It depends how much moisture is in the air but I don't think normal room temp air will hurt its purity too much. That would come down to what temperature it is and if it's humid enough to affect. The transformation of sodium nitrite to sodium nitrate typically occurs at elevated temperatures, usually above 320 degrees Celsius (608 degrees Fahrenheit). This process involves oxidation, converting sodium nitrite (NaNO₂) to sodium nitrate (NaNO₃). See this is the issue I've always had when it comes to the info that has changed so much about SN oxidizing into Nitrate on the forum, what specific conditions does SN oxidize into Nitrate and based on those conditions, how fast or how slow it'll be.
Chemical Stability: Stable at room temperature in closed containers under normal storage and handling conditions. Unstable if heated, may explode at temperatures greater than 533°C.
Conditions to Avoid: Ignition sources, dust generation, exposure to air, exposure to moist air or water, temperatures above 320°C.
Incompatibilities with Other Materials: Reducing agents, acids, amines, chlorates, permanganates, cyanides (e.g. potassium cyanide, sodium cyanide), metals as powders (e.g. hafnium, raney nickel), hypophosphites, sulfites, tannic acid, organic matter, antipyrine, ammonium salts, acetanilide, iodides, mercury salts,
moisture, air, activated carbon, vegetable astringents.