Airborne observations related to ozone depletion at polar sunrise

Airborne observations were conducted in the high Arctic (69°N–83°N) during April 6–16, 1992, in support of the Polar Sunrise Experiment. Measurements of temperature, O3, NOx aerosol particles, inorganic aerosol species, inorganic bromide, alkyl nitrate species, and several organohalogens (including bromoform) were made from an altitude of 30 m above ground level (agl) up to 7000 m msl (mean sea level). The average temperature profile shows a strong surface-based inversion up to about 500 m, an isothermal region up to about 1.5 km, and steadily decreasing from 1.5 to 6.8 km. Ozone mixing ratios were frequently found to be depleted from the surface up to various altitudes within the boundary layer (maximum altitude for ozone <10 parts per billion by volume (ppbv) was 375 m). The average ozone profile increases from <10 ppbv near the surface up to about 40 ppbv at 1 km, remaining approximately constant up to 5 km, and increasing with altitude thereafter as the stratospheric source becomes evident. NOx, including a possible peroxyacetyl nitrate (PAN) interference, was typically <50±20 parts per trillion by volume (pptv), and frequently below detection limit (20 pptv). Accumulation-mode aerosol particle number concentrations in the boundary layer were 100–200 cm−3, and although CN increased low over a polynya, there were indications of an absence of nucleation-mode particles in ozone depleted air in the boundary layer compared with the free troposphere. Inorganic gaseous bromide, bromoform (CHBr3) and dibromochloromethane (CHClBr2) all exhibited strong anticorrelations with O3. Gaseous nitrate (HNO3 plus possibly some contribution from PAN interference) ranged up to 110 pptv but was <40 pptv in 11 of 14 samples. With the exception of 1-propyl nitrate the C3–C6 alkyl nitrates correlated positively with ozone, as did the isomer ratio C3/C6. Organohalogens were measured using charcoal cartridges {C} and Tenax cartridges {T}. CHBr3 was similar by both techniques (medians of 1.83{C} and 1.60{T} pptv), and negative correlations with O3 were indicated by both sets of samples (R2 = 0.75{C} and 0.71{T}). CHClBr2 was also very close in both sets of samples (median of 0.25{C} and 0.22{T} pptv), however, a negative correlation with O3 was present only in the Tenax samples (R2 = 0.63). Ln(CHClBr2/CHBr3) correlated negatively with In (CHBr3) with a coefficient of determination of 0.75, and with higher In(CHBr3) approached the value indicated by Li et al. (this issue) for air immediately above seawater at 0°C (i.e., 0.032). CHClBr2/CHBr3 was higher in the free troposphere than in the boundary layer and possibly less variant with In(CHBr3), indicating either different source regions for these free troposphere organohalogens and/or, as Li et al. suggest, faster chemical destruction of CHBr3 relative to CHClBr2 in the free troposphere. The airborne organohalogen data and that from ice camp SWAN (Hopper et al., this issue) and Alert (Yokouchi et al., this issue) were combined to produce a vertical profile of CHBr3. CHBr3 exhibited a well-defined logarithmic decrease with increasing altitude, indicating a strong surface source, opposite to the average O3 profile. In general, the low-level airborne observations related to O3 depletion are very consistent with the observations at Alert, indicating that the many features of this phenomenon are ubiquitous.