysis to the daily radical budgetis significant enough (�24% of the sum) to warrant fullconsideration when modeling atmospheric production ofphotooxidants in the near-surface boundary layer.[22] It should be noted that HONO levels should be

highest near the ground since ground surfaces were itsdominant source, so the estimated radical production rateusing the data observed at the 4-m sampling height probablyrepresents an upper limit if taking the whole atmosphericboundary layer into consideration. Certainly, as HONOconcentrations decrease with height above the ground,HONO photolysis becomes a less important source; thus,the actual radical production would be smaller throughoutthe entire boundary layer. Flux and profile measurements ofHONO and other species are critical for assessing therelative radical production rates throughout the surfaceand mixed layer.

4.5. Photo-Steady State and Daytime HONO Sources

[23] Figure 6 shows the observed (solid line) and calculatedHONO based on photo-steady state (dashed line), assuming

Figure 4. Average diurnal variation of HONO (circles,upper panel), NOx (solid line, upper panel), and NOy

(dashed line, upper panel), along with UV intensity (dashedline, lower panel) and relative humidity (solid line, lowerpanel).

Figure 5. Daytime radical production from photolysis ofHONO (circles), HONO + HCHO (squares), and HONO +HCHO + O3 (triangles). The calculation is based on theconcentrations of O3 (diamonds) and HCHO (crosses)shown in this figure and on the HONO concentration andUV intensity in Figure 4.


[OH] was proportional to radical production (Figure 5) with amaximum [OH] of 5 � 106 molecules cm�3 at noon:

k5NOþ OH , HONO

jHONO

ðR5Þ

It is obvious that the ambient HONO concentration deviatedsignificantly from the photo-steady state with respect to OHand NO. The observed HONO concentration, substantiallyhigher than the expected for the photo-steady state, suggestsstrong sources for HONO during the daytime. To maintainthe observed concentration, a local HONO production rateof �220 pptv hr�1 around noontime would be needed. Theaverage nighttime HONO production rate was about 13pptv hr�1, estimated from Figure 4 between 2200–2400when the HONO accumulation rate reached a maximum.This production of HONO was probably due to NOx-H2Oreactions (R2 and R3) on surfaces. The maximum HONOproduction from the OH - NO reaction (R5) was calculatedto be about 40 pptv hr�1 around noon, using the observedNO concentration and assuming an OH concentration of5 � 106 molecules cm�3. Even after taking these twoHONO sources into consideration, there is still a 170-pptvhr�1 HONO source needed to account for the observedHONO levels.[24] During the early to midmorning hours, the release of

trapped HONO/nitrite from ground surfaces during and afterdew evaporation may be a major HONO source. From thelate morning on, however, we propose that HNO3 photol-ysis may be a major candidate for the missing HONOsource. HNO3 is a highly ‘‘sticky’’ species and its deposi-tion to the ground surface is an important pathway for NOy

removal from the atmosphere [e.g., Russell et al., 1993;Finlayson-Pitts and Pitts, 2000]. The adsorbed HNO3 mayaccumulate on the ground surfaces in the absence ofprecipitation, forming a thin layer of hydrated HNO3. Whenexposed to sunlight, the highly concentrated HNO3 absorbsthe UV portion of the sunlight and undergoes photolysis,which may follow similar photolytic pathways that are

known to occur in aqueous solutions [Mack and Bolton,1999]:

HNO3 adsð Þ þ hv ! HNO3½ �* adsð ÞðR6Þ

HNO3½ �* adsð Þ ! HNO2 adsð Þ þ O 3P� �

adsð ÞðR7Þ

HNO3½ �* adsð Þ ! NO2 adsð Þ þ OH adsð ÞðR8Þ

The yield ratio of R8 to R7 in aqueous solutions is about9:1. Assuming an average relative humidity � 60% in thesummer season during the measurement period, thereshould be several monolayers of water on the surfaces onthe grounds and vegetation leaves [Svensson et al., 1987].Thus the adsorbed NO2 would react rapidly with adsorbedH2O on the surface to produce HONO [Svensson et al.,1987]:

2NO2 adsð Þ þ H2O adsð Þ ! HONO adsð Þ þ HNO3 adsð ÞðR30Þ

The adsorbed HONO then is released into the air. Themechanism is consistent with the recent observation ofphotochemical production of HONO and NOx in snow-pack [Honrath et al., 1999, 2000; Zhou et al., 2001] andin the glass inlet manifold [Zhou et al., 2002]. Morelaboratory work on surface HNO3 photolysis, however, isneeded to verify and quantify this process.

4.6. A Case Study, 2–5 July

[25] The period of July 2–5 was interesting in that itillustrates the effects of NOx, dew and rain scavenging,and subsequent release of the HONO trapped in the dewor rain on observed HONO concentrations. Therefore, thisperiod will be examined in greater detail. Figure 7 displaysthe time series of wind speed and direction (top panel),relative humidity and UV radiation (2nd panel), NOx andNOy (3rd panel), and HONO and HNO3 (bottom panel)during this period. Rain during the overnight hours of July4–5 is shown as the solid lines in the 2nd panel of Figure7. Figure 8 shows the back trajectories for each of the fourdays ending at 1600 LT. Note that the trajectories on July3 and 4 were westerly and southwesterly, respectively, andindicated that boundary layer emission sources may haveaffected air quality at the site. On the other hand, thetrajectories on July 2 and 5 were more likely to originatefrom southern Canada.[26] Light northwesterly winds helped keep HONO

levels relatively low on July 2. During the late night ofJuly 2, the winds shifted more southwestly, advecting aNOx plume past the Pinnacle site, with a peak NOx

concentration of nearly 5 ppbv. High HONO concentra-tions between 100 and 140 pptv were recorded a fewhours later near midnight, in part as a result of heteroge-neous reactions between H2O and NOx. A significantamount of the newly formed HONO was subsequentlyscavenged by the dew droplets formed on the ground andvegetation surfaces as ambient water vapor pressurereached saturation around 0300 LT, resulting in a rapiddecrease in HONO concentration to about 60 ppt. As therelative humidity decreased during the morning hours ofJuly 3, the HONO scavenged by dew droplets during the

Figure 6. Observed (solid line with circles) and predictedphoto-steady state (dashed line with triangles) HONOconcentrations during the daytime.


prior night was released, resulting in high HONO concen-trations, up to 150 pptv, despite rapid photolysis under fullsunlight during the late morning hours and the earlyafternoon. This observation seemed to suggest that thesubsequent release of the trapped HONO/nitrite during andafter the dew evaporation was a relatively slow process.[27] July 4 was mostly overcast, so that photolytic loss

of HONO was low on this day. As a result, afternoonHONO concentrations exceeded 75–100 pptv. During thenight of July 4, a significant rain event (13 mm) occurred,removing most of the ambient HONO and HNO3. Duringthe morning hours of July 5, HONO concentrationsincreased from a few pptv to about 50 pptv, even thoughthe NOx concentrations were low on this day. Thissuggests that the rise in morning HONO concentrationsmay again be attributed in part to the release of the trapped

HONO that was scavenged by water droplets on theground surfaces overnight. Fairly strong midday winds(>4 m s�1) may have also contributed to the relatively lowHONO concentrations on this day.

5. Summary

[28] The average concentrations of HONO and HNO3 ata rural site in New York State from June 26 to July 14,1998 were 63 and 418 pptv, respectively. On average, theHONO/NOx ratio was about 7% at the site, while the sumof HONO and HNO3 accounted for about 20% of the NOy

concentrations. HONO photolysis appeared to be a majormechanism for ground-level radical production during themorning hours, and accounted for about 24% of the dailyradical production, comparable with HCHO photolysis.

Figure 7. The time series of wind speed and direction (top panel), UV intensity and relative humidity(2nd panel), NOx and NOy (3rd panel), and HNO3 and HONO (bottom panel) from July 2–5, 1998.


The observed HONO concentration deviated substantiallyfrom the photo-steady state with respect to OH and NO,suggesting strong sources during the daytime. Ground andvegetation surfaces were HONO sources for the overlyingatmosphere during most of the time, but also could besinks when dew droplets formed. The release of thetrapped HONO/nitrite from ground and vegetation surfa-ces, scavenged by dew droplets at night when relativehumidity was high, could contribute to high HONOconcentrations during the morning hours, partially offset-ting the photolytic loss after sunrise. We also propose thatphotolysis of adsorbed HNO3 on the ground and vegeta-tion surfaces may be an important daytime HONO source.HONO and HNO3 gradient or flux measurements [e.g.,Huebert and Robert, 1985; Huebert et al., 1988; Meyers etal., 1989] would be useful to quantify the strength of eachof the HONO sources and sinks related to the ground andvegetation surfaces, at this rural site.

[29] Acknowledgments. This research was supported by the NationalScience Foundation (NSF), ATM-9615748. The Pinnacle State Park meas-

urements and support for KD and JS provided by the Empire State ElectricEnergy Research Corporation, Contract #EP92-15. The use of trade andcompany names does not constitute an official endorsement of any productby the New York State Department of Health, the Atmospheric SciencesResearch Center, or the State University of New York at Albany. The authorswish to thank the staff at Pinnacle State Park for access to the monitoring site.

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��K. Civerolo, New York State Department of Environmental Conserva-

tion, 625 Broadway, Albany, NY 12233, USA.H. Dai, Wadsworth Center, New York State Department of Health,

Empire State Plaza, P.O. Box 509, Albany, NY 12201-0509, USA.K. Demerjian and J. Schwab, Atmospheric Sciences Research Center,

State University of New York at Albany, Albany, NY 12201-0509, USA.G. Huang, School of Public Health, State University of New York at

Albany, Empire State Plaza, P.O. Box 509, Albany, NY 12201-0509, USA.X. Zhou, Wadsworth Center, New York State Department of Health, and

School of Public Health, State University of New York at Albany, EmpireState Plaza, P.O. Box 509, Albany, NY 12201-0509, USA. ([email protected])
