ISSN 10248560, Atmospheric and Oceanic Optics, 2011, Vol. 24, No. 5, pp. 487–491. © Pleiades Publishing, Ltd., 2011. Original Russian Text © V.S. Kozlov, M.V. Panchenko, A.B. Tikhomirov, B.A. Tikhomirov, V.P. Shmargunov, 2011, published in Optica Atmosfery i Okeana.

OPTICAL INSTRUMENTATION

Effect of Relative Air Humidity on Photoacoustic Aerosol Absorption Measurements in the NearGround Atmospheric Layer V. S. Kozlov, M. V. Panchenko, A. B. Tikhomirov, B. A. Tikhomirov, and V. P. Shmargunov Zuev Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences, pl. Akademika Zueva 1, Tomsk, 634021 Russia Received December 27, 2010

Abstract—The paper discusses wintertime synchronous nearground measurements of the aerosol absorp tion coefficient at wavelengths of 532 and 1064 nm and the black carbon mass concentration by pulsed pho toacoustic (PA) spectroscopy and optical aethalometry, respectively. It was found that the signal of the pulsed PA spectrometer decreases monotonically, by 30–40% on average, as the relative air humidity increases from 30 to 90%. Analysis of the data has shown that the PA method is efficient for studying the absorption of laser radiation in the range of low humidity values, i.e., for measurements of the aerosol absorption coefficient of “dry” carbonaceous particles. Correctness of the aerosol absorption measurements for increased relative air humidity (60–90%) can be improved through a sensitivity correction (additional calibration) of PA spec trometers. DOI: 10.1134/S1024856011050101

INTRODUCTION The relative air humidity RH is one of the main atmospheric meteorological parameters. Changes in the microphysical and optical characteristics of atmo spheric aerosols are closely related to RH variations [1– 3]. As the relative humidity grows, the water content in the particle composition generally increases, leading to an increase in the aerosol scattering coefficient of optical radiation in the atmosphere [3]. The aerosol absorption of shortwave radiation in the atmosphere is primarily determined by carbon aceous (microcrystalline carbon) aerosol particles [4– 6]. The black carbon mass concentration Mc in the nearground midlatitude air layer generally varies from 0.1 to 10 μg/m3 [5, 6]. The aerosol absorption efficiency σ at a wavelength of λ = 550 nm is often characterized in –1 terms of σ(550) = k(550)/ M c = 10 m2 g–1 (see, e.g., [5, 7]) and the corresponding aerosol absorption coeffi cient k(550) = 1–100 Mm–1. Weak aerosol absorption in the atmosphere is masked by scattering effects. Therefore, an experi ment aimed to study the dependence of the aerosol absorption on RH on atmospheric paths is very difficult if at all. Characteristics of weak aerosol absorption in the atmospheric air are most often measured using filter ing measurement techniques [8]. To this end, carbon aceous aerosol particles are accumulated on a filter as atmospheric air is pumped through it; then, changes in the transmission of optical radiation by the filter are recorded to determine the aerosol absorption coeffi cient. Filtering methods overestimate the aerosol

absorption coefficient due to the distorting effect of scattering [9]. Changes in the air humidity further add to the filter measurement errors and, in particular, lead to recording of negative absorption [10]. Another commonly accepted method of data acquisition on aerosol absorption in air is laser photo acoustic (PA) spectroscopy (see, e.g., [10–14]). This method essentially consists in PA signal generation by aerosol during absorption of laser radiation and the corresponding measurement of the acoustic signal amplitude, which is proportional to the aerosol absorption coefficient. Presentday measurements successfully employ the resonance [10–13] and pulsed [14] PA spectroscopy. The PA method has a high sen sitivity: the minimum detectable absorption coeffi cient of spectrometers kmin [10–14] reaches ∼ 0.4 Mm–1. The molecular absorption in this case is weak and comes as a small addition to the aerosol absorption. Moreover, the PA method makes it possible to perform experiments for aerosol particles immediately residing in atmospheric air. These are key prerequisites for an efficient use of the PA method for studying weak non selective aerosol absorption. Despite the many works devoted to experimental study of aerosol absorption, the dependence of aerosol absorption on air humidity is still poorly understood. Radiation absorption by atmospheric aerosol and aerosol from wood combustion was measured in [10, 15], where a decrease in signal amplitude of a reso nance PA detector by 20–40% was observed as the rel ative air humidity increased from 10 to 90%. This study used the pulsed PA spectroscopy method to analyze the dependence of the absorption

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Aethalometr (Mc measurement)

Rotameter

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k ( RH ) σ =  Mc

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λ1 = 532 nm λ2 = 1064 nm

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PA signal recording Control of RH (%), T (K), and P (mbar) Computer Fig. 1. Blockdiagram of setup for studying the dependence of the aerosol absorption efficiency on relative air humidity.

efficiency of atmospheric aerosol at wavelengths of 532 and 1064 nm on the relative air humidity. RESULTS OF COMPLEX EXPERIMENT The experiment was performed in the winter period at the Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences (suburbs of Tomsk). Real atmospheric aerosol in the nearground (a) 15

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(b) 30 PA spectrometer λ = 532 nm Aethalometer

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Fig. 2. Time series of aerosol absorption coefficient k at wavelengths of (a) 1064 and (b) 532 nm and black carbon mass concentration Mc in atmospheric aerosol at a relative air humidity of RH = 11%. The last points in the plots were obtained after preliminary cleaning of air samples with blocks of aerosol filters.

Mc, µg/m3

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air layer was the object of the study. A block diagram of the experimental setup is presented in Fig. 1. Absorption of pulsed radiation of a YAG laser (λ = 532 and 1064 nm) was recorded with the help of a PA spectrometer with a time signal resolution [14]. The black carbon mass concentration was recorded with the help of an aethalometer, which measured the extinction of radiation at λ ∼ 900 nm by a layer of par ticles on the aerosol filter [14]. The analyzed air was collected into optical cells of the devices via air pipes at a height of 3 m above the Earth’s surface. The out door air temperature for the period of measurements was T < –10°C. Therefore, sootcontaining aerosol in air, heated to room temperature (25°C), can be con sidered “dry.” The air intake channel of the PA spec trometer was equipped with an air saturator, whose layout and principles of operation were presented in [3]. The air saturator ensured variations in the relative humidity in the range RH = 10–90%. The RH value was measured with the help of a DV2TS sensor of rel ative humidity and temperature (Microfor Company Ltd.) built in a PA cell. The air intake channels of the PA spectrometer and aethalometer were also equipped with AFAKhP aerosol filters (aerosol filter, see Fig. 1), switching to which allowed the studied air to be cleaned from aerosol (black carbon), thereby making it available for cleanair optical measurements. Figure 2 shows the time series according to simul taneous measurements of the mass concentration of black carbon making up the aerosol particles and the aerosol absorption coefficient at wavelengths of 532 and 1064 nm.

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k, Mm–1

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y = A + Bx 532 nm A = 1.98 + 0.38 B = 10.0 + 0.18 1064 nm A = 0.25 + 0.16 B = 4.97 + 0.08

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3 4 Mc, μg/m3 (b)

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y = A + Bx 532 nm A = 1.01 + 0.45 B = 9.46 + 0.21 1064 nm A = 0.69 + 0.14 B = 4.69 + 0.06

RH = 70%

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40 k, Mm–1

The correlation between the aerosol absorption coefficient (PA spectrometer measurements) and the black carbon mass concentration (aethalometer mea surements) for dry (RH = 11%) and saturated (RH = 70 and 80%) atmospheric aerosols is illustrated in Fig. 3. It is seen from Figs. 2 and 3 that, in the absence of black carbon aerosol (Mc = 0, i.e., when air samples are collected via aerosol filters), the PA spectrometer records a certain residual absorption. The residual absorption at λ = 1064 nm depends on the air humidity and increases from 0.25 Mm–1 for RH = 11% to 0.83 Mm–1 for RH = 80%. It was exper imentally determined [16] that molecular absorption at λ = 1064 nm in air is mainly determined by molec ular oxygen. In air free of water vapor, the relaxation of excited oxygen molecules and generation of acoustic the signal are primarily caused by O2–O2 and O2–N2 colli sions. This process is relatively slow and, as argued in [17], is characterized by a vibrational–translational relaxation time of τVT ∼ 10–2 s at an air pressure of P = 1 bar. The collisions of oxygen molecules with water vapor molecules are more effective at converting the excitation energy into translational energy of mole cules in atmospheric air and act to considerably (pre sumably, by several orders of magnitude) reduce the relaxation time τVT. The scientific literature contains data on H2O–H2O collisions: τVTP ∼ 10–9 s bar [18]. For collisions of excited water vapor molecules with oxygen and nitrogen molecules, τVTP is two orders of magnitude larger; however, it remains much less than τVT for the O2–O2 and O2–N2 collisions. Thus, water vapor catalyzes the conversion of energy of excited oxygen molecules into the translational energy of mol ecules, explaining the abovementioned rise in the PA signal amplitude with an increase in RH during radia tion absorption at λ = 1064 nm. The residual absorption at λ = 532 nm is somewhat larger than at λ = 1064 nm (see Fig. 3). We ascribe this absorption to NO2 molecules. The nitrogen dioxide absorption cross section at λ = 532 nm is S NO2 ≈ 1.4 × 10–19 cm2 [19]. The NO2 molecule concentration in the nearground air varies from 5 to 50 ppb [20], which corresponds to the change in the absorption coeffi cient at λ = 532 nm in the range from 1.5 to 15 Mm–1. On the whole, this absorption is larger than that observed in our measurements. However, considering the interaction between NO2 and water vapor mole cules, it is expected that the residual absorption should decrease after air samples are pumped through the sat urator, which is observed in our measurements (see Fig. 3). The aerosol absorption coefficient at λ = 1064 nm is half as large as that at λ = 532 nm. Within the exper imental errors, the parameter of linear regression B for these wavelengths differs twofold for all RH values (see Fig. 3), which corresponds to the theoretical spectral

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y = A + Bx 532 nm A = 1.35 + 0.45 B = 8.8 + 0.18 1064 nm A = 0.83 + 0.16 B = 4.45 + 0.06

3 4 Mc, μg/m3 RH = 80%

20

0

1

2

3 4 Mc, μg/m3

Fig. 3. Linear correlation between aerosol absorption coef ficient k at wavelengths of 532 and 1064 nm and black car bon mass concentration Mc in air for (a) dry and (b, c) sat urated atmospheric aerosol.

dependence of the efficiency of shortwave radiation absorption by atmospheric aerosol σ ∼ λ–1 [21]. Figure 4 presents the experimental dependences of the aerosol absorption efficiency at λ = 532 and

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variations in the aerosol absorption with an increase in air humidity; moreover, they are not sufficient for quantitative assessments of the parameter of conden sation activity with respect to absorption. Thus, cor rect measurements of the aerosol absorption coeffi cient require new models describing in more detail the processes of optical radiation energy conversion into acoustic signals, as well as development of instrumen tal methods accounting for changes in the sensitivity of PA spectrometers depending on the air humidity in a wide variability range.

σ/σmax 1.0 *

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Atmospheric aerosol 532 nm 1064 * 532 [10]

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0

20

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Fig. 4. Dependences of aerosol absorption efficiency for two laser wavelengths on relative air humidity in real atmo sphere.

1064 nm on the relative air humidity, which we obtained with the pulsed PA spectrometer and with the help of the resonance PA spectrometer in [10]. It is seen that the results obtained with different PA spectrometers qualitatively agree, and an increase in the relative air humidity from 60 to 90% leads to a decrease in the amplitude of the PA signal by 30–40%. The authors of [10, 15] argue that aerosol absorption efficiency σ decreases for saturated aerosols not due to an actual decrease in the aerosol absorption coeffi cient, but rather due to the specific performance of the PA method under the conditions of increased air humidity. When the PA method is used to measure radiation absorption by “moist” aerosol particles, the efficiency of acoustic signal generation is less than when a dry aerosol base is studied. The energy of radiation absorbed by black carbon making up the aerosol parti cles is lost to heating and evaporation of water mole cules, which “envelop” the particles, so that the sensi tivity of the PA spectrometer decreases. A second rea son for the decrease in the PA spectrometer signal with an increase in RH is considered in [15] to be due to the socalled “collapse of aerosol particles,” which leads to a decrease in aerosol absorption by 6%. The third reason may be a decrease in the concentration of aero sol particles due to their “trapping” in the saturator. Analysis of the data presented in this paper and available from the literature makes it possible to con clude that the PA methods used to estimate aerosol absorption have specific applicability limits and are predominantly efficient in studying laser radiation absorption by the dry base of atmospheric aerosol. In this context, the results presented in Fig. 4 cannot be considered to adequately reflect the actual pattern of

CONCLUSIONS Simultaneous nearground measurements of the aerosol absorption coefficient at wavelengths of 532 and 1064 nm (by the PA method) and the concentra tion of the black carbon mass in particles (by the aeth alometry method) have shown that these characteris tics have high linear correlations for relative air humidities in the range of 10–90%. We have studied the dependence of the signal of a pulsed PA spectrom eter on relative air humidity; we have found that, as the relative air humidity increases from 30 to 90%, the sig nal amplitude decreases by 30–40%, consistent with certain publications. The PA method is efficient in studying laser radia tion absorption in the low humidity range, i.e., absorption by “dry” carbonaceous particles. The sen sitivity of PA spectrometers should be corrected when used in measurements of aerosol absorption charac teristics under conditions of increased air humidity (60–90%). ACKNOWLEDGMENTS This work was partially supported by the Russian Foundation for Basic Research (project nos. 0305 64787 and 030506038). REFERENCES 1. E. F. Mikhailov, S. S. Vlasenko, A. A. Kiselev, and G. I. Ryshkevich, “Restructuring Factors of Soot Parti cles,” Izv. RAN. Fiz. Atmos. Okeana 34, 345–356 (1998). 2. I. Colbeck, L. Appleby, E. J. Hardman, and R. M. Har rison, “The Optical Properties and Morphology of CloudProcessed Carbonaceous Smoke,” Aerosol Sci. 21, 527–538 (1990). 3. M. V. Panchenko, M. A. Sviridenkov, S. A. Terpugova, and V. S. Kozlov, “Active Spectral Nephelometry as a Method for the Study of Submicron Atmospheric Aerosols,” Opt. Atmos. Okeana 17, 428–436 (2004). 4. G. V. Rozenberg, “On the Nature of Aerosol Absorp tion in the ShortWave Region of the Spectrum,” Izv. AN SSSR, Fiz. Atmos. Okeana 15, 1280–1292 (1979). 5. H. Moosmuller, W. P. Arnott, C. F. Rodgers, J. C. Chow, C. A. Frazier, L. E. Sherman, and D. L. Dietrich, “Photoacoustic and Filter Measurements Related to

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