6.3. Comparison of Ambient OC, EC, and TC Concentrations Measured During ACE-Asia to Those Measured in Previous Field Campaigns
 Concentrations (μg C m−3) of OC and EC as determined from samples collected on RFs conducted during ACE-Asia are summarized in Table 3. The average latitude, longitude, and altitude where the aerosol sample was collected and the standard deviation of these values are presented in Table 4. Concentrations of OC and EC ranged from 0.58 to 28.94 μg C m−3 and 0.20 to 1.80 μg C m−3, respectively. Samples were collected at altitudes from ≈50 m to 3000 m. Pooling the data for samples collected at all latitudes and longitudes but during sampling legs conducted at a nearly constant altitude, concentrations of OC and EC are shown versus altitude in Figures 10a and 10b. In these figures the horizontal bar represents the standard deviation of the of OC or EC measurement and the vertical arrow the range in aircraft altitude during sampling.
Figure 10. a. Ambient EC concentrations measured in ACE-Asia, Rubidoux, California [Kim et al., 2001], Pasadena, California [Mader et al., 2001], and on Lake Michigan downwind of Chicago, IL [Offenberg and Baker, 2000]. Data from literature plotted as the average (symbol) and the range (bars) of values observed in that study. b. Ambient OC concentrations measured during ACE-Asia, and in Pasadena, California [Mader et al., 2001]. Data from Pasadena plotted as the average (symbol) and the range (bars) of values observed in that study. Note: For ACE-Asia data where the ambient concentration was above the method detection limit (MDL), error bars in the horizontal direction correspond to the standard deviation for the measurement, error bars in the vertical direction correspond to the range of the aircraft altitude during the sampling period. For ACE-Asia data where the ambient concentration was below the MDL, the concentration corresponding to the MDL is presented; the true ambient concentration is likely below this value.
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Table 4. Navigational and Meteorological Dataa
|Date||Flight||Sample Time, UT||Average Latitude||(±)||Average Longitude||(±)||Average Altitude, m||(±)||Layer||RH, %||RSD, %||T, °C||RSD, %|
|2 April 2001||2||01:21:41–03:10:00||32.338||0.2672||132.622||0.2703||1300.4||999||multiple||38||58||8.2||61|
|4 April 2001||3||01:07:45–04:56:26||36.517||0.3622||133.279||0.0745||1665.7||1144||multiple||34||55||2.29||196|
|6 April 2001||4||00:22:31–04:13:12||33.063||0.2434||127.828||0.2963||1357.1||1255||multiple||43||67||6.41||112|
|8 April 2001||5||04:24:36–07:41:46||37.684||0.4169||133.586||0.1371||2515||968||multiple||44||44||1.95||58|
|8 April 2001||5||06:14:56–07:21:26||37.902||0.0601||133.735||0.0913||2885.8||202||dust||49||27||4.15||106|
|9 April 2001||6||02:38:40–05:32:10||37.874||0.3676||133.586||0.1557||2271.1||1111||pollution/dust||47||37||5.39||108|
|9 April 2001||6||03:54:20–04:54:10||38.056||0.1398||133.636||0.1551||2914.4||285||dust||37||33||1.99||72|
|9 April 2001||6||02:38:39–05:32:26||37.873||0.3692||133.586||0.1556||2273.3||1108||multiple||51||39||3.64||164|
|12 April 2001||7||02:40:07–03:48:38; 04:31:48–04:59:49||33.101||0.1343||127.687||0.2086||1024.3||1008||multiple||38||31||5.67||141|
|12 April 2001||7||03:48:48–04:31:38||33.069||0.0512||127.557||0.1059||919.9||10.5||mbl||47||11||5.61||3|
|13 April 2001||8||01:06:46–02:04:36; 03:58:27–04:13:28||32.358||0.1364||132.583||0.1412||47.8||5.7||mbl||35||8||16.5||2|
|13 April 2001||8||02:32:37–03:50:06||32.383||0.1247||132.618||0.1361||1231.5||5.9||mbl||45||16||5.85||5|
|17 April 2001||11||03:41:58–06:24:48||32.964||0.0734||128.136||0.1361||1774.1||1093||multiple||54||31||8.2||79|
|17 April 2001||11||04:55:08–05:29:08||32.946||0.0497||128.141||0.1052||1388.8||29.5||pollution/dust||53||5||10.7||4|
|19 April 2001||12||01:37:49–04:58:29||37.838||0.6239||133.711||0.3104||1242.7||1030||multiple||44||66||11.8||34|
|19 April 2001||12||03:14:59–04:41:19||37.628||0.3986||133.619||0.2181||1757.3||466||pollution/dust||27||40||9.97||24|
|19 April 2001||12||01:37:59–05:57:57||37.317||1.158||133.543||0.4418||1723.2||1286||multiple||56||36||10.6||44|
|20 April 2001||13||00:29:16–01:58:36||32.452||0.1346||132.683||0.1377||1129.7||683||multiple||24||86||15.6||14|
|23 April 2001||14||01:02:34–04:54:05||33.094||0.068||134.248||0.3101||852.4||719||multiple||49||52||12.2||23|
|23 April 2001||14||01:36:34–02:55:44||33.07||0.0563||134.177||0.2747||606||10.8||mbl||68||10||12.1||6|
|25 April 2001||15||02:50:03–06:50:44||32.454||0.1477||132.671||0.1546||1824.8||1168||multiple||69||15||9.09||43|
|26 April 2001||16||00:45:41–04:38:22||32.404||0.1555||132.616||0.1616||1283.4||1109||multiple||28||48||9.96||52|
|26 April 2001||16||01:34:51–02:41:41||32.456||0.1466||132.674||0.151||1084.5||5.5||pollution/mbl||21||9||9.36||4|
|27 April 2001||17||00:50:04–01:36:14; 02:12:24–03:12:24||34.018||0.0044||129.179||0.34||104.1||59.6||mbl||61||7||16.3||3|
|27 April 2001||17||01:38:24–02:10:14||34.016||0.0031||129.452||0.2537||456.1||6.1||pollution||36||32||14.1||3|
|27 April 2001||17||00:50:04–04:20:29||34.019||0.007||129.141||0.3454||426.2||412||pollution/mbl||49||38||13.1||18|
|28 April 2001||18||02:39:16–03:39:06||36.563||0.1185||133.007||0.0988||1099||3.4||pollution||49||9||12.1||2|
|28 April 2001||18||02:18:16–04:48:53||36.552||0.1255||133.031||0.1187||1510||977||multiple||44||41||8.5||61|
|1 May 2001||19||01:14:58–02:44:49||35.831||0.0035||134.441||0.2751||1238||23.6||pollution||54||25||13.5||4|
 Since it is not likely that adsorption of gaseous OC to filter surfaces would significantly affect measurements of the EC concentration, the concentrations of EC measured during ACE-Asia can be compared to values measured at other locations where filter pack samplers were used to collect aerosol samples. In addition, since the relative amounts of OC and EC measured for a given sample often depend on the instrument used for analysis [Hering et al., 1990; Turpin et al., 2000; Schmid et al., 2001], data are compared only among samples analyzed using a thermal-optical carbon analyzer. In Figure 10a, EC concentrations measured during ACE-Asia are compared to values measured during sampling campaigns conducted in other locations: on the ground in Rubidoux, California [Kim et al., 2001], on the roof of the Keck Engineering Laboratory at the California Institute of Technology in Pasadena, California [Mader et al., 2001] and aboard a ship on Lake Michigan, downwind of Chicago, IL [Offenberg and Baker, 2000]. Data from each study are presented as the average value (symbol) and the range (bars) of values observed. Moreover, for sampling events during ACE-Asia where the ambient concentration exceeded the method detection limit, the error bars in the horizontal direction correspond to the standard deviation for the measurement, and the bars in the vertical direction correspond to the range of the aircraft altitude during the sampling period. For sampling events during ACE-Asia in which the OC or EC concentrations were below the MDL, the concentration corresponding to the MDL is presented. This value can be considered as an upper estimate of the true ambient EC or OC concentration.
 As shown in Figure 10a, the EC concentrations measured during ACE-Asia were, on average, higher than those measured downwind of Chicago, nearly equal to those observed in Pasadena, CA, and generally lower than those observed in Rubidoux, CA. Since differences among OC/EC analysis methods and aerosol sampling techniques may complicate comparisons of measured OC concentrations made during different sampling campaigns, OC concentrations measured during ACE-Asia are compared only to values measured in Pasadena, CA in which the same denuder sampler sampling system was used. As shown in Figure 10b, the concentration of OC measured in Pasadena was, on average, higher than that observed during ACE-Asia. However, in one ACE-Asia sampling event (RF 17), the OC concentration was significantly higher than any OC concentration measured at the Pasadena location.
 As discussed previously the relative amounts of OC and EC measured for a sample is a function of both the sample collection method and OC/EC analysis method. If these methods differ between a set of experiments meaningful comparisons of OC and EC levels measured in the experiments may not be possible. Differences in the OC/EC analysis method between experiment generally will cause the biggest difference in the relative amounts of OC and EC measured for a given sample, however on the basis of total carbon (TC) different OC/EC analysis methods generally agree to within ≈10% [Hering et al., 1990; Turpin et al., 2000; Schmid et al., 2001]. Therefore with proper consideration of differences in the methods used to collect carbonaceous aerosols, it is sometimes possible to make meaningful comparisons of the TC levels measured during different experiments. The levels of TC measured using aircraft during the field projects: ACE-2, Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX), and INDOEX are currently available and these levels summarized in Table 5.
 Briefly ACE-2 was conducted over the subtropical North-East Atlantic 16 June–24 July 1997 [Raes et al., 2000]. During ACE-2 Schmeling et al.  measured OC and EC levels in dust layers as well as in the MBL during relatively clean conditions and when the MBL was influenced by anthropogenically generated aerosols. TARFOX was conducted in July of 1996 in a polluted region off the coast of Virginia [Novakov et al., 1997; Hobbs, 1999]. During TARFOX sampling was conducted at altitudes between 0.1 and 3.8 km, but the levels of TC in specific atmospheric layers were not reported. INDOEX was conducted over the tropical Indian Ocean during the Northern Hemisphere dry monsoon season. During February–March of 1999 aircraft measurements of carbonaceous aerosols were made in two polluted layers: the residual continental boundary layer (rCBL) and the MBL.
 During ACE-Asia, flights were purposely conducted in dust, pollution or the MBL, that is the sampling area, altitude and sampling times were not randomly chosen, and the sampling strategy employed was not intended to provide average OC and EC values for the entire study region; rather, the strategy was to identify the chemical composition of different layers of the atmosphere. Therefore it is best to make comparisons of TC levels between different field experiments (i.e., ACE-Asia versus INDOEX) on the basis of the type of atmospheric layer sampled (i.e., the MBL or pollution layer) rather than basing comparisons on the average TC levels measured during an entire individual experiment. The average or approximate levels of TC observed in specific atmospheric layers are summarized in Table 5 for ACE-2, INDOEX, and ACE-Asia. Also presented in Table 5 are the average levels of TC observed during TARFOX for samples collected between altitudes of 0.1 and 1 km and 1 and 3.8 km. It must be noted that the number of TC data for each experiment are different, and in some cases only a few data are available. It was not possible to use statistical techniques such as a paired T-test to determine whether, for a given type of layer, statistically significant differences in TC levels between field experiments were observed; nonetheless, some general comparisons can be made.
 Dust layers were sampled during both ACE-2 and ACE-Asia. The levels of TC in dust layers observed in ACE-2 were below the MDL of 1.3 μg C m−3. Considering the MDL, these levels were likely lower than the TC levels observed in dust layers sampled during ACE-Asia, where TC was greater than 2.5 and less than 3.7 μg C m−3. It is possible that the levels of organic material in the soils of the regions that are the source of the dust collected in ACE-Asia are greater than the levels in the source regions for ACE-2. It is also possible that the dust layers observed in ACE-Asia also contained some anthropogenically generated carbonaceous aerosol.
 The MBL was sampled during ACE-2, TARFOX, INDOEX, and ACE-Asia. When sampling the MBL under relatively clean conditions, the average and range of TC levels were nearly equal during ACE-2 and ACE-Asia (Table 5), suggesting that the levels of TC in remote background marine air may be similar. It is likely that the layers termed anthropogenically influenced MBL, altitude less than 1 km, MBL, and polluted-MBL, in ACE-2, TARFOX, INDOEX, and ACE-Asia, respectively; all describe a similar type of atmospheric layer. The level of TC for the one ACE-2 sample was below the average for INDOEX and TARFOX but within the range of values observed during those studies. The TC value for ACE-2 was significantly below the average level and range of TC levels observed in a similar layer in ACE-Asia. The levels of TC observed in this layer were greater during ACE-Asia than in similar layers sampled during INDOEX and TARFOX, however the ACE-Asia average value is influenced by an exceptionally high level during RF 17. On average it appears that the levels of TC in polluted MBLs were in the order ACE-2 < TARFOX ≈ INDOEX < ACE-Asia.
 As discussed previously, during ACE-Asia, layers present above the MBL were occasionally observed to be affected by anthropogenic aerosols. On the basis of back trajectories these layers were mostly observed to originate from the Mainland China and in this regard are probably similar in character to the layers termed rCBL sampled during INDOEX. For ACE-Asia an approximate average TC level in such a layer was 15 μg C m−3 and ranged from 7.6 to 30 μg C m−3. This approximate average is greater than the average value of 7.4 μg C m−3 observed during INDOEX. Moreover, compared to INDOEX, the range of TC values in this layer were greater and the maximum TC level more than twice the value observed during INDOEX.
 In general, for a given type of atmospheric layer, higher levels of TC were observed during ACE-Asia than were observed during ACE-2, TARFOX and INDOEX. Important parameters influencing the concentration of TC observed in a given layer, and which are not considered here, are the mixing height and amount of dispersion that occurred between point at which the TC was emitted and the location where sampling occurred. These factors, rather than only differences in the magnitude of the sources of TC in these regions, might also explain why, for a given type of atmospheric layer, some differences in the TC levels were observed between different field experiments, i.e., INDOEX versus ACE-Asia.