Gas phase UV absorption spectra for a series of alkyl sulfides

Absorption cross-sections have been measured for a series of alkyl sulfides between 209.0 and 235.0nm. Measurements are reported for dimethylsulfide ((CH3)2S,h6-DMS) and its deuterated isotopomer ((CD3)2S,d6-DMS), methylethylsulfide (CH3SC2H5, MES), diethylsulfide ((C2H5)2S, DES), dipropylsulfide ((C3H7)2S, DPS) and dibutylsulfide ((C4H10)2S, DBS)

Loss of isoprene and sources of nighttime OH radicals at a rural site in the U.S.: Results from photochemical models

A one-dimensional Lagrangian model for atmospheric transport and photochemistry has been developed and used to interpret measurements made at Pellston, Michigan, during the summer of 1998. The model represents a moving vertical column of air with vertical resolution of 25 m near the ground. Calculations have been performed for a series of trajectories, with representation of emissions, vertical mixing, and photochemistry for a 3-day period ending with the arrival of the air column at Pellston. Results have been used to identify causes of the observed decrease in isoprene at night, to investigate causes of high nighttime OH. Significant OH can be generated at night by terpenes if it is assumed that some fast-reacting monoterpenes are emitted at rates comparable to inventory emissions for terpenes. However, this nighttime OH is confined to a shallow surface layer (0–25 m) and has little impact on nighttime chemistry. The observed decrease in isoprene at night can be reproduced in models with low OH, and is attributed primarily to vertical dilution. There is also evidence that transport from Lake Michigan contributes to low nighttime isoprene at Pellston. Model results compare well with measured isoprene, NOx, and with isoprene vertical profiles. Significant model-measurement discrepancies are found for OH, HO2, methylvinylketone, and formaldehyde

BrO and inferred Bry profiles over the western Pacific: Relevance of inorganic bromine sources and a Bry minimum in the aged tropical tropopause layer

We report measurements of bromine monoxide (BrO) and use an observationally constrained chemical box model to infer total gas-phase inorganic bromine (Br) over the tropical western Pacific Ocean (tWPO) during the CONTRAST field campaign (January-February 2014). The observed BrO and inferred Br profiles peak in the marine boundary layer (MBL), suggesting the need for a bromine source from sea-salt aerosol (SSA), in addition to organic bromine (CBr). Both profiles are found to be C-shaped with local maxima in the upper free troposphere (FT). The median tropospheric BrO vertical column density (VCD) was measured as 1.6×1013 molec cm-2, compared to model predictions of 0.9×1013 molec cm-2 in GEOS-Chem (CBr but no SSA source), 0.4×1013 molec cm-2 in CAM-Chem (CBr and SSA), and 2.1×1013 molec cm-2 in GEOS-Chem (CBr and SSA). Neither global model fully captures the C-shape of the Br profile. A local Br maximum of 3.6 ppt (2.9-4.4 ppt; 95 % confidence interval, CI) is inferred between 9.5 and 13.5 km in air masses influenced by recent convective outflow. Unlike BrO, which increases from the convective tropical tropopause layer (TTL) to the aged TTL, gas-phase Br decreases from the convective TTL to the aged TTL. Analysis of gas-phase Br against multiple tracers (CFC-11, H2O-O3 ratio, and potential temperature) reveals a Br minimum of 2.7 ppt (2.3-3.1 ppt; 95 % CI) in the aged TTL, which agrees closely with a stratospheric injection of 2.6 ± 0.6 ppt of inorganic Br (estimated from CFC-11 correlations), and is remarkably insensitive to assumptions about heterogeneous chemistry. Br increases to 6.3 ppt (5.6-7.0 ppt; 95 % CI) in the stratospheric >middleworld> and 6.9 ppt (6.5-7.3 ppt; 95 % CI) in the stratospheric >overworld>. The local Br minimum in the aged TTL is qualitatively (but not quantitatively) captured by CAM-Chem, and suggests a more complex partitioning of gas-phase and aerosol Br species than previously recognized. Our data provide corroborating evidence that inorganic bromine sources (e.g., SSA-derived gas-phase Br) are needed to explain the gas-phase Br budget in the upper free troposphere and TTL. They are also consistent with observations of significant bromide in Upper Troposphere-Lower Stratosphere aerosols. The total Br budget in the TTL is currently not closed, because of the lack of concurrent quantitative measurements of gas-phase Br species (i.e., BrO, HOBr, HBr, etc.) and aerosol bromide. Such simultaneous measurements are needed to (1) quantify SSA-derived Br in the upper FT, (2) test Br partitioning, and possibly explain the gas-phase Br minimum in the aged TTL, (3) constrain heterogeneous reaction rates of bromine, and (4) account for all of the sources of Br to the lower stratosphere.Peer Reviewe