Abstract. Nitrous Acid (HONO) plays an important role in tropospheric chemistry as a precursor of the hydroxyl radical (OH), the most important oxidizing agent in the atmosphere. Nevertheless, the formation mechanisms of HONO are still not completely understood. Recent field observations found unexpectedly high daytime HONO concentrations in both urban and rural areas, which point to unrecognized, most likely photolytically enhanced HONO sources. Several gas-phase, aerosol, and ground surface chemistry mechanisms have been proposed to explain elevated daytime HONO, but atmospheric evidence to favor one over the others is still weak. New information on whether HONO formation occurs in the gas-phase, on aerosol, or at the ground may be derived from observations of the vertical distribution of HONO and its precursor nitrogen dioxide, NO 2 , as well as from its dependence on solar irradiance or actinic flux.Here we present field observations of HONO, NO 2 and other trace gases in three altitude intervals (30-70 m, 70-130 m and 130-300 m) using UCLA's long path DOAS instrument, as well as in situ measurements of OH, NO, photolysis frequencies and solar irradiance, made in Houston, TX, during the Study of Houston Atmospheric Radical Precursor (SHARP) experiment from 20 April to 30 May 2009. The observed HONO mixing ratios were often ten times larger than the expected photostationary state with OH and NO. Larger HONO mixing ratios observed near the ground than aloft imply, but do not clearly prove, that the daytime source of HONO was located at or near the ground. Using a pseudo steady-state (PSS) approach, we calculated the missing daytime HONO formation rates, P unknown , on four sunny days. The NO 2 -normalized P unknown , P norm , showed a clear symmetrical diurnal variation with a maximum around noontime, which was well correlated with actinic flux (NO 2 photolysis frequency) and solar irradiance. This behavior, which was found on all clear days in Houston, is a strong indication of a photolytic HONO source.[HONO]/[NO 2 ] ratios also showed a clear diurnal profile, with maxima of 2-3 % around noon. PSS calculations show that this behavior cannot be explained by the proposed gas-phase reaction of photoexcited NO 2 (NO * 2 ) or any other gas-phase or aerosol photolytic process occurring at similar or longer wavelengths than that of HONO photolysis. HONO formation by aerosol nitrate photolysis in the UV also seems to be unlikely. P norm correlated better with solar irradiance (average R 2 = 0.85/0.87 for visible/UV) than with actinic flux (R 2 = 0.76) on the four sunny days, clearly pointing to HONO being formed at the ground rather than on the aerosol or in the gas-phase. In addition, the observed [HONO]/[NO 2 ] diurnal variation can be explained if the formation of HONO depends on solar irradiance, but not if it depends on the actinic flux. The vertical mixing ratio profiles, together with the stronger correlation with solar irradiance, support the idea that photolytically enhanced NO 2 to HONO conversion on the gr...

Abstract. Megacities are major sources of anthropogenic fossil fuel CO 2 (FFCO 2 ) emissions. The spatial extents of these large urban systems cover areas of 10 000 km 2 or more with complex topography and changing landscapes. We present a high-resolution land-atmosphere modelling system for urban CO 2 emissions over the Los Angeles (LA) megacity area. The Weather Research and Forecasting (WRF)-Chem model was coupled to a very high-resolution FFCO 2 emission product, Hestia-LA, to simulate atmospheric CO 2 concentrations across the LA megacity at spatial resolutions as fine as ∼ 1 km. We evaluated multiple WRF configurations, selecting one that minimized errors in wind speed, wind direction, and boundary layer height as evaluated by its performance against meteorological data collected during the CalNex-LA campaign (May-June 2010). Our results show no significant difference between moderate-resolution (4 km) and high-resolution (1.3 km) simulations when evaluated against surface meteorological data, but the highresolution configurations better resolved planetary boundary layer heights and vertical gradients in the horizontal mean winds. We coupled our WRF configuration with the Vulcan 2.2 (10 km resolution) and Hestia-LA (1.3 km resolution) fossil fuel CO 2 emission products to evaluate the impact of the spatial resolution of the CO 2 emission products and the meteorological transport model on the representation of spatiotemporal variability in simulated atmospheric CO 2 concentrations. We find that high spatial resolution in the fossil fuel CO 2 emissions is more important than in the atmospheric model to capture CO 2 concentration variability across the LA megacity. Finally, we present a novel approach that employs simultaneous correlations of the simulated atmospheric CO 2 fields to qualitatively evaluate the greenhouse gas measurement network over the LA megacity. Spatial correlations in the atmospheric CO 2 fields reflect the coverage of individual measurement sites when a statistically significant number of sites observe emissions from a specific source or location. We conclude that elevated atmospheric CO 2 concentrations over the LA megacity are composed of multiple fine-scale plumes rather than a single homogenous urban dome. Furthermore, we conclude that FFCO 2 emissions monitoring in the LA megacity requires FFCO 2 emissions modelling with ∼ 1 km resolution becausePublished by Copernicus Publications on behalf of the European Geosciences Union. 9020 S. Feng et al.: LA megacity GHG modelling system coarser-resolution emissions modelling tends to overestimate the observational constraints on the emissions estimates.

Nitrous acid (HONO) often plays an important role in tropospheric photochemistry as a major precursor of the hydroxyl radical (OH) in early morning hours and potentially during the day. However, the processes leading to formation of HONO and its vertical distribution at night, which can have a considerable impact on daytime ozone formation, are currently poorly characterized by observations and models. Long-path differential optical absorption spectroscopy (LP-DOAS) measurements of HONO during the 2006 TexAQS II Radical and Aerosol Measurement Project (TRAMP), near downtown Houston, TX, show nocturnal vertical profiles of HONO, with mixing ratios of up to 2.2 ppb near the surface and below 100 ppt aloft. Three nighttime periods of HONO, NO₂ and O₃ observations during TRAMP were used to perform model simulations of vertical mixing ratio profiles. By adjusting vertical mixing and NO_x emissions the modeled NO₂ and O₃ mixing ratios showed very good agreement with the observations. <br><br> Using a simple conversion of NO₂ to HONO on the ground, direct HONO emissions, as well as HONO loss at the ground and on aerosol, the observed HONO profiles were reproduced by the model for 1–2 and 7–8 September in the nocturnal boundary layer (NBL). The unobserved increase of HONO to NO₂ ratio (HONO/NO₂) with altitude that was simulated by the initial model runs was found to be due to HONO uptake being too small on aerosol and too large on the ground. Refined model runs, with adjusted HONO uptake coefficients, showed much better agreement of HONO and HONO/NO₂ for two typical nights, except during morning rush hour, when other HONO formation pathways are most likely active. One of the nights analyzed showed an increase of HONO mixing ratios together with decreasing NO₂ mixing ratios that the model was unable to reproduce, most likely due to the impact of weak precipitation during this night. <br><br> HONO formation and removal rates averaged over the lowest 300 m of the atmosphere showed that NO₂ to HONO conversion on the ground was the dominant source of HONO, followed by traffic emission. Aerosol did not play an important role in HONO formation. Although ground deposition was also a major removal pathway of HONO, net HONO production at the ground was the main source of HONO in our model studies. Sensitivity studies showed that in the stable NBL, net HONO production at the ground tends to increase with faster vertical mixing and stronger NO_x emission. Vertical transport was found to be the dominant source of HONO aloft

[1] Ozone (O 3 ) and secondary fine particles come from the atmospheric oxidation chemistry that involves the hydroxyl radical (OH) and hydroperoxyl radical (HO 2 ), which are together called HO x . Radical precursors such as nitrous acid (HONO) and formaldehyde (HCHO) significantly affect the HO x budget in urban environments. These chemical processes connect surface anthropogenic and natural emissions to local and regional air pollution. Using the data collected during the Study of Houston Atmospheric Radical Precursors (SHARP) in spring 2009, we examine atmospheric oxidation chemistry and O 3 production in this polluted urban environment. A numerical box model with five different chemical mechanisms was used to simulate the oxidation processes and thus OH and HO 2 in this study. In general, the model reproduced the measured OH and HO 2 with all five chemical mechanisms producing similar levels of OH and HO 2 , although midday OH was overpredicted and nighttime OH and HO 2 were underpredicted. The calculated HO x production was dominated by HONO photolysis in the early morning and by the photolysis of O 3 and oxygenated volatile organic compounds (OVOCs) in the midday. On average, the daily HO x production rate was 24.6 ppbv d À1 , of which 30% was from O 3 photolysis, 22% from HONO photolysis, 15% from the photolysis of OVOCs (other than HCHO), 14% from HCHO photolysis, and 13% from O 3 reactions with alkenes. The O 3 production was sensitive to volatile organic compounds (VOCs) in the early morning but was sensitive to NO x for most of afternoon. This is similar to the behavior observed in two previous summertime studies in Houston