Abstract. Agricultural soils are the most important anthropogenic source of nitrous oxide to the atmosphere. We observed large shifts with time in the emission rate (from 170 to 16 ng N cm '2 h '•) and in/Sl•lxl of N20 emitted (from -46%o to +5%o relative to atmospheric N2) from a ureafertilized and irrigated agricultural field in Mexico. We calculated overall instantaneous enrichment factors for the sampling period, which suggest that the microbial N20 production shifts from nitrification (week 1) to denitrification (week 2). Isotopic signatures of N20 emissions were not always in accord with other proxies (such as NOfN20 emission ratio or water-filled pore space) Used to estimate the relative importance of nitrification and denitrification as N20 sources. These observations strongly suggest that the soil surface emissions integrate processes occurring at different depths in the soil and a decoupling of NO and N20 production in this system.

Measurements of hydroxyl (OH) and hydroperoxy (HO 2 ) radicals were made during the Mexico City Metropolitan Area (MCMA) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project during March 2006. These measurements provide a unique opportunity to test current models of atmospheric RO x (OH + HO 2 + RO 2 ) photochemistry under polluted conditions. A zero-dimensional box model based on the Regional Atmospheric Chemical Mechanism (RACM) was constrained by 10-min averages of 24 J -values and the concentrations of 97 chemical species. Several issues related to the RO x chemistry under polluted conditions Correspondence to: S. Dusanter (sdusante@indiana.edu) are highlighted in this study: (i) Measured concentrations of both OH and HO 2 were underpredicted during morning hours on a median campaign basis, suggesting a significant source of radicals is missing from current atmospheric models under polluted conditions, consistent with previous urban field campaigns. (ii) The model-predicted HO 2 /OH ratios underestimate the measurements for NO mixing ratios higher than 5 ppb, also consistent with previous urban field campaigns. This suggests that under high NO x conditions, the HO 2 to OH propagation rate may be overestimated by the model or a process converting OH into HO 2 may be missing from the chemical mechanism. On a daily basis (08:40 a.m.-06:40 p.m.), an analysis of the radical budget indicates that HONO photolysis, HCHO photolysis, O 3 -alkene reactions and dicarbonyls photolysis are the main radical sources. O 3 Published by Copernicus Publications on behalf of the European Geosciences Union. 6656 S. Dusanter et al.: HO x model/measurement comparison for MCMA-2006photolysis contributes to less than 6% of the total radical production.

The detailed molecular composition of laboratory generated limonene ozonolysis secondary organic aerosol (SOA) was studied using ultrahigh-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Approximately 1200 molecular formulas were identified in the SOA over the mass range of 140 to 850 Da. Four characteristic groups of high relative abundance species were observed; they indicate an array of accretion products that retain a large fraction of the limonene skeleton. The identified molecular formulas of each of the groups are related to one another by CH₂, O and CH₂O homologous series. The CH₂ and O homologous series of the low molecular weight (MW) SOA (<i>m/z</i> < 300) are explained with a combination of functionalization and fragmentation of radical intermediates and reactive uptake of gas-phase carbonyls. They include isomerization and elimination reactions of Criegee radicals, reactions between alkyl peroxy radicals, and scission of alkoxy radicals resulting from the Criegee radicals. The presence of compounds with 10–15 carbon atoms in the first group (e.g. C₁₁H₁₈O₆) provides evidence for SOA formation by the reactive uptake of gas-phase carbonyls during limonene ozonolysis. The high MW compounds (<i>m/z</i> > 300) were found to constitute a significant number fraction of the identified SOA components. The formation of high MW compounds was evaluated by molecular formula trends, fragmentation analysis of select high MW compounds and a comprehensive reaction matrix including the identified low MW SOA, hydroperoxides and Criegee radicals as building blocks. Although the formation of high MW SOA may occur via a variety of radical and non-radical reaction channels, the combined approach indicates a greater importance of the non-condensation reactions over aldol and ester condensation reaction channels. Among these hemi-acetal reactions appear to be most dominant followed by hydroperoxide and Criegee reaction channels

Abstract. A simple box model is developed which accounts for the natural and anthropogenic sources and sinks of tropospheric nitrous oxide. Stable isotopic terms are included as well to investigate what additional insight they might provide concerning the overall picture of the global nitrous oxide budget. It is seen that fractionation associated with ultraviolet photolysis in the stratosphere plays a key role in balancing the isotopic budget. It is also noted that increased production due to human influence should have a unique isotopic signature which should provide observable differences between preindustrial air trapped in polar tim or ice and modem air. This build-up of isotopically light, anthropogenic nitrous oxide can be thought of as an N20 "Suess effect" and should be observable in time series measurements of clean baseline atmospheric samples. High-precision records of such changes could lead to a better estimate of the preindustrial N20 isotopic signature as well as provide insight into the pathways of the anthropogenic sources.

Recent studies showed that photochemical breakdown (photodegradation) of plant material accounts for a substantial portion of litter decomposition and subsequent trace gas release in ecosystems under high radiative load and low precipitation. In the absence of solar radiation, thermal degradation may also cause trace gas release at temperatures below the ignition point. These observations suggest that the abiotic processes of photodegradation and thermal degradation of plant litter may be important in understanding global trace gas budgets. In a laboratory incubation study, we performed a simultaneous carbon (C) accounting of CO 2 , CO, and CH 4 produced as a byproduct of photodegradation and thermal degradation of six different plant litter types that varied in chemical composition. The patterns of trace gas release during photodegradation and thermal degradation differed considerably across the six plant materials, suggesting that chemical composition of litter may influence the rates of abiotic degradation. There was a strong positive correlation between the rates of trace gas release during photodegradation and temperature. A significant portion of trace gases were produced during low temperature (< 100°C) thermal degradation of litter in the absence of solar radiation, which was also positively correlated to temperature. In addition, both thermal degradation and photodegradation occurred in the absence of O 2 . This indicates that the mechanism formerly accepted as photo-oxidation may only be one of several photodegradation processes. We speculate that the direct breakdown of chemical groups such as carboxyl, carbonyl, and methoxyl groups may result in CO 2 , CO, and CH 4 release. We suggest that the combined processes of thermal and photodegradation of litter may be a previously under accounted source of Cbased trace gases from terrestrial systems.