Isoprene and its oxidation products, methacrolein and methylvinyl ketone, at an urban forested site during the 1999 Southern Oxidants Study

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. D8, PAGES , APRIL 27, 2001 Isoprene and its oxidation products, methacrolein and methylvinyl ketone, at an urban forested site during the 1999 Southern Oxidants Study C. A. Stroud, l,2,3 J. M. Roberts, l,2 P. D. Goldan, 1 W. C. Kuster, 1 P. C. Murphy,l,2 E. J. Williams, l,2 D. Hereid, l,2, D. Parrish, 1 D. Sueper, l,2 M. Trainer, 1 F. C. Fehsenfeld, l,2 E. C. Apel,4 D. Riemer,4,5 B. Wert,4 B. Henry,4 A. Fried,4 M. Martinez-Harder,6 H. Harder,6 W. H. Brune,6 G. Li,7 H. Xie,7 and V.L. Young7 Abstract. Isoprene (ISOP) and its oxidation products, methacrolein (MACR) and methyl vinyl ketone (MVK), were measured at an urban forested site in Nashville, Tennessee, as part of the 1999 Southern Oxidants Study (SOS). Hourly observations were performed at Cornelia Fort Airpark for a 4 week period between June 13 and July 14. At the midday photochemical peak (1200 local standard time, LST), average mixing ratios of isoprene, MACR, and MVK were 410 parts per trillion by volume (pptv), 240 pptv, and 430 pptv, respectively. Median isoprene, MACR, and MVK mixing ratios were 400 pptv, 200 pptv, and 360 pptv, respectively, at 1200 LST. An emissions inventory calculation for Davidson County, encompassing Nashville, suggests that MACR and MVK were produced predominately from isoprene oxidation rather than direct combustion emissions. The observations are compared with results from two chemical models: a simple sequential reaction scheme and a one-dimensional (l-d) numerical box model. The daytime ratios of MVK/ISOP and MACR/ISOP varied in a systematic manner and can be reproduced by the analytical solution of the sequential reaction scheme. Air masses with more photochemically aged isoprene were observed during SOS 1999 at Cornelia Fort ( hours) compared to the SOS 1990 canopy study at Kinterbish ( hours). This is consistent with the proximity of the tower inlets to the forest canopies during both campaigns. Isoprene had a chemical lifetime of 20 min at the average observed midday HO mixing ratio of 8 x 106 molecules/cm3. As a result, significant conversion of isoprene to its oxidation products was observed on the timescale of transport from the dense forest canopie surrounding Nashville. The systematic diurnal behavior in the MVK/MACR ratio can also be simulated with a 1-D photochemical box model. General agreement between the observations of MACR and MVK during SOS 1999 with the two chemical models suggests we have a comprehensive understanding of the first few stages of isoprene oxidation in this urban forested environment. 1. Introduction the HO-initiated oxidation of isoprene, in a NOx-rich environment, has been observed in laboratory chamber studies Biogenic emissions of isoprene represent a significant to produce carbon monoxide, formaldehyde, methacrolein source of reduced carbon to the troposphere affecting (MACR), methyl vinyl ketone (MVK), peroxyacetic nitric photochemistry on local, regional, and global scales anhydride (PAN), peroxymethacrylic nitric anhydride [Fehsenfeld et al., 1992]. The oxidation of isoprene results in (MPAN), methyl furan, hydroxynitrates, and the production of a wide range of stable products. For example, hydroxycarbonyls [Carter and Atkinson, 1996]. However, these studies were done with initial chamber mixing ratios of isoprene and NO x which are orders of magnitude larger than typically observed in the troposphere. It is uncertain if this may affect the branching ratios and product yields in isoprene 1Aeronomy Laboratory, NOAA, Boulder, Colorado 2Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, Colorado. 3Now at Department of Earth and Atmospheric Science, York University, North York, Ontario, Canada. 4Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado. SNow at Rosenstiel School of Marine and Atmospheric Science, Miami, Florida. 6Department of Meteorology, Pennsylvania State University, University Park, Pennsylvania. 7Department of Chemical Engineering, Ohio University, Athens, Ohio. Copyright 2001 by the American Geophysical Union. Paper number 2000JD /01/2000JD oxidation mechanism. An understanding of the isoprene oxidation mechanism in the troposphere and its accurate representation in computer models is of fundamental importance due to isoprenefs role in the production of ozone. The high reactivity of isoprene renders it a major precursor to 0 3 formation in regional [Trainer et al., 1987; Roberts et al., 1998], suburban [Starn et al., 1998], and urban environments [Chameides et al., 1988; Biesenthal et al., 1997]. Isoprene's contribution to 0 3 production has been estimated through the measurement of specific oxidation products such as MVK [Biesenthal et al., 1997], MACR [Starn et al., 1998], and MPAN [Williams et al.,

2 8036 STROUD ET AL.: ISOPRENE AND ITS OXIDATION PRODUCTS 1997; Roberts et al., 1998]. The Southern Oxidants Study (SOS) results of Roberts et al. [1998] suggesthat on average ~25% of the 0 3 production in the region surrounding Nashville, Tennessee, can be attributed to isoprene oxidation. This is consistent with the results of Starn et al. [1998], who observed an isoprene contribution to 0 3 production of 28% for a period when a power plant plume impacted the forested site in suburban Nashville. Furthermore, for certain periods with high NO x over heavy forest, the study of Roberts et al. [1998] observed an isoprene contribution to 0 3 production above background as large as 75%. Thus it is clear that isoprene is a very important precursor to 03 formation. For forested cities it is critical to address the impact of reductions in anthropogenic volatile organic compounds (VOCs) on oxidant levels. If the emissions of reactive biogenic VOCs are large in an urban area so that 0 3 formation is rendered NO x- limited, then reductions in anthropogenic VOCs will be ineffective at reducing oxidant levels. Several studies have been done in rural, forested environments to characterize the products of isoprene oxidation. The first two reported studies by Pierotti et al. [1990] and Martin et al. [1991] arrived at contradictory results. Martin et al. [1991] observed generally higher levels of MVK than MACR during the daytime. By contrast, Pierotti et al. [1990] observed generally higher levels of MACR than MVK during the daytime. Further studies by Montzka et al. [ 1993; 1995] were in agreement with Martin et al. [ 1991] and could be reproduced with a one-dimensional (l-d) photochemical model. However, few studies have been performed in more polluted environments. Two such studies observed little variation in the diurnal pattern of the MVK/MACR ratio [Biesenthal et al., 1997; Biesenthal and Shepson, 1997]. This is in contrast to the rural studies and may be attributed to the importance of direct emissions of MVK and MACR in urban environments. During the summer of 1999 we performed measurements of isoprene and its oxidation products, as a part of the continuing efforts of the Southern Oxidants Study. These observations were performed at an urban forested site in Nashville, Tennessee. The city of Nashville is fairly isolated from other major cities and is surrounded by heavy forest. Thus it is an ideal location to study isoprene photochemistry and its contribution to 0 3 formation for an urban environment. In this paper, we discuss the details of the measurement technique used and whether our present understanding of isoprene photochemistry can explain the observed interrelationships between isoprene, MACR, and MVK in this environment. flushed with 8 cm3 STP of He to remove 02, 03, and water vapor within the cartridge and transfer lines. The preconcentrated VOCs were then thermally desorbed from the 2. Experimental Approach Tenax TA at a temperature of 140øC and transferred in a He stream (10 cm3min -1 STP for 5 min) to the cryofocusing unit Analytical Procedures After the VOCs were transferred to the cryofocusing unit, the Measurements of C2-C 3 alcohols, C2-C 4 carbonyl Tenax cartridge was cleaned at 200øC for 30 min under a 10 compounds (including methacrolein and methyl vinyl ketone) cm3min-1 STP He flow and then cooled in preparation for the and C5 hydrocarbons (including isoprene) were performed next run under a 0.5 cm3min -1 STP He flow. every hour using an automated instrument based on solid The cryofocusing unit was constructed from a glass-lined sotbent preconcentration, cryofocusing, a gas chromatographic stainless steel loop (1.6 mm OD and 0.50 mm ID) and placed (GC) separation, and flame ionization detector (FID). A within the neck of a dewar filled with liquid nitrogen. The schematic representation of the instrument is shown in Figure liquid nitrogen level could be controlled electronically at the 1. The solid sotbent preconcentrator was constructed of a glass top of the dewar for cryofocusing as described by Goldan et tube (12 cm long, 4 mm ID) filled with g of Tenax TA al., [1995]. The liquid nitrogen level was then lowered, and the (35/60 mesh). Deactivated fused silica wool was used as end low-volume cryo loop was heated resistively to 140øC for plugs for the cartridge. The cartridge was enclosed within a VOC injection into the GC. The two stainles steel valves, used PFA Valve Pump Inlet from Manifold Vent He Tenax TA Trap ~ øC ] I MFC -' Standard Gwl MFC ' Zero Air Water [ Bubbler To GC/FID Cryo Loop Valve A: Ambient/Cal Valve B: Tenax Sample Valve C: GC Injection Figure 1. Schematic diagram of the system used to measure volatile organic compounds (VOCs). copper block for temperature control. The copper block was cooled with thermoelectric units and heated with an embedded heater cartridge. For sampling, the cartridge was cooled to a few degrees above ambient dew point temperatures. It was observed in the laboratory that sampling moist standards at cartridge temperatures below the standard dew point temperature resulted in the condensation of water vapor on the cartridge. Sampling large amounts of water had several adverse affects on instrument performance such as retention time shifts, poor GC peak shape, and irreversible adsorption of carbonyl compounds within the capillary column. These affects were likely due to the presence of a film of liquid water within the GC column. A measured volume of air, typically 100 to 400 cm3 STP, was drawn with a 5 min acquisition time through the cartridge to concentrate VOCs from the air sample. Sample volumes were varied with ambient humidities. Periods of lower ambient humidity enabled us to use cooler Tenax trap temperatures, sample larger volumes, and thus achieve lower instrument detection limits. The sample flow rate was measured with a MKS mass flow controller. After sampling, the cartridge was

3 STROUD ET AL.: ISOPRENE AND ITS OXIDATION PRODUCTS 8037 to switch the flow of VOCs within the instrument, were maintained at 100øC (Tenax sample valve B and GC injection valve C in Figure 1). The VOCs were separated with a combination 60 m Stabilwax capillary column (1 gm film thickness, 0.32 mm ID) and a 40 m DB-1 capillary column (1 gm film thickness, 0.32 mm ID). The DB-1 capillary column was necessary because it was observed that 2-methylpropanal coeluted with acetone on the Stabilwax column. The capillary columns were enclosed within a Shimadzu GC-14A oven. The temperature program was held at 40øC for 10 min followed by a ramp at 2øC/min to 100øC. After elution of the C4 carbonyls, the GC oven was ramped quickly to 160øC to clean the GC column in preparation for the next run. Helium was used as the GC carrier gas at a flow rate of 2.4 cm3min -1 STP. The VOCs were quantified with a flame ionization detector (FID) maintained at 200øC. Ultrazero air and H2 were supplied to the FID from compressed gas cylinders (Scott-Marrin and Air Liquide). A hydrocarbon trap was used to clean the zero air (Restek). Ultrapure grade 5.0 He was also supplied to the sorbent cartridge and the GC column from a compressed gas cylinder (Air Liquide). A hydrocarbon, moisture, and 02 trap were used to clean the He gas. Helium, H2, and zero air flows within the instrument were measured with MKS mass flow controllers. All the functions of the sampler and GC were controlled through a laptop computer running in-house software (written in Visual C++). The in-house software also displayed and stored the operational parameters and data from the instrument in formats compatible with commercial packages. Data analysis software was used to calculate the cartridge sample volumes from the stored sample flow data (Igor Pro) and integrate the stored chromatographic data (Turbochrom). Calibration standards were generated by dynamic dilution of primary gas standards at the sampling site. The primary standards consisted of two compressed gas cylinders containing C2-C 4 carbonyl compounds in the 1-10 ppmv range (including methacrolein and methyl vinyl ketone) and one National Institute of Standards and Technology (NIST)- traceable pressurized canister containing hydrocarbons in the ppbv range (including isoprene). The primary carbonyl standards were prepared gravimetrically by established techniques in Acculife-treated aluminum cylinders [Apel et al., 1998]. A dilution system based on mass flow controllers was used to generate calibration mixtures in the low ppbv range (0.5-5 ppbv). The dilution system also contained an in-line water bubbler to generate standards simulating ambient humidities. A Pd catalyst heated to 325øC was necessary downstream of the water bubbler to remove carbonyl compounds introduced from the water bubbler. Calibration mixtures were introduced at the three-way PFA solenoid valve (Valve A in Figure 1) and therefore passed through the entire instrument. Laboratory calibrations performed with dry and moist air showed that for some alcohols and carbonyl compounds lower responses were observed with moist air; however, these losses were less than 10%. Calibration factors used during the campaign were based on the humidified standards and thus compensated for any slight loss associated with sampling humidified ambient air. Calibrations were typically performed every other day. FID response showed no detectable drift over the month long campaign, so average calibration factors were applied to each species over the whole data set. The uncertainty of these measurements depends on the accuracy of the cylinder standards at preparation, the stability of the standards over time, and the performance of the dilution system. The carbonyl compounds and alcohols were prepared in cylinders 8 months prior to the field campaign. These cylinders were again calibrated against standards at the National Center for Atmospheric Research (NCAR) laboratory 5 months prior to the campaign. The C2-C 4 carbonyl compounds were all within 6% of the original calibration. The C2-C 3 alcohols had experienced more considerable loss with ethanol decreasing by 22% over the 13 month period. The reproducibility of repeated calibrations (100 cm3 STP sample sizes and mixing ratios of 2 ppbv) resulted in relative standard deviations of less than 7% for all the C2-C 4 carbonyl compounds. Detection limits for the C4 carbonyl compounds and isoprene were 0.05 ppbv and 0.03 ppbv, respectively, for a 400 cm3 STP air sample. Overall measurement uncertainties for the C4 carbonyl compounds and isoprene are estimated at 15%, due to the combination of systematic and random errors Site Description and Sampling Procedures The measurements were performed between June 13 and July 14, 1999, at an urban, forested site in Nashville, Tennessee. The urban island of Nashville encompasses a diameter of-20 km with a landcover consisting of a mix of grassland, trees, water, and urban structures (buildings and roads). The countryside surrounding Nashville is heavily forested with hardwood deciduous trees and to a lesser extent conifers. The site was at the Cornelia Fort Airpark approximately -7 km northeast of downtown Nashville; a location where hydrocarbons were of biogenic and anthropogenic origin and NOx was influenced by urban and power plant emissions. Plate 1 illustrates the position of several SOS measurement sites and coal-burning power plants in the Nashville vicinity, superimposed on a map of isoprene fluxes (based on the biogenic emissions inventory system (BEIS-2) emissions inventory model of Geron et al. [1994]). A 10 m tower in an open pasture field was used as the platform for the chemical measurements. The nearest cluster of isopreneemitting trees was along the Cumberland river at a distance of -200 m from the tower. An exhaust fan was used to draw VOCs down a 3.0 inch ID glass manifold to a mobile trailer. The inlet of the glass manifold extended 2 m above the tower platform. The calculated residence time of gases in the tower manifold was <10 s. A 3 m PFA transfer line (1/4 inch OD) with an airflow of 1 standard liter per minute (slpm) was used to sample off the manifold and into the GC/FID. No traps to scrub 0 3 were used within the instrument. Laboratory tests with 130 ppbv of 0 3 showed no depletion of isoprene or production of MACR or MVK within the instrument when compared to i u[,lcitc standards run without 0 3. This was likely due to the low sample volumes collected and the fact that 0 3 was displaced from the Tenax trap and transfer lines with a He stream prior to Tenax trap heating Ancillary Measurement Techniques Fast response measurements of O 3 were performed by 0 3- NO chemiluminescence and calibrated based upon a UV absorption instrument (E. Williams, personal communication, 2000). Measurements of NO x and NO y were also performed with a NO-O3 chemiluminscence instrument with a Xe lamp for NO2 conversion to NO and a Mo convertor for NOy species

4 STROUD ET AL.' ISOPRENE AND ITS OXIDATION PRODUCTS conversion to NO [Williams et al., 1998]. HO was measured by laser-induced fluorescence as described by Brune et al. [1998]. HO detection limits were estimated at 106 molecules cm-3 for a 1 min average. Measurements of HCHO were performed by tunable diode laser absorption spectroscopy as described by Fried et al. [1998]. Spectroradiometry measurements were used to derive photolysis rate coefficients as described by Shetter et al. [1999]. Measurements of other C2-C8 hydrocarbons were also performed by GC/FID (V. Young, personal communication, 2000). 3. Results and Discussion 3.1. Characterization of the Cornelia Fort Site: Chemical Time Series for the Entire Study In this study, an extensive data set has been produced covering a 4 week period. In Figure 2, we present the observations for O 3, NO x, and NO y for the entire measurement period. Three photochemically active periods can be highlighted where afternoon O3 levels were above 100 ppbv: June 20-22, June 30, and July 5-8. For the period between June 20-22, afternoon NO x levels were 1-10 ppbv. NO x peaked for the whole campaign during this time period at 145 ppbv on June 21 at 0715 LST. For the period between July 5-8, afternoon NO x and NOy levels were typically 1-10 ppbv and 5-15 ppbv. The exception was the D.L. Isoprene 0.012I MACR I I - D.L,-. MVK Figure 3. Statistical data for isoprene, MACR, and MVK. The full rectangles encompass all the measurements for the entire study; the hatched region indicates the central two thirds of the observations. The solid bar indicates the average, while the broken bar indicates the median. The measurements of isoprene and MACR were occasionally below the instrument detection limit. 100,' 4 o Z 'l'l'11111'lll'lllll'111'l'l'l'lll'l'111'l'l' 'l'l'lll'l -Illll,lll'111' I' I'1 '1' I'1'1' Ill :l'lll'l Illlll'l' Illll'llllllL 6/21/99 7/1/99 7/11/99 Date afternoon of July 6 at 1600 LST when NO x and NO y peaked at 10 ppbv and 27 ppbv, respectively, during which time the Cumberland power plant plume intercepted the site. These observed NO x levels suggesthat urban and suburban Nashville can be considered a high NO x environment where peroxy radical reactions with NO dominate the photochemistry. As a result, MACR and MVK were major products of isoprene oxidation in Nashville, as opposed to low NOx conditions when organic peroxides are favored. Figure 3 presents the statistical results of over 600 ambient air measurements of isoprene, MACR, and MVK during the study. The isoprene mixing ratio ranged from below detection limit (47 samples) to a maximum of 6.6 ppbv on July 6 just before sunrise. The MACR mixing ratio ranged from below detection limit (four samples) to a maximum of 2.8 ppbv on July 3 at 1900 LST. The MVK mixing ratio ranged from ppbv to a maximum of 2.6 ppbv observed on June 27, also in the early evening at 1900 LST. The average levels of isoprene were lower by a factor of 2 compared to a Nashville suburban site [Starn et al., 1998] and a factor of 10 lower than a rural site in Alabama [Montzka et al., 1993]. This was due to the short lifetime of isoprene and its chemical removal on the timescales of transport from the forest canopies surrounding Nashville to the site. The average levels of MACR and MVK observed at Cornelia Fort were similar to measurements from other rural and suburban sites in the southeast [Montzka et al., 1993, 1995; Starn et al., 1998]. This is a reflection of the longer lifetime of these oxidation products and thus their more spatially uniform distribution in the southeast United States. In Figure 4, we presenthe isoprene, MACR, and MVK time series data for the entire measurement period. For the Figure 2. 03, NOx and NOy time series data for the entire photochemically active period between June 22-24, midday study period. The lighter shading corresponds to daytime isoprene, MACR, and MVK levels were ppbv, measurements between LST ppbv, and ppbv, respectively. For the period

5 STROUD ET AL.' ISOPRENE AND ITS OXIDATION PRODUCTS berland Hendersonville Dickson CFA Gallatin 36 Nashville... Youth... i i! 1! 35 i! Longitude (øw) Isoprene Flux x 1014mol-m-2-s- 1 Plate 1. A map of Nashville and the surrounding countryside. The colorshow the BEIS-2 isoprene emission estimate for the month of July, projected on a 20 x 20 km grid. The triangles show the location of major coalburning power plants, and the circles show the location of SOS measurement sites.

6 STROUD ET AL.: ISOPRENE AND ITS OXIDATION PRODUCTS calculation. A diurnal average CO emission rate of moles/s has been estimated for Davidson county which encompasses Nashville on a 20 km x 20 km grid [ Harley et al., 2001]. Emission factors of MACR and MVK, relative to CO, were recently inferred from measurements during the winter in Toronto, a metropolitan city with strong CO emissions, but relatively sparse biogenic isoprene emissions [Biesenthal and Shepson, 1997]. Therefore the observed MACR and MVK were likely driven by direct mobile emissions, especially in the wintertime when biogenic sources are inactive. The observed emission factors were 7.3 x 10-5 moles MACR/mole CO and 1.4 x 10-4 moles MVK/mole CO. Applying these factors to the Nashville CO emission rate yields anthropogenic MACR and MVK emission estimates of moles/s and moles/s respectively. Summertime, biogenic isoprenemissions of 13 moles/s have also been estimated for Davidson county using the BEIS- 2 model [Geron et al., 1994]. Applying the 0.23 and 0.32 product yields for MACR and MVK production from HOinitiated isoprene oxidation in a NOx-rich environment [Carter and Atkinson, 1996], we obtain chemical production 2 r I I ] I I [ I I [! I [ [ I l estimates of 3.0 moles/s and 4.2 moles/s, respectively. Comparing the anthropogenic emission rates with in situ production rates, we find that anthropogenic emissions are less ß than 0.4% of in situ production for both MACR and MVK. Qualitatively, the measurements of isoprene, MACR, MVK, 2 and CO observed in Nashville and Toronto support this 0.1 conclusion. In Nashville during SOS 1999, average MACR, I I I I I I I I I I I I MVK, and CO levels were 420 pptv, 570 pptv, and 300 ppbv, /23/99 ' 7/ /99 ' 7/1 /99 respectively. In Toronto, at CO levels of-300 ppbv, MACR Date and MVK averages were considerably smaller at 25 pptv and 50 pptv. This comparison provides further evidence that Figure 4. Isoprene, MACR, and MVK time series data for the isoprene chemistry was the driving factor for the MACR and entire study period. The lighter shading corresponds to MVK observed at Cornelia Fort during SOS daytime measurements between LST. Tailpipe emissions of isoprene are slightly smaller than MACR emissions (-1.5 times smaller on a per mole basis) [Harley et al., 1999]. Thus tailpipe isoprene emissions of moles/s can be estimated for Davidson county. This can between July 5-8, midday isoprene, MACR, and MVK levels were generally higher, in the range ppbv, be compared with biogenic isoprenemissions of 13 moles/s ppbv, and ppbv, respectively. for Davidson county. Thus biogenic emissions of isoprene To address whether isoprene is an important contributor to dominate automobile emissions for Nashville by a factor of more than These emissions inventory calculations peroxy radical production, a comparison of the relative HOinitiated oxidation rates for measured chemical species was suggest biogenic sources for isoprene and in situ chemical performed. Figure 5 is a compa,rison of the values of production for MACR and MVK dominate Nashville during the summertime. Thus the observed relationships between ki[species]i where k i is the rate coefficient for HO reaction at 298 K for a sample at 1200 LST on July 6, The isoprene, MACR, and MVK should be addressed with our importance of isoprene as a reactive gas during this understanding of isoprene biogenic emissions and photochemically active period is clearly seen in this urban photochemistry. forested environment. Isoprene oxidation products (HCHO, MVK, and MACR) are also highly reactive species and 3.3. Diurnal Trends of Isoprene, MACR, contributed significantly to the production of peroxy radicals MVK, and Related Species and hence 0 3. The diurnal patterns of VOCs at ground sites are affected by a number of chemical and meteorological factors. During the 3.2. Emissions Inventory Calculations: Sources of Isoprene, daytime, reactions with HO dominate the oxidation of VOCs MACR, and MVK in Davidson County at Cornelia Fort. Surface heating results in convective mixing Automobile emission studies have identified isoprene, and elevated boundary layer heights, so that air samples are MACR, and MVK as minor products of fuel combustion more representative of the lower troposphere over a wide [Harley et al., 1992]. Before discussing the chemical region. At night a shallow and poorly mixed nocturnal relationships between isoprene, MACR, and MVK, we estimate boundary layer (NBL) accentuates the influence of local for Nashville the importance of combustion emissions emissions and surface deposition [Andronache et al., 1994; compared to biogenic emissions and in situ photochemical Mahrt et al., 1998] production. Our approach is through an emissions inventory The diurnal trend in isoprene is shown in Figure 6a. The

7 , STROUD ET AL.' ISOPRENE AND ITS OXIDATION PRODUCTS ] July 6, :00 CST _- 1' I 0,',,, - HCHO Isoprene CO CH3CHO NO 2 MVK CH 4 MACR Propanal Ethanol Ethene Toluene Figure 5. Relative HO reactivity for species measured on July 6 at 1200 LST assuming a temperature of 298 K. The mixing ratios (ppbv) used in the calculation are shown above each bar. ozone photolysis rate coefficient J(O1D) is included as an indicator of available daytime actinic flux leading to hydroxyl radical formation. Both arithmetic mean and median isoprene mixing ratio along with the standard deviation of the mean are included. The standard deviation of the mean reflects O > i! i! i i, i i :00 06:00 12:00 18: 00 00:00 Time (LST) 20x 10 '6 15 4'" 5 Figure 6. The diurnal trends in (a) isoprene, J(O1D) and (b) water vapor. The average and median isoprene mixing ratios for each diurnal hour are shown with squares and circles, respectively. The vertical bars indicate one standard deviation about the mean. 0 the day-to-day variability and not the instrument measurement uncertainty. There is large variability about the mean isoprene values with the variability being greater during the nighttime than daytime. This variability is not restricted to isoprene as other nonbiogenic hydrocarbons, such as i-pentane, also exhibited large variability from night to night. The variability seen in the chemical measurements likely reflects many factors related to nighttime meteorology [Bange and Rot& 1999]. In contrast to the convective boundary layer, the stable NBL is characterized by low horizontal wind speeds at the surface and a maximum in horizontal wind speed at the capping inversion. This so-called "low-level jet" at the capping inversion causes wind shear which can intermittently create turbulence and short bursts of vertical mixing. Thus the capping inversion is variable in height and stability. Since the capping inversion defines the height into which emissions are mixed, the variability in the height from night to night and stability on a given night can affect the measured VOC concentrations. A shallow, poorly mixed NBL also accentuates the spatial and temporal variability associated with VOC emissions surrounding the site. During the daytime, greater atmospheric mixing results in mixing ratios that are more uniform throughout the boundary layer and more representative of emissions averaged over a wider footprint. A large difference in magnitude and trend was also observed between mean and median isoprene levels at night. For example, between midnight and 0400 LST the mean isoprene levels were observed to increase from 390 pptv to 520 pptv, while the median isoprene mixing ratio were observed at lower levels and decreased from 140 pptv to 100 pptv. Upon closer inspection of the nighttime isoprene data, the high mean isoprene levels are skewed by several nights with unusually large and sustained isoprene levels. For the purposes of describing a "typical" isoprene diurnal pattern, it is more representative to use the median mixing ratios. The median nighttime ( LST) and daytime ( LST) isoprene mixing ratios observed at Cornelia Fort were 0.20 and 0.42 ppbv, respectively. The trend in the median isoprene throughout the nighttime is similar to rural canopy studies

8 8042 STROUD ET AL.' ISOPRENE AND ITS OXIDATION PRODUCTS [Goldan et al., 1995], as median isoprene decayed throughout the night to a minimum at 0400 LST, just before sunrise, which was 0500 LST at Cornelia Fort. At sunrise the median isoprene mixing ratio increased due to the dependence of biogenic isoprene emissions on actinic flux and temperature [Sanadze, 1957; Monson and Fall, 1989; Harley et al., 1999]. The trend in the median isoprene increased until 0900 LST and then decreased to a minimum at midday hours. The observed midmorning decrease in median isoprene is related to boundary layer vertical mixing and photochemistry. Figure 6b is the diurnal trend in the median water vapor mixing ratio. The water vapor mixing ratio increased at sunrise, consistent with increased evaporation due to surface heating. However, at 0730 LST, the water vapor began to decrease. This is due to the growth of the convective boundary layer and entrainment of drier air from the above residual layer. Thus the water vapor trend is useful in determining when vertical mixing may affect the other chemical measurements. The timing of the midmorning isoprene decrease is consistent with the water vapor decrease. The morning increase in HO mixing ratio followed very closely the morning increase in the J(O1D) ozone photolysis rate coefficient. The largest increase in HO mixing ratio occurred between the hours of LST (W. H. Brune, personal communication, 2000). The chemical lifetime of isoprene by HO reaction derived from the average observed midday HO mixing ratio of 8 x 106 molecules/cm 3 was very short (-20 min) at 298 K. Thus the midday suppression in isoprene was due to a combination of boundary layer vertical mixing and HO-driven photochemistry. Trainer et al. [1987] used a 1-D photochemical model, with explicit vertical transport and photochemistry, to study the vertical distribution of isoprene. Since the chemical loss of isoprene is dominated by reaction with HO, any change in HO will affect the ground-level isoprene mixing ratio and the vertical gradient of isoprene. The spatial and temporal variability of HO in the atmosphere will depend on numerous factors, among which are the NO x mixing ratio. For NO x levels below 1 ppbv the mixing ratio of HO increases with NO x due to the increasing conversion of HO 2 to HO by NO. The HO mixing ratio reaches a maximum at a NOx mixing ratio of a few ppbv and then decreases as the reaction of HO with NO2 becomes the dominant loss term. Trainer et al. [1987] showed that the isoprene vertical gradient was dependent on NOx mixing ratios in the range ppbv. The steepest isoprene vertical gradient was observed for the largest NO x mixing ratio of 3.0 ppbv which corresponds to the maximum in the HO mixing ratio. Thus the sharp isoprene vertical gradient was attributed to chemical loss by HO on the timescales of vertical transport. At Cornelia Fort we were in a high-nox regime within the urban island of Nashville. Thus, during midday hours, we would also expect to be in a high-ho regime with rapid isoprene chemical loss on the timescales of transport from the dense forest canopies surrounding Nashville. The fairly uniform isoprene mixing ratios observed during midday hours ( LST) are consistent with results from a recent study in which tethered balloon vertical profiles of isoprene were performed in the convective boundary layer [Greenberg et al., 1999]. The fairly constant isoprene mixing ratios observed during midday in both the SOS 1999 and the Greenberg et al. [1999] study are in contrasto canopy studies [Goldan et al., 1995] which show a continuous increase in isoprene throughout the morning and afternoon. The , 20x mo :00 06:00 12:00 18:00 00:00 Time (LST) x ,2, Figure 7. The diurnal trends in (a) MVK, and (b) MACR, along with J(O1D). The average and median mixing ratios for each diurnal hour are shown with circles and squares, respectively. The vertical bars indicate one standard deviation about the mean. The average J(O1D) for each diurnal hour is shown with triangle symbols. continuous increase throughouthe day in the canopy studies is due to the proximity of the sampling inlet to the forest, as short distances limit the importance of the boundary layer height and photochemistry on the measured isoprene mixing ratios. Rural canopy studies are also in a lower-nox regime than Cornelia Fort, thus midday HO mixing ratios would be lower at rural sites which would further limit the importance photochemistry on measured isoprene at the forest canopy. The median isoprene mixing ratio at Cornelia Fort increased significantly between 1400 LST and sunset, due to a combination of continuing emissions, a decrease in HO-driven photochemistry, and a decrease in vertical mixing. This interpretation is supported by the HO diurnal profile which shows HO decreased significantly throughout the afternoon starting at 1330 LST (W. H. Brune, personal communication, 2000) and by observations from the boundary layer wind profiler which, typically, shows maximum vertical mixing heights between LST. The large increase in mean isoprene at 1900 LST is driven by several evening cases where a shallow NBL formed at sunset. The diurnal trend in water vapor can again yield insight into the initial time for development of the NBL. The median water vapor began to increase at 1730 LST, an hour before sunset at 1830 LST. The water vapor trend illustrates when a shallow inversion developed and concentrated evaporative emissions. The increase in mean isoprene between LST is 5

9 STROUD ET AL.: ISOPRENE AND ITS OXIDATION PRODUCTS 8043 consistent with residual emissions of isoprene being concentrated into a developing NBL. The diurnal profile for the first generation isoprene oxidation products, MVK and MACR, are shown in Figures 7a and 7b. Both MACR and MVK were observed at higher mixing ratios during the nighttime (2000-0:00 LST) than daytime ( LST). Nighttime median MACR and MVK levels were 0.41 ppbv and 0.49 ppbv; higher than the daytime median levels of 0.25 ppbv and 0.44 ppbv. Mean values followed the same trend as median values, but were slightly higher. This trend is in contrast to rural canopy studies which show higher levels of MACR and MVK during the daytime [Montzka et al., 1993]. As with isoprene, larger variability was also observed during the nighttime compared to daytime. The largest increase in median MVK mixing ratio was typically observed in the early morning and early evening hours. These peak times reflect the combination of rapid production from isoprene oxidation and reduced vertical mixing in the Diurnal Time (LST) atmosphere. Another interesting aspect of these observations, compared Figure 8. The diurnal trend in the MVK/MACR ratio. The to rural canopy studies, is that after the early evening increase average observed ratios during SOS 1999 are shown with in MACR and MVK, both remain at high levels throughouthe triangles. Vertical bars represent one standard deviation about the mean ratio. The solid line is the result of the 1-D model night. Rural studies have observed a consistent decrease in study of Montzka et al. [1993]. MACR and MVK throughout the night with the lowest levels observed just before dawn [Montzka et al., 1993]. This argues that nighttime chemical oxidation and/or deposition of these products was slower at the urban Cornelia Fort site than at rural forested sites. dynamics. The minimum in the observed nighttime ratio is 1.1, while the maximum in the observed afternoon ratio is 2.0. The The lowest median mixing ratios of MACR and MVK observed at midday hours can be rationalized by the elevated boundary layer mixing heights and the fast photochemical destruction rates in the early afternoon. This interpretation is supported by the work of Montzka et al. [1995], who were able to observe a relationship between MACR and MVK mixing ratios and mixing layer depth. During the SOS 1992 study the levels of MACR and MVK were observed to decrease by a factor of 2, as the boundary layer height increased from m. Higher boundary layer heights result in MACR and MVK being diluted into a larger mixing volume. The MACR and MVK chemical lifetimes by HO loss for the observed average midday HO mixing ratio of 8 x 106 molecules/cm3 are 1.0 hours and 1.9 hours, respectively. Thus the first-stage oxidation products of isoprene are also highly reactive species, and like isoprene, the combination of dilution in a well developed boundary layer and fast photochemistry likely explain the observed midday suppression in MACR and MVK. MACR was also observed at lower levels than MVK 3.4. A Framework to Study the Photochemistry of Isoprene and Its Products, MACR and MVK To study isoprene oxidation and its importance in determining the observed levels of MACR and MVK, it is instructive to look at the diurnal trend in the MVK/MACR ratio shown in Figure 8. One striking observation is the reduced nighttime variability in the product ratio compared to the individual measurements. This reflects the fact that taking the ratio of two related products removes much of the variability associated with nighttime boundary layer ß 00:00 04:00 08:00 12:00 16:00 20:00 00::00 MVK/MACR product ratio expected from the reaction of HO with isoprene is 1.4. MACR is also lost faster than MVK, so a daytime ratio slightly larger than 1.4 would be expected. The 1-D model of Montzka et al. [1993] simulated a daytime ratio of 2.0; identical to the SOS 1999 daytime maximum. At night the isoprene ozonolysis reaction results in a product ratio of Furthermore, MVK reacts faster with O 3 than MACR, so a ratio slightly less than 0.41 would be expected from pure ozonolysis reactions at steady state. The 1-D model of Montzka et al. [1993] run over the full diurnal profile showed the product ratio decreasing to a value of 1.0 before sunrise; again very similar to the SOS 1999 nighttime data. The agreement between the 1-D model and the observed MVK/MACR ratio suggest that the levels of MVK and MACR were, indeed, dominated by isoprene chemistry and that our current knowledge of the first few steps of isoprene oxidation can account for the Nashville observations. Another method of interpreting the relationship between during the daytime. This reflects the higher yield of MVK MACR and MVK is through their direct correlation. Figure 9 is compared to MACR from HO-driven isoprene photochemistry the regression of MVK against MACR for daytime ( and the faster daytime HO-initiated loss rate for MACR LST) and nighttime ( LST). The observed compared to MVK [Carter and Atkinson, 1996]. correlation is strong (daytime R2-0.90, nighttime R2-0.85). The upper slope along the daytime points is -2, while the lower slope along the nighttime points is -1, consistent with the results of Montzka et al. [1993]. Several urban studies (Vancouver and Toronto) have observed little diurnal variation in the MVK/MACR ratio. A ratio of 2.0 has been observed during both the daytime and nighttime [Biesenthal et al., 1997; Biesenthal and $hepson, 1997]. These other urban studies suggested that automobile emissions, especially at night in a shallow NBL, were influencing the observed ratios. Nighttime ratios observed in Nashville decreased to --1, consistent with the 1-D photochemical model, providing further support that for this

10 8044 STROUD ET AL.' ISOPRENE AND ITS OXIDATION PRODUCTS o A 2:1 o expected. In the canopy the rate of production of MACR and MVK will also be much larger than their rate of destruction. However, as the air parcel is advected from the canopy, isoprene is chemically processed to products. It is possible to derive an expression for the time rate of change in the [MACR]/[ISOP] ratio as a function of [HO], rate coefficients, and the time available for chemical processing. Assuming the following HO-driven isoprene oxidation mechanism from the laboratory chamber studies discussed by Carter and Atkinson [1996], o o ISOP + HO --> 0.23 MACR MACR + HO --> products MVK + HO --> products MVK kl=l.0x (1) k2=3.3 x (2) k3=1.9 x 10 -ll (3) 0.5 o MACR (ppbv) Figure 9. The correlation between MVK and MACR. The data are divided between daytime ( LST) and nighttime ( LST) points. The lines are 1:1 and 2:1 through the origin. environment, chemical production from isoprene dominates over direct automobile emissions. The ratios of MACR/ISOP and MVK/ISOP also yield useful information related to the photochemical age of isoprene in an air mass. Directly above a forest canopy, isoprene levels will be dominated by local, fresh emissions, and small ratios would be where ks are in units of cm3 molecules-1 sec-1, and initially only isoprene present (at forest canopy t = 0 and [MACR]/[ISOP] -- 0), the expression derived is as follows: [MACR] [ISOPI 0'23kl (k2-k ) (1-- e (kl -- k2) [OHlavgt) It should be emphasized that this expression is purely chemical, based on the above isoprene oxidation scheme and does not include any mixing processes which might affect the observed ratio during transport. An analogous expression can also be derived for the MVK/ISOP ratio. As shown in Figure 10, it is possible to derive a trend line by plotting one ratio versus another. Times were assigned to points on the trend line using the observed average daytime [HO]avg=6 x 106 molecules/cm3 between 0900 and 1600 LST. Included on this plot are ambient ratios from the Cornelia Fort site during SOS 1999 and the Kinterbish site during SOS The tower inlet for the Kinterbish site was only a few meters above the forest canopy. The data include only daytime observations between LST hr o 0.6hr 0.8 hr o 1 hr 1.2 hr o 2 & 0.4 hr hr 6 0.1hr } i [MACR] / [ISOP] Figure 10. The correlation between the MVK/ISOP and the MACR/ISOP ratio for the daytime measurements ( LST) during SOS 1990 (triangles) and SOS 1999 (circles). The solid line is the result of the isoprene sequential reaction scheme model. Isoprene photochemical ages between hours are derived along the modeline based upon the daytime average observed HO mixing ratio ( LST). 5

11 STROUD ET AL.: ISOPRENE AND ITS OXIDATION PRODUCTS 8045 The Kinterbish data lie slightly above the predicted line but in good agreement considering the uncertainties in the observations and the sequential reaction scheme model. The isoprene photochemical ages, derived from comparing the ambient ratios with the predicted line, are between hours, with the bulk of the points near 0.2 hours. A recent 1-D canopy model study reached similar results with isoprene being observed just above the canopy level with a calculated residence time of several minutes between the time of emission and detection [Makar et al., 1999]. The Cornelia Fort daytime data follow the HO-driven kinetic trend line very well. The photochemical ages derived were between 0.3 and 1.6 hours with the bulk of the points near 0.7 hours. A comparison of the derived daytime photochemical ages between Kinterbish and Cornelia Fort is strong evidence that, indeed, the Cornelia Fort site was being impacted by air masses with more chemically processed isoprene. This is consistent with Cornelia Fort being a greater distance from the forest canopies than Kinterbish. The chemical processing of isoprene at these two sites would also depend on HO mixing ratios which, in turn, depends on the NOx mixing ratio. NO x levels of a few ppbv, typical of Cornelia Fort during midday hours, correspond to peak HO mixing ratios and peak rates for photochemical conversion of isoprene to its products. Thus longer tranport times and higher HO mixing ratios contribute to the larger MVK/ISOP and MACR/ISOP ratios observed at Cornelia Fort. The placement of the photochemical ages along the predicted line depends on the average HO mixing ratio used in the calculation of the MVK/ISOP and MACR/ISOP ratios. The HO mixing ratio will be variable along an air mass trajectory. Thus photochemical ages derived by comparison of ambient ratios with the calculated line should be interpreted with caution. However, we can ask the question whether a typical daytime isoprene photochemical age at Cornelia Fort is consistent with the proximity of site to the forest canopies. Assuming a wind speed of 3 rn/s and 0.7 hours corresponds to a horizontal transport scale of 8 km. This distance is physically realistic. The site is in an open field with few trees in the immediate vicinity. However, just surrounding metropolitan Nashville, there are large sources of isoprene from dense forest canopies. The distance between Cornelia Fort and the dense forest canopies surrounding Nashville varies with wind direction, but is of the order of- 10 km. While the MVK/ISOP and MACR/ISOP ratios in an air mass will move along the predicted line with a rate which depends on the HO mixing ratio, the position of the calculated line will be independant of the HO mixing ratio. The position of the predicted line will be dependent on the rate coefficients and branching ratios in the isoprene oxidation mechanism. Thus the agreement between the predicted line and the observations provides a critical test of the applicability of the laboratory-derived isoprene oxidation mechanism to the atmosphere. 4. Conclusions Isoprene and its oxidation products were measured at the Cornelia Fort site as part of the SOS 1999 summer field intensive. These measurements enabled us to study isoprene photochemistry in an urban environment surrounded by heavy forest. Previous urban studies have shown that combustion emissions of isoprene, MACR, and MVK were important sources. However, an emissions inventory calculation for Davidon county suggests that in this environment combustion emissions are of negligible importance. Isoprene was produced predominantly from biogenic emissions, and MACR and MVK were produced predominantly from in situ photochemistry. It is interesting to study isoprene photochemisty in a high- NOx environment and contrast the observations with the rural canopy measurements performed during SOS 1990 and These previous studies have compared the diurnal trend in the MVK/MACR ratio with a 1-D photochemical model to address our understanding of the operative chemistry. A similar diurnal trend in the MVK/MACR ratio was observed at this urban, forested site compared to rural canopy studies and the 1- D photochemical model. Afternoon ratios maximized at 2.0 consistent with HO-driven isoprene oxidation. Nighttime ratios minimized at 1:1 consistent with a decrease in the importance of HO and an increase in the importance of O 3 as a nighttime oxidant. Thus the current isoprene chemistry in 1-D photochemical models can explain the MVK/MACR ratios observed at this urban forested site. A striking difference between these urban measurements and the rural canopy studies is in the magnitude of the observed daytime MVK/ISOP and MACR/ISOP ratios. Larger productto-isoprene ratios were observed at Cornelia Fort than the Kinterbish site. However, this is consistent with our understanding of these product-to-parent hydrocarbon ratios as a measure of the photochemical age of an air mass. The observation of air masses more photochemically processed in isoprene at Cornelia Fort can be attributed to two factors: (1) a higher-no x regime at Cornelia Fort which increases afternoon HO mixing ratios and oxidation rates, and (2) larger distances between the tower inlet at Cornelia Fort and the forest canopies. A novel approach was investigated to study the MVK/ISOP and MACR/ISOP ratios with a sequential reaction scheme model. The photochemical ages of isoprene derived for Cornelia Fort and Kinterbish were and hours, respectively. The sequential reaction scheme model provides another tool to address our understanding of the isoprene chemistry. The agreement between the observe daytime ratios and the sequential reaction scheme model provides further confidence that our understanding of the first few stages of HO-initiated isoprene oxidation is complete. Incorporating this proven isoprene photochemisty into 3-D urban airshed models and then comparing the magnitude of the MVK/ISOP and MACR/ISOP ratios derived from airshed models with ambient observations may be an important way to evaluate the treatment of isoprene emissions and transport in the more sophisticated models. Isoprene and its oxidation products, MACR and MVK, have been shown to be very important sinks for HO at midday hours. Thus, isoprene's contribution to the instantaneous 0 3 production rate at this urban location is significant. Since these observations suggest isoprene was significantly converted to its products on the timescale of transporto this urban site, isoprene may also make an important contribution to the 03 already formed in the air masses sampled. The observations described here will be useful in future modeling studies which address the importance of isoprene to oxidant levels at forested cities in the southeastern United States. Acknowledgements. C.A.S. thanks the National Science and Engineering Research Council of Canada for a doctoral fellowship. This

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Martinez-Harder, Department of Meteorology, Pennsylvannia State University, University Park, PA (brune@essc.psu.edu; harder@essc.psu.edu) F. C. Fehsenfeld, P. D. Goldan, D. Hereid, W. C. Kuster, P. C. Murphy, D. Parrish, J. M. Roberts, D. Sueper, M. Trainer, and E. J. Williams, Aeronomy Laboratory, NOAA, 325 Broadway, Boulder, CO (fcf@al.noaa.gov; pgoldan@al.noaa.gov; wkuster@al.noaa.gov; pcmurphy@al.noaa.gov; parrish@al.noaa.gov; jr@al.noaa.gov; sueper@al.noaa.gov; trainer@al.noaa.gov; eric@al.noaa.gov) G. Li, H. Xie, and V. L. Young, Department of Chemical Engineering, Ohio University, Athens, OH (gi240491@oak.cats.ohiou.edu; youngv@oak.cats.ohiou.edu) D. Riemer, Rosenstiel School of Marine and Atmospheric Science, 4600 Rickenbacker Causeway, Miami, FL (driemer@rsmas.miami.edu) C. A. Stroud, EATS Department, York University, North York, Ontario, Canada M3J 1P3. (cstroud@yorku.ca) (Received June 29, 2000; revised September 12, 2000; accepted September 21, 2000.)