3506B24 Final Report

In the monthly progress reports (attached), certain features of the above 2002/03 measurement period had been discussed in detail by describing general synoptic conditions on a case by case basis, and linking them to individual meteorological and ambient pollutants observations. It was specially focused on the periods when intensive meteorological, trace gas concentration, fine PM composition and POC speciation measurements were conducted at OLC, which is presented in the following. First, the general meteorological conditions encountered during each of the intensive measurement episodes are described in relation to Figure 40 and Table A7, using some of the most important meteorological parameters, trace gas and individual PM_2.5 species.

The discrete sampling for PM_2.5 composition and POC speciation analyses at OLC was tested during the period 1/20 to 23 2003. Note, that 1/20 was a Monday National Holiday commemorating Dr. Martin Luther King’s birthday, and was chosen in anticipation of relatively clean, low traffic, less polluted atmospheric background conditions. The region was under a warming trend with increasing moisture carried from the southwest, after hitting record low temperatures early morning 1/18. The movement of a strong northwesterly front brought again dry and cold air into the region reaching new record low temperatures on the 23^rd and 24^th. Lighter winds under initially westerly directions, veering over northerly to southerly flow caused a continuous warming of the region until the end of January. The background sampling period fell into the first warming period with increased moisture and cloud cover and region-wide precipitation by the 22^nd. No prescribed burns were conducted nor did any wild fires occur on the military base between the 16^th and the 26^th. The trace gas and PM_2.5 concentrations depicted therefore did not show any indications of direct impact from nearby biomass burning activity.

In preparation of the prescribed burning of Training Areas (TA) K-14 and K-20 on Fort Benning’s military installation, scheduled for the week of February 3^rd, the HVS and PCM background samples were taken on Sunday, 02/02/03 and five 5-h burn event samples were taken beginning at noon on Wednesday, 02/05/03, when a total of 937 acres were burned on TAs K-14 and K-20, ~28 km to the ENE from OLC (see Table A2). The synoptic weather condition during this period was characterized by a low pressure system associated with an area of low clouds moving northeast out of AL into and through central GA with winds increasing from Feb 3^rd to 4^th. The dynamic front produced showers and elevated thunder storms that moved rapidly through northern GA. In the wake of this front, colder air moved into the region from the north, causing below normal temperatures across the State, as freezing temperatures were reached at OLC early morning of the 5^th. The prescribed burns of K-14 and K-20 were started shortly before noon that day.

The burn activities were focused on four days of the second and the fourth week of March each. On 3/10, combined 535 acres of the TA Q3 and Q6, ~15 and 17 km to the ESE, were burned, followed by TA J3 and D4 on 3/11 with 721 acres total and 14 and 23 km away to the E, respectively. Larger differences between the daily minimum and maximum air temperature and relative humidity levels were observed on average during the 2^nd (10-16) and 4^th week (24-30) of March, indicating overall less cloud coverage and generally drier conditions, allowing the conduct of prescribed burning as mentioned above. The two burn weeks were also characterized by elevated nighttime PM_2.5, CO and NOy pollution levels compared to the non-burn weeks (3-9, and 17-23). Average nocturnal O₃ minima were lower during the burn weeks, corresponding to increased titration under elevated pollution loads, compared to the less polluted non-burn weeks. An overall increasing warming trend was observed during the month, with increasing average daily maxima in Photosynthetic Active Radiation (PAR), possibly leading to more intense atmospheric photochemical activity and higher average O₃ maxima; the average maximum O₃ increased from ~35 ppbv during the 1^st week to ~55 ppbv during the last week.

The main feature of this month was the lifting of an upper level low out of the Gulf of Mexico around the 17^th, leaving widespread rain across South and Central Georgia. In the following days, a warm front moved north, leaving behind unstable conditions with an upper level ridge moving over the Atlantic. The conditions became increasingly unstable as the wedge broke down and the upper level jet moved across the SE-US, until drier and cooler air moved into the region on the 20^th. With the warming trend in the second half of the month, a regional build-up of O₃ was observed, while NOy levels seemed highly variable and subject to more local influences. CO and NOy levels at OLC seemed elevated relative to the other FAQS sites, especially at nights after prescribed burns. The levels detected at night between the 23^rd and 24^th , before the prescribed burning of training areas K-22 and M-4, ENE from OLC, coincided with easterly flow and average 457 and 14 ppbv CO and NOy, respectively, and a linear regression of slope 18 and intercept 200 ppbv at r² = 0.44. Even though the peak CO concentration of 1140 ppbv occurred at 0900, the peak NOy was only 33 ppbv, pointing to a mostly non-vehicular source and the likelihood of an impact from a biomass burning plume of different origin.

This month was characterized by a periodic scattered showers and thunderstorms across the State. A particularly large system with persistent high winds moved through the region between April 7 and 10, causing PM_2.5 and O₃ values to drop equally at all FAQS sites. The following dry period was characterized by mostly clear skies, convective flow [local winds with daytime highs and nighttime lows], regional build-up in PM_2.5 and daytime O₃ maxima. The occurrence of the large PM_2.5 peak at the Macon SBP site (see Fig. 38) was due to a large pick-nick event held by the American Cancer Society over the April 11 weekend. The regional distribution and homogeneity of fine PM mass concentration is again apparent, showing a close dependence on weather conditions and frontal movements, with therefore close relation to atmospheric pressure. During this month, low pressures were associated with scattered showers across the region, with precipitation being the most effective removal mechanism for atmospheric PM. The early morning period on the 15^th was characterized by a rapid brief increase in O₃, wind speed, and air temperature in conjunction with a sharp decline of relative humidity, indicating a dynamic disturbance in the nocturnal BL and the down-mixing of drier, warmer air from the reservoir air layer aloft, containing regionally distributed ozone from the previous day.

Two smaller TA (W4 and B3) with a combined burn area of 329 acres were burned to the S and SE on 4/28, whereas the 285 acres on 4/29 were more in easterly directions. The winds were relatively low from veering directions. Despite the large differences in local near surface air mass transport all sites seem to be subject to higher average fine mass loadings from southerly directions. A more regional subsidence of air occurred continuing the seasonal warming trend.

This month was characterized by a record rain fall across the State. Only the last week of May saw drier conditions, with PM_2.5 averages (and O₃ maxima) rising region-wide, as seen at all sites in Fig. 38. The fine PM mass concentration appears again regionally distributed and homogeneous as previously observed. With the wide spread rain fall earlier, the first three weeks showed regionally low [PM_2.5]. Significant increases occurred region-wide under increasingly clear skies and stagnant conditions on the 23^rd and 24^th, here indicated by the increasing differences in daily minima and maxima ambient temperatures and relative humidity, combined with regionally low wind speeds.