Analysis of Data from International Consortium for Atmospheric Research on Transport and
4.4. PILS-IC Measurements
4.5.2. Case Study: Power Plant Plume in Cloud (Flight 5 on 9 August 2004)
On 9 August 2004 (1709–2216 UT), the TO sampled the Conesville Power Plant plume in cloud (Figure 4.5a). Located south of Cleveland, this plant emits SO2 and NOx
at rates of 1.31x108 kg/yr and 2.16x107 kg/yr, respectively, and VOCs at 1.21x105 kg/yr (http://www.emissionsonline.org/nei99v3/plant/pl44374x.htm, 1999). The plume was transported northeast (~ 230°) at wind speeds between 6 and 12 m/s. Figure 4.5b shows the TO altitude, LWC, and aerosol number concentration. The TO flew a stair-step pattern between altitudes of 1500 and 2200 m downwind of the plant between 1815 and 2140 UT. LWC measurements reached values as high as 0.94 g/m3.
Typical FSSP cloud droplet size distributions showed a number concentration mode ranging from 7.7 to 9.0 μm with a geometric standard deviation ranging between 1.2 and 1.5. All instruments were sampling the total aerosol through the main inlet, which when used in cloud, as noted earlier, contained a mixture of interstitial aerosol, particles that contain significant amounts of water but are not activated (usually < 2 μm), and droplet residuals. The DACAD measured a consistent bimodal size distribution with increasing distance downwind of the power plant in cloud, with a smaller mode that grew from 25 to 42 nm (Figure 4.6). The aerosol number concentration outside of cloud and close to the power plant was dominated by particles with Dp < 40 nm. A cloud- processing mode in the range between 100 and 200 nm was measured throughout the flight. The hygroscopic growth factor for the larger mode ranged between 1.16 and 1.20 at the relative humidities shown in Figure 4.6. The growth factors are low relative to that for pure ammonium sulfate particles (1.17 compared to 1.44 for pure ammonium sulfate
at 77% RH), suggesting that the sampled aerosol was composed of less hygroscopic components.
Figure 4.5c shows a time series of SO42-, various organic ions, and the volume concentration, from the DACAD, of the total aerosol sampled (Dp < 1 μm). There is significant growth in SO42-, oxalate, and the aerosol volume concentration in the plume proceeding downwind of the power plant. The aerosol volume concentration and the SO42- loadings are correlated and close in magnitude, suggesting that most of the volume growth was a result of the conversion of SO2 to SO42-. The ammonium-to-sulfate molar ratio was between 1.1 and 1.6 for most of the flight downwind of the plant, indicating that there was insufficient ammonia to neutralize the relatively high level of sulfuric acid.
This high level of acidity can also explain why virtually no NO3- was detected (< 0.3 μg/m3). The aerosol volume concentration reached a maximum close to 40 km downwind of the plant (2043 UT), the point at which the highest SO42- and oxalate loadings occurred (18.90 and 0.61 μg/m3, respectively). Oxalate grew with increasing distance from the plant as well, representing between 0.5 and 3.1% of the total water- soluble mass for the majority of the flight and 2.6% in the vial with its maximum mass.
Out-of-plume, oxalate concentrations dropped below 0.1 μg/m3, and SO42- dropped to between 1 and 5 μg/m3.
Aqueous-phase precursors to oxalate were only identified in cloud, offering support to the hypothesis of an aqueous formation pathway. Glyoxylate, the anion of glyoxylic acid, was measured in two successive vials, at a level on the order of 0.06 μg/m3, while oxalate was measured as 0.26 μg/m3 and 0.39 μg/m3 in these two vials.
Two other vials contained 0.03 μg/m3 glyoxylate, while oxalate was at 0.19 μg/m3 and
0.36 μg/m3. Pyruvate, the anion of pyruvic acid, which is thought to be a precursor of glyoxylic acid (Lim et al., 2005), was measured at a level between 0.09 and 0.10 μg/m3. Glutarate, the anion of the glutaric acid (C5 dicarboxylic acid), reached a maximum level of 0.24 μg/m3. One vial contained 0.05 μg/m3 of malonate, the anion of malonic acid (C3
dicarboxylic acid). Longer-chain dicarboxylic acids such as glutaric acid are oxidized in the aqueous phase leading to smaller dicarboxylic acids, ultimately reaching oxalic acid (Kawamura and Sakaguchi, 1999). The aforementioned data alone do not reveal the relative contribution from the decay of longer-chain acids such as glutarate and the oxidation of glyoxylate to the production of oxalate mass; this will be explored subsequently.
Since the concentrations of SO42- and oxalate are correlated and they are both typically found in the same mode in the submicrometer aerosol size range, it is hypothesized that oxalate is formed in the aqueous phase, for which the presence of key aqueous intermediates to oxalic acid provides evidence. The mass concentrations of SO42- and oxalate were higher in the Flight 5 total aerosol samples as compared to the total aerosol and droplet residual measurements in clouds uninfluenced by local pollution sources from other flights. This is hypothesized to be a result of longer cloud processing times for sampled particles during Flight 5 since the clouds were more abundant and closely packed. Also, some of the highest LWC values from the entire mission were observed, leading to increased partitioning of gases (specifically SO2, organic precursors to oxalic acid, and oxidants) into the droplets, yielding higher mass production rates of SO42- and oxalate. The highly turbulent nature of the lower atmosphere on 9 August 2004 may have also promoted the re-activation of evaporated droplets, via cycling in- and
out-of-cloud, to produce increasing amounts of SO42- and oxalate. We note that although the SO2 concentrations were measured to be higher in plume compared to non-plume conditions, the concentrations of volatile organic carbon (VOC) species were essentially similar, which highlights the importance of non-VOC factors in oxalic acid production.