A second, but different NBL case Seasonal Variation Diurnal Variation Continuous Tower-Based Ozone Measurements L.C. Patrick 1, S.J. Oltmans 2, J.C. Williams 1, A.E. Andrews 2, J. Kofler 1, and D.E. Wolfe 2 1 Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder 80309; , Fax: , 2 NOAA Earth System Research Laboratory, 325 Broadway, Boulder, CO Introduction As well as its role as a greenhouse gas, tropospheric ozone plays a pivotal part in the chemistry of the lower atmosphere. Photochemical and chemical reactions involving ozone drive many nocturnal and diurnal chemical processes. Metropolitan area ozone concentrations are frequently monitored, but continuous measurement of background concentrations found in rural areas is also necessary. In order to understand background levels and influences of local ozone sources, we installed two monitors at NOAA’s Boulder Atmospheric Observatory in Erie, Colorado. Continuous measurements were taken at the surface (8 meters AGL) and at a 300-meter height on the tower starting in July An Interesting Case Continuous Ozone Analyzers Future plans Based on UV absorption, the instrument uses a catalyst (blue can) to create a zero reference, measures its absorption, and then measures absorption of the same volume of unscrubbed air. Its precision is 1.5 ppbv. At 8 m – We installed an elevated inlet with 25 ft of Teflon- FEP tubing leading to a TEI Ozone Analyzer, that was housed inside the building near the tower. The analyzer is based on UV absorption and has a reputation for particularly accurate measurements. In May of 2009, we installed three more ozone monitors at KWKT tower in Moody, TX as part of a nighttime continental jet study. These monitors are coupled with met systems and will provide information about the jet, boundary layer, and ozone transport in the area. At 300 m – We mounted a modified 2B Technologies Ozone Monitor, housed in a stainless-steel enclosure. Added: fans, large particle filter, zeroing cycle, computer control board, improved catalyst (MnO 2 ), power supply and upgraded pump. We used HYSPLIT (a single-particle Lagrangian trajectory model) back trajectories and measurements of other constituents to theorize where our ozone values originated and what may have happened to the air mass en route. March 29, :00 pm MST - The surface instrument (8m – blue line) read values over the EPA’s 80 ppbv daily maximum. Values in excess of 60 ppbv are very unusual for March, but this event is even more interesting because it was on a Sunday and pollution from I-25 should be very low. And why didn’t the 300-m monitor measure the same value? The extent of ozone formation, which is dependent on VOCs and nitrogen oxides in the presence of sunlight, varies diurnally and seasonally. The surface analyzer (blue line) tracks a larger daily range of values than does the 300-m tower instrument. In the relatively stable environment of the nocturnal boundary layer (NBL), ozone mixing ratios near the surface can drop significantly, even during high ozone days, as seen here in July. The monitor at 300 m usually lies out of the NBL and tracks a smaller range of mixing ratios diurnally. 300-meter values rarely reach 0 ppbv. On day 236, there is another rare case in which the NBL likely reaches above the 300-m monitor. Unlike the July event, ozone values fall near 0 ppbv on both analyzers. Again, wind speeds near the 300- m tower level are low and indicative of those usually found within the NBL. The difference between the two days? In this case, there is also high relative humidity, likely causing reaction with ozone. On day 208, the NBL appears to extend higher than usual and is above the 300-m monitor, noted by the well-mixed (similar) values that night. Wind speeds that night near the 300-m level are low and indicative of those usually found within the NBL. Probability distribution functions (PDFs) are a useful way to understand the range of values seen due to seasonal variation. The y-axis represents the probability as a percentage of a given O 3 mixing ratio (x-axis). July – The surface always has a wider range of values, especially in summer. The PDF shows a bimodal Probability (%) Nov – As days have less sunlight, the diurnal range of mixing ratios becomes smaller and NBL values are often 0 ppbv near the surface. The 300-m mixing ratios rarely reach zero, but the max values become lower and the PDF for this month becomes thinner. 300 m ___ 8 m ___ population of the mixing ratios. The low peak at ppbv is indicative of the nighttime values and the high peak at 65 ppbv represents a typical daytime value in July. Jan – The 300-m analyzer measures very low values at night, now sometimes reaching 0 ppbv. Due to the long tail, a mean value would not be a representative value. Mar – At the surface and 300-m, the range of mixing ratios widens again as hours of sunlight increase. Ozone no longer drops to 0 ppbv at night. O 3 ppbv Mar 29 ~3:00 MST 8 m (AGL) 300 m ___ 8 m ___ Studying nitrogen oxides would provide additional information about nighttime processes in and out of the NBL. Instruments are available to measure NO, NOx (NOx = NO + NO 2 ) and NO 2. Plans include testing these instruments on the BAO tower in the near future and possibly adding them to the suite of instruments at KWKT towers in Texas to study the nighttime continental jet. HYSPLIT back trajectories indicate that day 87 and day 89 (March 29) were atypical in that warm air was flowing from the south. In the diagram, the flows cross I-70 and West Denver. The trajectory for 04/02 shows a typical flow. CO and CO 2 mixing ratios measured at BAO showed the presence of urban pollution. The CO/CO 2 signatures are suggestive of automotive and urban pollution rather than industrial pollution. CO and CO 2 values confirm what the ozone analyzers measured: the pollution reached the bottom layers, but didn’t reach the top of the tower at 300m. 300 m ___ 8 m ___