Baseline Aerosol Observations
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**This is a section of NOAA's Climate Monitoring and Diagnostics Laboratory Summary Report No. 22 from 1993.
The aerosol monitoring program at BRW, MLO, and SPO continued during 1993 as in previous years. CN concentration was measured continuously with TSI (butanol-based) CN counters at BRW, MLO, and SPO. Daily calibration points were provided by Pollak CN counters at all stations. Aerosol scattering extinction coefficient (ssp) at 450-, 550-, 700-, and 850-nm wavelengths was measured continuously at BRW, MLO, and SPO with four-wavelength nephelometers. Aerosol absorption coefficient has been measured continuously using aethalometers at BRW since April 1988, MLO since April 1990, and at SPO during December 1986-December 1991.
Figure 1 shows daily geometric means of CN concentration (lower portion of each plot), ssp (middle portion of each plot), and Ångström exponent (upper portion of each plot) at the BRW, MLO, and SPO stations for 1993. Two independent values of Ångström exponent (a) were calculated from the 450-, 550-, and 700-nm channels of ssp data using the relation a = -Dlog ssp/Dlogl, where a is Ångström exponent and l is wavelength. These averages were calculated only if data for all three wavelengths were available. A graphical presentation of the monthly geometric means of the entire data record for BRW, MLO, and SPO is shown in Figure 2. Monthly geometric means of the 1993 aerosol data are listed in Table 1.
TABLE 1. Monthly Geometric Means of CN concentration (cm-3) and ssp (m-1) at 450, 550, and 700 nm for BRW, MLO, and SPO During 1993
|ssp(450)||1.22 x-5||9.62 x-6||1.59 x-5||9.86 x-6||5.03 x-6||7.09 x-7||2.33 x-6||2.51 x-6||3.28 x-6||5.70 x-6||3.57 x-6||1.10 x-5|
|ssp (550)||1.13 x-5||8.85 x-6||1.45 x-5||8.38 x-6||4.24 x-6||7.71 x-7||2.16 x-6||2.59 x-6||3.51 x-6||5.97 x-6||3.37 x-6||1.07 x-5|
|ssp (700)||8.74 x-6||6.81 x-6||1.10 x-5||5.73 x-6||2.82 x-6||5.90 x-7||1.44 x-6||2.25 x-6||3.16 x-6||5.39 x-6||2.55 x-6||8.79 x-6|
|ssp (450)||1.08 x-6||9.88 x-7||2.21 x-6||2.26 x-6||2.11 x-6||1.38 x-6||1.62 x-6||8.20 x-7||1.16 x-6||1.09 x-6||5.35 x-7||5.75 x-7|
|ssp (550)||8.71 x-7||8.17 x-7||1.79 x-6||1.69 x-6||1.74 x-6||1.14 x-6||1.28 x-6||5.86 x-7||8.39 x-7||7.99 x-7||3.76 x-7||4.55 x-7|
|ssp (700)||6.97 x-7||6.21 x-7||1.43 x-6||1.22 x-6||1.39 x-6||8.84 x-7||9.58 x-7||4.03 x-7||5.59 x-7||5.51 x-7||2.84 x-7||3.38 x-7|
|ssp (450)||5.01 x-7||3.26-7||2.03 x-7||1.06-7||2.03 x-7||1.85 x-7||2.31 x-7||2.88 x-7||2.67 x-7||3.03 x-7||2.32 x-7||1.87 x-7|
|ssp (550)||3.91 x-7||2.77-7||1.64 x-7||9.70-8||1.55 x-7||1.73 x-7||2.14 x-7||2.30 x-7||2.22 x-7||2.47 x-7||1.95 x-7||1.63 x-7|
|ssp (700)||3.46 x-7||2.27-7||1.36 x-7||7.95-8||1.48 x-7||1.28 x-7||1.62 x-7||1.74 x-7||1.95 x-7||2.09 x-7||1.60 x-7||1.11 x-7|
A compact exponential format is used for ssp such that 1.22 x-5 = 1.22 x 10-5.
The BRW data in Figure 1 show a ssp maximum of about 3 X 10-5 m-1 during spring, typical of the well-known Arctic haze. Minimum values of ssp below 10-6 m-1 occurred in July-August. The BRW long-term record shown in Figure 2 clearly shows this annual cycle in ssp, with springtime monthly means of about 10-5 m-1 and summertime monthly means of about 10-6 m-1. The BRW CN record shows a more variable semiannual cycle with a maximum that usually coincides with the maximum in ssp and a secondary maximum in late summer or early fall. The 1993 annual geometric mean for CN is 195 cm-3 (compared with 228 cm-3 for 1992) and the annual mean for ssp (550 nm) is 4.85 x 10-6 m-1 (compared with 4.62 x 10-6 m-1) for 1992. Note that the G.E. counter CN record is shown as a solid line and the TSI counter CN record is shown as a dashed line. The individual squares plotted on the CN graph are monthly means of Pollak counter observations. These are shown separately because the TSI and Pollak counter give independent data sets, whereas the G.E. counter was calibrated using the Pollak counter data. Gaps in the data are apparent because of the averaging process that excludes data if local pollution is evident or if wind direction is not from the clean air sector. The BRW aerosol data set was presented by Bodhaine  and Quakenbush and Bodhaine .
The MLO ssp data shown in Figure 1 are typical of the long-term record, with the highest values in April and May, and lower values in fall and winter. Large events are apparent in the springtime, caused by the long-range transport of Asian desert dust in the upper troposphere to the vicinity of Hawaii. As discussed in the 1988 Summary Report [Elkins and Rosson, 1989], ssp values were generally higher since the installation of the new nephelometer in 1985 and have not reached the low values expected in winter. The MLO CN record shown in Figure 1 is typical, giving an annual geometric mean concentration of 381 cm-3 (compared with 375 cm-3 during 1992); the annual mean of ssp (550 nm) is 9.20 x 10-7 m-1 (compared with 8.58 x 10-7 m-1 during 1992). Note that all MLO aerosol data presented here are in the form of geometric means during 0000-0800 HST (1000-1800 UT) in order to include data for nighttime downslope wind conditions only. The MLO data set was presented by Massey et al. .
The SMO 1993 CN data are not presented because of problems associated with the sampling stack during that year. The SMO nephelometer was removed from service in March 1991. Future plans for the SMO aerosol program include a new humidity-controlled, size-controlled sampling system and a new multi-wavelength nephelometer.
The SPO ssp and CN data are shown in Figure 1. These data show a strong annual cycle reaching a maximum in the austral summer and a minimum in the austral winter, similar to previous years. The ssp data show events due to the transport of seasalt particles in winter but fairly clean values in the fall. Referring to the long-term data set for SPO shown in Figure 2, the ssp data generally show intermediate values in the austral summer and fall, and large events, sometimes exceeding 10-6 m-1, in late winter. These large aerosol events are caused by the transport of seasalt in the upper troposphere from stormy regions near the Antarctic coast to the interior of the continent. The SPO 1993 annual means were about 90 cm-3 (69 cm-3 for 1992) for CN and 2.00 x 10-7 m-1 (4.41 x 10-7 m-1 for 1992) for ssp (550 nm). The complete SPO data set was presented by Bodhaine and Shanahan .