Total-column nitrogen dioxide (NO2) data collected by a ground-based sun-tracking spectrometer system (Pandora) and an photolytic-converter-based in-situ instrument collocated at NASA’s Langley Research Center in Hampton, Virginia were analyzed to study the relationship between total-column and surface NO2 measurements. The measurements span more than a year and cover all seasons. Surface mixing ratios are estimated via application of a planetary boundary-layer (PBL) height correction factor. This PBL correction factor effectively corrects for boundary-layer variability throughout the day, and accounts for up to ≈75 % of the variability between the NO2 data sets. Previous studies have made monthly and seasonal comparisons of column/surface data, which has shown generally good agreement over these long average times. In the current analysis comparisons of column densities averaged over 90 s and 1 h are made. Applicability of this technique to sulfur dioxide (SO2) is briefly explored. The SO2 correlation is improved by excluding conditions where surface levels are considered background. The analysis is extended to data from the July 2011 DISCOVER-AQ mission over the greater Baltimore, MD area to examine the method’s performance in more-polluted urban conditions where NO2 concentrations are typically much higher.

Abstract. Several previous studies highlight pressure (or equivalently, pressure altitude) discrepancies between the radiosonde pressure sensor and that derived from a GPS flown with the radiosonde. The offsets vary during the ascent both in absolute and percent pressure differences. To investigate this problem further, a total of 731 radiosonde/ozonesonde launches from the Southern Hemisphere subtropics to northern mid-latitudes are considered, with launches between 2005 and 2013 from both longer term and campaign-based intensive stations. Five series of radiosondes from two manufacturers (International Met Systems: iMet, iMet-P, iMet-S, and Vaisala: RS80-15N and RS92-SGP) are analyzed to determine the magnitude of the pressure offset. Additionally, electrochemical concentration cell (ECC) ozonesondes from three manufacturers (Science Pump Corporation; SPC and ENSCI/Droplet Measurement Technologies; DMT) are analyzed to quantify the effects these offsets have on the calculation of ECC ozone (O 3 ) mixing ratio profiles (O 3MR ) from the ozonesonde-measured partial pressure. Approximately half of all offsets are > ±0.6 hPa in the free troposphere, with nearly a third > ±1.0 hPa at 26 km, where the 1.0 hPa error represents ∼ 5 % of the total atmospheric pressure. Pressure offsets have negligible effects on O 3MR below 20 km (96 % of launches lie within ±5 % O 3MR error at 20 km). Ozone mixing ratio errors above 10 hPa (∼ 30 km), can approach greater than ±10 % (> 25 % of launches that reach 30 km exceed this threshold). These errors cause disagreement between the integrated ozonesonde-only column O 3 from the GPS and radiosonde pressure profile by an average of +6.5 DU. Comparisons of total column O 3 between the GPS and radiosonde pressure profiles yield average differences of +1.1 DU when the O 3 is integrated to burst with addition of the McPeters and Labow (2012) above-burst O 3 column climatology. Total column differences are reduced to an average of −0.5 DU when the O 3 profile is integrated to 10 hPa with subsequent addition of the O 3 climatology above 10 hPa. The RS92 radiosondes are superior in performance compared to other radiosondes, with average 26 km errors of −0.12 hPa or +0.61 % O 3MR error. iMet-P radiosondes had average 26 km errors of −1.95 hPa or +8.75 % O 3MR error. Based on our analysis, we suggest that ozonesondes always be coupled with a GPS-enabled radiosonde and that pressure-dependent variables, such as O 3MR , be recalculated/reprocessed using the GPS-measured altitude, especially when 26 km pressure offsets exceed ±1.0 hPa/±5 %.

Key Points (shortened to less than 140 characters each, and changed as suggested by Reviewer #2): • In spring and summer 2020, stations in the northern extratropics report on average 7% (4 nmol/mol) less tropospheric ozone than normal. • Such low tropospheric ozone, over several months, and at so many sites, has not been observed in any previous year since at least 2000. • Most of the reduction in tropospheric ozone in 2020 is likely due to emissions reductions related to the COVID-19 pandemic.