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...