ALBERT

Description: This study discusses an analysis of combined airborne and ground observations of particulate nitrate (NO3(p)) concentrations made during the wintertime DISCOVER-AQ (Deriving Information on Surface Conditions from COlumn and VERtically resolved observations relevant to Air Quality) study at one of the most polluted cities in the United States Fresno, CA in the San Joaquin Valley (SJV) and focuses on developing an understanding of the various processes that impact surface nitrate concentrations during pollution events. The results provide an explicit case-study illustration of how nighttime chemistry can influence daytime surface-level NO3(p) concentrations, complementing previous studies in the SJV. The observations exemplify the critical role that nocturnal chemical production of NO3(p) aloft in the residual layer (RL) can play in determining daytime surface-level NO3(p) concentrations. Further, they indicate that nocturnal production of NO3(p) in the RL, along with daytime photochemical production, can contribute substantially to the buildup and sustaining of severe pollution episodes. The exceptionally shallow nocturnal boundary layer (NBL) heights characteristic of wintertime pollution events in the SJV intensify the importance of nocturnal production aloft in the residual layer to daytime surface concentrations. The observations also demonstrate that dynamics within the RL can influence the early-morning vertical distribution of NO3(p), despite low wintertime wind speeds. This overnight reshaping of the vertical distribution above the city plays an important role in determining the net impact of nocturnal chemical production on local and regional surface-level NO3(p) concentrations. Entrainment of clean free-tropospheric (FT) air into the boundary layer in the afternoon is identified as an important process that reduces surface-level NO3(p) and limits buildup during pollution episodes. The influence of dry deposition of HNO3 gas to the surface on daytime particulate nitrate concentrations is important but limited by an excess of ammonia in the region, which leads to only a small fraction of nitrate existing in the gas phase even during the warmer daytime. However, in the late afternoon, when diminishing solar heating leads to a rapid fall in the mixed boundary layer height (BLH), the impact of surface deposition is temporarily enhanced and can lead to a substantial decline in surface-level particulate nitrate concentrations; this enhanced deposition is quickly arrested by a decrease in surface temperature, which drops the gas-phase fraction to near zero. The overall importance of enhanced late-afternoon gas-phase loss to the multiday buildup of pollution events is limited by the very shallow nocturnal boundary layer. The case study here demonstrates that mixing down of NO3(p) from the RL can contribute a majority of the surface-level NO3(p) in the morning (here, 80%), and a strong influence can persist into the afternoon even when photochemical production is maximum. The particular day-to-day contribution of aloft nocturnal NO3(p) production to surface concentrations will depend on prevailing chemical and meteorological conditions. Although specific to the SJV, the observations and conceptual framework further developed here provide general insights into the evolution of pollution episodes in wintertime environments.

Description: A comprehensive group of reactive nitrogen species (NO, NOz, HN03, HOzN02, PANs, alkyl nitrates, and aerosol-NO3) were measured over North America during July/August 2004 from the NASA DC-8 platform (0.1 - 12 km). Nitrogen containing tracers of biomass combustion (HCN and CH3CN) were also measured along with a host of other gaseous (CO, VOC, OVOC, halocarbon) and aerosol tracers. Clean background air as well as air with influences from biogenic emissions, anthropogenic pollution, biomass combustion, convection, lightning, and the stratosphere was sampled over the continental United States, the Atlantic, and the Pacific. The North American upper troposphere (UT) was found to be greatly influenced by both lightning NO, and surface pollution lofted via convection and contained elevated concentrations of PAN, ozone, hydrocarbons, and NO,. Observational data suggest that lightning was a far greater contributor to NO, in the UT than previously believed. PAN provided a dominant reservoir of reactive nitrogen in the UT while nitric acid dominated in the lower troposphere (LT). Peroxynitric acid (H02N02) was present in sizable concentrations peaking at around 8 km. Aerosol nitrate appeared to be mostly contained in large soil based particles in the LT. Plumes from Alaskan fires contained large amounts of PAN and aerosol nitrate but little enhancement in ozone. A comparison of observed data with simulations from four 3-D models shows significant differences between observations and models as well as among models. We investigate the partitioning and interplay of the reactive nitrogen species within characteristic air masses and further examine their role in ozone formation.

Description: Measurements of CH2O from a tunable diode laser absorption spectrometer (TDLAS) were acquired onboard the NASA DC-8 during the summer 2004 INTEX-NA (Intercontinental Chemical Transport Experiment - North America) campaign to test our understanding of convection and production mechanisms in the upper troposphere (UT, 6-12-km) over continental North America and the North Atlantic Ocean. Point-by-point comparisons with box model calculations, when MHP (CH3OOH) measurements were available for model constraint, resulted in a median CH2O measurement/model ratio of 0.91 in the UT. Multiple tracers were used to arrive at a set of UT CH2O background and perturbed air mass periods, and 46% of the TDLAS measurements fell within the latter category. At least 66% to 73% of these elevated UT observations were caused by enhanced production from CH2O precursors rather than direct transport of CH2O from the boundary layer. This distinction is important, since the effects from the former can last for over a week or more compared to one day or less in the case of convective transport of CH2O itself. In general, production of CH2O from CH4 was found to be the dominant source term, even in perturbed air masses. This was followed by production from MHP, methanol, PAN type compounds, and ketones, in descending order of their contribution. In the presence of elevated NO from lightning and potentially from the stratosphere, there was a definite trend in the CH2O discrepancy, which for the highest NO mixing ratios produced a median CH2O measurement/model ratio of 3.9 in the 10-12-km range. Discrepancies in CH2O and HO2 in the UT with NO were highly correlated and this provided further information as to the possible mechanism(s) responsible. These discrepancies with NO are consistent with additional production sources of both gases involving CH3O2 + NO reactions, most likely caused by unmeasured hydrocarbons.

Description: We use observations from two aircraft during the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) campaign over the eastern United States and North Atlantic during summer 2004, interpreted with a global 3-D model of tropospheric chemistry (GEOS-Chem) to test current understanding of the regional sources, chemical evolution, and export of nitrogen oxides. The boundary layer NO(x) data provide top-down verification of a 50% decrease in power plant and industry NO(x) emissions over the eastern United States between 1999 and 2004. Observed 8-12 8 km NO(x) concentrations in ICARTT were 0.55 +/- 36 ppbv, much larger than in previous United States aircraft campaigns (ELCHEM, SUCCESS, SONEX). We show that regional lightning was the dominant source of this NO(x) and increased upper tropospheric ozone by 10 ppbv. Simulating the ICARTT upper tropospheric NO(x) observations with GEOS-Chem require a factor of 4 increase in the model NO(x) yield per flash (to 500 mol/flash). Observed OH concentrations were a factor of 2 lower than can be explained from current photochemical models, and if correct would imply a broader lightning influence in the upper troposphere than presently thought.An NO(y)-CO correlation analysis of the fraction f of North American NO(x) emissions vented to the free troposphere as NO(y) (sum of NO(x) and its oxidation products PAN and HNO3) s shows observed f=16+/-10 percent and modeled f=14 +/- 8 percent, consistent with previous studies. Export to the lower free troposphere is mostly HNO3 but at higher altitudes is mostly PAN. The model successfully simulates NO(y) export efficiency and speciation, supporting previous model estimates of a large U.S. contribution to tropospheric ozone through NO(x) and PAN export.