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Field observations show daytime HONO levels in urban, rural and remote environments are greater than those expected at photostationary state, that is, balance between production by \(NO+OH\) reaction and loss by UV-photolysis and OH-oxidation. Studies have interpreted measurements of \([HONO]_{Obs} > [HONO]_{PSS}\) - or equivalently, the rate of HONO loss exceeding that of production - as evidence of a missing, sunlight-driven HONO source. Formation rate inferred from assuming photostationarity indicate a significant source of HONO, which photolyzes to yield OH. Moreover, depending on the mechanism, it may represent a pathway by which deposited nitrogen oxides are repartitioned back into the atmosphere in reactive form. The accumulation of HONO beneath the nocturnal boundary layer initiates photochemistry in the early morning prior to other \(HO_x\) precursors. Previous studies have estimated nighttime HONO production rate by attributing the increase in \(HONO:NO_x\) solely to heterogeneous HONO formation, while treating \(NO_x\) as an invariant. Moreover, because ambient \(HONO:NO_x\) exceed what is observed in automobile exhaust, combustion sources are discounted. In May of 2009, we observed HONO and \(NO_2\) mixing ratios in Houston, Texas during the SHARP campaign. We demonstrate – using a chemical box model – that photostationary state during daytime is not fully established. The reaction/transport time since emission from automobiles is short relative to the lifetime of HONO. The result of assuming PSS is a drastic over-estimation of the magnitude of the so-called missing HONO source. At night, we show that \(NO_x\) oxidation and emission are significant, thus, \(NO_x\) cannot be treated as a conservative tracer to infer secondary HONO production. Nearly-continuous observations of HONO and \(NO_2\) at Harvard Forest from December 2010 to December 2011 reveal daytime HONO levels that are comparable to what is expected from just known chemistry and much lower than has been reported in similar environments by different measurement techniques. Moreover, HONO fluxes are always below detection limit, indicating daytime HONO production contributes negligibly to the \(HO_x\) and \(NO_x\) budgets of the overlying atmosphere at Harvard Forest. Nighttime HONO enhancement is observed, but high night-to-night variability in \(HONO:NO_2\) that is not reasonably explained by the trends in HONO and \(NO_2\) fluxes, suggest a non-\(NO_2\), non-ground/canopy-surface related HONO source. / Engineering and Applied Sciences

<p> Methane (CH₄) is one of the most important greenhouse gases after water vapor and carbon dioxide due to its high concentration and global warming potential 25 times than that of CO₂(based on a 100 year time horizon). Its atmospheric concentration has more than doubled from the preindustrial era due to anthropogenic activities such as rice cultivation, biomass burning, and fossil fuel production. However, the rate of increase of atmospheric CH₄ (or the growth rate) slowed from 1980 until present. The main reason for this trend is a slowdown in the trend of CH₄sources. Measuring stable isotopes of atmospheric CH₄ can constrain changes of CH₄sources. The main goal of this work is to interpret the CH₄ trend from 1978-2010 in terms of its sources using measurements of CH₄ mixing ratio and its isotopes. </p><p> The current work presents the measurements and analysis of CH₄ and its isotopes (&delta;¹³C and &delta;D) of four air archive sample sets collected by the Oregon Graduate Institute (OGI). CH₄ isotope ratios (&delta;¹³C and &delta;D) were measured by a continuous flow isotope ratio mass spectrometer technique developed at PSU. The first set is for Cape Meares, Oregon which is the oldest and longest set and spans 1977-1999. The integrity of this sample set was evaluated by comparing between our measured CH₄ mixing ratio values with those measured values by OGI and was found to be stable. Resulting CH₄ seasonal cycle was evaluated from the Cape Meares data. The CH₄ seasonal cycle shows a broad maximum during October-April and a minimum between July and August. The seasonal cycles of &delta;¹³C and &delta;D have maximum values in May for &delta;¹³C and in July for &delta;D and minimum values between September-October for &delta;¹³C and in October for &delta;D. These results indicate a CH₄ source that is more enriched January-May (e.g. biomass burning) and a source that is more depleted August-October (e.g. microbial). In addition to Cape Meares, air archive sets were analyzed from: South Pole (SPO), Samoa (SMO), Mauna Loa (MLO) 1992-1996. The presented &delta;D measurements are unique measured values during these time periods at these stations. </p><p> To obtain the long-term in isotopic CH₄ from 1978-2010, other datasets of Northern Hemisphere mid-latitude sites are included with Cape Meares. These sites are Olympic Peninsula, Washington; Monta&ntilde;a de Oro, California; and Niwot Ridge, Colorado. The seasonal cycles of CH₄ and its isotopes from the composite dataset have the same phase and amplitudes as the Cape Meares site. CH₄ growth rate shows a decrease over time 1978-2010 with three main spikes in 1992, 1998, and 2003 consistent with the literature from the global trend. CH₄ lifetime is estimated to 9.7 yrs. The &delta;¹³C trend in the composite data shows a slow increase from 1978-1987, a more rapid rate of change 1987-2005, and a gradual depletion during 2005-2010. The &delta;D trend in the composite data shows a gradual increase during 1978-2001 and decrease from 2001-2005. From these results, the global CH₄ emissions are estimated and show a leveling off sources 1982-2010 with two large peak anomalies in 1998 and 2003. The global average &delta;¹³C and &delta;D of CH₄ sources are estimated from measured values. The results of these calculations indicate that there is more than one source which controls the decrease in the global CH4 trend. From 1982-2001, &delta;¹³C and &delta;D of CH₄ sources becomes more depleted due to a decrease in fossil and/or biomass burning sources relative to microbial sources. From 2005-2010, &delta;¹³C of CH₄ sources returns to its 1981 value. There are two significant peaks in &delta;¹³C and &delta;D of CH₄ sources in 1998 and 2003 due to the wildfire emissions in boreal areas and in Europe.</p>

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