Environ. Sci. Technol. 2005, 39, 8847-8852
Measurements of OH Reactivity and Photochemical Ozone Production in the Urban Atmosphere Y A S U H I R O S A D A N A G A , * ,†,‡,§ AYAKO YOSHINO,‡ SHUNGO KATO,‡ AND Y O S H I Z U M I K A J I I * ,‡ Japan Science and Technology Agency, 4-1-8, Honcho, Kawaguchi, Saitama 332-0012 Japan, and Department of Applied Chemistry, Faculty of Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397 Japan
FIGURE 1. A fundamental reaction cycles in the urban atmosphere. which are produced in the photolysis of O3 with solar UV (λ < 329 nm).
Measurements of OH reactivity (i.e., OH loss rates) in the troposphere are essential for discussing tropospheric OH photochemistry. In July and August 2003, we observed the total OH reactivity in a suburban area of Tokyo. More than 90% of the data of the measured OH loss rates were higher than the calculated values with simultaneously measured concentrations of various trace species even though the rate coefficient of the OH + NO2 reaction was measured by us. We concluded that this discrepancy is due to the existence of unmeasured volatile organic compounds (VOCs). We estimated the potential of the photochemical ozone production in the case of including the unknown species as VOCs and excluding the missing sink, respectively. When the unknown species were included as VOCs, the potential increases from 32% to 88%. This result indicates the photochemical production rates of ozone in the urban air are substantially greater than expected.
1. Introduction OH radicals play a central role in tropospheric photochemistry. They oxidize most of the trace gaseous species and contribute to the removal of these gases. Comprehensive understanding of production and loss processes of OH radicals is required to investigate the tropospheric photochemistry in detail. In the urban atmosphere, an abundance of many kinds of anthropogenic species such as CO, NOx (NO and NO2), and volatile organic compounds (VOCs) are emitted so that the photochemical reaction mechanisms involving OH radicals are complex (1). Figure 1 shows a summary of the photochemical reaction cycles in the urban atmosphere. As for the production processes, OH is produced primarily by a reaction of water vapor with excited oxygen atoms (O1D), * Address correspondence to either author. Phone: +81-72-2549325 (Y. S.); +81-426-77-2834 (Y. K.); fax: +81-72-254-9325 (Y. S.); +81-426-77-2837 (Y. K.); e-mail:
[email protected] (Y. S.);
[email protected] (Y. K.). † Japan Science and Technology Agency. ‡ Tokyo Metropolitan University. § Present address: Department of Applied Chemistry, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531 Japan. 10.1021/es049457p CCC: $30.25 Published on Web 10/15/2005
2005 American Chemical Society
O3 + hν f O2 + O(1D)
(1)
O(1D) + H2O f 2OH
(2)
Although these reactions are the main OH production processes in the global atmosphere, the photolysis of nitrous acid and formaldehyde, as well as ozone-alkenes reactions, are also important OH sources in an urban area.
HONO + hν f OH + NO
(3)
HCHO + hν f H + HCO
(4)
H + O2 + M f HO2 + M
(5)
HCO + O2 f HO2 + CO
(6)
HO2 + NO f OH + NO2
(7)
O3 + alkene f f f OH + products
(8)
The loss processes of OH are more complicated because of its high reactivity. OH reacts with CO, CH4, NO2, SO2, and VOCs.
OH + CO(+ O2) f HO2 + CO2
(9)
OH + CH4(+ O2) f CH3O2 + H2O
(10)
OH + NO2 + M f HNO3 + M
(11)
OH + SO2 f f f H2SO4
(12)
OH + VOCs(+ O2) f RO2 + products
(13)
In the clean atmosphere, the loss processes of OH are mainly controlled by CH4 and CO because of low concentrations of NOx and VOCs. Meanwhile, VOCs and NOx govern the OH lifetime in the urban atmosphere. The discussion on the loss processes of OH radicals in the urban atmosphere is very difficult because of the many kinds of VOCs. Lewis et al. identified more than 500 kinds of VOCs in urban air (2). Considering this result, the total OH loss rates (LOH) are expressed by the following equation: VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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LOH ) -(kCO[CO] + kCH4[CH4] + kO3[O3] + kNO[NO] + n
kNO2[NO2] + kSO2[SO2] +
∑k i
VOCi[VOCi]
kothers[others])[OH]
+
(n > 500) (14)
This equation indicates that comprehensive measurements of VOCs and rate constants for the reaction of OH with various kinds of VOCs are required in order to estimate the OH loss rates. In addition, these measurements and rate constants might be insufficient to estimate the total OH reactivity because unknown OH reaction partners would exist in the urban air. Instead of the various VOCs observations, the direct measurements of total OH reactivity are very effective to discuss tropospheric OH chemistry (3-5). An assessment of OH loss processes becomes possible by measuring the total OH loss rates in the absence of the comprehensive measurements of VOCs. OH production rates can also be estimated indirectly by the simultaneous measurements of total OH reactivity and OH concentrations, assuming the steady state of OH.
d[OH] ) POH - k[OH] ) 0 dt
(15)
FIGURE 2. CO, O3, NO, and NOx mixing ratios in Augus 2003.
where POH and k represent the OH production rate and the measured first-order decay rate of OH radicals, respectively. Recently, a measurement system of the total OH reactivity has been developed by use of the flow-reactor technique (3). The OH reactivity was measured in the urban atmosphere during the SOS (Southern Oxidant Study; 4, 6) and PMTACSNY (PM2.5 Technology Assessment and Characterization Study-New York; 7, 8) field campaigns. However, the knowledge of the total OH reactivity is very limited and further investigations are required to understand the reaction mechanisms involving OH in the urban area. We have developed a novel instrument for measuring OH reactivity in the troposphere, using a laser-induced pump and probe technique (9). OH radicals are produced artificially in the photolysis of ozone using a UV laser and then the OH decay rate is measured by the time-resolved laser-induced fluorescence (LIF) technique. The observations of total OH loss rates were conducted at Tokyo Metropolitan University (TMU), a suburban area, in July and August 2003 (10). In this article, we present the observational results in detail. We also discuss photochemical ozone production in the urban atmosphere using a newly proposed parameter.
2. Experimental Section 2.1. Site Description. The observations were performed at Tokyo Metropolitan University (latitude, 35°37′03′′N; longitude, 139°23′09′′E, 130 m asl), which is located about 40 km west of central Tokyo (10). Figure 2 shows the interdiurnal variation of O3, CO, NO, and NOx mixing ratios in TMU in August 2003. The observational site is affected by local anthropogenic activity. The observations were carried out on July 29 and August 4, 11, and 20. 2.2. Measurement System of Total OH Reactivity. The total OH reactivity was measured by using a laser-induced pump and probe technique. Figure 3 shows a schematic drawing of the measurement system for OH reactivity. The details of the instrument are described elsewhere (9). Briefly, ambient air was introduced into a flow tube placed just above the fluorescence detection cell. A pulsed 266-nm laser beam with a low repetition rate (0.5-10 Hz) was irradiated through 8848
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FIGURE 3. Schematic diagram of the measurement system of OH reactivity by using a laser pump and probe technique. the center of the flow tube axially. OH radicals are generated by the 266-nm laser light via the following reactions:
O3 + hν f O2 + O(1D)
(16)
O(1D) + H2O f 2OH
(17)
A fourth harmonic of flash-lamp-pumped Nd:YAG laser (Quanta-Ray INDI-40, Spectra-Physics) was used as the light source to generate OH. The energy of the YAG laser can be varied up to 30 mJ/pulse by controlling the flash-lamp energy. The OH reacts with trace species in the reaction tube and the OH concentrations decrease with time after irradiating the laser pulse. Decay of the OH concentration after the 266-nm laser pulse is measured by an LIF technique. The air in the reaction tube was introduced into a low-pressure (∼300 Pa) fluorescence detection cell through an orifice (1-mm diameter), which was settled at the center of the radial cross section of the flow tube. The A-X(0,0) Q1(2) line of OH near 308 nm was excited by use of a tunable frequency-doubled dye laser (Scanmate, Lambda Physik) pumped by a second harmonic of the solid-state pulsed Nd:YVO4 laser with a repetition rate
FIGURE 4. Timing schematic of the 308-nm and 266-nm lasers. of 10 kHz (YHP40-532Q, Spectra-Physics). The resonant fluorescence was detected using a dynode-gated photomultiplier tube (R2256P, Hamamatsu). The output signals from the photomultiplier tube were recorded by a photon counting method. The energy of the 308-nm laser pulse was monitored outside the detection cell using a photodiode (S1226-5BQ, Hamamatsu) to correct the sensitivity fluctuated by the laser power. The time profile of the OH decay was observed by timeresolved LIF measurements after the irradiation of the laser pulse. The interval of pulse trains of the 308-nm laser operated at 10 kHz (corresponding to 100 µs) was used as a clock for measurements of the OH decay rates. Figure 4 shows the timing schematic of the OH fluorescence, the pump, and probe laser. The time-resolved LIF signals were integrated (typically 500 times). The time profile of the OH decay can be expressed by the following formula:
[OH] ) [OH]0 exp(-ktott)
(18)
where ktot is the pseudo-first-order decay rate of OH and t is the time after the 266-nm laser irradiation. The time profile measured by LIF is fit to a single-exponential curve and then the decay rate is determined by this fitting expression. Figure 5a shows examples of the measured decay profile of zero air and ambient air. The zero air was generated by passing compressed ambient air through a Hot Pt oven (623 K) and purafil-charcoal filters to remove most of OH reaction partners. In the zero air, the concentrations of CO, NOx, and VOCs were less than 10 ppbv (parts per billion by volume), 50 pptv (parts per trillion by volume), and 10 pptv, respectively. After the 266-nm laser irradiation, time series of OH signals show two decay components as shown in Figure 5a. The slower decay reflects the true OH reactivity, OH diffusion, and turbulence in the flow tube. Although the reason of the fast decay cannot be specified, turbulence by shock wave arisen by the laser shot would be considered as a possible cause. In Figure 5b, the same decay profile as shown in Figure 5a is shown in a logarithmic scale. The slower decay is linear so that the OH decay can be fit to a single exponential. A weighted fit was used to determine the decay rate. The OH decay in the zero-air would be due to the escape processes of OH by diffusion and turbulence. The zero-air decay rate was approximately 3.5 s-1. The OH self-reaction can be neglected (decay rate
