Ozone production in the upper troposphere and the influence of aircraft during SONEX: Approach of NO(x)-saturated conditions

During October/November 1997, simultaneous observations of NO, HO2 and other species were obtained as part of the SONEX campaign in the upper troposphere. We use these observations, over the North Atlantic (40-60°N), to derive ozone production rates, P(O3), and to examine the relationship between P(O3) and the concentrations of NO(x) (= NO + NO2) and HO(x) (= OH + peroxy) radicals. A positive correlation is found between P(O3) and NO(x) over the entire data set, which reflects the association of elevated HO(x) with elevated NO(x) injected by deep convection and lightning. By filtering out this association we find that for NO(x)>70 pptv, P(O3) is nearly independent of NO(x), showing the approach of NO(x)-saturated conditions. Predicted doubling of aircraft emissions in the future will result in less than doubling of the aircraft contribution to ozone over the North Atlantic in the fall. Greater sensitivity to aircraft emissions would be expected in the summer

Seasonal differences in the photochemistry of the South Pacific: A comparison of observations and model results from PEM-Tropics A and B

A time-dependent photochemical box model is used to examine the photochemistry of the equatorial and southern subtropical Pacific troposphere with aircraft data obtained during two distinct seasons: the Pacific Exploratory Mission-Tropics A (PEM-Tropics A) field campaign in September and October of 1996 and the Pacific Exploratory Mission-Tropics B (PEM-Tropics B) campaign in March and April of 1999. Model-predicted values were compared to observations for selected species (e.g., NO2, OH, HO2) with generally good agreement. Predicted values of HO2 were larger than those observed in the upper troposphere, in contrast to previous studies which show a general underprediction of HO2 at upper altitudes. Some characteristics of the budgets of HOx, NOx, and peroxides are discussed. The integrated net tendency for O3 is negative over the remote Pacific during both seasons, with gross formation equal to no more than half of the gross destruction. This suggests that a continual supply of O3 into the Pacific region throughout the year must exist in order to maintain O3 levels. Integrated net tendencies for equatorial O3 showed a seasonality, with a net loss of 1.06×1011 molecules cm-2 s-1 during PEM-Tropics B (March) increasing by 50% to 1.60×1011 molecules cm-2 s-1 during PEM-Tropics A (September). The seasonality over the southern subtropical Pacific was somewhat lower, with losses of 1.21×1011 molecules cm-2 s-1 during PEM-Tropics B (March) increasing by 25% to 1.51×1011 molecules cm-2 s-1 during PEM-Tropics A (September). While the larger net losses during PEM-Tropics A were primarily driven by higher concentrations of O3, the ability of the subtropical atmosphere to destroy O3 was ∼30% less effective during the PEM-Tropics A (September) campaign due to a drier atmosphere and higher overhead O3 column amounts. Copyright 2001 by the American Geophysical Union

Observing entrainment mixing, photochemical ozone production, and regional methane emissions by aircraft using a simple mixed-layer framework

In situ flight data from two distinct campaigns during winter and summer seasons in the San Joaquin Valley (SJV) of California are used to calculate boundary-layer entrainment rates, ozone photochemical production rates, and regional methane emissions. Flights near Fresno, California, in January and February 2013 were conducted in concert with the NASA DISCOVER-AQ project. The second campaign (ArvinO3), consisting of 11 days of flights spanning June through September 2013 and 2014, focused on the southern end of the SJV between Bakersfield and the small town of Arvin, California - a region notorious for frequent violations of ozone air quality standards. Entrainment velocities, the parameterized rates at which free tropospheric air is incorporated into the atmospheric boundary layer (ABL), are estimated from a detailed budget of the inversion base height. During the winter campaign near Fresno, we find an average midday entrainment velocity of 1.5g cmg sg'1, and a maximum of 2.4g cmg sg'1. The entrainment velocities derived during the summer months near Bakersfield averaged 3g cmg sg'1 (ranging from 0.9 to 6.5g cmg sg'1), consistent with stronger surface heating in the summer months. Using published data on boundary-layer heights we find that entrainment rates across the Central Valley of California have a bimodal annual distribution peaking in spring and fall when the lower tropospheric stability (LTS) is changing most rapidly.Applying the entrainment velocities to a simple mixed-layer model of three other scalars (O3, CH4, and H2O), we solve for ozone photochemical production rates and find wintertime ozone production (2.8g ±g 0.7g ppbg hg'1) to be about one-third as large as in the summer months (8.2g ±g 3.1g ppbg hg'1). Moreover, the summertime ozone production rates observed above Bakersfield-Arvin exhibit an inverse relationship to a proxy for the volatile organic compound (VOC)g :g NOx ratio (aircraft [CH4] divided by surface [NO2]), consistent with a NOx-limited photochemical environment. A similar budget closure approach is used to derive the regional emissions of methane, yielding 100 (±100)g Ggg yrg'1 for the winter near Fresno and 170 (±125)g Ggg yrg'1 in the summer around Bakersfield. These estimates are 3.6 and 2.4 times larger, respectively, than current state inventories suggest. Finally, by performing a boundary-layer budget for water vapor, surface evapotranspiration rates appear to be consistently g1/4 g 55g % of the reference values reported by the California Irrigation Management Information System (CIMIS) for nearby weather stations

Photochemical production of ozone and emissions of NOx and CH4 in the San Joaquin Valley

Midday summertime flight data collected in the atmospheric boundary layer (ABL) of California's San Joaquin Valley (SJV) are used to investigate the scalar budgets of NOx , O3, and CH4, in order to quantify the individual processes that control near-surface concentrations, yet are difficult to constrain from surface measurements alone: These include, most importantly, horizontal advection and entrainment mixing from above. The setting is a large mountain- valley system with a small aspect ratio, where topography and persistent temperature inversions impose strong restraints on ABL ventilation. In conjunction with the observed time rates of change this airborne budgeting technique enables us to deduce net photochemical ozone production rates and emission fluxes of NOx and CH4. Estimated NOx emissions from our principal flight domain averaged 216 (33) t d-1 over six flights in July and August, which is nearly double the California government's NOx inventory for the surrounding three-county region. We consider several possibilities for this discrepancy, including the influence of wildfires, the temporal bias of the airborne sampling, instrumental interferences, and the recent hypothesis presented by Almaraz et al. (2018) of localized high soil NO emissions from intensive agricultural application of nitrogen fertilizers in the region and find the latter to be the most likely explanation. The methane emission average was 438 Gg yr-1 (143), which also exceeds the emissions inventory for the region by almost a factor of 2. Measured ABL ozone during the six afternoon flights averaged 74 ppb (D 9:8 ppb). The average midafternoon ozone rise of 2.8 ppb h-1 was found to be comprised of-0:8 ppb h-1 due to horizontal advection of lower O3 levels upwind,-2:5 ppb h-1 from dry deposition loss,-0:5 ppb h-1 from dilution by entrainment mixing, and 6.9 ppb h-1 net in situ photochemical production. The O3 production rates exhibited a dependence on NO2 concentrations (r2 D 0:35) and no discernible dependence on methane concentrations (r2 0:02), which are correlated with many of the dominant volatile organic compounds in the region, suggesting that the ozone chemistry was predominantly NOx-limited on the flight days. Additionally, in order to determine the heterogeneity of the different scalars, autocorrelation lengths were calculated for potential temperature (18 km), water vapor (18 km), ozone (30 km), methane (27 km), and NOx (28 km). The spatially diffuse patterns of CH4 and NOx seem to imply a preponderance of broad areal sources rather than localized emissions from cities and/or highway traffic within the SJV

A low-cost system for measuring horizontal winds from single-engine aircraft

The implementation and accuracy of a low-rate (~1 Hz) horizontal wind measurement system is described for a fixed-wing aircraft without modification to the airframe. The system is based on a global positioning system (GPS) compass that provides aircraft heading and a ground-referenced velocity, which, when subtracted from the standard true airspeed, provides estimates of the horizontal wind velocity. A series of tests was performed flying "L"-shaped patterns above the boundary layer, where the winds were assumed to be horizontally homogeneous over the area bounded by the flight (approximately 25km2). Four headings were flown at each altitude at a constant airspeed. Scaling corrections for both heading and airspeed were found by minimizing the variance in the 1-s wind measurements; an upper limit to the error was then computed by calculating the variance of the corrected wind measurements on each of the four headings. A typical uncertainty found in this manner tends to be less than 0.2ms-1. The measurement system described herein is inexpensive and relatively easy to implement on single-engine aircraft. © 2014 American Meteorological Society

Simulating land-atmosphere coupling in the Central Valley, California: Investigating soil moisture impacts on boundary layer properties

Soil moisture links hydrologic and atmospheric processes and impacts important properties of the atmospheric boundary layer via turbulent land-atmosphere exchange. Research on land-atmosphere interactions and their impacts on the simulated boundary layer in semi-arid regions with substantial irrigation is relatively sparse. We use the Weather Research and Forecasting (WRF) model to evaluate the influence different land surface models (LSMs) and planetary boundary layer (PBL) schemes have on the performance of simulations through comparisons with multi-scale observations during a fifteen-day summertime period during 2016. The focus region for this study is the Central Valley (CV), California, which receives little to no rain in the summer and relies on widespread irrigation for agriculture. Results demonstrate that the LSM drives the differences between simulations, showing only minor variations with changing the PBL scheme. Simulations using the RUC (Rapid Update Cycle) and PX-NO (Pleim-Xu without soil moisture and soil temperature nudging) LSMs generated better comparisons with observed PBL depths. Contrasting RUC however, PX-NO better simulates surface fluxes and humidity, whereas Noah (Noah Unified) and Noah-MP (Noah Multiparameterization) simulate better temperatures despite relatively poor surface flux performance. For most quantities, indirect soil nudging in PX (Pleim-Xu) did not improve results compared to PX-NO, which may be related to soil moisture initialization, the nudging dataset, or a need for model improvements in arid regions. Despite these variations in performance statistics across simulations and quantities, we show that potential evapotranspiration (ETo) has robust performance statistics across simulations. This suggests that ETo depends more strongly on net radiation, which performs relatively well across simulations, than on wind, temperature, and humidity, and indicates a further disconnect between ETo and latent heat fluxes in WRF simulations. Finally, we suggest strategies to obtain the necessary observations to better understand the multi-scale dynamics in the CV and drive subsequent model development

Using the tracer flux ratio method with flight measurements to estimate dairy farm CH4 emissions in central California

Tracer flux ratio methodology was applied to airborne measurements to quantify methane (CH4) emissions from two dairy farms in central California during the summer. An aircraft flew around the perimeter of each farm measuring downwind enhancements of CH4 and a tracer species released from the ground at a known rate. Estimates of CH4 emission rates from this analysis were determined for whole sites and major sources within a site (animal housing and liquid manure lagoons). Whole-site CH4 flux rates for each farm, Dairy 1 (6108±821kgCH4dg-1, 95% confidence interval) and Dairy 2 (4018±456kgCH4dg-1, 95% confidence interval), closely resembled findings by established methods: ground-based tracer flux ratio and mass balance. Individual source emission rates indicate a greater fraction of the whole-site emissions come from liquid manure management than animal housing activity, similar to bottom-up estimates. Despite differences in altitude, we observed that the tracer release method gave consistent results when using ground or air platforms.

Intercomparison of lidar, aircraft, and surface ozone measurements in the San Joaquin Valley during the California Baseline Ozone Transport Study (CABOTS)

The California Baseline Ozone Transport Study (CABOTS) was conducted in the late spring and summer of 2016 to investigate the influence of long-range transport and stratospheric intrusions on surface ozone (O3) concentrations in California with emphasis on the San Joaquin Valley (SJV), one of two extreme ozone non-attainment areas in the US. One of the major objectives of CABOTS was to characterize the vertical distribution of O3 and aerosols above the SJV to aid in the identification of elevated transport layers and assess their surface impacts. To this end, the NOAA Earth System Research Laboratory (ESRL) deployed the Tunable Optical Profiler for Aerosol and oZone (TOPAZ) mobile lidar to the Visalia Municipal Airport (36.315 N, 119.392E) in the central SJV between 27 May and 7 August 2016. Here we compare the TOPAZ ozone retrievals with co-located in situ surface measurements and nearby regulatory monitors and also with airborne in situ measurements from the University of California at Davis-Scientific Aviation (SciAv) Mooney and NASA Alpha Jet Atmospheric eXperiment (AJAX) research aircraft. Our analysis shows that the lidar and aircraft measurements agree, on average to within 5ppbv, the sum of their stated uncertainties of 3 and 2ppbv, respectively