Citation: Talbot, R., H. Mao, and B. Sive (2005), Diurnal characteristics of surface level O3 and other important trace gases in ... [3] In addition to O3, other trace gases subject to surface .... located 20 km west of Interstate 95, but the prevailing.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, D09307, doi:10.1029/2004JD005449, 2005
Diurnal characteristics of surface level O3 and other important trace gases in New England Robert Talbot, Huiting Mao, and Barkley Sive Institute for the Study of Earth, Oceans, and Space, Climate Change Research Center, University of New Hampshire, Durham, New Hampshire, USA Received 21 September 2004; revised 3 December 2004; accepted 16 February 2005; published 11 May 2005.
[1] Data obtained from spring 2001 to summer 2003 in New England by the Atmospheric
Investigation, Regional Modeling, Analysis and Prediction (AIRMAP) program were used to document the diurnal characteristics of O3, CO2, NO, and during selected intervals hydrocarbon and oxygenated species. The diurnal cycles of O3 and oxygenated species showed a monotonic rise in mixing ratio following sunrise (replenishment) that was mirrored by nighttime removal (depletion) under the nocturnal inversion. The median depletion rate of O3 was 4.9 ppbv h1 compared to a replenishment rate of 6.2 ppbv h1. The significant and rapid loss of O3 at night combined with an anthropogenic hydrocarbon signature dominated by a vehicular source led us to the hypothesis that nocturnal O3 depletion represented the combined effects of dry deposition and titration by NO released from mobile sources. Nighttime removal of O3 averaged 31 ppbv (median of 27 ppbv), with 11 ppbv due to dry deposition and 20 ppbv loss by titration with NO and NO2. The seasonally averaged diurnal cycles of O3 and NO were very similar from year to year, indicating that although there was large variability in the daily levels of these species, their sources/sinks were quite consistent. Moreover, CO2 and selected hydrocarbons exhibited a diurnal cycle opposite to that of O3, with the highest mixing ratios occurring at night. The diurnal cycles of oxygenated compounds such as methanol, acetaldehyde, methyl ethyl ketone, acetone + propanal, methyl vinyl ketone + methacrolein were investigated for a 2 day time period in July 2003. Our data are among the first to illustrate the diurnal cycle of these compounds. We used these species to demonstrate the importance of vertical mixing in driving the diurnal cycle of ground level O3 in New England. Day/night ratios ranged from 2.3 for acetone + propanal to 11 for methyl vinyl ketone + methacrolein. Deposition velocities of 0.5–1 m s1 were estimated for these species, which are significantly higher than values used in many models. Such efficient removal may have important implications for the chemical impact of these species, at least on a regional scale. Citation: Talbot, R., H. Mao, and B. Sive (2005), Diurnal characteristics of surface level O3 and other important trace gases in New England, J. Geophys. Res., 110, D09307, doi:10.1029/2004JD005449.
1. Introduction [2] The mixing ratio of O3 in near-surface air exhibits significant diurnal variation at many low-elevation continental locations, often peaking in the afternoon followed by reduced values at night that frequently reach single digits or zero [Gu¨sten et al., 1998; Hastie et al., 1993]. Observations show that the nighttime depletion occurs in the stable layer beneath the nocturnal inversion [Stutz et al., 2004; Galbally, 1968]. The rates of nocturnal O3 depletion at many locations appear to be consistent with its nighttime loss by dry deposition [Harrison et al., 1978; Garland and Derwent, 1979; Colbeck and Harrison, 1985; Shepson et al., 1992], but titration by nitric oxide (NO) may also be important near urban areas [Berkowitz et al., 1995; Gu¨sten et al., 1998]. Copyright 2005 by the American Geophysical Union. 0148-0227/05/2004JD005449
The consequences of O3 titration in rural areas remain an open question although it has been considered in a few studies [Harrison et al., 1978; Shepson et al., 1992; Hastie et al., 1993]. [3] In addition to O3, other trace gases subject to surface deposition can be depleted at night, such as reported for formic and acetic acids [Talbot et al., 1988], peroxyacetyl nitrate [Shepson et al., 1992], nitric acid [Lefer et al., 1999], and methanol [Goldan et al., 1995; Riemer et al., 1998]. Furthermore, species such as acetone, methanol, and isoprene appear to have diurnal cycles driven by biogenic emissions in daytime and various removal processes at night [Goldan et al., 1995]. In contrast, a- and b-pinene are kept at low levels during daytime due to rapid attack by the OH radical and then build up at night within the nocturnal inversion layer [Goldan et al., 1995; Riemer et al., 1998]. [4] In the early morning hours the stable surface inversion layer that was established during nighttime is eroded
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due to solar heating, allowing vertical mixing processes to subsequently transport O3 and other species from the remnant (i.e., the previous days) boundary layer down to the surface [Shepson et al., 1992; Neu et al., 1994]. Field measurements at three sites in the Swiss plateau combined with a turbulence model showed that 50 – 70% of the maximum O3 mixing ratios on a day following nocturnal depletion were contributed by downward mixing of the air masses from the remnant layer with the rest coming from in situ chemical production and advection [Neu et al., 1994]. In rural areas removed from large pollution sources, the occurrence of the nocturnal low-level jet can potentially enhance O3 and its precursors in the remnant boundary layer, increasing the relative importance of this source for surface O3 and other trace gases [Zhang et al., 2001]. For example, in the northeastern United States, air masses in the highly polluted northeast urban corridor extending from Washington, D.C., to New York can influence O3 levels at the surface over New England on the following day [Mao and Talbot, 2004a]. [5] Ozone is the most abundant nighttime oxidant in the troposphere, so its nocturnal depletion and that of associated oxygenated compounds can have important ramifications for atmospheric chemistry including: (1) controls on regional chemical budgets, (2) controls on radical concentrations both at night (e.g., NO3) and in the early daylight hours (e.g., HOx), (3) identification of vertical mixing processes, (4) an influence on the relationship between O3 and CO [Mao and Talbot, 2004b], and (5) a potential impact on the amount of O3 and other trace gases available for intercontinental transport. Furthermore, the occurrence of depleted nighttime O3 can complicate the interpretation of regional background levels of O3 [Mao and Talbot, 2004c]. To date, the majority of studies on diurnal characteristics of surface O3 and oxygenated compounds have focused on the summertime period. Thus little information is available to describe the seasonality in occurrence of the nocturnal inversion and associated removal processes of trace gases. Such information is crucial for improved model simulations on regional and global scales. [6] The Atmospheric Investigation, Regional Modeling, Analysis and Prediction (AIRMAP) program is conducting continuous measurements of important trace gas and aerosol species in the New England atmosphere with high temporal resolution (http://www.airmap.unh.edu/). In this paper we assess the rates of depletion and replenishment of O3 and other trace gases, determine their seasonality, and make a first-order regional assessment of O3 loss mechanisms. To document the extent of the O3 depletion across New England, we utilized O3 data from the Environmental Protection Agency (EPA) AIRNOW data archive.
2. Observational Methods [7] The AIRMAP program is conducting atmospheric chemistry measurements on a continuous year-round basis with a time resolution varying from 1 to 60 min for 180 species. In this study we used one minute averaged records primarily from Thompson Farm (TF) located near Durham, New Hampshire at 24 m elevation above sea level (43.11N, 70.95W) and a smaller suite of species from Castle Springs (CS, Moultonborough, New Hampshire, 43.75N,
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71.35W) at 406 m elevation. Both of these sites are surrounded by a mixed hardwood/pine forest setting, but it is considerably more dense at CS [Ollinger et al., 1998]. The prevailing winds are from the west to southwest quadrant, with calm conditions predominating at night, especially in the warmer months. The most polluted conditions generally occur under strong southwesterly synoptic flow bringing air from the northeast corridor to eastern New England [Mao and Talbot, 2004a, 2004b]. The TF site is located 20 km west of Interstate 95, but the prevailing meteorological conditions inhibit direct contamination of the site by vehicular emissions. [8] At each site ambient air is brought into the building housing the instruments using a 10 cm OD PFA Tefloncoated aluminum manifold operated at 1500 standard liters per minute. The inlets to the manifolds are located 15 m above ground level, just above the surrounding canopy. A suite of standard meteorological measurements (Davis, Inc.) are conducted about 1 m above the inlet. All the instruments are controlled using National Instruments LabView hardware with custom software. The data record utilized here covers the time period from April 2001 through September 2003, with almost complete coverage over the 2.5 year record for O3, CO, and NO. We report in companion papers the measurement details for O3, CO, NO, NOy, and meteorological parameters at this site [Mao and Talbot, 2004a, 2004b, 2004c]. The data plots shown in this paper correspond to universal time (UT). Local time at our sampling sites is UT – 4 hours in the summer (daylight saving interval) and UT – 5 hours at other times of the year. [9] In addition to these species, we use selected segments of data for CO2, and key hydrocarbon, halocarbon and oxygenated species. Carbon dioxide was measured with 1-min time resolution using a Licor model 7000 differential infrared absorption instrument. The instrument was zeroed and calibrated on a 12 and 14 hour time basis respectively. Calibration standards were obtained from S. Marrin with a mixing ratio near 388 ppmv CO2 (National Institute of Standards and Technology certified, ±1%). Hydrocarbon and halocarbon compounds were measured with one hour time resolution using a combination of cryogenic trapping and capillary columns coupled with flame ionization and electron capture detectors (B. C. Sive et al., Development of a novel cryogen-free concentration system for measurements of volatile organic compounds, submitted to Analytical Chemistry, hereinafter referred to as Sive et al., submitted manuscript, 2005). [10] A Proton Transfer Reaction – Mass Spectrometer (PTR – MS) from Ionicon Analytik [Lindinger et al., 1998] was used for fast response measurements of oxygenated volatile organic compounds (OVOCs), nonmethanehydrocarbons (NMHCs), dimethyl sulfide, and acetonitrile at the TF site since July 2003. The instrument was run with a drift tube pressure of 2 mbar and an electric field of 600V, while continuously stepping through a series of 30 masses. Of the 30 masses monitored, 6 masses were used for diagnostic purposes while the other 24 masses corresponded to the VOCs of interest. The dwell time for each of the 24 masses was 10 seconds s, yielding a total measurement cycle of 4 min. The system was zeroed every hour for 4 cycles by passing the flow through a catalytic converter (0.5% Pd on
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Figure 1. One minute averaged data for O3 at TF from 1 June to 1 October 2003. alumina at 425C) to determine the system background signals. [11] Calibrations for the PTR-MS were conducted using three different high-pressure cylinders containing synthetic blends of selected NMHCs and OVOCs at the part per billion by volume (ppbv) level (Apel-Riemer Environmental, Inc.). Each of the cylinders used for the calibrations has an absolute accuracy of
