Acid deposition and photochemical smog are urban air pollution problems, and they remain localized as long as the sulfur, nitrogen, and hydrocarbon pollutants are confined to the lower troposphere (below about 1-kilometer altitude) where they are short-lived. If, however, the contaminants are rapidly transported to the upper troposphere, then their atmospheric residence times grow and their range of influence expands dramatically. Although this vertical transport ameliorates some of the effects of acid rain by diluting atmospheric acids, it exacerbates global tropospheric ozone production by redistributing the necessary nitrogen catalysts. Results of recent computer simulations suggest that thunderstorms are one means of rapid vertical transport. To test this hypothesis, several research aircraft near a midwestern thunderstrom measured carbon monoxide, hydrocarbons, ozone, and reactive nitrogen compounds. Their concentrations were much greater in the outflow region of the storm, up to 11 kilometers in altitude, than in surrounding air. Trace gas measurements can thus be used to track the motion of air in and around a cloud. Thunderstorms may transform local air pollution problems into regional or global atmospheric chemistry problems.

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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...

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[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

Abstract. We have developed a new nested-grid mercury (Hg) simulation over North America with a 1/2 • latitude by 2/3 • longitude horizontal resolution employing the GEOSChem global chemical transport model. Emissions, chemistry, deposition, and meteorology are self-consistent between the global and nested domains. Compared to the global model (4 • latitude by 5 • longitude), the nested model shows improved skill at capturing the high spatial and temporal variability of Hg wet deposition over North America observed by the Mercury Deposition Network (MDN) in [2008][2009]. The nested simulation resolves features such as higher deposition due to orographic precipitation, land/ocean contrast and and predicts more efficient convective rain scavenging of Hg over the southeast United States. However, the nested model overestimates Hg wet deposition over the Ohio River Valley region (ORV) by 27 %. We modify anthropogenic emission speciation profiles in the US EPA National Emission Inventory (NEI) to account for the rapid inplume reduction of reactive to elemental Hg (IPR simulation). This leads to a decrease in the model bias to −2.3 % over the ORV region. Over the contiguous US, the correlation coefficient (r) between MDN observations and our IPR simulation increases from 0.60 to 0.78. The IPR nested simulation generally reproduces the seasonal cycle in surface concentrations of speciated Hg from the Atmospheric Mercury Network (AMNet) and Canadian Atmospheric Mercury Network (CAMNet). In the IPR simulation, annual mean gaseous and particulate-bound Hg(II) are within 140 % and 11 % of observations, respectively. In contrast, the simulation with unmodified anthropogenic Hg speciation profiles overestimates these observations by factors of 4 and 2 for gaseous and particulate-bound Hg(II), respectively. The nested model shows improved skill at capturing the horizontal variability of Hg observed over California during the ARCTAS aircraft campaign. The nested model suggests that North American anthropogenic emissions account for 10-22 % of Hg wet deposition flux over the US, depending on the anthropogenic emissions speciation profile assumed. The modeled percent contribution can be as high as 60 % near large point sources in ORV. Our results indicate that the North American anthropogenic contribution to dry deposition is 13-20 %.

Photochemical modeling and analysis of field data have been used to evaluate the effects of convective clouds on tropospheric trace gas chemistry. Observations were made during a 1985 field campaign over the rural south-central United States. Meteorological data and measurements of CO, NO, NOy, 03, and hydrocarbons were collected in air surrounding and inside clouds during and immediately following cloud convection. A one-dimensional photochemical model has been used to calculate 03 production potential before and after cloud redistribution of 03 precursor gases. Four distinct types of convective events have been analyzed. Fair weather cumulus clouds increase 03 production in a layer immediately above the boundary layer (to 4 km in the case studied). Outflow from deeper convection can cause enhanced 03 production in the upper troposphere hundreds of kilometers downstream from the clouds. A comparison of trace gas profiles measured in and around a large cumulonimbus during dissipation shows 03 production in the upper troposphere may be increased fourfold by convection relative to undisturbed air. Convective enhancement of 03 production for the entire tropospheric column is about 50%. Compared to nonurban continental regions with no convection, the rate of 03 production potential in air processed by convection is up to 3-4 times greater. Catalysis of 03 production becomes more efficient when NO becomes more dilute after being transported from the boundary layer to the free troposphere. Free tropospheric NO may also be enhanced by lightning, adding to 03 production, particularly when sufficient hydrocarbons are transported to such locations. INTRODUCTION Convective clouds are effective in rapidly transporting trace gases (e.g., CO, NOx, and hydrocarbons) generated in the boundary layer to the free troposphere [Dickerson et al., 1987; Ching and Alkezweeny, 1986; Greenhut, 1986]. Models [e.g., Gidel, 1983; Chatfield and Crutzen, 1984; Costen et al., 1988; Cho et al., 1989] that simulate this vertical transport indicate that convective clouds are capable of transporting and redistributing significant quantities of trace species in the atmosphere. These analyses, however, did not consider the consequences for ozone production resulting from the redistributed trace gases.During convective transport, trace gas concentrations become more dilute through turbulent mixing, and ozone can be produced at a greater rate because NOx catalyzes ozone production more efficiently on a per molecule basis at low concentrations (but greater than 5-10 ppt) than at higher concentrations [Liu et al., 1987]. Therefore photochemical production of ozone can be enhanced during and following convective activity, not only near convection but also in areas downwind of the convective cell. We have reported trace gas observations conducted during and after a series of convective events in June 1985 over rural areas of the south-central United States [Dickerson etPaper number 90JD00777. 0148-0227/90/90JD-00777505.00 Pickering et al., 1988Pickering et...

We perform global-scale inverse modeling to constrain present-day atmospheric mercury emissions and relevant physiochemical parameters in the GEOS-Chem chemical transport model. We use Bayesian inversion methods combining simulations with GEOS-Chem and ground-based Hg-0 observations from regional monitoring networks and individual sites in recent years. Using optimized emissions/parameters, GEOS-Chem better reproduces these ground-based observations and also matches regional over-water Hg-0 and wet deposition measurements. The optimized global mercury emission to the atmosphere is 5.8 Gg yr(-1). The ocean accounts for 3.2 Gg yr(-1) (55 % of the total), and the terrestrial ecosystem is neither a net source nor a net sink of Hg-0. The optimized Asian anthropogenic emission of Hg-0 (gas elemental mercury) is 650-1770 Mg yr(-1), higher than its bottom-up estimates (550-800 Mg yr(-1)). The ocean parameter inversions suggest that dark oxidation of aqueous elemental mercury is faster, and less mercury is removed from the mixed layer through particle sinking, when compared with current simulations. Parameter changes affect the simulated global ocean mercury budget, particularly mass exchange between the mixed layer and subsurface waters. Based on our inversion results, we re-evaluate the long-term global biogeochemical cycle of mercury, and show that legacy mercury becomes more likely to reside in the terrestrial ecosystem than in the ocean. We estimate that primary anthropogenic mercury contributes up to 23 % of present-day atmospheric deposition