Aircraft and ground-based measurements of hydroperoxides during the 2006 MILAGRO field campaign

International audienceMixing ratios of hydrogen peroxide and hydroxymethyl hydroperoxide were determined aboard the US Department of Energy G-1 Research Aircraft during the March 2006 MILAGRO field campaign in Mexico. Ground measurements of total hydroperoxide were made at the T1 site at Universidad Technologica de Tecámac, about 35 km NW of Mexico City. In the air and on the ground, peroxide mixing ratios near the source region were generally near 1 ppbv, much lower than had been predicted from photochemical models based on the 2003 Mexico City study. Strong southerly flow resulted in transport of pollutants from the T0 to T1 and T2 surface sites on several flight days. On these days, it was observed that peroxide concentrations slightly decreased as the G-1 flew progressively downwind. This observation is consistent with low or negative net peroxide production rates calculated for the source region and is due to the very high NOx concentrations above the Mexico City plateau. However, relatively high values of peroxide were observed at takeoff and landing near Veracruz, a site with much higher humidity and lower NOx concentrations

The time evolution of aerosol composition over the Mexico City plateau

International audienceThe time evolution of aerosol concentration and chemical composition in a megacity urban plume was determined based on 8 flights of the DOE G-1 aircraft in and downwind of Mexico City during the March 2006 MILAGRO field campaign. A series of selection criteria are imposed to eliminate data points with non-urban emission influences. Biomass burning has urban and non-urban sources that are distinguished on the basis of CH3CN and CO. In order to account for dilution in the urban plume, aerosol concentrations are normalized to CO which is taken as an inert tracer of urban emission, proportional to the emissions of aerosol precursors. Time evolution is determined with respect to photochemical age defined as ?Log10 (NOx/NOy). The geographic distribution of photochemical age and CO is examined, confirming the picture that Mexico City is a source region and that pollutants become more dilute and aged as they are advected towards T1 and T2, surface sites that are located at the fringe of the City and 35 km to the NE, respectively. Organic aerosol (OA) per ppm CO is found to increase 7 fold over the range of photochemical ages studied, corresponding to a change in NOx/NOy from nearly 100% to 10%. In the older samples the nitrate/CO ratio has leveled off suggesting that evaporation and formation of aerosol nitrate are in balance. In contrast, OA/CO increases with age in older samples, indicating that OA is still being formed. The amount of carbon equivalent to the deduced change in OA/CO with age is 56 ppbC per ppm CO. At an aerosol yield of 5% and 8% for low and high yield aromatic compounds, it is estimated from surface hydrocarbon observations that only ~9% of the OA formation can be accounted for. A comparison of OA/CO in Mexico City and the eastern U.S. gives no evidence that aerosol yields are higher in a more polluted environment

Chemical Evolution of a Power-Plant Plume

Measurements made from the DOE G-1 aircraft were used to calculate the rate and efficiency of O{sub 3} production downwind of an isolated, coal-fired power plant. The plume was transected 12 times at distances ranging to 65 km from its source (corresponding to an age of {approx}4 h assuming constant wind velocity). For NO{sub x}, a loss rate of 0.5 h{sup -1} was calculated. If reaction with OH was the sole loss mechanism, then an [OH] = 1.6 x 10{sup 7}molec/cm{sup 3} is inferred, which is {approx}2-3X values calculated using a box model constrained by observations. Possible explanations for this discrepancy are discussed. O{sub 3} production per molecule of NO{sub x} approached 6-8 after the plume had aged >3h. Peak O{sub 3} concentrations were 15 ppbv above background. Dilution appears to limit the peak O{sub 3} concentration despite the high production efficiency. Hydrocarbon samples indicate high levels of VOC reactivity ({approx}8 s{sup -1}) in the plume. The number concentration of accumulation mode particles increases significantly with plume age indicating a rapid formation of aerosol mass

Dependence of ozone production on NO and hydrocarbons in the troposphere

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95525/1/grl10419.pd

Influence of Oil and Gas Emissions on Summertime Ozone in the Colorado Northern Front Range

Tropospheric O 3 has been decreasing across much of the eastern U.S. but has remained steady or even increased in some western regions. Recent increases in VOC and NO x emissions associated with the production of oil and natural gas (O&NG) may contribute to this trend in some areas. The Northern Front Range of Colorado has regularly exceeded O 3 air quality standards during summertime in recent years. This region has VOC emissions from a rapidly developing O&NG basin and low concentrations of biogenic VOC in close proximity to urban-Denver NO x emissions. Here VOC OH reactivity (OHR), O 3 production efficiency (OPE), and an observationally constrained box model are used to quantify the influence of O&NG emissions on regional summertime O 3 production. Analyses are based on measurements acquired over two summers at a central location within the Northern Front Range that lies between major regional O&NG and urban emission sectors. Observational analyses suggest that mixing obscures any OPE differences in air primarily influenced by O&NG or urban emission sector. The box model confirms relatively modest OPE differences that are within the uncertainties of the field observations. Box model results also indicate that maximum O 3 at the measurement location is sensitive to changes in NO x mixing ratio but also responsive to O&NGVOC reductions. Combined, these analyses show that O&tp://esrl. noaa.gov/csd, FRAPPNG alkanes contribute over 80% to the observed carbon mixing ratio, roughly 50% to the regional VOC OHR, and approximately 20% to regional photochemical O 3 production

TRACE GAS MEASUREMENTS IN PHOENIX, ARIZONA (1998).

The DOE Atmospheric Chemistry Program, and the Arizona Department of Environmental Quality (DEQ) conducted a field program in the Phoenix Metropolitan area in the late spring of 1998. The experiment was composed of a linked set of aircraft and surface measurements designed to characterize the chemical and meteorological processes leading to ozone episodes. The existing network of Arizona DEQ sites in Phoenix was utilized to document ground level concentrations of ozone and its precursors. West of the downtown area, a site (Usery Pass) was set up for the detailed characterization of the mature Phoenix urban plume. Detailed measurements in the source region were made at several sites in downtown Phoenix. The DOE G-1 aircraft, equipped with a comprehensive array of instruments to characterize atmospheric trace gas and aerosol composition, flew over the region at various times during the day. All times in the following discussion are local standard time (LST). Morning flights were typically made between 08:00 and 12:00 upwind, to measure background concentrations, and over the Phoenix source region, to characterize the sources of ozone precursors. Afternoon flights over the Phoenix source region and downwind between 15:00 and 18:00 were made to examine the chemical properties and physical distribution of the photochemically aged urban plume. The aircraft flights typically included an atmospheric sounding to circa 3 km upwind and over Phoenix in the morning, and downwind in the afternoon. A total of 22 flights were made on 14 different days during the one month program. The motivation for conducting the program was to examine ozone formation rates and efficiencies in an environment where the pollutant mix is dominated by vehicle emissions, where the contribution of biogenic hydrocarbons to ozone formation is thought to be low, and where processing conditions are different than they are in the Eastern US. The latter includes significant differences in atmospheric humidity, solar intensity, and boundary layer heights. The objective of this paper is to describe general features of the chemical and meteorological data collected during the program and to present results from an aircraft case study when O{sub 3} concentrations were among the highest measured during the entire study

Trace gas measurements in Phoenix, Arizona (1998)

The DOE Atmospheric Chemistry Program, and the Arizona Department of Environmentel Quality (DEQ) conducted a field program in the Phoenix Metropolitan area in the late spring of 1998. The experiment was composed of a linked set of aircraft and surface measurements designed to characterize the chemical and meteorological processes leading to ozone episodes. The existing network of Arizona DEQ sites in Phoenix was utilized to document ground level concentrations of ozone and its precursors. West of the downtown area, a site (Usery Pass) was set up for the detailed characterization of the mature Phoenix urban plume. Detailed measurements in the source region were made at several sites in downtown Phoenix. The DOE G-1 aircraft, equipped wih a comprehensive array of instruments to characterize atmospheric trace gas and aerosol composition, flew over the region at various times during the day. All times in the following discussion are local standard time (LST). Morning flights were typically made between 08:00 and 12:00 upwind, to measure background concentrations, and over the Phoenix source region, to characterize the sources of ozone precursors. Afternoon flights over the Phoenix source region and downwind between 15:00 and 18:00 were made to examine the chemical properties and physical distribution of the photochemically aged urban plume. The aircraft flights typically included an atmospheric sounding to circa 3 km upwind and over Phoenix in the morning, and downwind in the afternoon. A total of 22 flights were made on 14 different days during the one month program. The motivation for conducting the program was to examine ozone formation rates and efficiencies in an environment where the pollutant mix is dominated by vehicle emissions, where the contribution of biogenic hydocarbons to ozone formation is thought to be low, and where processing conditions are different than they are in the Eastern US. The latter includes significant differences in atmospheric humidity, solar intensity, and boundary layer heights. The objective of this is to describe general features of the chemical and meteorological data collected during the program and present results from an aircraft case study when O{sub 3} concentrations were among the highest measured during the entire study

Ozone production rate and hydrocarbon reactivity in 5 urban areas: A cause of high ozone concentration in Houston

Observations of ozone (O3) and O3 precursors taken from aircraft flights over Houston, TX, Nashville, TN; New York, NY; Phoenix, AZ, and Philadelphia, PA show that high concentrations of reactive volatile organic compounds (VOCs) in the Houston atmosphere lead to calculated O3 production rates that are 2 to 5 times higher than in the other 4 cities even though NOx concentrations are comparable. Within the Houston metropolitan area, concentrations of VOCs and O3 production rates are highest in the Ship Channel region; the location of one of the largest petrochemical complexes in the world. As a consequence the concentration of O3 in the Houston metropolitan area has recently exceeded 250 ppb, the highest value observed in the U.S within the past 5 years

Sensitivity of ozone production rate to ozone precursors

The photochemical equations describing O3 formation in the lower troposphere contain 2 major sink terms for free radicals; combination reactions and reactions with NOx. Knowing the fraction of radicals removed by reactions with NOx, termed LN/Q, allows one to predict the sensitivity of O3 production to NO and VOCs. We derive an analytic formula that gives LN/Q in terms of readily measured O3 precursors and test this formula using constrained steady state calculations based on field observations gathered in Phoenix, Arizona. The formula quantifies well‐known results regarding the effects of dilution, oxidation, and the production of oxidants on the transition from VOC to NOx sensitive behavior as an air parcel is advected away from an urban source