Christine Haman

Vertically Resolved Measurements of Nighttime Radical Reservoirs in Los Angeles and Their Contribution to the Urban Radical Budget

Photolabile nighttime radical reservoirs, such as nitrous acid (HONO) and nitryl chloride (ClNO(2)), contribute to the oxidizing potential of the atmosphere, particularly in early morning. We present the first vertically resolved measurements of ClNO(2), together with vertically resolved measurements of HONO. These measurements were acquired during the California Nexus (CalNex) campaign in the Los Angeles basin in spring 2010. Average profiles of ClNO(2) exhibited no significant dependence on height within the boundary layer and residual layer, although individual vertical profiles did show variability. By contrast, nitrous acid was strongly enhanced near the ground surface with much smaller concentrations aloft. These observations are consistent with a ClNO(2) source from aerosol uptake of N(2)O(5) throughout the boundary layer and a HONO source from dry deposition of NO(2) to the ground surface and subsequent chemical conversion. At ground level, daytime radical formation calculated from nighttime-accumulated HONO and ClNO(2) was approximately equal. Incorporating the different vertical distributions by integrating through the boundary and residual layers demonstrated that nighttime-accumulated ClNO(2) produced nine times as many radicals as nighttime-accumulated HONO. A comprehensive radical budget at ground level demonstrated that nighttime radical reservoirs accounted for 8% of total radicals formed and that they were the dominant radical source between sunrise and 09:00 Pacific daylight time (PDT). These data show that vertical gradients of radical precursors should be taken into account in radical budgets, particularly with respect to HONO.

Seasonal Variability in the Diurnal Evolution of the Boundary Layer in a Near-Coastal Urban Environment

AbstractBoundary layer height is estimated during a 21-month period in Houston, Texas, using continuous ceilometer observations and the minimum-gradient method. A comparison with over 60 radiosondes indicates overall agreement between ceilometer- and radiosonde-estimated PBL and residual layer heights. Additionally, the ceilometer-estimated PBL heights agree well with 31 vertical profiles of ozone. Difficulty detecting the PBL height occurs immediately following a frontal system with precipitation, during periods with high wind speeds, and in the early evening when convection is weakening, a new stable surface layer is forming, and the lofted aerosols detected by the lidar do not represent the PBL. Long-term diurnal observations of the PBL height indicate nocturnal PBL heights range from approximately 100 to 300 m throughout the year, while the convective PBL displays more seasonal and daily variability typically ranging from 1100 m in the winter to 2000 m in the summer.

Nocturnal loss of NOx during the 2010 CalNex-LA study in the Los Angeles Basin

The chemical removal of NOx at night in urban areas remains poorly constrained due to uncertainties in the contribution of various loss pathways and the impact of the suppressed nocturnal vertical mixing. Here we present long-path differential optical absorption spectroscopy observations of nocturnal vertical concentration profiles of O3, NO2, and NO3 in the lower atmosphere (33–556 m above ground level) measured during the CalNex-LA 2010 study. Positive nocturnal vertical gradients of O3 and NO3 and negative gradients of NO2 were observed during the night. Relatively short lifetime of nocturnal NO3 (less than 1000 s) and high nighttime steady state N2O5 mixing ratios (up to 2 ppb) indicated active nocturnal chemistry during CalNex. Comparison of modeled and observed altitude-resolved NO3 loss frequencies shows that hydrolysis of N2O5 on aerosols was the dominant loss pathway of NO3 and NOx. Based on this argument, the nocturnal loss rates of NOx, L(NOx), at different altitudes and averaged over the lowest 550 m of the atmosphere were calculated. The nocturnally averaged L(NOx) ranged between 0.8 and 1.3 ppb h−1 for the lower atmosphere with the L(NOx) for the first 8 days at about 1 ppb h−1. This number is close to the one previously determined in Houston in 2009 of ~0.9 ppb h−1. Comparisons between daytime NOx loss due to the OH + NO2 reaction and nighttime L(NOx) show that during CalNex, nocturnal chemistry contributed an average of 60% to the removal of NOx in a 24 h period in the lower atmosphere.

Relationship between boundary layer heights and growth rates with ground‐level ozone in Houston, Texas

Measurements and predictions of ambient ozone (O3), planetary boundary layer (PBL) height, the surface energy budget, wind speed, and other meteorological parameters were made near downtown Houston, Texas, and were used to investigate meteorological controls on elevated levels of ground-level O3. Days during the study period (1 April 2009 to 31 December 2010 for measurements and 15 April 2009 to 17 October 2009 for modeled) were classified into low (LO3) and high ozone (HO3) days. The majority of observed high HO3 days occurred in a postfrontal environment. Observations showed there is not a significant difference in daily maximum PBL heights on HO3 and LO3 days. Modeling results showed large differences between maximum PBL heights on HO3 and LO3 days. Nighttime and early morning observed and modeled PBL heights are consistently lower on HO3 days than on LO3 days. The observed spring LO3 days had the most rapid early morning PBL growth (~350 m h−1) while the fall HO3 group had the slowest (~200 m h−1). The predicted maximum average hourly morning PBL growth rates were greater on HO3 (624 m h−1) days than LO3 days (361 m h−1). Observed turbulent mixing parameters were up to 2–3 times weaker on HO3 days, which indicate large-scale subsidence associated with high-pressure systems (leading to clear skies and weak winds) substantially suppresses mixing. Lower surface layer ventilation coefficients were present in the morning on HO3 days in the spring and fall, which promotes the accumulation of O3 precursors near the surface.

Inorganic and black carbon aerosols in the Los Angeles Basin during CalNex

We evaluate predictions from the Community Multiscale Air Quality (CMAQ version 4.7.1) model against a suite of airborne and ground-based meteorological measurements, gas- and aerosol-phase inorganic measurements, and black carbon (BC) measurements over Southern California during the CalNex field campaign in May/June 2010. Ground-based measurements are from the CalNex Pasadena ground site, and airborne measurements took place onboard the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Navy Twin Otter and the NOAA WP-3D aircraft. BC predictions are in general agreement with observations at the Pasadena ground site and onboard the WP-3D, but are consistently overpredicted when compared to Twin Otter measurements. Adjustments to predicted inorganic mass concentrations, based on predicted aerosol size distributions and the AMS transmission efficiency, are shown to be significant. Owing to recent shipping emission reductions, the dominant source of sulfate in the L.A. Basin may now be long-range transport. Sensitivity studies suggest that severely underestimated ammonia emissions, and not the exclusion of crustal species (Ca^(2 +), K^+, and Mg^(2 +)), are the single largest contributor to measurement/model disagreement in the eastern part of the L.A. Basin. Despite overstated NO_x emissions, total nitrate concentrations are underpredicted, which suggests a missing source of HNO_3 and/or overprediction of deposition rates. Adding gas-phase NH_3 measurements and size-resolved measurements, up to 10 μm, of nitrate and various cations (e.g. Na^+, Ca^(2 +), K^+) to routine monitoring stations in the L.A. Basin would greatly facilitate interpreting day-to-day fluctuations in fine and coarse inorganic aerosol.