In situ measurements of HNO3, NOy, NO, and O3 in the lower stratosphere and upper troposphere
Introduction
Nitric acid (HNO3), which is a principal reservoir for the reactive nitrogen oxides in the upper troposphere (UT) and lower stratosphere (LS), is important in the photochemistry of these regions of the atmosphere. HNO3 is photochemically linked to NOx (NOx=NO+NO2), which is directly involved in processes that control ozone (O3) abundance. The production of O3 in the UT is partially controlled by NOx, and in the LS, NOx catalyzes O3 destruction.
In situ measurements of HNO3, NOy, NO, and O3 are necessary to characterize and understand the photochemistry of the UT and LS. The accuracy of atmospheric models has been assessed by comparing observations with model simulations of the partitioning of the reactive nitrogen reservoir (NOy=NO+NO2+HNO3+PAN+⋯) (e.g. Gao et al., 1999). In situ HNO3 measurements are essential for directly determining NOy partitioning. While NOy has been measured in many regions of the atmosphere (Murphy et al., 1993), there are few in situ measurements of HNO3 in the UT and LS (e.g. Talbot et al., 1999). Correlations between O3 and NOy or HNO3 have been used to identify the sources of NOy in the UT and LS (Murphy et al., 1993), characterize stratospheric and tropospheric air masses (Schneider et al., 1999; Talbot et al., 1997), and examine photochemical O3 production in the troposphere (Ridley et al (1994), Ridley et al (1998), Trainer et al., 1993). Recent work has focused on reconciling differences between measured and modeled HNO3/NOx ratios (Jaegle et al., 1998; Hauglustaine et al., 1996) to explain the origins of NOx in the UT. Understanding the origins and fate of NOx in the UT is important for determining anthropogenic contributions to O3 production (Jaegle et al., 1998). For example, the effects of NOx emissions from aircraft upon O3 production in the UT have been summarized recently (IPCC, 1999).
The measurements, reported here, were obtained over a wide altitude range (7−18 km) using a suite of fast time-response instruments. They provide a unique data set to examine NOy partitioning and correlations between the measured species for the purpose of understanding photochemical processes in the UT, LS, and near-tropopause region.
Section snippets
Instrumentation and sampling
HNO3, NOy, NO, and O3 were measured in the UT and LS on flights originating from Ellington Field, Texas (30°N) aboard the NASA WB-57 aircraft during the NASA Atmospheric Chemistry and Combustion Emissions Near the Tropopause (ACCENT) mission in September 1999. Measurements of NO and NOy were made with a chemiluminescence detector (Ridley et al., 1994), O3 was measured using a dual-beam ultraviolet absorption photometer (Proffitt et al., 1989), and HNO3 was measured with a new chemical
NOy partitioning
The partitioning of the reactive nitrogen reservoir, NOy, is examined using measurements of HNO3, NOy, and NO, and modeled NO2. In the LS observed here, HNO3 accounts for the majority of NOy. Above the tropopause in the LS, O3 values are greater than 110 ppbv. For O3 mixing ratios between 400 and 1000 ppbv, HNO3 comprises approximately 75% of NOy, as shown in Fig. 2a. In the UT, NOx is generally the most abundant NOy species. Here, HNO3 accounts for less than half of NOy. While the NOy budget can
Correlations between measured species
The range of NOy partitioning observed in the UT and LS can be further understood by examining correlations between long-lived species.
Discussion
These new measurements of HNO3, NOy, NO, and O3 in the LS and UT, made at midlatitudes during the fall, will be useful for constraining photochemical processes in the UT, including the importance of acetone and the effects of aviation emissions. The signature of the upper tropospheric O3 production associated with NOx-hydrocarbon photochemistry that also produces PAN is evident in this air mass where there is a positive correlation between (NOy−NOx−HNO3) and O3. Additionally, the results
Conclusions
NOy partitioning and correlations between HNO3, NOy, N2O, and O3 are examined for measurements obtained in the UT and LS at midlatitudes in the Fall. Both NOy and HNO3 are strongly correlated with O3 in the UT and LS. In the LS, HNO3 is the majority of NOy, and HNO3+NOx accounts for 80–100% of NOy. In the UT, HNO3 is less than half of NOy, and HNO3+NOx accounts for 40–100% of NOy. In the LS, NOy and HNO3 are anticorrelated with N2O. The correlations of HNO3 and NOy with O3 and N2O are compared
Acknowledgments
We thank the NASA JSC WB-57 pilots and ground crew. Partial support was provided by the NASA Atmospheric Effects of Aviation Project. JAN thanks H. Selkirk for helpful discussions. BAR thanks the NOAA Climate and Global Change Program for partial support of instrument fabrication. NCAR is supported by the National Science Foundation.
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