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.

We investigate the role of the spatial absorption profile within bulk heterojunction small molecule solar cells comprising a 50 nm ZnPc:C60 active layer. Exploiting interference effects the absorption profile is varied by both the illumination wavelength and the thickness of an optical spacer layer adjacent to the reflecting electrode. The fill factor under 1 sun illumination is observed to change from 43 to 49% depending on the absorption profile which approximately equals the charge‐carrier generation profile. It is shown by varying the mixing ratio between ZnPc and C60 that the importance of the generation profile is correlated with the imbalance of mobilities. Therefore, it is concluded that non‐geminate recombination is the dominating loss mechanism in these devices. Numerical drift‐diffusion simulations reproduce the experimental observations showing that charge carrier extraction is more efficient if charge carriers are generated close to the contact collecting the less mobile charge carrier type. Furthermore, this effect can explain the dependence of the internal quantum efficiency measured at short circuit on wavelength and implies that the spectral mismatch for a given solar simulator and device depends on the applied voltage.

We present a novel parameterization method to convert multi-axis differential optical absorption spectroscopy (MAX-DOAS) differential slant column densities (dSCDs) into near-surface box-averaged volume mixing ratios. The approach is applicable inside the planetary boundary layer under conditions with significant aerosol load, and builds on the increased sensitivity of MAX-DOAS near the instrument altitude. It parameterizes radiative transfer model calculations and significantly reduces the computational effort, while retrieving ~ 1 degree of freedom. The biggest benefit of this method is that the retrieval of an aerosol profile, which usually is necessary for deriving a trace gas concentration from MAX-DOAS dSCDs, is not needed.

The method is applied to NO₂ MAX-DOAS dSCDs recorded during the Mexico City Metropolitan Area 2006 (MCMA-2006) measurement campaign. The retrieved volume mixing ratios of two elevation angles (1° and 3°) are compared to volume mixing ratios measured by two long-path (LP)-DOAS instruments located at the same site. Measurements are found to agree well during times when vertical mixing is expected to be strong. However, inhomogeneities in the air mass above Mexico City can be detected by exploiting the different horizontal and vertical dimensions probed by the MAX-DOAS and LP-DOAS instruments. In particular, a vertical gradient in NO₂ close to the ground can be observed in the afternoon, and is attributed to reduced mixing coupled with near-surface emission inside street canyons. The existence of a vertical gradient in the lower 250 m during parts of the day shows the general challenge of sampling the boundary layer in a representative way, and emphasizes the need of vertically resolved measurements

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