I'm conducting an experiment in an open system, so the temperature and pressure is equal to that of the atmosphere at that time. However, it is not equal to STP conditions of $273 \; \mathrm{K}$ and $1 \; \mathrm{bar}$.
How do I state my conditions concisely without it being ambiguous with STP? Is simply 'under atmospheric conditions' clear enough?
If you really mean "the atmospheric conditions at the time of the experiment" you should measure them and record them. Otherwise, the results are not reproducible - unless of course the atmospheric conditions are irrelevant, in which case you don't need to mention them at all!
In particular, even if you are working in a lab which isolates you from weather-related temperature changes, your results will depend on the altitude of your location, which has a significant effect on air pressure.
It would be better to correct the conditions "at the time" to some standard values. You could use "Normal temperature and pressure" (NTP) as defined by NIST in the USA to be 20C and 760mm Hg.
Alternatively you could use the International Standard Atmosphere (ISA) conditions for your altitude. At sea level that corresponds to 15C and 760mm Hg. The temperature and pressure both change for different altitudes.
Other options could be the IUPAC's SATP (Standard Atmospheric Temperature and Pressure) which is 25C and 100kPa pressure (750.06mm Hg) or the EPA (USA Environment Protection Agency) standard of 25C and 760mm Hg.
The old joke about a different industry comes to mind: "Standards are very important in computing - that's why there are so many different ones" ;)
