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Abstract. Nitrous acid (HONO) fluxes were measured above an agricultural field site near Paris during different seasons. Above bare soil, different crops were measured using the aerodynamic gradient (AG) method. Two LOPAPs (LOng Path Absorption Photometer) were used to determine the HONO gradients between two heights. During daytime mainly positive HONO fluxes were observed, which showed strong correlation with the product of the NO 2 concentration and the long wavelength UV light intensity, expressed by the photolysis frequency J (NO 2 ). These results are consistent with HONO formation by photosensitized heterogeneous conversion of NO 2 on soil surfaces as observed in recent laboratory studies. An additional influence of the soil temperature on the HONO flux can be explained by the temperature-dependent HONO adsorption on the soil surface. A parameterization of the HONO flux at this location with NO 2 concentration, J (NO 2 ), soil temperature and humidity fits reasonably well all flux observations at this location.

Croplands mainly act as net sources of the greenhouse gases carbon dioxide (CO₂) and nitrous oxide (N₂O), as well as nitrogen oxide (NO), a precursor of troposheric ozone. We determined the carbon (C) and nitrogen (N) balance of a four-year crop rotation, including maize, wheat, barley and mustard, to provide a base for exploring mitigation options of net emissions. The crop rotation had a positive net ecosystem production (NEP) of 4.4 +/- 0.7 Mg C ha(-1) y(-1) but represented a net source of carbon with a net biome production (NBP) of -1.3 +/- 1.1 Mg C ha(-1) y(-1). The nitrogen balance of the rotation was correlated with the carbon balance and resulted in net loss (-24 +/- 28 kg N ha(-1) y(-1)). The main nitrogen losses were nitrate leaching (-11.7 +/- 1.0 kg N ha(-1) y(-1)) and ammonia volatilization (-9 kg N ha(-1) y(-1)). Dry and wet depositions were 6.7 +/- 3.0 and 5.9 +/- 0.1 kg N ha(-1) y(-1), respectively. Fluxes of nitrous (N₂O) and nitric (NO) oxides did not contribute significantly to the N budget (N₂O: -1.8 +/- 0.04; NO: -0.7 +/- 0.04 kg N ha(-1) y(-1)) but N₂O fluxes equaled 16% of the total greenhouse gas balance. The link between the carbon and nitrogen balances are discussed. Longer term experiments would be necessary to capture the trends in the carbon and nitrogen budgets within the variability of agricultural ecosystems

Abstract. Terrestrial ecosystems represent a major sink for ozone (O 3 ) and also a critical control of tropospheric O 3 budget. However, due to its deleterious effects, plant functioning is affected by the ozone absorbed. It is thus necessary to both predict total ozone deposition to ecosystems and partition the fluxes in stomatal and non-stomatal pathways. The Surfatm-O 3 model was developed to predict ozone deposition to agroecosystems from sowing to harvest, taking into account each deposition pathways during bare soil, growth, maturity, and senescence periods. An additional sink was added during senescence: stomatal deposition for yellow leaves, not able to photosynthesise but transpiring. The model was confronted to measurements performed over three maize crops in different regions of France. Modelled and measured fluxes agreed well for one dataset for any phenological stage, with only 4 % difference over the whole cropping season. A larger discrepancy was found for the two other sites, 15 % and 18 % over the entire study period, especially during bare soil, early growth and senescence. This was attributed to site-specific soil resistance to ozone and possible chemical reactions between ozone and volatile organic compounds emitted during late senescence. Considering both night-time and daytime conditions, non-stomatal deposition was the major ozone sink, from 100 % during bare soil period to 70-80 % on average during maturity. However, considering only daytime conditions, especially under optimal climatic conditions for plant functioning, stomatal flux could represent 75 % of total ozone flux. This model could improve estimates of crop yield losses and projections of tropospheric ozone budget.