We determined the infrared optical constants of nitric acid trihydrate, nitric acid dihydrate, nitric acid monohydrate, and solid amorphous nitric acid solutions which crystallize to form these hydrates. We have also found the infrared optical constants of H2O ice. We measured the transmission of infrared light through thin films of varying thickness over the frequency range from about 7000 to 500 cm−1 at temperatures below 200 K. We developed a theory for the transmission of light through a substrate that has thin films on both sides. We used an iterative Kramers‐Kronig technique to determine the optical constants which gave the best match between measured transmission spectra and those calculated for a variety of films of different thickness. These optical constants should be useful for calculations of the infrared spectrum of polar stratospheric clouds.

[1] We determined microbial decomposition of dissolved organic carbon (DOC) over 3.7 year long dark bioassays of six Swedish lake waters. The overall lost DOC fraction was similar in clearwater lakes (34.8 AE 2.4%) and in brownwater lakes (37.8 AE 1.9%). Reactivity continuum modeling revealed that the most labile DOC fraction, degrading at rates >0.01 d À1 , was larger in the clearwater lakes (11.1 AE 1.2%) than in the brownwater lakes (0.8 AE 0.1%). The initial apparent first-order decay coefficient k was fivefold larger in the clearwater lakes (0.0043 AE 0.0012 d À1 ) than in the brownwater lakes (0.0009 AE 0.0003 d À1). Over time, k decreased more steeply in the clearwater lakes than in the brownwater lakes, reaching the k of the brownwater lakes within 5 months. Finally, k averaged 0.0001 d À1 in both lake categories. In the brownwater lakes, colored dissolved organic matter (CDOM) absorption decayed with an initial k twice as large (0.0018 AE 0.0008 d À1 ) as that of DOC. The initial k was inversely correlated with initial specific UV absorption and CDOM absorption and positively correlated with initial tryptophan-like fluorescence as proxy for autochthonous DOC. Exposure to simulated sunlight at the end of the incubations caused loss of color in the clearwater lakes and loss of DOC in the brownwater lakes, where subsequent mineralization was also stimulated. The DOC lost in the absence of photochemical processes fell below previously reported watershed-scale losses in Sweden by 25% at most. This suggests that a major part of the in situ DOC loss could potentially be attributed to dark reactions alone.

The emissions of carbon dioxide (CO 2 ) from inland waters are substantial on a global scale. Yet the fundamental question remains open which proportion of these CO 2 emissions is induced by sunlight via photochemical mineralization of dissolved organic carbon (DOC), rather than by microbial respiration during DOC decomposition. Also, it is unknown on larger spatial and temporal scales how photochemical mineralization compares to other C fluxes in the inland water C cycle. We combined field and laboratory data with atmospheric radiative transfer modeling to parameterize a photochemical rate model for each day of the year 2009, for 1086 lakes situated between latitudes from 55°N to 69°N in Sweden. The sunlight-induced production of dissolved inorganic carbon (DIC) averaged 3.8 ± 0.04 g C m À2 yr À1 , which is a flux comparable in size to the organic carbon burial in the lake sediments. Countrywide, 151 ± 1 kt C yr À1 was produced by photochemical mineralization, corresponding to about 12% of total annual mean CO 2 emissions from Swedish lakes. With a median depth of 3.2 m, the lakes were generally deep enough that incoming, photochemically active photons were absorbed in the water column. This resulted in a linear positive relationship between DIC photoproduction and the incoming photon flux, which corresponds to the absorbed photons. Therefore, the slope of the regression line represents the wavelength-and depth-integrated apparent quantum yield of DIC photoproduction. We used this relationship to obtain a first estimate of DIC photoproduction in lakes and reservoirs worldwide. Global DIC photoproduction amounted to 13 and 35 Mt C yr À1 under overcast and clear sky, respectively. Consequently, these directly sunlight-induced CO 2 emissions contribute up to about one tenth to the global CO 2 emissions from lakes and reservoirs, corroborating that microbial respiration contributes a substantially larger share than formerly thought, and generate annual C fluxes similar in magnitude to the C burial in natural lake sediments worldwide.

This study combines Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD) measurements of nitric‐acid/ice films representative of type I polar stratospheric clouds (PSCs). Using this combination of techniques, we are able to correlate the FTIR spectra with measurements of the film stoichiometry. The results confirm the assignments for amorphous nitric‐acid/ice films and for crystalline nitric acid trihydrate (NAT), dihydrate, and monohydrate proposed by Ritzhaupt and Devlin (1991). In addition to these films, we observe a new high temperature nitric‐acid/ice film which we attribute to a more stable structure of NAT with fewer defects. When low temperature crystalline NAT is heated, such as during TPD or slow annealing, the IR absorption spectrum irreversibly changes around −88° to −75°C. Future IR absorption measurements of PSCs in the atmosphere should be compared with IR spectra of both amorphous and crystalline nitric acid/ice, including both forms of NAT.

Tropical nitrogen (N) deposition is projected to increase substantially within the coming decades. Increases in soil emissions of the climate-relevant trace gases NO and N 2 O are expected, but few studies address this possibility. We used N addition experiments to achieve N-enriched conditions in contrasting montane and lowland forests and assessed changes in the timing and magnitude of soil N-oxide emissions. We evaluated transitory effects, which occurred immediately after N addition, and long-term effects measured at least 6 weeks after N addition. In the montane forest where stem growth was N limited, the first-time N additions caused rapid increases in soil N-oxide emissions. During the first 2 years of N addition, annual N-oxide emissions were five times (transitory effect) and two times (long-term effect) larger than controls. This contradicts the current assumption that N-limited tropical montane forests will respond to N additions with only small and delayed increases in soil N-oxide emissions. We attribute this fast and large response of soil N-oxide emissions to the presence of an organic layer (a characteristic feature of this forest type) in which nitrification increased substantially following N addition. In the lowland forest where stem growth was neither N nor phosphorus (P) limited, the first-time N additions caused only gradual and minimal increases in soil N-oxide emissions. These first N additions were completed at the beginning of the wet season, and low soil water content may have limited nitrification. In contrast, the 9-and 10-year N-addition plots displayed instantaneous and large soil N-oxide emissions. Annual N-oxide emissions under chronic N addition were seven times (transitory effect) and four times (long-term effect) larger than controls. Seasonal changes in soil water content also caused seasonal changes in soil N-oxide emissions from the 9-and 10-year N-addition plots. This suggests that climate change scenarios, where rainfall quantity and seasonality change, will alter the relative importance of soil NO and N 2 O emissions from tropical forests exposed to elevated N deposition.